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Thread: Voltage spikes to induce false readings

  1. #1
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    Voltage spikes to induce false readings

    I have a natural gas compressor with a control panel (Remvue) and and ignition panel (Murphy) a few feet away. The rem panel has an analog out ran through a AI barrier, into a 1 pair beldon cable, into tray and up to a pressure sensor. Through the transducer, and back to the panel for control. The shielded beldon is ran in the same 4" inst tray as the ignition wires coming from the Murphy. These cables have metal mesh along the outside and hold 250volts that pulse to each spark plug. With that being said, I want to know if a pulsed voltage can cause a different reading (being a resistance) on the beldon? The Analog wires (4-20) use resistance to measure the pressure from the transducer. I can simulate 159 kPa with a 1.3 kohm resister on the rem. The shutdowns are at 15 kPa and for some reason we have a spike every so often up to 200 kPa. I guess I'm wondering if a spiking 250 voltage can cause a different resistance in my 1 pair beldon, thinking that it's actually seeing a higher kPa than the transmitter is sending. We shut the transmitter in and still got a high reading every so often. I hope this is enough info, but ask if you need more about the situation.

  2. #2
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    I suppose it is possible but 4-20 mA signals are low impedance and as a current controlled signal fairly resistant to noise of all types.

    Electrical "noise" and "bad" grounds are often blamed for erratic operation of all kinds of things, but IME it is rare to find that is the actual cause.

    Just where are you putting the 1.3 kOhm resistor?

    Most of these circuits are 24VDC and that would give you less than 2 mA of signal if you put it into the 4-20 mA circuit.
    Last edited by petersonra; 05-20-12 at 01:29 PM.
    Bob

  3. #3
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    We first thought the irrational reading were from the AI card and it was changed, that didn't fix it, so we moved to change the barrier, transmitter, and re calibrated the raw costs in the panel. We did find the counts were out by a few thousand which translates to a few kPa. But on a 15 kPa shutdown, a few is alot. After everything was replaced, we were still having the spikes come in, this is where we thought maybe the back pane of the input card was toast. It's only the one imput seeing the spikes. The resister was put in the input card to simulate a signal. The 159 kPa held for an hour without moving at all. Since we did that, I don't think the card is the problem. The resister was removed and cable re terminated spikes follow.

  4. #4
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    I did make sure the shield was isolated on the transmitter end and tightly connected to the terminal in the panel.

  5. #5
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    Quote Originally Posted by Electchicken View Post
    ... The resister was put in the input card to simulate a signal. The 159 kPa held for an hour without moving at all. Since we did that, I don't think the card is the problem. The resister was removed and cable re terminated spikes follow.
    Connect that resistor at the transducer end of the signal cable. If you get spikes, you'll know they are being induced on the cable. If not, the spikes are being introduced by or through the transducer.
    I'll never get there. No matter where I go, I'm always here.

  6. #6
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    o'scope?

    If you have a dual trace oscilloscope, you could look at the spark wire with one trace and the pulse wire with the other and see if there is a match. Probably trigger on the spark wire.

  7. #7
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    Quote Originally Posted by Electchicken View Post
    I did make sure the shield was isolated on the transmitter end and tightly connected to the terminal in the panel.
    While this is good practice, most times it does not make any real difference.
    Bob

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