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Pressure Sensor for water level measurement

Discussion in 'General Electronics Chat' started by premkumar9, Apr 1, 2015.

  1. premkumar9

    premkumar9 Member

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    Hello,
    I wanted to measure water level up to 50 Mts with 5 cm accuracy. Can I use a pressure sensor for this purpose? I tried a pressure sensor with built in amplifier got from one supplier. I kept it at a depth of 2.5Mts of water and monitored the output throughout day time. I found a variation of 1.5% in the output voltage. What could be the reasons for such variations? Could it be due to the atmospheric pressure variation? Is there any way to compensate it?
    Your help will be highly appreciated.
     
  2. MCU88

    MCU88 Member

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    Is the water in an tank with known size? Have you considered using ultrasonic technology to measure water level where tiny echos are transmitted and received?
     
  3. premkumar9

    premkumar9 Member

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    Water in a dam. 50 Meters.
     
  4. dave

    Dave New Member

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  5. koolguy

    koolguy Active Member

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    Hello,
    There are many ultra sonic sensor are not good in this situation, i think because it sensor get wet due to evaporation.
    may be something like Electro magnet Reed switch?
     
  6. alec_t

    alec_t Well-Known Member Most Helpful Member

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    Have you checked the specs of available pressure sensors? You are asking for a resolution of 0.1%. I'd be surprised if any one sensor type would be linear to that accuracy, so multiple calibration points would probably be needed. This link describes various non-linearity components.
     
  7. JimB

    JimB Super Moderator Most Helpful Member

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    Yes it is usually an expensive pressure transmitter that will give you 0.1% accuracy.

    Can you tell us which pressure transmitter you are using?

    Whether the Ptx will respond to atmospheric pressure depends on the type and configuration. More information required.

    JimB
     
  8. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    While you do not mention the manufacturer or model number of the pressure sensor you used there are many variables that can effect your measurement over time. Temperature? Changes in atmospheric pressure? The uncertainty of the instrument used to measure the pressure sensor output? The excitation voltage applied to the sensor? Then you have the data sheet specifications of the sensor itself.

    You can have the 0.1% accuracy you desire but it requires, as was mentioned, more expensive pressure transmitters. Here is an example of an Omega Manufacture Smart Pressure Transmitter, High Stability, Low Drift pressure transducer. Just the transducer will cost about $2,300 USD and if we add a quality process meter we easily will exceed $3,000 USD. That is just an example, there are other manufacturers like Rosemount who make similar units in precision and cost.

    So what this comes down to is I can buy a pressure transducer/transmitter for about $25 USD or I can buy a pressure transducer/transmitter for about $2,500 USD depending on my needs. They will both measure pressure but one does a much, much better job but cost much, much more. :)

    What you are seeing is not at all unusual. I would start with a differential type sensor and work from there based on your uncertainty needs.


    Ron
     
  9. jpanhalt

    jpanhalt Well-Known Member Most Helpful Member

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    There should be some reference point on the top or face of the dam that doesn't move. Can't you measure referenced to that?

    John
     
  10. cowboybob

    cowboybob Well-Known Member Most Helpful Member

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    Your stated accuracy requirements are severe.

    Given the depth of the water, you need a pressure sensor in the ≈70 PSI range (0.445lb per column-foot).

    This site: http://www.astsensors.com/submersible_level_sensors/AST4500__AST4510

    has sensors that, I believe, meet your needs (accuracy, barometric pressure compensation, etc), but are very expensive, especially with their associated electronics.

    If I may ask, why the need for such accuracy (a 5cm depth differential would be overcome with a stiff wind "piling" up surface water)? jpanhalt's suggestion sounded inexpensive and easily implemented, if you're willing to accept a reduced accuracy.
     
  11. premkumar9

    premkumar9 Member

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    Thank you all for your replies. Normally Alerts come to my inbox when somebody replies. This time alerts came only for first 2 replies. So I didn't check the other replies. Now only the replies came to my notice. I will go through them in detail and revert. Once again thank you all.
     
  12. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    Hi,
    Many depth pressure sensors have a built in air vent pipe within the cable to the transducer.
    This ensures that the differential between the water pressure side of the sensor diaphragm and the atmospheric side compensates for changes in barometric pressure.
    The 2.5Bar depth transducer [~25mtrs] I used cost around ~£120, you can get 5Bar sensors [~50mtrs], which is the depth that you have specified.

    The 2.5Bar transducer gave water depth accuracy of +/-5cms.

    What maximum range of water depth 'change' do you expect to see in the dam water.???

    E
     
  13. premkumar9

    premkumar9 Member

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    30 meters
     
  14. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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  15. Externet

    Externet Member

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    For the original question; yes, a pressure transducer can be used. But I think that attaching a measuring tape to a dam wall would be much simpler, reliable, accurate, precise and long lasting.
     
  16. premkumar9

    premkumar9 Member

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    The data is to be sent to the server. So electronic measurement is required.
     
  17. premkumar9

    premkumar9 Member

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    I can fix an another pressure sensor at fixed location near the dam and take the readings. Can I use this readings to compensate the pressure variation due to atmospheric pressure change? Is this atmospheric pressure change the main reason for the drift in sensor output?
     
  18. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    You could use an external atmospheric pressure sensor to correct for changes in the readings, but the 'drift' may be due to other causes in the equipment being used

    It should be easy enough to test using the atmospheric pressure sensor,, record both the readings from the submerged sensor and the atmospheric sensor over a few hours,
    observing the actual water 'height' using a semi immersed calibrated water level board.

    Do you follow that OK.?
    E
     
  19. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    The datasheet Eric linked to had, pressure, density @ temperature and G.

    I THINK if one were to look at relative height, you would need a ported sensor or one related to differential pressure. So, i would think you would need calm waters.
    Acceleration due to gravity varies. See: http://www.blazelabs.com/f-u-massvariation.asp. Density will vary with temperature. If then sensor has any smart, the temperature would be measured at the sensor. The temperature may not be the same over the measured distance.

    Further remember that the pressure REPORTED by the weather bureau is NORMALIZED to sea level even if your 5000 ft up.

    You have a lot of potential error sources because this isn't a tank on the ground.

    Now, I'm wondering if you could do a differential sort of measurement to a known depth (floating sensor).

    Say the floating sensor is 1 foot under water, but it reports a value of 13". Your sensor reports 26' The density of the water may change with temperature and depth.
    So, you sort of normalize it.

    These are just ideas.I haven't worked in the field. I have had hydraulics and relativity in class, but don't remember much.
     
  20. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    You may get a better understanding by reading this PDF.
    E
     

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  21. premkumar9

    premkumar9 Member

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    I understand like this. The actual pressure sensor output from the sensor immersed in water at a known depth and the output voltage from a sensor at fixed position has to be compared over a period of time to understand their relationship. From this a correction factor can be worked out. Am I right?
     

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