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Old 04-28-2018, 10:56 PM   #1
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Default I will preface this by saying this is probably a stupid question.

Does putting four thermocouple wires in one terminal block (as in two wires in, two wires out) cause issues with temperature readings? I was at a customer's site recently and they were having some issues with one of their temperature signals. The first TC was reading the temperature correctly and each panel was getting a steady mV signal from it. The second TC was also reading temperature correctly but for some reason the reading was fluctuating ± 30ºf at the two panels that it sent the signal towards. I had to leave before I could completely address the issue. I'm gonna be back on their site in May and I wanna have some ideas of where to look first. Anyway, I noticed that they had a few TB where two TC wires went into and two TC wires went out of the TBs. My thinking is that for the signal to be as erratic as it was, given that we're dealing with mV here, minor changes in resistance are gonna wreak havoc on the signal. Is it common for TC wires to be used this way? I've only ever seen one TB per TC wire before so is this a potential issue or am I barking up the wrong tree?
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Old 04-28-2018, 11:00 PM   #2
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Is the terminal block an actual thermocouple block? And is it the right one for the TCs you're using?
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Old 04-28-2018, 11:11 PM   #3
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Is the terminal block an actual thermocouple block? And is it the right one for the TCs you're using?
x2

If it's an ordinary terminal block, you can generally get away with one and make up for it in the scaling/correction factor (if at all). More than one and you're asking for issues unless it's a genuine TC block of a type matched to the thermocouple type.

Fluctuating 30 degrees on either side of the expected reading seems to me, however, to be more like a failing thermocouple or a bad transmitter/display unit/temp control/PLC TC card or whatever you're reading the measurement out on.

Worth a check to make sure the TC extension lead wire is of the type matched to the thermocouple also.
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Old 04-29-2018, 01:35 PM   #4
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My first thought. Replace TC.
I have used regular TB's with TC's and RTD's.
However, they do make special TB's for this application. That is what I would use today if I was in this process.
Make sure you pick the correct TB's as TC's come is several flavors.
I really liked this type of process work BTW.
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Old 04-29-2018, 08:56 PM   #5
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Fluctuating 30 degrees on either side of the expected reading seems to me, however, to be more like a failing thermocouple or a bad transmitter/display unit/temp control/PLC TC card or whatever you're reading the measurement out on.
I checked the signal being output by the TC with my 724 and it was reading ambient temperature accurately without any fluctuations. The wire they're using for the TC was the correct kind but I didn't consider if the TB are for TC use. I will say, though, that the second TC is using the same TB as the first TC and the readings from it are steady.
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Old 05-06-2018, 01:58 PM   #6
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I checked the signal being output by the TC with my 724 and it was reading ambient temperature accurately without any fluctuations. The wire they're using for the TC was the correct kind but I didn't consider if the TB are for TC use. I will say, though, that the second TC is using the same TB as the first TC and the readings from it are steady.
Did you ever get this straightened out? Its not the TB unless the connection there is not good.
We used regular TB's for years. But we always calibrated the controllers after the install. And then every so often afterwards.
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Old 05-07-2018, 10:59 PM   #7
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Did you ever get this straightened out? Its not the TB unless the connection there is not good.
We used regular TB's for years. But we always calibrated the controllers after the install. And then every so often afterwards.
I haven't been back to their site yet, I have to repair and calibrate an ST98 (or return it to the manufacturer) first. I'm thinking the problem is probably a loose connection somewhere, maybe a hidden splice.
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Old 05-08-2018, 04:16 PM   #8
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Default Jump term block

For testing just remove wires from block and connect them with wire nuts.
If problem goes away buy correct block. If not return wires to term block and keep looking.
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