Air & liquid pressure measurement

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Air & liquid pressure measurement

Postby ZKT » Thu Jan 25, 2007 2:51 pm

The need arises to measure air and liquid pressures in the range 0-4 psi. with a PIC mcu. I have never seen sensors in this range. Any ideas ?
:)
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Postby hooziewhatsit » Thu Jan 25, 2007 4:52 pm

go to www.freescale.com and peruse their 'sensor' products. You should find all their pressure sensors in there.

Lots to choose from that should interface nicely to the PIC.

-Dave
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Postby ZKT » Fri Jan 26, 2007 7:31 am

Perfect
Thanks
:)
Small problem
Freescales explaination of the diferent types of pressure transducers does not satisfy my courosity.
What are the differences in gauge, differential, absolute types? Or does it really matter. My app needs to sense the air pressure being aplied to a spray nozzle and adjust the compressor speed accordingly. And same for liquid.
Thanks again
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Postby brennen » Fri Jan 26, 2007 8:20 am

Absolute pressure takes into account air pressure as well as any other pressures on whatever you're measuring. Gauge pressure is simply the pressure above atmospheric (for instance the pressure inside a tire). Differential pressure generally uses two pressure sensors and takes the difference of them.
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Postby oPossum » Fri Jan 26, 2007 8:21 am

The pressure that will be reported as 0 psi (or kP)...

Gauge - ambient pressure is 0 psi
Differential - same pressure on both ports is 0 psi - leave one port exposed to ambient and you have a gauge sensor
Absolute (psia/kPa) - complete vacuum is 0 psi
Last edited by oPossum on Fri Jan 26, 2007 8:28 am, edited 1 time in total.
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Postby emf » Fri Jan 26, 2007 8:26 am

ZKT wrote:Freescales explaination of the diferent types of pressure transducers does not satisfy my courosity.


I tried to figure this stuff out a while back, and the datasheets aren't much help unless you know the terminology. I found this page helpful:
http://www.aeroconsystems.com/electronics/Pressure_transducer_basics/Transducer_primer.htm

Another way to find sensors that fit your needs is to use Digikey's online catalog. It lets you filter the search results by the pressure range you're trying to measure. A lot of listings are in kPa so know your range in both units.
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Postby ZKT » Fri Jan 26, 2007 1:08 pm

Thanks one and all.
I always search DK first- musta overlooked it.
This outta do ithttp://rocky.digikey.com/WebLib/Motorola/Web%20Data/MPX2050%20SERIES.pdf
Next is to interface it to a PIC ADC I guess. :?:
The idea is to control a 120VAC compressor motor with a pic
And hense the airflow to the device.
It would be a simple matter with a triac.
But a stepper motor connected to the pot shaft is a little too Rube Goldberg.
Any ideas ?
Oh, one other thing. I found an old blood pressure monitor in the junkbox. It has a pressure transducer in it and appears conected to a 74HC02, Quad 2-input NOR gate. Only 2 connections to it and there is no change in voltage ref to gnd or across the pins as the cuff pumps up. And it works. Strange.
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Postby emf » Fri Jan 26, 2007 2:08 pm



That would work. It might be worth the extra $5 for the MPX5050 series, which gives scales the output up to something you can feed the ADC directly.

The idea is to control a 120VAC compressor motor with a pic
And hense the airflow to the device.
It would be a simple matter with a triac.
But a stepper motor connected to the pot shaft is a little too Rube Goldberg.


If you're just looking for on/off control, solid state relays are easy to deal with. If you're looking to control the speed of the motor, I don't know the best way to do it. A lot of motors don't like being connected to a light dimmer, though, so try it with someone else's compressor first if you decide to go that route :-)
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Postby ZKT » Fri Jan 26, 2007 3:05 pm

Good Idea
http://www.digikey.com/scripts/dksearch/dksus.dll?KeywordSearch
is is then. Thiis is a diff type. great accuacy isnt needed. Just need to keep air pressure 3<=P<=5 psig. Type`s not gonna matter is it.?
This isnt a whopping big compressor. 0.36-0.49 cfm at abovementiopned pressure. Thats about what a little 12vdc car tire inflator puts out. the compressor motor has to be rated for continous operation and the speed adjustable.( to keep the pressure within limits). I drawing a blank here HeLLLLLLLLPPPPP.
:lol:
A 6vdc virbator type compressor like in the BP monitor might work. Couldnt it be pulse width modulated to control the air output?
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Postby ZKT » Fri Jan 26, 2007 3:20 pm

ZKT wrote:Good Idea
http://www.digikey.com/scripts/dksearch/dksus.dll?KeywordSearch
is is then. Thiis is a diff type. great accuacy isnt needed. Just need to keep air pressure 3<=P<=5 psig. Type`s not gonna matter is it.?
This isnt a whopping big compressor. 0.36-0.49 cfm at abovementiopned pressure. Thats about what a little 12vdc car tire inflator puts out. the compressor motor has to be rated for continous operation and the speed adjustable.( to keep the pressure within limits). I drawing a blank here HeLLLLLLLLPPPPP.
:lol:
A 6vdc virbator type compressor like in the BP monitor might work. Couldnt it be pulse width modulated to control the air output?

I was thinking 120 VAC induction motor cause theyre often continous duty and easy to control with a triac.
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Postby SOI_Sentinel » Fri Jan 26, 2007 3:31 pm

Motors tend to suck at this kind of control, actually. Too much energy. Single phase is the most difficult to control. I don't know if a triac will give you the control you seek.

Of note, you might have more luck if you look into proportional valves. This is the more conventional method of controlling flow, but I can't say it will fit into your budget.
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Postby ZKT » Fri Jan 26, 2007 3:34 pm

thanx Ill check it out. and your blog too.
Lookin at opto isolators triac output at DK

continuing with the triac idea-http://rocky.digikey.com/WebLib/Sharp/Web%20Data/s2s5.pdf
Opto isolator triac output
PIC pwm`s Opto isolator triac which controls larger triac, which controls ac induction motor speed. Whadda ya think ?

First, bolt on a few fiberglass Sidewinder lookalikes. ...

LMFAO pleased to meet ya !!!
You got goood ideas. Ill reead more later.
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Postby emf » Fri Jan 26, 2007 8:33 pm

ZKT wrote:Type's not gonna matter is it.?

Your link didn't come through, but I'm sure you could make almost anything specified for that pressure range work.

This isnt a whopping big compressor.


Ah... not one of those 25 gallon monsters they sell at Home Depot then? Oh well...
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Postby ZKT » Sat Jan 27, 2007 4:24 am

Sorry to dissapoint ya. But the blower motor is 125VAC- maybe aven 1/8 hp. And there`s gonna be a pump in the system later too. :lol:
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