irratic fluctuations on humidity temp sensor

I have been trying to trouble shoot a problem with a dual temp humidity sensor. The readings are displayed on a panel veiw, I have tried changes the set points on the screen to meet the prosess variable, but the values never match up. one minute they are totally off, and the next there relatilely close. Some days the temp will be bang on and the humidity will be out to lunch. I have inspected the sensor, and it was replaced, but to no avail.The cable is sheilded, and goes back to a Allen Bradley 504 analog module, but what should I be looking for?
DM

temp/humidity

I had quite a number of erratic problems with a temp/humidity sensor at a thing called the CoolZone. He had installed the system with a several thousand dollar greenhouse controller, to turn water mist nozzles on and off on hot days.

The maintenance chap who hired us told us it was a voltage output; we hooked it up to a Micrologix 1000 w. analog and calibrated it.

Several service calls later, to adjust s caling and offsets, he told us it was current output, then told us it was RTD.

I told him that he had been wasting our time with all these calls when he didn't even know what sensor 'twas, after he had been trashing our program and PLC and saying that he, and the end customer, were losing mney and getting irate at us.

We eventually climbed up on the roof and opened the sensor box to check for ourselves. We found the shiny new sensor wrapped in a yellow rubber sleeve tagged "Remove before installation". Arrgghhh!

We had the last laugh as we notified the end customer that his contractor had royalled f'dup, and wasted the greenhouse controller because the installer didn't do his job.

We ended up buying a industrial wall-mount thermostat, and just using it as a digital input to the PLC.

Bottom line:
- check that the sensor is right for the analog card - current vs. voltage, or ??

- check that the analog card is set up for the sensor - DIP switches, jumpers, initialization values in the O registers.

- try a hot/cold test on it, if your sensor is rated for it - dip it in ice water and check the output at 0C/100%RH, and then boiling water to check the output at 100C, or use some other known temperatures.

RT

the first thing is to make sure that the shield on the cable is grounded but ONLY at one end ...

after that ... well, we'll see ...

I'm sure you'll get many excellent suggestions from a lot of truly knowledgeable people ... but if all else fails, you might try this "old-timer's" trick ... sometimes it's the ONLY thing that will work especially when the signal is "good" for a few seconds and then goes "haywire" for a few seconds and then quickly settles back down again ...

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the value or wattage of the resistor is not critical ... it's just there to "bleed off" any stray voltage from the transmitter's power supply ... I learned this from a genuine old-timer who would literally put his finger on the analog input screw ... if the erratic signal suddenly settled down, then he'd install one of these "bleeder" resistors ...

once you get the problem solved, please be sure to come back and let us know what solution actually worked for your system ...