# What to call a switch that CLOSES when air pressure is above, say, 45 psi? **URL:** \ **Category:** Factual Questions\ **Created:** [September 6, 2011, 3:47pm UTC](https://boards.straightdope.com/t/what-to-call-a-switch-that-closes-when-air-pressure-is-above-say-45-psi/595398 "2011-09-06T15:47:50Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [September 6, 2011, 3:47pm UTC](https://boards.straightdope.com/t/what-to-call-a-switch-that-closes-when-air-pressure-is-above-say-45-psi/595398/1 "2011-09-06T15:47:50Z")
I want a switch that opens when the compressed air pressure drops below a setpoint and closes when it rises above setpoint. This is the OPPOSITE of what normal air compressor switches do. I want to disable an electric air heater in the event that the flow stops, to prevent burnout. A little hunting around finds loads of switches that close at low pressure and open at high pressure, but not what I want. Does this have a special name or application I can hunt for? ---
**Author:** ![Darth\_Sensitive](https://avatars.discourse-cdn.com/v4/letter/d/58956e/32.png) [@Darth\_Sensitive](https://boards.straightdope.com/u/Darth_Sensitive)\ **Post date:** [September 6, 2011, 3:52pm UTC](https://boards.straightdope.com/t/what-to-call-a-switch-that-closes-when-air-pressure-is-above-say-45-psi/595398/2 "2011-09-06T15:52:10Z")
[Redneck WAG]Install one of those high pressure switches backwards so that “high” is atmospheric and “low” is inside?[/RWAG] ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [September 6, 2011, 4:02pm UTC](https://boards.straightdope.com/t/what-to-call-a-switch-that-closes-when-air-pressure-is-above-say-45-psi/595398/3 "2011-09-06T16:02:41Z")
It ought to be called a pressure sensor, I’d think. ---
**Author:** ![Baracus](https://avatars.discourse-cdn.com/v4/letter/b/2bfe46/32.png) [@Baracus](https://boards.straightdope.com/u/Baracus)\ **Post date:** [September 6, 2011, 4:17pm UTC](https://boards.straightdope.com/t/what-to-call-a-switch-that-closes-when-air-pressure-is-above-say-45-psi/595398/4 "2011-09-06T16:17:53Z")
> [@Darth\_Sensitive](#): > > [Redneck WAG]Install one of those high pressure switches backwards so that “high” is atmospheric and “low” is inside?[/RWAG] Or hook it up normal to a normally-closed relay to operate the heater. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [September 6, 2011, 4:20pm UTC](https://boards.straightdope.com/t/what-to-call-a-switch-that-closes-when-air-pressure-is-above-say-45-psi/595398/5 "2011-09-06T16:20:00Z")
> [@Darth\_Sensitive](#): > > [Redneck WAG]Install one of those high pressure switches backwards so that “high” is atmospheric and “low” is inside?[/RWAG] Sure, if you can find one that can be set to _minus_ 45 psi… I think what you want is a pressure switch that acts as a double-throw (SPDT or DPDT) switch; this will allow you to wire it either way (normally closed or normally open). A quick google search for “SPDT pressure switch” returned a number of results ([example](http://www.processinstrumentation.net/pressure-switches.html)), though I have no idea if these meet your other requirements (cost, size, environment, etc). ---
**Author:** ![cornflakes](https://avatars.discourse-cdn.com/v4/letter/c/9d8465/32.png) [@cornflakes](https://boards.straightdope.com/u/cornflakes)\ **Post date:** [September 6, 2011, 4:20pm UTC](https://boards.straightdope.com/t/what-to-call-a-switch-that-closes-when-air-pressure-is-above-say-45-psi/595398/6 "2011-09-06T16:20:30Z")
SPDT pressure switches exist. [Here](http://www.dwyer-inst.com/Product/Pressure/SinglePressure/Switches/SeriesA9/Specs) is one type from Dwyer, though the pressure limits may be too low. You might want to call Grainger and ask them for help. ---
**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\ **Post date:** [September 6, 2011, 4:21pm UTC](https://boards.straightdope.com/t/what-to-call-a-switch-that-closes-when-air-pressure-is-above-say-45-psi/595398/7 "2011-09-06T16:21:44Z")
> [@Napier](#): > > I want a switch that opens when the compressed air pressure drops below a setpoint and closes when it rises above setpoint. This is the OPPOSITE of what normal air compressor switches do. > > I want to disable an electric air heater in the event that the flow stops, to prevent burnout. > > A little hunting around finds loads of switches that close at low pressure and open at high pressure, but not what I want. > > Does this have a special name or application I can hunt for? Air compressors used in construction work this way… no? When my compressor gets the tank to 120 PSI, it shuts down. The pressure forces the ‘on/off’ switch up (up is ‘off’), which kills the juice to the motor like any other switch (by cutting the circuit). ---
**Author:** ![Snnipe\_70E](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)\ **Post date:** [September 6, 2011, 4:22pm UTC](https://boards.straightdope.com/t/what-to-call-a-switch-that-closes-when-air-pressure-is-above-say-45-psi/595398/8 "2011-09-06T16:22:25Z")
> [@Napier](#): > > I want a switch that opens when the compressed air pressure drops below a setpoint and closes when it rises above setpoint. This is the OPPOSITE of what normal air compressor switches do. > > I want to disable an electric air heater in the event that the flow stops, to prevent burnout. > > A little hunting around finds loads of switches that close at low pressure and open at high pressure, but not what I want. > > Does this have a special name or application I can hunt for? YOu want a normally open pressure switch. An electric air heater? and a compressed air swirch? Are you talking about heating process air or space heating air? ---
**Author:** ![boytyperanma](https://avatars.discourse-cdn.com/v4/letter/b/4491bb/32.png) [@boytyperanma](https://boards.straightdope.com/u/boytyperanma)\ **Post date:** [September 6, 2011, 4:34pm UTC](https://boards.straightdope.com/t/what-to-call-a-switch-that-closes-when-air-pressure-is-above-say-45-psi/595398/9 "2011-09-06T16:34:42Z")
> [@Philster](#): > > Air compressors used in construction work this way… no? He’s looking for the opposite. A closed switch means it is allowing power through. An open switch means it does not allow power through. On a regular pressure switch when it drops below a set pressure the switch closes and powers the compressor till it hits the top set point which it then opens cutting power to the compressor. I haven’t run into any commonly available switches for **Napier** ’s purpose. With his goal however you could easily wire up what you want with a contacter and a standard pressure switch. The combined cost will likely be less then a specialized switch. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [September 6, 2011, 4:35pm UTC](https://boards.straightdope.com/t/what-to-call-a-switch-that-closes-when-air-pressure-is-above-say-45-psi/595398/10 "2011-09-06T16:35:15Z")
> [@Philster](#): > > Air compressors used in construction work this way… no? > > When my compressor gets the tank to 120 PSI, it shuts down. The pressure forces the ‘on/off’ switch up (up is ‘off’), which kills the juice to the motor like any other switch (by cutting the circuit). What you’re describing is a switch that closes below a pressure setpoint (to complete a circuit and power the compressor motor), and opens above a pressure setpoint (to shut down the motor). OP wants a switch that _opens_ below a pressure setpoint (low pressure evidently an indicator of stagnant flow that requires shutting down a heater). ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [September 6, 2011, 4:39pm UTC](https://boards.straightdope.com/t/what-to-call-a-switch-that-closes-when-air-pressure-is-above-say-45-psi/595398/11 "2011-09-06T16:39:53Z")
It is called a pressure switch. You can get the NO, NC, form C, etc. (form C is a common with one NC and one NO contact) Take your pick: > **[McMaster-Carr](https://www.mcmaster.com/#standard-pressure-switches/=dy28j3)** > > McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [September 6, 2011, 8:31pm UTC](https://boards.straightdope.com/t/what-to-call-a-switch-that-closes-when-air-pressure-is-above-say-45-psi/595398/12 "2011-09-06T20:31:20Z")
Thanks! One’s on the way from McMaster - Carr. Snnipe, I’m heating process air. If somebody unplugs the air hose, my heater element will overheat and fail, unless this switch disconnects it from the power. ---
**Author:** ![Myglaren](https://avatars.discourse-cdn.com/v4/letter/m/f1d935/32.png) [@Myglaren](https://boards.straightdope.com/u/Myglaren)\ **Post date:** [September 6, 2011, 9:09pm UTC](https://boards.straightdope.com/t/what-to-call-a-switch-that-closes-when-air-pressure-is-above-say-45-psi/595398/13 "2011-09-06T21:09:10Z")
Thermostat in close proximity to the heating element? ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [September 6, 2011, 10:34pm UTC](https://boards.straightdope.com/t/what-to-call-a-switch-that-closes-when-air-pressure-is-above-say-45-psi/595398/14 "2011-09-06T22:34:07Z")
> [@Napier](#): > > Thanks! One’s on the way from McMaster - Carr. > > Snnipe, I’m heating process air. If somebody unplugs the air hose, my heater element will overheat and fail, unless this switch disconnects it from the power. The pressure switch will work to protect from loss of inlet air, but a flow switch is better, as the heater may overheat if the outlet is blocked, (dead ended) which will not trip the pressure switch on the input. Flow switches are more expensive than pressure switches so I’ve never won this argument, but maybe you will have better luck! ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [September 7, 2011, 6:30pm UTC](https://boards.straightdope.com/t/what-to-call-a-switch-that-closes-when-air-pressure-is-above-say-45-psi/595398/15 "2011-09-07T18:30:39Z")
> [@Myglaren](#): > > Thermostat in close proximity to the heating element? No. It’s further downstream where accuracy matters. This makes control harder, but I wrote a program to fit FOLDP models to step response studies and try a variety of tuning rules for PID parameters, and have a pretty good control loop now. There’s another component with a much longer lag time, so this particular loop does not set the time before settling anyway. It would have been nicer to cascade two loops, but this is good enough. ---
**Author:** ![Cheshire\_Human](https://avatars.discourse-cdn.com/v4/letter/c/5f8ce5/32.png) [@Cheshire\_Human](https://boards.straightdope.com/u/Cheshire_Human)\ **Post date:** [September 8, 2011, 5:27pm UTC](https://boards.straightdope.com/t/what-to-call-a-switch-that-closes-when-air-pressure-is-above-say-45-psi/595398/16 "2011-09-08T17:27:38Z")
> [@Kevbo](#): > > It is called a pressure switch. You can get the NO, NC, form C, etc. (form C is a common with one NC and one NO contact) > > Take your pick: > [McMaster-Carr](http://www.mcmaster.com/#standard-pressure-switches/=dy28j3) Exactly the company I was about to link to and suggest you search. They’re one of my favorite companies for finding and buying odd stuff like this. Bookmark their home page for future searches of this “oddity level”. 😉 ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [September 10, 2011, 8:13pm UTC](https://boards.straightdope.com/t/what-to-call-a-switch-that-closes-when-air-pressure-is-above-say-45-psi/595398/17 "2011-09-10T20:13:01Z")
> [@Cheshire\_Human](#): > > Exactly the company I was about to link to and suggest you search. They’re one of my favorite companies for finding and buying odd stuff like this. Bookmark their home page for future searches of this “oddity level”. 😉 Yeah. Actually, I tried them first, and then Grainger and a few others, before I posted. I don’t know what I did wrong, but I didn’t find ANYTHING. I actually order from McMaster-Carr several times a week and have a personal account as well! By the way, the switch I would up ordering from them just arrived and it looks perfect, very nice. I find that they occasionally fill orders later on the day I requisition them, meaning I process it, then the PA processes it, then McM-C processes it, then UPS (or whoever) processes it, then our receiving department processes it, and finally I get an email and run downstairs and pick the thing up. It’s amazing.