Do cheap, low pressure (~0.25psi) pneumatic components exist?

Question

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.

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?

Answer 1

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.