# Pressure sensor without using MEMS chips [closed]

I'd like to figure out how to make a pressure sensor for very low cost (essentially disposable), mass produced piece of medical equipment.

The problem: this can't use any parts which aren't already available in huge quantity. In other words only parts with > 100k and preferably > 1M in distributor stock. This probably rules out the obvious approaches of using MEMS pressure sensors for cell phones (or does it? any super-high volume MEMS devices?). Probably most specialty parts are out; commodity parts only (resistors, capacitors, generic op amps, generic transistors). It is okay to fabricate components from raw materials (eg - a force sensitive resistor from conductive foam) if the materials are readily available. It helps if all the parts used are extremely cheap (bom cost < $10). What the sensor should do is sense pressure differences. It needs to pick up changes of <1cmH2O (100 Pa) in pressure, and it needs to have response times <1 second. For this 0.1 cmH2O precision and readings every 0.1 second are great specs. The typical operating pressure is 0 to 40 cmH2O. The physical form factor of what this connects to would be a short 20mm tube - as short as possible; 20mm x 20mm is good - with tapered connectors on both sides. I assume this would need a mechanical component like a diaphragm or piston which is somehow interfaced to a PCB. It's okay if this doesn't have a very accurate zero, and other specs like linearity, drift, offset, scale error are non-essential as long as roughly 1cmH2O changes can be detected between 0 and 40 cmH2O. It is okay if each sensor needs an individual calibration. Idea #1: diaphragm the position of which is read out using some version of capacitive touch - read out capacitance Idea #2: diaphragm which compresses a force sensitive resistor made of anti-static foam - read out resistance Idea #3: conductive diaphragm which touches a different number of conductive pads on the pcb depending on how far it's deflected - read out which pads are shorted together Idea #4: conductive diaphragm the end to end resistance of which is read out depending on stretch Bounty if rather than an idea, you build a working prototype. Huge bounty if you do it within the next day or two. • You are a day late and a dollar short. A bunch of people thought we were going to be critically short of vents like a month ago. And if the curve had not deflected a bit, probably we would be by now (or soon) But there are serious companies with serious money jumping on it now. I don't think there will be any shortage. Maybe in countries like Brazil you might be able to help people. Not in the US though. – mkeith Apr 2 '20 at 4:59 • I'm not familiar with mass production of sensors but, how come you are considering valid to use conductive foam as a sensor but discarding proper sensors in quantities less than 100k? I would think that it would be far easier to just take the dozens of sensors that are available in the low hundreds of thousands and adapt the interfaces/calibration. So instead of having to make a million conductive foam pressure sensors you just design a board that accepts different "proper" sensors.. no? Am I missing something? – Wesley Lee Apr 2 '20 at 5:00 • In that case liability is much less of a concern. – mkeith Apr 2 '20 at 6:01 • Can the moderator also close all topics based on covid 19, before somebody reports that this platform is giving advice on how to make medical equipment without the proper standards! If find another user assisting people in this way again I will personally report you because this is not the platform for giving someone advice on something that is so serious. I have lost someone from this virus and these people who answered are definitely not helping the FDA or CDC to stop this virus! – Joey Apr 2 '20 at 16:55 • @Joey I'm really sorry you lost someone. FYI - I'm a medical device engineer; I've designed FDA cleared devices which are being used right now by the tens of thousands. When you are at the hospital - someone very much like me made all those devices you see; and they probably asked people for advice, very possibly right on here. – Alex I Apr 2 '20 at 18:26 ## 2 Answers Well, Digikey has some absolute pressure sensors with SPI and I2C interfaces, going for less than$1.50 and about 40,000 in hand.Resolution is 150 Pa.

But here's the catch on homemade pressure sensors - don't do it.

Look. Just don't. You're talking about the most critical sort of application - pressure fault alarm on a ventilator. This is NOT the place for jury-rigged sensors. If you try this sort of kludge, you will open yourself to grotesque levels of legal liability. And the fact that you are not manufacturing for profit is unlikely to save you.

• Understood. Legalities are a bit different outside the US, but - I think more to the point, you're right that a kludge is going to take time (and a number of iterations) to get right. 40k stock is just not gonna do it though. I'm looking now and there are MEMS devices with close to a million stock. Might be okay there. – Alex I Apr 2 '20 at 3:45
• @AlexI - Dude, you are missing the point. Homemade sensors suck. I don't care what sort of "ingenious" design you come up with, you're not going to get unit-to-unit consistency. And that will kill people in this particular case. – WhatRoughBeast Apr 2 '20 at 3:47
• Do they suck more than literally no sensor? Because that's what was probably gonna happen here. You should see the disposable $1 plastic gauges that EMTs use all the time. Okay, peace - I'm looking at MEMS which are available in large volume. I still do want to know how to do a kludge. – Alex I Apr 2 '20 at 3:57 • @AlexI "Do they suck more than literally no sensor" At 100Pa? Yeah they probably do due to the opportunity cost of wasting time on a sensor that's probably not going to work. Probably just better off using that time to build more ventilators. I don't know what pressure sensors ventilators traditionally use, but the pressure sensor I have for use in a UAV was made to be used in ventilators. It has a range of 500Pa, cost$140, and is a thermal type sensor which is basically a temperature sensor and heat source...but the devil is in the details. – DKNguyen Apr 2 '20 at 4:32
• The legal liability is off topic here. And off context because in the emergency, gov agencies, let alone developing country agencies, don't care much about that. The most important is to make clear to the users that this is not a certified device and that you can't be liable for malfunction. The "use at your own risk" statement. Of course it will better than no sensor. – Fredled Apr 2 '20 at 9:37

The world-wide market for pressure sensors is well in excess of ten billion units annually. I don't think finding a supplier with 1 million units available will be very difficult, there are likely tens millions of units surplused by the recent recession in addition to normal stock.

• Okay - thanks! Could you please let me know what some of the highest volume devices are? It is possible a lot of this is normally ordered from the factory with the usual long lead times (12-16 weeks). I'm just not seeing that much distributor stock. I'll start calling up manufacturers next. – Alex I Apr 2 '20 at 4:45
• I don't think the ventilator market will be in the millions. It's rather in the tens of thousands, Not so many people need it. – Fredled Apr 2 '20 at 9:38
• @Fredled A lot of knowledgeable people think otherwise: ventsforall.com "740,000 estimated ventilator deficit in the US alone". That's was written by a dozen MDs - critical care and epidemiology. – Alex I Apr 2 '20 at 18:18
• @AlexI Let's hope you are wrong. So far in Europe ventilator shortage is in small numbers. – Fredled Apr 2 '20 at 23:54