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I am working a design project (a bubble CPAP machine) for one of my classes and though the device is pretty simple, finding components for it is a nightmare. Essentially, a pump feeds air through a tube whose end is submerged in water. The deeper the tube is submerged, the more pressure builds in the circuit.

Simple Bcpap Schematic

The two main specifications are a flow rate between 0-8L/m and pressures between 0-~12cmH2O. Thankfully controlled flowmeters for this range exists and are pretty cheap. However, finding a good pressure source has proven difficult. Ive found some companies that produce devices nearly ideal for this (WM7040 blower fan), but they require intermittent pausing.

From the design, I dont think that the pressure of the blower matters too much--when the pressure exceeds the depth of water the air will bubble out, preventing pressure buildup (I'm assuming my reasoning makes sense). However, for the sake of lower power consumption, I would think that a pressure output of the same order of magnitude would be best--so able to pump 8L/m at ~1psi. But I can't find anything that matches this--everything I find has either too great a flow rate with too little pressure or vice versa.

Something else I've been trying to find is a pressure relief valve for pressures of ~20cmH2O as a safety feature in case of occlusion (and also better performance). I imagined these would be cheap initially, but its nearly impossible to find any below 10PSI, way more than I need. Anything in the realm of <1psi costs hundreds of dollars. Similarly, Ive looked into pressure reducing valves and its a similar story--nothing below 1psi.

One thing I can think of is the components that I'm seeing are ultra-precise, thus costing a lot more. I dont need this--im only looking for components in the same ballpark as my specifications. The pressure generator only needs to produce enough flow and exceed the maximum pressure I want, I dont really care if its able to deliver a precise flow rate (since this will likely get controlled by the flowmeter).This is my first time ever building anything so I'm assuming I'm doing something wrong--I've spent hours upon hours browsing different sites but everything is either way too expensive or off from the specifications I want. Do such components just not exist?

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  • $\begingroup$ Shopping questions are off-topic since parts recommendations get obsoleted very quickly and make the answers rather useless to future readers. But I sympathize; Pressure sensors in that range run 50-150CAD. If this is an electronics capstone project then it shouldn't be difficult for you to make a sensored, digitally controlled valve as a substitute though it won't meet safety requirements if it a safety valve. But if this is a mechanical project then it won't be feasible for you to go out of your way to implement a micro-processor controlled solenoid valve and sensor. $\endgroup$
    – DKNguyen
    Nov 29, 2023 at 21:54
  • $\begingroup$ Also, I don't know what type of relief valves you are looking at, but have you looked at one time burst valves? They might be cheaper. Since this is just a student project, if you have access to a shop you might be able to make a crude, not-so-accurate burst valve for low pressures. For example, clamping a pre-tensioned thin film of material between two blocks that fits over the end of a tube. If you experiment with tensioning the film to the same size each time for mounting, but start with different sizes of material to stretch you should have some degree of consistency. $\endgroup$
    – DKNguyen
    Nov 29, 2023 at 22:03
  • $\begingroup$ @DKNguyen Ill change the question a little in case someone is able to answer with a type of component rather than one specific product. I am trying to make the project as mechanical as possible rather than relying on microcontrollers, but this is difficult to do with zero experience. I am thinking of maybe using a piezo diaphragm+comparator+solenoid valve to try to achieve this but I dont think its going to work very well $\endgroup$
    – user62783
    Nov 29, 2023 at 22:04
  • $\begingroup$ @DKNguyen Im looking for a component that is reusable--I dont think the relief valve is necessary at all since the water should serve as a pressure relief valve but having the extra safety feature would be nice (there's also research suggesting it might be beneficial for the devices performance). $\endgroup$
    – user62783
    Nov 29, 2023 at 22:07
  • $\begingroup$ Natural gas is typically regulated to between 4" and 8" of H2O in a domestic installation. It may be useful to look at components intended for that application. For instance there are diaphragm pumps intended to boost natural gas flow at relatively high flow rates that are used to increase the BTU rating of wok burners. Similarly, aquarium componentry for large aquaria is getting up to your flow rates and pressures. Plenty of valves, one-way restrictors, pressure breaks etc in that space. $\endgroup$
    – elchambro
    Nov 30, 2023 at 22:11

1 Answer 1

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enter image description here

Try making your own pressure relief valve. It seems to me that a weighted ball to seal off a pipe end might do the job. You would need a ball of weight, $ w = \frac 1 4 \pi d^2 \times P $ where $ w $ is the weight in pounds, $d$ is the pipe diameter and $P$ is the desired pressure.

For 1 psi on a 0.5" pipe this would work out at $ w = \frac 1 4 \pi \times 0.5^2 \times 1 = 0.19 \text{lbs}$.

If the ball isn't heavy enough then add a cylinder of metal above it. If the ball is too heavy then add a balance and counterweight.

You might need a soft rubber seal at the end of the pipe to prevent leakage. An o-ring might suffice.

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  • $\begingroup$ Great idea--I was thinking of using a spring or something but this seems much, much easier and a good option since it doesn't need to be very precise. $\endgroup$
    – user62783
    Nov 29, 2023 at 22:51
  • $\begingroup$ also curious: how did you make that image? $\endgroup$
    – user62783
    Nov 30, 2023 at 6:36
  • $\begingroup$ I used an ancient (licenced) copy of FastCAD. Inkscape would be an excellent alternative and it's free. $\endgroup$
    – Transistor
    Nov 30, 2023 at 9:41

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