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Reading Pressure — Parallax Forums

Reading Pressure

Carl PCarl P Posts: 5
edited 2009-09-03 05:06 in BASIC Stamp
Hello all I'm working on a project I need to measure the pressure on a refrigerant circuit . I have a pressure transducer with a 0-5V output 0-500 psi how can I covert the ADC to a psi # I would appreciate any help this is my first time using ADC

Comments

  • stamptrolstamptrol Posts: 1,731
    edited 2009-09-02 14:00
    Hi Carl,

    Welcome to the Forums.

    By far, the simplest way to proceed is to get an LTC1298 dual channel ADC chip. Its a 12-bit unit and available with sample code from Parallax or you can get the chip from your favorite supplier and download the sample code from the Products page.

    Cheers,

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    Tom Sisk

    http://www.siskconsult.com
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  • Carl PCarl P Posts: 5
    edited 2009-09-02 14:56
    Thank you Tom for the help it looks like i will be using that as my ADC the only problem that remains is how to I take that output and reference it to my 0-500 psi . For example the output might be 347 from the ADC which converts to lets say 125 psi ( just made those numbers up as example )

    Thanks again
  • Carl HayesCarl Hayes Posts: 841
    edited 2009-09-02 15:40
    I would run some experiments. Write a small program to read the thing repeatedly, with DEBUG to tell you what you're getting. Then put some pressure on the thing, hand-recording the readings at various pressures. From that you can make a calibration curve, or derive an algebraic expression for the pressure as a function of the reading from the chip.

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  • stamptrolstamptrol Posts: 1,731
    edited 2009-09-02 16:27
    Carl,

    As Carl H pointed out, you can run a set of tests and get some representative points.

    If its a commercial transducer, you'll probably be able to use a formula like:

    pressure = adnum* (multiplier) + offset

    For instance, say 500 psi drove the a/d to fullscale(4096). The multiplier would be 500/4096 or about 0.122. Now, the only snag is that the Stamp works in whole integer numbers so you'd have to use a technique of dividing by 8; or multiplying by 10 then dividing by 82. The offset would be something like (-15) to take atmospheric pressure into account.

    Cheers,

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    Tom Sisk

    http://www.siskconsult.com
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  • LloydsLloyds Posts: 75
    edited 2009-09-02 17:37
    Carl,
    A lot of good suggestions. I just ran thru the same situation using an 8 bit converter. I collected some data (measured pressure vs converter output count) and entered it into an excel spreadsheet, and then had excel plot a graph. Luckily, mine was a linear function.
    I used the excel "trendline" and "show formula" function and it gave the conversion formula.

    My formula was psi = ADcount * 16.38 + 120. As Tom said, the integer math won't handle the 0.38 part, so I wrote the fromula as:

    psi = (ADcount * 38)/100 + (ADcount * 16) + 120.

    There will be a little bit of truncation error when you divide by 100, but it will probably be insignificant for your application.

    Your application is interesting. I made a transducer from a little surface mount wheatstone bridge pressure sensor and it worked pretty well. The hardest part was getting the 4 wires for the bridge out of thru the wall of the pressure vessel without any leakage.
    Regards,
    Lloyd
  • Carl PCarl P Posts: 5
    edited 2009-09-02 19:28
    Thank you all for your input, my pressure transducer will be in on friday and i will get to work I will keep the post updated as i move along
  • sam_sam_samsam_sam_sam Posts: 2,286
    edited 2009-09-02 22:14
    Carl P
    Thank you all for your input, my pressure transducer will be in on Friday·· ·Can you post a Link to this Pressure Transducer

    ·and i will get to work I will keep the post updated as i move along········ ·Can I ask you what you are builting



    Sound like· every nice Project

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    Sam
  • Carl PCarl P Posts: 5
    edited 2009-09-02 22:41
    Sure Sam

    The Pressure Transducer
    [noparse][[/noparse]url]http://sensing.honeywell.com/index.cfm?ci_id=140301&la_id=1&pr_id=122351[noparse][[/noparse]url]
  • Carl PCarl P Posts: 5
    edited 2009-09-02 22:47
    Sorry Sam

    I dont have the hang of the url attachment yet

    im building a refrigerant monitoring controller for a garbage plant Im not sure how far i can take it but im going to give it my best

  • sam_sam_samsam_sam_sam Posts: 2,286
    edited 2009-09-02 23:01
    Carl P

    If I can help you with any coding let me know

    Post what you want it to and let see what happens



    Thanks for the Link···· This link really helped a lot for a project at work need a pressure switch like that

    The Glass icon is what you use to link a site

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    ··Thanks for any·idea.gif·that you may have and all of your time finding them smile.gif

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    Sam

    Post Edited (sam_sam_sam) : 9/2/2009 11:46:50 PM GMT
  • FranklinFranklin Posts: 4,747
    edited 2009-09-03 05:06
    The sensor you linked to is only good to 10 bar (145 psi)

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    - Stephen
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