Pneumatic pressure sensor?

Hello all,

I'm working on a project that utilizes pneumatics, and I'd like some info on pressure sensors. Basically I'm attempting to build an air chamber out of PVC pipe that I'll pressurize with a small emergency-type compressor to only a few PSI (maybe 4 or 5). Ultimately I'll dump the air using a valve of some kind (maybe a sprinkler system valve). I'd like to be able to turn off the compressor once the chamber has reached it's ideal pressure (probably with a relay or some such device), and then recharge after I've dumped the air.

Anyone know where I can find a device that'll let me measure the pressure with perhaps a change in resistance or something? I've been using Google to search for "Pressure Sensor" and such things, but all the devices I'm seeing cost in the 20-100 dollar range. Surely there's cheaper alternatives out there. I could probably time the charging to get results that are ballpark, but I'd like more precise PSI (resolution to around .1 PSI if possible).

Has anyone done anything like this before, and can you point me to a good source for parts?

Thanks, Aneal K.

Have a look at the sort of air pressure switch used in domestic washing machines to detect when the drum is full - they should be pretty cheap. They measure the increased air pressure in the space above the water.

You need to get a pressure transducer, they can be bought off ebay for a decent price.

They output an analog voltage if I am correct which could be read by the arduino micro controller. Make sure its 5v or less or else you will need a voltage divider (2 resistors).

I tried to get one of these a while ago but never really needed it for my pneumatics project.

Here is one on ebay for $10 plus shipping, now I don't know what type of air connector is on this thing (npt ect..) so go look for the data sheet. If its a weird one get an adapter.

http://cgi.ebay.com/SPAN-Pressure-Transducer-READOUT-LR050-250-psi_W0QQitemZ380110456544QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item58805946e0

Here is another which I think is nicer, as all of the specs are there. 1/4in npt input and a 3 wire output .5dc to 4.5v but its $50.

http://cgi.ebay.com/NOSHOK-0-150-PSI-Hall-Effect-Pressure-Transducer_W0QQitemZ370200905219QQcmdZViewItemQQptZLH_DefaultDomain_0?hash=item5631b18603

Do you really need an analog sensor?

Not knowing exactly what your project is, let me make an analogy with a home heating system...

Most thermostats are "digital" sensors. They don't tell the boiler the room temperature, they have just two outputs: "Need heat"/"Warm enough". Two output states=digital. That doesn't mean that I can't have my home at 60, 65 or 70... I can adjust the trip point of the sensor.

For your project, if you want an inexpensive solution, maybe something Rube Goldberg would work/ be "fun"(?)...

You mentioned that you only needed slight pressures measured.

For this a U-tube liquid column manometer could be used.

(See... http://en.wikipedia.org/wiki/Pressure_measurement .. and skip down to "liquid colum" section if what I mean not obvious)

Detecting the rise of the liquid past a point of your choice could be done with an LED/phototransitor pair.

Not the simplest answer... but very inexpensive, which seemed to be in your design requirements! And if the idea doesn't suit your project, perhaps it will be useful to another reader of the thread.

For an overview of all you ever could want to know about pressure sensors...

http://en.wikipedia.org/wiki/Pressure_sensor

An inexpensivepressure transducer can be found in the $6 digital tire pressure guages at Walmart. It probably has a higher range than you would like. I think I've seen low range ones used in barometers and HVAC setups.

You can check out Freescale Semiconductor at http://freescale.com

They have pressure transducers with analog outputs that should easily encompass the range you're looking for, just make sure that the package you're ordering is what you're looking for, they sell everything from bare chips to encased with hose barbed outputs.

http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=MPXx5050&webpageId=M98716&nodeId=01126990368716&fromPage=tax

+1 for freescale. They have sensors with barbed fittings for your type of application.

-j

A hint when using pressure sensors. If measuring a gas pressure the sensor should be mounted higher then the vessel being measured so condensation or other liquids will self-drain to the vessel. If measuring a liquid pressure, the sensor should be mounted below the vessel so entrained gas will migrate out the sensor tubing back to the vessel.

Lefty

This might work for you (datasheet can be found online):

http://www.goldmine-elec-products.com/prodinfo.asp?number=G17180

$8.95 ea

Although the spec notes that "burst pressure" is 5 psi, so it may be too small for your application...

so it may be too small for your application...

Yes, much too sensitive a sensor for what you are measuring. It's measurement range is 0 to 4 inches of H2O which is like 0 to 0.14 psig.

Lefty