4 - 20 ma Pressure Transducer "unstable" with IF4

[Chris Elston 79]

Granted I've got a cheap pressure transducer and it seems the raw values coming back from this device seem unstable to me. I've got the sensor connected to a 1762-IF4 card. Actually I got four of them. They all act the same except one dead one...LOL anyway... The transducer range is: 0 - 500 PSI. At no pressure, I get a raw value of about 1400 - 1800 fluctions, which I offset this value to a zero point and then run it through scaling, this spits out a PSI value of fluctions of about -1 to 3 PSI. The next day my raw value seems to jump around, it might climb to 1800-2200, still a 400 value difference but now my offset is wrong and my sensor jumps around like 6-9 psi all the time at no pressure. So I got to thinking...and I remembered something a long time ago... When you guys are running analog current sensors, do you use a LINER DC power supply? I've got a Sola DC switcher. Could this be why my sensor is so jumpy and all over the place? It should be alot more stable than this I think. What gets me is from day to day, it settles at a different "zero" point...which aggravates me. I've got shielded cable and everything like that seems good to me. Even a short lead direct to the card holding the sensor in my hand yields the same "jumpy" results.

[finfin 0]

I would start by measuring the output current of the instrument with a meter to get some feeling that The card is measuring correctly. Assuming it is; I would read the specs on the instrument to see if this is acceptable variation and also to determine the power supply requirements. You might also want to track the power supply while this variation is occurring.

[Bill Linne 0]

We see this type of thing when more than one power supply is connected to an input module. In other words, if you have transmitters with 4-20ma outputs that derive their power from more than one source (either they are 4-wire instruments or their loop-power comes from more than one power supply) interference can result. Our "cure" is to add a loop-powered isolator in front of the offending source. Many companies make the isolators, but my favorite is AGM. They are very stable and have a seven-year warranty. http://www.agmelectronics.com/SCGraphic.htm If you can, try connecting one of the "unstable" instruments to a module all by istelf and see what happens. Bill

[Ken Moore 23]

I've been using the Sola switched power supplies for analog devices several years, never had a problem. So the fact that it is a switched power supply is not your problem. A bad power supply could be it, but I doubt it. I agree with others, use a meter and watch the device output. Sounds like the devices are the root cause, not the card. Do you have an analog meter (most people don't), they respond so much faster than the digital ones, you can see the needle jumping all over the place on a noisy signal. Speaking of noise, how are the transducers wired? Twisted and shielded?

[Chris Elston 79]

Yup, twisted and shielded. I think it's just the crappy transducers I have. They seem a bit unstable to me. I don't think I will buy any more of them. On a side note.... I just bought a Balluf cylinder LVTD, I hope that bad boy behaves...it cost $1400. If grandma was right, "you get what you pay for", I shouldn't have any problems with that bad boy.