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BX-08AD-1 Anolog Card to PTD25-20-1000H 2-wire pressure sensor

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  • BX-08AD-1 Anolog Card to PTD25-20-1000H 2-wire pressure sensor

    I am having trouble getting a good 0 reading on my sensor input in my BRX plc. I am using two PTD25-20-1000H AD pressor sensors and both units have different values at what should be 0 psi. One sensor is reading 30 psi the other is reading 8-10 psi. Initially, I thought it maybe I did not adjust for absolute pressure but not I am wondering if I have a wiring issue or some current interference

    I have properly programmed and scaled the sensors input to give me the correct high end reading (1,000psi), the issue only lies when there is no pressure being applied to the system and the sensors should read 0 psi. I checked and also segregated the pneumatic circuit and there is no pressure in the system.

    I have the 2-wire sensors wired as follows: the Brown wires are wired together to a common +24VDC, and the white wires go into the input card at I0+ and I1+. The Input card is separately wired to 24VDC. Any help would be greatly appreciated, thanks!


  • #2
    You could use the data monitor to see WX1 and/or RX1 values to see if they are steady and what signal is being received.
    The small amount of signal may be from the resolution of the sensor.
    The PTD25-20-1000H sensor is accurate to +/- 0.75% of full scale or +/- 7.5 psi.
    It would be interesting to see if the reading was correct at 100 psi.

    I have some transducers that are 10,000 psi and have some very small output at no pressure.
    Usually calibrate and scale the sensor individually based upon its ma output vs pressure.

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    • #3
      I would stick a meter on the inputs and see what's there.

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      • #4
        Hi Roger,

        I am using the data monitor and am reading about 6 x 10^36 psi when the system is at 0 psi. Using the data logger and a calibrated digital pressure sensor for reference, I pressurized the system to 300 psi according to the pressure gauge and read 270 psi on the PLC. If I try and calibrate I notice that my scale is off and that I am even more under pressure at 1,000 psi. I might try and reprogram the inputs scalable range from an input max range of 0-32767 to a lesser value like 0-32000

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        • #5
          Changing those values is similar to what I normally do.
          You can use RX1 and change the scaling to match the real physical pressure with hard coded entries
          or
          take the raw WX1 value as input in the scale instruction.
          The value of the input at zero is the zero value tag
          The value of the input at full scale if possible is the max value tag
          Then the scaled value for this sensor would be 0-1000 or add a decimal 0-1000.0
          Output to a tag to be used

          Change the input scale tags to match the pressure low and high.
          The input scale instruction will go off the end, so it could be calibrated with less than full pressure.
          It will not be perfectly linear and be off more the lower the calibration pressure is from the maximum pressure.

          The RX1 scale feature can clamp to zero and full scale.
          If you dont want any slight overshoot while using the scale instruction, an extra line of code can rewrite the output tag to one of the limits if it is outside the limit.

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          • #6
            Thank you, I was able to get the sensors working in a good range, I calibrated them by adjusting the Max input value and added some code so each max pressure range I needed was on its own rung and calibrated so. Also, I noticed my 24VDC supply was supplying around 22VDC so that may have helped the sensors max WX range some.

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