The Covid-19 Steampunk Hat Thread

[J. Wilhelm]

I believe you are on the right track....

SNIP

You MAY even find them very cheap at local thrift stores, or thru "used medical supply" houses.

so if you fill with air at ~ 2000 psi I guarentee you will get more than 30 minutes of airflow

you will still need a valve and or regulator, and a high pressure pump,
AND you need to make sure the air from the pump (any pump) is "oil-free"
but common cannula hoses work from there to your needle.

hope this helps
prf marvel

I'm cracking up while watching this video. But here's a practical solution for the specific problem I have of reducing pressure.

In this 1920s inspired video  , Aneer explains the nature of a two stage pressure regulator. This is exactly what I need to "decant gas" from O[2 kpsi] to O[0.1 kpsi] 

Working Principle - Two Stage Pressure Regulator

These things are really expensive, typically on the order of $500. I found a beer keg CO₂ two stage regulator (daisy chained) for $136.49.  It reduces from 3000 to 60 psi (actually stops at 55 psi before the release valve is activated). Let's say I can safely get 50 psi out of it, that is 344 kPa, or the back pressure ratio is 29%,which is below the maximum of 53 when choked flow is no longer possible (assuming the pressure of the basketball pin outlet matches ambient pressure - I don't even know if that is true, but I can check.

https://www.webstaurantstore.com/micro-matic-642-battery-premium-series-double-gauge-dual-primary-co2-low-pressure-regulator-battery-with-2-shut-offs/379642BATTRY.html

Since the mass flow rate is linearly proportional to the stagnation pressure, and I defined a 10-fold human respiration rate at P_o=78 psi, then it follows that you'd get a ratio of V/V_min=10(50/78)=6.4 times the minimum respiration rate of a human being. Just enough to allow a person to run while wearing the mask... Assuming I can sustain choked flow. This is nearly ideal for what I need to do...

Maybe I can pull this off, if I can get a cheap enough tank and two stage regulator for the right price. I worry about the cost and weight of the system, though.. I'm not in the best of financial circumstances,especially after a self audit I made on my expenses. Came to the conclusion I'm barely making enough money to pay for all of my bills, minus food. So I'm literally eating my savings right now. *sigh*

I'll be exploring the "compress as you go" paradigm as well. It might turn out to cheaper.

Either way, I'll Charleston my way into making one of these systems...

[Prof Marvel]

Greetings J

quick note, I am literally running to try to compress some sleep, then go get tires, car battery, oil, food, Rx
blahg bala, chase Quigong Healing DVD's and then get winter preperations started.... oooh more concrete... and windows,
and flushing water heaters...

look into the used Medical Oxygen tank regulators. I got a couple at thrift stores for ~ $5 each.
they fit on all standard O2 tanks and are designed to regulate oxy flow from ~ 2k psi at in the tank down the the "trickle" needed at the canula or mask.
at full blast I can hear them hiss and the oxy flow feels cold, but not too high a pressure like from my 125psi  nailgun compressor.

hope this helps
prf mumbles

[J. Wilhelm]

Given that compression of tanks in the order of 3 kpsi will be difficult anyway (need a special compressor.), I'm concurrently looking at other options.

A liquid air generation device is not out of the question, for the simple fact that air becomes very compact at a density of about 0.8kg /m³. The main issue with liquid air is insulation, and also the fact that the nitrogen content of air will boil off before oxygen starts to boil, so it'd be very difficult to insure that you can get a stream of gas that is not all nitrogen or all oxygen. It seems freezing air is good for separating the gases..

Making Liquid Nitrogen From Scratch!

The other option is to go back to the idea of compression on a continuous basis. Most compressors, however are noisy, vibrate a lot and don't last very long due to the heat and wear of their components. So I've been looking at the principle of compression in rotary devices, to see if I can find a viable quiet compression device. Turbine style compressors are really not that efficient,compared to piston type compressors. But there is one device that we know of that has the same efficiency as a piston: the Wankel engine.

There's are a couple of technical papers on the use of Wankel engines as compression devices, and I'm going to be looking at miniature wankel models as a possible compression device.

https://iopscience.iop.org/article/10.1088/1757-899X/232/1/012065

https://www.google.com/url?q=https://docs.lib.purdue.edu/cgi/viewcontent.cgi%3Farticle%3D3303%26context%3Dicec&sa=U&ved=2ahUKEwjD3vrwlPTrAhWFVc0KHbxeDDwQFjACegQIBBAB&usg=AOvVaw31rT9R-Gc_71mzfYkj67Mn

[J. Wilhelm]

It looks like I'm out of luck with rotary compressors. The Wankel type compressor is at the moment just a concept on paper. It's not available commercially anywhere as far as I can tell, even though it's quite viable. The current Wankel compresor concept is being designed for aerospace applications such as compressors aboard satellites. A quick look at what is available reveals a variety for the reverse, that is, compressed air motors. So at first, I thought I could get a small air motor and run it in reverse:

Rotary Di Pietro compressed air motor used for automotive applications
https://www.engineair.com.au/


A more conventional compressed air motor for rotary tools and the like

Unfortunately the motors are not reversible, at least not efficiently. Thermodynamics' Second law prevents the reverse operation without gross inefficiencies. Besides, the large Di Pietro motor, as it's much smaller air tool cousins would both contaminate the air supply with oil.

So I continued searching, and found that a company in Scotland invented a "screw type compressor" that is very efficient and is rated up to 10 bar (1000 kpsi). Their design was also developed for the aerospace industry in satellite applications.

Vert Rotor's 10 bar rotary compressor
http://vertrotors.com

Looks like an ideal solution with no oil lubrication, until you realize that the Vert Rotor compressor requires water injection to work. Sadly, the result is a drink cooler size box around the compressor.

So it looks like I'm stuck with piston type compressors, or noisy diaphragm-bellows style compressors like my hand held unit, which is good at most for 15 minutes before it needs to shut down due to heat. If I want a quiet portable compressor, I might actually need a foot operated compressor! It sounds ridiculous, but I'm thinking of having a foot operated compressor attached to my shoe, so I can supply air while I walk!   It would definitely be quiet... Though you would get tired quickly.  

What was that Mr. Bailey said about bellows?  

A more realistic solution is a worm drive or rack and pinion driven by an electric motor to run a bicycle frame inflator. For about $15 at the wall in the mart it is rated to 120 psi and includes its own pressures gauge:

[J. Wilhelm]

So I'm going to step back a bit from fancy math and do some practical experimentation here. I bought the bicycle frame pump above and ran some simple calculations. The basic air mass flow rate for a human being is 6 L/min in a state of rest and 36 as a maximum in an agitated state. Whatever the speed or pressure of the gas, in the end you have to supply 6 Liters of expanded (1 atmosphere) air. How does that feel on your face? How hard is it to push the pump ? Do you need an air tank at all? Are there simple tanks you can make/find for 100 psi or less?  Is that possible with a tiny pump like that?

I attempt to answer those questions today.

The frame pump is entirely made from plastic. It is made from a two-cylinder telescopic tube with two settings: low volume - high pressure, and high volume - low pressure. Air comes in through a small opening near the pressure gauge. In the high volume setting, by blocking the outlet of the pump with my thumb, it's actually hard to get much above 30 psi, because the area of the piston inside at that pressure provides a large reaction force. But using the low volume setting (locking the outer cylinder and using only the inner cylinder), I can easily get up to 50 psi momentarily (the piston leaks a little so pressure goes down immediately after the stroke - it's the job of the tyre valve to keep the air in the tyre, so it doesn't matter if the pump's piston leaks a little).

Most importantly, the piston is almost 100% quiet and the operation very smooth with no vibration. It gives me confidence that getting to 60 psi would probably be trivial. Getting to 80 psi looks entirely doable with a simple pulley and electric motor. The pump is rated 120 psi - and it's just a plastic and rubber piston!! The outer cylinder wall is transparent Lexan/Acrylic or similar and no more that 1.2mm thick!

It seems to me that a simple pulley-piston setup would be able to pull it off with a flywheel frequency of a few cycles per second. Let us assume that air enters the cylinder of the pump at ambient pressure and temperature. The travel of the inner cylinder is just 11.5 cm. The inner diameter of the inner cylinder is about 1.7 cm (estimated). This translates to a volume of ~2.61 X10^-5 m³ of air at 1 atmosphere. Since we have 1000 liters in one cubic meter, then that corresponds to 0.0261 L/stroke. If we divide the minimum flow rate of 6 L/min by the volume of 0.0261 liters per stroke we get the number of strokes per minute to get 6 liters which is ~230 strokes/min, and if we divide again by 60, we get the number of strokes or cycles per second, which is 3.83 strokes per second to pump the 6 liters of air. This does not take into account pressure or temperature at all. The needle in the pressure gauge hardly moves when I pump air through the needle at this rate. These numbers are just for getting the required air supply of expanded air.

Given the physics of the problem, that seems to me to be an entirely achievable goal. Getting a rate 6 times that for the maximum minute respiration rate (36 L / min) seems to me will probably be a challenge, but at least I know I can get the minimum respiration rate with this pump. May no be good for jogging, but if you're riding the bus, shopping in the supermarket, or waiting in line it seems doable. As an experiment I tried pumping 4 cycles per second with my hand - it's not difficult, but your arm will get tired in about 20 seconds, and let me tell you, it's quite a stream of air on your face coming from that needle! The stream of air does come out noticeably cooler, but not much below ambient.

Again I'm not paying attention to cooling at all or pressure at the moment, its all near 1 atmosphere at ambient temperature, because I realize that we are in Fall, and Winter is upon us. It may not matter if by November I don't have a cooling system ready. But my idea is to pursue the development of the system WITH the Bell-Coleman refrigeration cycle, using with a flywheel and piston - its the quietest most efficient setup I can fathom, because other practical compressors are too noisy and rotary compressors are expensive/difficult/non existent yet. All in all it will be a very Victorian setup   I would prefer to get my hands on a screw type compressor, but the water requirement is ridiculous. With more time, I may try to build a Wankel compressor if I can get my hands on a working small-scale die cast model.

I'm actually very disappointed that the engineering community has not given us a quiet, cheap, practical compressor for something as simple as 100 psi. It just sounds ridiculous to me in the 21st century. To give you a feel for the kind of pressures we're talking about, a 2 liter soda bottle packs about 40 psi of pressure at room temperature! That's more than a car tyre which is between 25 and 35 psi !! Actually most sources I researched agree on the 40 psi maximum for a plastic soda bottle when filled with soda, and about 100-150 psi for a maximum failure pressure.

Air Compressor vs 2 Liter Pop Bottle


I don't even need to buy a tyre or steel pressure tank! I can just make a pressure vessel out of a soda bottle, or make my own using PVC pipes. But I would much prefer to use PVC pipes as I can adjust the size to what I want, they're much thicker, less prone to puncturing, and one can use tyre valves like these, which I can find at my local Wall of Mart:

So this knowledge save me a lot of complications by allowing me to build stagnation tanks with simple PVC pipes and off the shelf fittings. I would use two valves, one for inflating the tank and another for bleeding the tank.

Do you even need the tank? Well, that depends on how hard the pump needs to work to keep the pressure up to 80 psi. I can tell you it will have to work a lot harder than 4 cycles per second. And for 80 psi the material will probably need to be good enough to deal with the heat generated during compression. The mass flow rate I calculated for a stagnation tank at 80 psi was 10 times that mass flow rate, and that's not paying attention to how much you need in excess to start building up pressure. At 4 cycles per second you will get the basic mass flow rate for respiration, but no cooling at all because the pressure will not increase much throughout the system - speed is close to zero. Likewise with little compression the tank will not heat up by much. If this is Winter and you don't care about choking the needle and expanding a stream of air at Mach 1 or above air to cool, one way to go is to just build a small tank with the sole purpose of stabilizing the air flow coming from the pump, to avoid getting a single puff of air for every cycle in the flywheel/stroke of the cylinder.

I think I will build a small PVC pressure tank and will engineer a pulley system with a DC motor and see what happens... I still need to estimate the electrical consumption and size of batteries, but I imagine one can buy large power tool batteries.

[J. Wilhelm]

Just thinking aloud. I've discovered that there was in fact one Wankel type compressor called the "Ogura Wankel Compressor" patented in Japan in the 1980s, and patented by Sumio Oota in 1981. The compressor was built in Japan as an automotive air conditioning compressor for Land Rover.

https://patents.google.com/patent/JPS5877191A/en

The Wankel compressor is still about the size of a piston type automotive compressor though. So too big and heavy.

Even more interesting, Wankel superchargers have been made, and even seat belt tensioner motors with explosive charges (!) have been implemented by Volkswagen and Mercedes Benz!

On the other hand, the smallest Wankel engine I have found is for model aircraft. There are two manufacturers at least making model aircraft engines. Unfortunately they're very expensive  ($300-$750 used! ).

I'm even thinking of designing and building my own compressor. Based not on the Wankel rotor, but a close relative, the Reuleaux Triangle or a different more generalized "Curve of Constant Width. I may use a rubber belt and avoid the 8-shape chamber of the Wankel engine

The compression chamber is almost a perfect square, but the rotation of the center of the triangle is not circular. It may not matter if I use a rubber belt with teeth. There's a valved inlet and a direct outlet at each corner of the square. Who knows? If I manage to design one and make it work, I may patent it. Why not?

The Reuleaux and Wankel Triangles have a few issues, most notably seals at the vertices that wear out. I could use a different shape... You can design a generalized shape with curved vertices that still has a constant width.

[J. Wilhelm]

I've been quiet about this project for some time, because I was trying to figure out exactly what kind of idea my brain had generated, and whether it was useful beyond this project.

I think I'm 90% convinced I can make a rotary pump out of common materials, like plastic or steel plate. I have come up with a preliminary design for a rotor based on a modified Reuleaux triangle within the square compression chamber, which I think would be simpler than the Wankel rotor design. I'm even fairly certain I can make some kind of pump for low pressures, equivalent to the bicycle pump frame in the immediate future this November, and out of craft wood board, even.

Given my current lack of resources, I think I'll build a wooden prototype, with the idea of providing low pressure air at 6 L/min. Simultaneously, I'll see if I can design a plastic or metal equivalent for 60 psi. Perhaps by 3D printing through some online service. Given the nature of the design, and the evidence from the bicycle pump materials for the pressure range below 120 psi, I think this project is quite doable, it's just that I was stuck with the pump problem. The trick was finding an adequate "rotary piston," but the concept is already well proven.

Going the way of scuba equipment did not appeal to me due to the cost of filling relatively large tanks thati can't see anyone carrying around. You need a special high pressure pump. Not something likely to find at home depot or be able to sell to anyone. You need a high pressure regulator, a bulky air tank to carry around. You're not allowed to liquefy the air because it's components will boil off at different temperatures... So the only option is to pump filtered air continuously, and then I realized how poor low pressure pumps can be. I think a rotary pump solves all of the issues surrounding air pumps. Once I solve the problem, the project will take off.

I'm debating whether to post the sketch I have on paper, because I've discussed the pump design with my mother who works in patent law (she's not a patent lawyer herself, but works for firms involved in patent law litigation). I'll see if I can be less cryptic and post something once I've built the pump prototype in wood.

I think I have something good in my hands. My instincts tell me the process is fairly simple,the forces involved can be handled by a variety of common materials, and the math is putting all relevant fluid mechanics quantities within an order of magnitude to achieve all I need. Low pressure can give you a minimum ventilation, and as you go past a certain pressure, the cooling effect is generated, peaking once the needle is choked. Which should be somewhere less than 60 psi.

The design is not singularly unique, so there's a good chance it already exists, and more likely there's something equivalent, like the Ogura/Oota Wankel compressor, so I'll have to research those patents. But it'd be nice to findoout that by serendipity designs like these have not been exploited yet for everyday applications...

[J. Wilhelm]

We got hit with fairly cool weather yesterday, and that just injected a sense of urgency into the project. The place where I work has no insulation, and thus I'll likely be working in 10 C or below conditions indoors. Because of COVID-19 I have to keep the location ventilated, which means temperatures will likely drop to near zero Celsius indoors. Or I can keep doors closed and risk having an unacceptable viral load in the air. The respirator mask project will be brought to completion regardless of cost as long as I can afford it.

A bit more progress on the pump design and build. Design wise, I chose to make the body of the pump out of plastic and wood with a plastic rotor. The reason is that humidity could make the compression chamber and rotor swell of they're made of wood. I've obtained an 11mm thick plastic cutting board and cut out a rotor for the new pump. It's not a phenomenally accurate cut, but the rotor looks the part and it will rotate in a chamber lined with a flexible sealing material.

For the input and output valves, I've managed to cobble together a design from brass and steel parts. Mostly as a demonstrator. At the very low pressures I intend to run this pump, I doubt that I need such strong valves but they are very compact (less than 1 inch in diameter) and should work at near zero pressures, just as well as 100 psi or likely 10 times higher, even. This gives me hope the compressor can be designed for much higher pressures that the ones I've been discussing.

I wish I could share more about the design, like photos and descriptions, but I really think I've got something here whose usefulness will outlive the COVID pandemic. If this works, I'm definitely filing for a patent, and I'll take it from there. The number of uses for this pump could be very large, and I'm convinced the design could be made to work for very high pressures, just by changing the materials and scaling the mechanisms to the desired volume.

At the moment:

1. The compressor will be little more than a fan, temporarilly. There are some mechanical sticking points, so far as transfer of power is concerned. I should add that my design is not a Roeleaux Triangle, but a variation on it, and that makes its properties not well defined or catalogued. I could have defaulted to a Wankel rotor (which is not a Roeleaux triangle either), and which is better documented, but the design is less efficient as far as a compressor design is concerned. In my design you can even configure the compressor as a two-stage compressor. The advantage of the Wankel design is that mechanical transfer of power to the rotor is easy and mathematically simple. Most rotary pumps I've seen (papers quoted above) default to the Wankel design.

2. I have two different ways to transfer power to the rotor, which are well founded mechanically and mathematically, and a third method that would look closer to that used for the Wankel rotor, but is mathematically difficult to resolve. The problem being that all curves of constant width used as rotors inside regular polygons or other geometric shapes (including Roeleaux Polygons) are very strange beasts, with very interesting properties, and their kinetics are not even fully understood by mathematicians as far as developing exact equations for motion anywhere inside the perimeter of the curve (I'm not joking! There are unresolved mathematical theorems about these odd pieces of horrible Non-Euclidean geometry). The approach is to use mathematical approximations and space mapping, like engineers do, which I'm sure Wankel did when developing his design.

3. I don't have time to develop a mathematical theorem, so I'll use a simpler, if clunkier method involving a universal joint to run the rotor with a battery operated screwdriver. I can also use an electric drill if I need more power and speed. As the immediate goals are passed (making a respirator capable of giving you 6L/min of uncooled air, I will investigate increasing the speed of the rotor (new materials may be needed), to add a stagnation chamber (tank) and increase pressure above 30 psi, then start I'll start investigating the Bell Compton effect..

Stay tuned

 

Cheers,
J. Wilhelm.

[J. Wilhelm]

I've started the build of the pump, and i realized that some leftovers from previous projects would come in handy.

1. Rather than buy rubber for gaskets, I realized that I could use leftover leather from my Luftschiffengel uniform spats/gaiters. The leather is needed because the gaskets need to be flexible and yet slippery at the same time, and I was having a hard time figuring out what material to use. Neoprene foam is too "grabby" and rubber parts would need oil as lubrication. As it turns out, the smooth side of the leather fits the bill without the need of lubrication.

2. The body of the pump will re-use the pine wood rounds that were a leftover from the Victorian Boombox Mk. III project. I'm chuffed to see that the diameter of the wooden rounds is perfect for the pump. I didn't even need to cut wood! The best part is that the pump housing will look the same as the mask.

3. The pump is built like a "sandwich" in layers, and the thickness of the pine craft wood is perfect for the pump layers and the rotor housing. I will use the same type of  pocket screws I used for the Boombox project. Between that and the stain an varnish it should look quite steampunk

4. I also realized that I could use transparent vinyl tubing for both parts of the pump and also conduits for air to be pumped to the needle. The inner diameter of the vinyl tubing allows me to build the valves and just insert the needle into the tubing, so there is no need for threaded brass pipe adapters, etc. Perhaps I just need some miniature pipe clamps to hold the pipes in place. Because of this, the construction of the (low pressure) pump will be very swift and have a very low count of parts. I'm sure there will be issues cropping up (leakage), but at least at low pressures, the system will demonstrate its viability.  

5. The vinyl tubing is rated at 55 psi. Not high enough that I would feel confident to take it all the way up to choking the needle (because the minimum pressure would probably be in that range), but it should not matter at low pressures without the Bell-Compton refrigeration cycle kicking in. Theoretically if I beef up the housing and I beef up my one way valves and then trade the transparent vinyl tubing for something else like fiber-reinforced vinyl (200 psi) with threaded brass joints, then I could use the same pump to take it all the way to 80 psi

A cool touch would be to use the stainless steel wrapped water pipe extensions. Now that would look very Steampunk!

Sorry for being so cryptic. When I have the pump assembled I may take a picture, as the patentable part of the
 mechanism will be hidden from view. The assembly will probably be stained and varnished as the facemask was.

[Prof Marvel]

Fascinating!

Keep on Keeping On!

watching with breathless anticipation!

prf mvl

[J. Wilhelm]

Fascinating!

Keep on Keeping On!

watching with breathless anticipation!

prf mvl

Thank you, Prof. Marvel.

I also have a second rotor design in the works, mainly because I have to resolve the transmission of torque problem, and also because it may be difficult or impossible to patent. The more research o do, the more I realize that many people have come up with similar designs for pumps. In the last couple of days I've been looking at a class of screw-type compressors called Moineau pumps, which were invented in France in the 1930s. These are used for incompressible fluids like water, oil, slurry, cement etc. And they have cross sectioned based on cycloid curves. Very interesting stuff. Some of the more innovative pumps advertised today are actually rehashed 1930s designs. So little originality, and all of those rotors are already patented, and the patents are still active.

[J. Wilhelm]

The first compressor pump/testbed has been built and is in the testing phase. There are a lot of variables that I had not considered, so more than a prototype, this is a testbed for different configurations. Like all rotary pumps, achieving good compression is very tricky. The pump alone does not make a compressor. You don't achieve compression until you have one way valves. Think of this as a piston. The piston is not a compressor until you have a one way valve preventing pressurized back-flow into the cylinder.

The first rotary compressor pump testbed has been built. Trials are under way

The pump is quiet, but needs torque to move the rotor and I'm trying to reduce friction. Performance at this point seems very low, but I attribute that to imperfect seals. The type of seals I use affect friction, which in turn affects energy spent and noise . It's a vicious circle. A panoply of materials is being tested for both the rotor and the cavity, in the hope of achieving an ideal friction and seal balance for the pressure range. As the design pressure increases, you will need stronger, more rigid materials and a much higher level of precision. Achieving that ideal balance is very tricky. The pump will need to be all-metal for very high pressures, but for pressures below 100 psi, plastic probably will be OK.

For practicality sake I'm not above replacing this pump with the bicycle pump if I need to build a system in a hurry. The bicycle pump has demonstrated it's ability to perform the task (6 L/min at 4 cycles /s), but the device will be big and wobbly, probably suitable only to be carried in a backpack at best, assuming the box is small enough. My goal is to have a device as small as possible. I could use a PC fan instead instead of a compressor for Winter, but that would require a much larger diameter tube than the 1/4 inch vinyl tubing I'm using. With a fan I'd need a relatively large diameter tube, like a vacuum cleaner tube, and the system would not be upward compatible with the Bell-Compton refrigeration cycle for the artificial climate in Summer.

The goal is to connect the compressor to the mask with 1/4 inch vinyl tubing

[J. Wilhelm]

*Sigh*

Fixing to settle for a much shorter simpler reciprocating piston compressor, just in an effort to have a functional respirator in January.

My problem, as I've written before is fitness of materials and tolerances. I'm still working on the rotary pump shown in the post above, and I wholeheartedly believe in it for high pressures, but I'm facing a number of issues the way I built this prototype, all of which is related to manufacturing tolerances and materials. It seems rotary variable volume pumps are exquisitely sensitive to tolerances. Gases are prone to leaking quite easily and seals in compression devices are critical. This may be something I need someone else to manufacture for me.

In the meantime the good old piston will work without issues, I hope.

 I've been mulling over the alternative to use more "Aerospace" or "Gas Physics" industry type solutions, more akin to my educational background. I can always go to a centrifugal compressor, which is well known, well documented, and used in industry. The only thing is that thermodynamically, you're taking one whole step down the efficiency ladder below pistons. For high pressures you need extremely fast rotation and large intakes of air, typically not found outside of gas turbine engines. For tiny applications you will pay for inefficiency in amp hours of battery per flux of air pumped.

The upside is that the torque required to operate the pump could be much lower and suitable for use with a high speed electric motor without any need for gears. It could be quit by default. Who knows? Maybe the efficiency turns out to be better, simply because you're dealing with lower forces, less friction. Tolerances are also not as critical for a slow device (we're not talking about a jet engine compressor here). But reaching higher pressures will be an interesting challenge if I go that route.

This tiny 5cm blower will not give you any level of compression, but it's a step in that direction. I'm wondering if it could muster blowing air through the narrow tubes (enough pressure). As advertised it moves about 100 L/min in unrestricted ambient pressure. I can see hooking this to a plastic bottle to serve as a stagnation chamber. As the air comes to rest it increases its pressure. I don't need much to give me 6 L/min, which is about 0.2 ft³/min. And then see what happens. About $7, it uses 0.14A at 5 volts.

Should be quiet enough. What do you think?

[Banfili]

Goodness me, I did miss a lot of interesting topics while in hospital!!

[J. Wilhelm]

Goodness me, I did miss a lot of interesting topics while in hospital!!

Mostly it's me dabbling and tinkering with a bit of maths here and there... But it looks like progress will resume this weekend.


I have taken receipt of a pair of centrifugal fans. two days ago. I immediately tested them out. These are two miniature  5 cm wide, 5.0V brushless computer fans. The 5V rating is critical because it allows me to use USB power bank batteries for smartphones, which I think would be an easy and excellent battery pack.

Fan Specs:

Dimensions: 50mm x 50mm x 15mm
Rated Voltage: DC 5V
Connector:USB
Bearing Type: Sleeve bearing
Rated Current: 0.12±10%Amp
Noise: 25.7±10%dBA
Air flow: 2.91CFM±10%
Weight: 40g/pcs

At 2.9 ft³/min this yields an airflow of about 82 L/min which seems very high, if I just take it at face value

I don't know if these give me 82 L /min, but I can tell you that the flow is several times higher than the maximum flow I get with the piston running at 4 cycles per second. So let's say 10 times, then it'd be about 60 L/min at least, and that certainly seems realistic. I put them to the test right away, because I need to know how much they can compress air. The problem being that if I want the system to be upgradeable to a high pressure air con system it needs to at least use the same kind of tubes that will hold the basketball needle, and that's without saying anything about whether it can build enough pressure to pass any air through the needle.

What you want is to build enough pressure in a stagnation chamber (pill bottle below) to overcome the drag of a fully developed flow in the vinyl tube. The larger the stagnation chamber the better to make sure the flow comes to rest and all kinetic energy in the flow is converted to pressure energy. As the flow velocity goes to zero, you maximize the pressure in the stagnation chamber (hence the name "stagnation chamber").

At low pressure and velocity say M < 0.3 the airflow behaves more like an incompressible liquid than a gas.
Hence we can compare to a developing flow of water in a pipe. Net viscous drag force is proportional to the length of tube

 I expected that these would be poor compressors compared to the piston, because that is a thermodynamic fact. Vane rotor compressors such as turbine compressors are one whole step lower in efficiency than pistons, which is why a piston internal combustion engine is always more efficient than a turbine jet engine in converting fuel to kinetic/mechanical energy. The reason turbine engines are better in aircraft is that compared to piston engines, turbine engines are very light relative to the power they unleash. To match the power of a modern jet engine you need an absolute gigantic monster of an engine block, which is prohibitively heavy, and thus kills any and all thermodynamic efficiency you had with the piston cycles.

Similarly, in this case I found that while the piston engine is every efficient, the way that the piston is made gives you a heavy cumbersome mechanism with issues such as high friction forces, heat, reciprocating oscillations, etc. My rotating compressor pump is basically nothing more than a rotating piston, and made from leather and wood and items like that I found it ridiculously difficult to bring down friction and achieve good compression (leaky seals), unless I make the pump out of some highly polished metal. In other words, you waste most of your energy just overcoming friction with a geared electric motor, making the piston's "high thermodynamic efficiency" a moot point ( the overall compressor's thermodynamic efficiency is paired down to nothing, in other words).

As expected the centrifugal fans are very quiet and relatively frictionless. As I wrote I 'd expect the flow rate from the nozzle of the fan to be over 60 L/min - it's hard to measure, I'd need a little anemometer). So I'm throwing math away for a second and just doing things by tinkering. I know the "compressor" is very poor. What I found is that performance drops dramatically as you close the diameter of the air outlet. At the diameter of the needle, barely any air comes out. Not even close to the amount of air the bicycle pump can push through the needle and more like the fish tank air pump, I'd say.

The centrifugal fan is by definition a compressor, as it has the right geometry (a spiraling horn opening in the direction of the air, alternatively called an "involute casing") but it simply can't push air hard enough to pump it through the needle. So I tried "measuring" the flow without the needle, and I found that the 6 mm internal diameter of the vinyl tube with just a short length is probably giving me more than 26-40 L/ min (I'm guessing here - very unscientific, but I now have a feel for what 60-80L/min feels on my face). Even at 26-40 L/min that is already 4 times to  6.7 times the minimum respiration rate. So if I'm lucky (high end) the setup shown above with one single fan is already giving me the maximum respiration rate of a human being, Enough to run around the block.

As a compressor, the device is poor, but the problem is made a bit worse by the fact that the vanes are inclined in the opposite direction when compared to an actual centrifugal compressor direction. Centrifugal pumps can gave vanes that are perpendicular, inclined toward the direction of the flow (shown above) or inclined in reverse to the direction of the flow. Paradoxically, it is the reverse orientation of the vanes that gives you the highest thermodynamic efficiency and highest compression potential. The forward facing vanes give you the highest output velocity and the lowest thermodynamic efficiency. So these little buggers were designed to maximize speed and not pressure and the manufacturer doesn't care about efficiency. But there's nothing I can do other than 3D printing my own rotors. This will have to do.

It looks like these little centrifugal blowers/fans are just the thing I need. They're quiet enough and I trust I can use a battery. I'm going to try using a smartphone power bank and see how long it lasts. The goal would be to at least get one hour out of it, for high risk situations such as riding the bus in close proximity to others.

If I really wanted to 3D print my own, then I'd have to use all these nice maths which I did study in college, but I'm not sure if it;s even worth my time to dive into this subject. It just seems that every step I take turns into an engineering project. Being an engineer doesn't help that way   Ignorance is bliss.

https://en.wikipedia.org/wiki/Centrifugal_compressor

The thought has crossed my mind that I could use a metal canister, such as a thermos bottle, as a stagnation chamber and add and LED UV light inside the stagnation chamber to kill as many viriii as I can. Impossible to tell how effective that would be, but it's an interesting thought. Another even more outlandish idea is to find a UV laser and force the airflow to pass through a "laser gate" zapping the killer SARS bugs along their way.

Assuming that real world horrors don't stop me, I'll be working on this during the weekend...

Cheers!

[J. Wilhelm]

Having a system that at least is capable of blowing the minute respiration rate, gives me cofidence to go ahead with the project. One of the requirements of the project is that it should be powered by a rechargeable battery with enough charge for at least on hour or better. The task is easily accomplished with a power bank for smartphones. I dug up this USB rechargeable power bank that I never use, and is rated 5 V at 2000 mAh  (2Ah)which is a bit lower than  my smartphone. The actual rating sticker of the air blower is 0.14 A which means that at full charge it would drain the power bank in 14 hours, which of course doesn't mean that I would get 14 hours of use it just means time to total drain. I don't know exactly how many minutes or hours it would yield a usable airflow rate, but I guess it's safe to assume itd be more than 30-45 minutes (a typical bus ride duration). How long it will last will be determined through experimentation.

The next question is how to introduce air into the mask.I have run a number of scenarios in my mind and decided that the most flexible thing to do is to drill two holes to fit the vinyl pipe through the lower "jaw" of the mask. This arrangement gives me the flexibility of choosing more than one method and is upward compatible with the high pressure needle system in the future, should I ever build the Bell Compton cooling device.  I found a couple of PEX pipes that are the exact same outer diameter as the vinyl pipe. The main purpose is to use the rigid pipes to drill small holes and experiment with different air delivery nozzles such as multiple perforations, slots, etc.

A primary concern with wearing a mask is in avoiding condensation from forming on the visor. Your respiration contains a lot of moisture and you will get significant amount of fogging and water condensation in the mask, especially in Winter when the weather is very cold, as I found out with my other facemask. Cleaning moisture from the outside is easy with a handkerchief (or miniature windshield wipers  ), but the wiping the inside is impossible unless you take the mask off (or you have another set of mini windshield wipers   ).

How bad is the problem? The filter on my WWI style mask tends to get waterlogged if I wear the mask the whole day long in a very humid day (ie. rainy day), as I have had to do at work for up to 10 hours. At the end of a humid day the filter stops working because air can't come in anymore, and if you exhale the mask lifts off your face! The amount of water condensation inside the mask is quite significant, with me having to take off and drain accumulation of water in the mask at least 5 times per day in winter weather. In summer the problem is not so bad, in spite of heavy sweat in sweltering conditions. In Spring and Fall this situation will tend not to happen. Condensation accumulation is made much worse while running and wearing the mask as your respiration rate goes from 6 to 36 L/min. All of this results in me having to change the filter after a couple of days in Winter to avoid breathing mold, something that people who wear professional respirators should consider!!

I expect that circulation of air under pressure behind the plastic visor will mitigate this situation greatly, but condensation and fog blocking your view will still be a major issue unless I deal with it now. To that end you must blow any incoming air toward the faceshield in front of your eyes, like a defroster on the windshield of an automobile. The air must be directed across the face from one side to the other horizontally using the plastic visor as a wall to bend the airflow to be extracted on the other side. This is where the PEX pipes come in handy:

One possible solution is a push-pull approach where one fan blows air into the mask, and another fan extracts air out from the mask. The other approach is to prepare for condensation accumulated at the bottom of the visor by using a rope "wick" to extract any accumulated moisture. Most likely both approaches will be implemented simultaneously So the mask "jaw" would in fact have a "beard" made of one or more strands of rope! I'm thinking this would be a fun feature of the mask. I can replace those white pipes for transparent vinyl tubes or even copper pipes if I want to keep the Steampunk look.

So this is where I'm at. I will be experimenting with different configurations while I figure out a suitable air filter and pump box.  

Cheers!

[J. Wilhelm]

So it looks like the project has accelerated. One of the main issues to tackle in this project is the type of filter used. I debated with myself what to use, when I realized that it'd be very practical for me to use the same filter cartridges that I used for my facemask. Especially since I already know how to make them and I have a template, it wouldn't be any different than the filters I'm already making now, except that I'd put two filters in series instead of only one. So I went to the hardware shop seeking the same square gutter to round drain pipe adapter (4 x 3), and I sought 4" PVC pipes or adapters which could serve as a "bottle" or container for the system.

What I found was the 4x3 adapter, plus a pipe drain with screw cap, a PVC couple (not used) and a 2-1/2 inch cap. At $18 it was far more money than I wanted to spend on what essentially is a bottle, but it will soon become apparent that this combination was extremely purposeful. Starting with the fact that the untrimmed square gutter adapter allows for two filter cartridges

At a minimum of 8 mm thickness, the 4 inch pipe screw cap is actually very heavy (not my preference) but it allows easy access to the interior of the "bottle" for cleaning upgrades, etc, in conjunction with the threaded drain. The reverse of the cap has a square depression which when paired with the 2.5 inch round cap gives a tiny stagnation chamber for the centrifugal blower (herein referred to as a "compressor" based on its fuction), That way the compressor remains attached to the screwcap and you can just service it or exchange the compressor just by unscrewing the drain cap.

I might add that the compressor is attached by way of latex glue , which should be strong enough, and yet removable with a knife, thus allowing replacement of the compressor. The little remaining space around the stagnation chamber could serve to install switches and/or UV LEDs which I'm very strongly considering for use in the interior of the bottle. The reason is that I have a feeling that I may need more protection than just two filters, and I'm very mindful of the fact that blowing a constant stream of air could present an inherent risk of blowing more virus particles on my face than what I would inhale just by wearing my present mask. The filters, after all, are not perfect. If I calculate their effectiveness using 3M specs at face value, then the single filter module should be 94.5% effective, close to an N 95 mask. But even an N 95 mask is not perfect, and less so if you're blowing a constant stream of air through it.

Which brings me to the next subject: a potentiometer to adjust the speed of the compressor. Given that the compressor only uses 0.14 A, it is quite feasible to add a potentiometer to regulate the speed of the fan. The potentiometer would de constantly draining the battery, but with a sufficiently high base resistance, that drain could be quite minimal. Something like a 10K potentiometer, perhaps. The other thing to do is to improve on the quality of the filters. Currently I'm using grade 1900 3M house AC filters, but there are higher grades available, and because these filters will not become waterlogged they'd last longer than the filters I'm using. Which in turn brings me back to the topic of UV LEDs. It should be not that hard to install UV LEDs inside, run from the same battery. The light would be installed in the cap. pointing toward the filters. The advantage being that the filters are being constantly decontaminated as they're being used. This is important because it can kill loose virii particles from saliva drops which dry up on the filter. I can see running the device on at night for 15 20 minutes in a clean enviroinment just to disinfect the filters before shutting down

The blue marks on the plastic betray the final decoration of the device. As I did with my mask I will try the same "Dutch Delft / Mexican Talavera Cobalt blue folkloric decorations with PVC cement as I used on my umbrella holder and the other facemask. Pretending this is a ceramic pot of some sort is probably the only way to steampunk white PVC, and I happen to like the results. But first I will have to reinforce the joint, between the two plastic adapters because there is only a very thin strip of PVC cement joining both halves of the bottle. I would up not using the PVC couple because it was far too heavy. At 4mm thickness, the coupler is actually heavier than the all the other PVC parts put together. Instead I will cut a few round strips off the couple and glue/fuse them to the joint you see above and below.

I have a feeling that the blue cement is actually UV reactive, and it acquires a deep bright color when exposed to fluorescent and certain LED light as you can see above. I have a love-hate relationship with this type of decoration. I love it as it is very reminiscent of my childhood memories and yet the technique is horrendously difficult to perform using PVC cement. The reason for using the PVC cement is that it is basically liquid PVC and when cures it is as strong as the pipe itself, it fuses completely into the white plastic, and it can't be scratched off nor will crack. After 3 years my umbrella holder is looking like new, save the dirt and scratches acquired in the process. It really is worth the pain to apply!

I have another day off tomorrow, Monday, so I'll see if I advance some more and post it here. Feel free to chime in, and my the way sorry for the spelling errors. I'm quite sure I have plenty on this post!   Will clean them up tomorrow.  

Cheers!

Adm. J. Wilhelm

[J. Wilhelm]

There's some encouraging news here. Research studies in Israel seem to indicate that UV-c light rays can kill 99.9% of SARS Cov 2 virus particles in about 30 seconds! That's extremely encouraging. Now I'm wondering how I can adapt this technology to the "bottle" of the respirator mechanism. Even if it can't kill 99% reducing a significant amount before of particles reaching the compressor and even after would be a great idea.

https://www.jpost.com/health-science/tel-aviv-research-999-percent-of-covid-19-germs-dead-in-30-seconds-with-uv-leds-653315

https://www.medicalnewstoday.com/articles/study-reveals-uv-led-lights-effectively-kill-the-human-coronavirus

[J. Wilhelm]

I'm experimenting with using a window weather strip as a means to weather proof the clear plastic visor lens. If I can make it watertight then I will only need to attach the plastic lens with few screws. When you use glue or a calking type material, it's possible to damage the clear plastic, and the wooden frame. This would make it easy for me to replace the visor, maybe even get a smoked or polarized visor for Summer.

The "bottle" on the left is meant to dangle by an over-the-shoulder strap. The filter should be facing down, to avoid getting wet in the rain and make it as difficult for virus particles to blow into the filter. The clear vinyl tubing is attached under pressure. You can always have a second longer tube to fit the occasion. The bottle is small enough to fit under a baggy jacket or inside my backpack, for added insulation against viciated public transportation air. When I'm working in front of a computer I have the option to disconnect the USB cable from the battery and connect it to the computer to save battery time. Pf course I can always have several charged power banks at hand, and one of them should run on standard AA batteries, just in case.

The next steps are cutting and attaching the visor lens, adjusting the weather proof seal (there's a couple of spots on my temples where it's not closing, so it needs some padding) and testing to see of the compressor can still work under various lengths of tubing (it does have an effect) and vearious types of nozzles and exhaust filters. If it can't push air through all of that, I'll buy a second drain cap and install both fans or even a larger blower. It's a matter of experimentation now with, nozzles, sealing methods, blowers, batteries, switches and potentiometers. And, of course, trying to get the UV-C LED lights which I do want. Should I need a bigger power bank, there's plenty of options on the Internet. It's not cheap, but I think I' can see the light at the end of the tunnel for a first prototype

Cheers  

J W

[J. Wilhelm]

So I've found a suitable UV-C LED strip for the project. It was a bit more difficult to find than I imagined, because the market is flooded with cheap imitations running UVB and even UVA LED lights, which means that if you really want to sanitize the surface of any object or the enclosed air, you will have to ignore about 80 to 90% of all the products out there advertised as sanitizing UV light. The other aspect is that I wanted it to be powered by a 5v source so it can be run from the same USB power bank.

One of the issues that is important to know is that UV light is invisible to the human eye but it's harmful to the human eye as well. So it's a matter of safety to be able to "see" where the UV light is shining. LED light by nature produces a very narrow bandwidth of wavelengths (ie its a very pure color of light) and so it's impossible for a true UV LED to be visible unless the UV element is complemented with a visible Far-UVA element, meaning a purple light like a black light in the 400 nm range, either in the same LED module, or an a separate module. In this case the flexible strip I found has alternating UVA and UVC modules on the strip, which should give me a visible light so I can at least guess where UVC light is shining. The idea is for the strip to be completely enclosed in the can and the bank of filters, so the UV light can't harm you or your clothing.

The exact UV wavelength is also a bit of an issue. The ideal light wavelength to kill pathogens is around 250-260 nm. But I was hoping to find LED elements producing light in the "Far-UVC," 222 nm range, which means a higher frequency /lower wavelength than most UV C lights out there. The reason is that the latest research from just a few years ago has found 222 nm is still harmful to virus proteins but not DNA or cellular creatures. The theory is that if you can just deactivate the proteins surrounding the virus, it's safer for humans and less likely to damage the skin, eyes and cause cancer. Of course that means it kills less pathogens. Research shows that this extremely high frequency Far-UVC light could be used safely for illumination in public places.

The problem is that the efficiency of UVC lamps drops dramatically below 250 nm and above 280 nm, and the lamps are as much as 100 times less efficient (lumen vs watt) in the 222 nm range, plus the 222 nm elements are much more expensive and harder to find. So I settled for this 270 nm + 400 mm strip above, as a good cost /efficiency compromise. The 270 nm UV light could be harmful to me, but it will be safe to handle if the light is enclosed inside the cannister. I just need to be careful, which is why I insist on having the purple black light elements. Otherwise it's unsafe to experiment without special UV goggles.

[J. Wilhelm]

I'm getting ready to trim the visor. This polycarbonate material is very ductile and it doesn't crack no matter what I do. But it's also prone to scratching, which meant using a layer of masking tape on both sides to protect the surface.. I'll remove the masking tape only when major work on the facemask is completed.

The visor or lens will be attached with four screws near the edges of the wood frame. My idea is to make the visor removable so I can make modifications or if I have to replace the lens again.

EDIT:

I've also started looking at configurations. May be time to pull out the old fluid mechanics equations again. The flow through the pipe is extremely sensitive to the length of the vinyl tube and the inner diameter of the tube. I'm finding that the airflow rate decreases dramatically by the time that it reaches the mask using a 29 inch length of tube instead of the 10 inch rigid tube shown in the picture below. That is not unexpected, but since I had not done any calculations, I didn't expect to drop do dramatically by only 20 inches of pipe length.

As the velocity profile of the air becomes "fully developed" inside the tube, the drag increases with the length of the tube. That is the problem with low pressure systems, the drag is too high compared to the pressure feeding the tube, in spite of the blower moving a decent amount of air. The result is an ever slowing column of air moving through the tube. If the airflow is turbulent, then it's worse, because that increases the apparent viscosity, hence the drag inside the tube dramatically. I've noticed that even bending the tube a bit will affect the airflow (that's one of the reasons I wanted a high pressure system).

One fix is to increase pressure by adding the second blower or a bigger blower. But I want to save as much battery for the UV LEDs as possible. Rather than do that first, I will increase the diameter of the tube to bring down the drag, in other words, delay the formation of viscous drag across the whole section of the air column in the tube. What I don't want is to have a tube that is as wide as a vacuum hose, because at that point the mask becomes unwieldy to carry around. That was the main point of the higher pressure system (besides refrigeration). The tube hookup sprouting from the cannister can accept 3 different diameters of tubing. I will get a length of the next size vinyl tube I can get.

[J. Wilhelm]

A bit more work on the mask.

Having tested the flow of air with the slightly wider hose, I found it did improve things a bit, but the flow rate is most sensitive to the length of the hose. I've trimmed the length to less than 20 inches. I've also installed the visor with 4 flanged screws to the side posts of the wooden frame over the insulation strip.

The back of the mask needed two rubber seals to close the gap between my jaw and the wood frame. I ended up using pipe insulation foam, which has a cylindrical shape and makes an ideal "gasket." The rubber seal and gasket are comfortable on the face.

So that is the minimum work I needed to do to start trials on the airflow inside the visor. I can see that while there is airflow through the mask, I'll most likely need to increase the pressure in the line. I can feel the flow in front of my face, but my respiration (inhalation) may overtaking the strength of the air pressure. The exhaust from the mask is (the second hole in the "jaw") is noticeable but only when I exhale. I would like to feel a constant exhaust of air, as opposed to only when I exhale. When I get it right, the should always be an exhaust of air from the facemask, regardless of whether I'm inhaling or exhaling. The pressure on the mask right now is on the order of my own respiration. I'm just shy of where I need to be.

[MWBailey]

Looks like this is really moving along!

Also giving me ideas for a mostly-unrelated project that I'm currently trying to figure out.

Keep up the good work!

[J. Wilhelm]

Looks like this is really moving along!

Also giving me ideas for a mostly-unrelated project that I'm currently trying to figure out.

Keep up the good work!

Thank you!

Not without its setbacks, though. I'm having a hard time increasing the pressure on the mask. The impellers are way too sensitive to 1) viscous drag on the hose 2) *anything* that could be remotely construed as an obstacle to the flow.

I tried making a two stage compressor by bolting the two blowers back to back and hooking a bit of hose from the output of one to a PVC cap over the inlet of the second one. Looked really cool, but the flow was actually reduced instead of increasing!

Failed attempt at a two stage compressor.

Very disappointed. I looked at the situation and realized that I most likely the impellers of the blowers rely on a phenomenon called "air entrainment."  Without giving a lecture on fluid mechanics, what I can say is that a core of air will pull a ring of air around it into a fan, propeller, impeller, what have you, so you have a greater input of air that just allowed by the diameter of the fan, propeller or impeller. If you use a duct around a fan or in this case a "bell" around the impeller input, you're actually depriving the device from additional entrained air, thus the net flow of air is reduced.

The only way around that is to put one impeller inside a bottle with a perforation connecting to the hose, and then have the bottle be filled by the second blower. But o don't have the room to make that two stage device.

I'm currently scratching my head trying to make use of both blowers. Short of using the "bottled compressor in canister" method, I'm also trying to hook two compressors into the stagnation chamber... Which is also yielding mediocre results, as the blowers are not blowing directly into the output tube

Failed parallel single stage blowers. The output of one blower is 90 degrees away from the other

The threaded cap and stagnation chamber are now battered, along with the two little blowers. I'll have to buy another threaded cap and start over again. So I'm mulling my options. My guess is that putting both blowers side by side with their output pointing directly in line to a bell nozzle connected to the hose, similar to the inlet the fan inside a wind tunnel (in this case à tiny acoustic port from a pair of discarded computer speakers.. .

The reason this is all happening is because the pressure handled by the blowers is very low, just slightly over the viscous drag (friction) of air inside 20 inches of hose at that flow rate. The flow inside the PVC "stagnation chamber" is actually complicated. You probably have turbulent flow, which dramatically increases the apparent viscosity (hence drag) of the air inside the hose. It's a mess. If the flow was faster the turbulence would be delayed along the line. If the pressure was higher, you'd overcome the drag in the hose. *bleh*

I'll figure it out before pulling the equations to guesstimate pressures (it does me no good to know the exact pressure, because these blowers is all I have), or going ballistic with a full DNS Navier Stokes simulation with some software like fluent, etc. to try to device an aerodynamic solution to the complex flow inside. It shouldn't be so complicated, but it is.

[J. Wilhelm]

Double Trouble.

The simplest arrangement I can find that these finicky "compressors" will accept for the moment. If this doesn't work then I need to increase the size of the blower or enlarge the can and make multiple stagnation chambers to see if I can chain both units to make a two stage compressor (it's probably easier and more efficient to just get a larger blower).

This time I found a lighter and cheaper threaded cap at about $3. If the price is low enough, one can just replace the whole cap and blowers when something fails. It's a fairly easy project.

Looks like this is really moving along!

Also giving me ideas for a mostly-unrelated project that I'm currently trying to figure out.

Keep up the good work!

Do share your project with us!