RTD fluctuations

[mcpaytas]

What would cause an RTD (several actually) to fluctuate wildly? By wildly I mean 40 - 100 degree jumps every few seconds. I am using 3 wire RTD's hooked to a terminator IO talking to a DL205 and a WPLC running TnD. The cable being used is STP. thanks! Edited November 29, 2006 by mcpaytas

[geniusintraining]

mcpaytas, Is this a new application or a old one that just started to do this?, do they all come back to the same input card?

[mcpaytas]

This is a new application. they all come back to the same input card on a terminator IO which is a slave to a DL205. The RTDs (5 of them) are fluctuating anywhere from 50deg to 100deg in temperature. We have grounded all of the cable shielding. We do have a VFD and 75hp vent fan in the area, would that have a negative effect?

[geniusintraining]

sounds like it may be hardware... have you tried posting your question here http://forum1.automationdirect.com/cgi-bin/Ultimate.cgi they have techs that watch there board (not that they dont here) but you may get a fast reply, sorry I could not help

[Ken Moore]

Sounds like noise to me, I've seen the same type of thing before. When you say shield are grounded, exactly how are they grounded? You could try disconnecting all but one of the RTD's, see if you still have the swings. Try each one by it's self, I'm not familiar with AD hardware, but one noisey RTD could be causing all the reading to skew.

[mcpaytas]

I'll give that a try and get back to you. Thanks!

[rlp122]

It could be noise. Or it could be the card is actually bad. I know that the thermocouple cards will do this same thing if the voltage between the channels exceeds 5 volts. The resultant differential pops a circuit designed to cancel common mode noise causing the readings to spike at random intervals. I do not know if the RTD card has the same circuit or not.

[mcpaytas]

Thanks for the reply. I am going to call AD and see if they have an answer. I will post the resolution here. If I find one :)