TC connections, best practice?

[Ken Moore 23]

To make a long story short, I have TC's connected via I.S. zenner barriers to an AB SLC 5/04. About once a week, one of the TC's reads open channel. Have to shutdown the system, purge the area, and then open a large NEMA 7 enclosure (about 100 bolts), unland the TC's from the barriers then reland them. Is there anything on the market that can be used to minimize this? I dis-like TC's but this application would not work with RTD's, (too cold).

[TConnolly 13]

Have you considered using an IS transmitter? At the very least it may help get the connection out where its easier to get to. BTW, what kind of a TC is it? Are you suffering from an oxidation problem? (I've got my own current TC struggles as you may have seen from my thread over at the other forum.)

[Ken Moore 23]

It's a type "T" TC measuring a cryogenic liquid at about -150F. It may be an oxidation problem, hard to tell, there is not any physical indications. Everything looks fine. Oxidation is one possible answer, by relanding the wires, we get a "new" connection. There are Five pumps on a skid, so at least one fails per week. So the average time between failures for a particular TC is around 5-weeks. Could be long enough for oxidation to occur. I'll look into some type of protective coating.

[TConnolly 13]

Type T is Cu/Cu-Ni. The copper could be oxidizing. The law of intermediate metals would suggest that a very thin tinning of the wires would not introduce any CJ error because the thin solder coat would be uniform in temperature. So I would suggest tinning the ends of the TC wire and see if the problem disappears.

[BudMan 1]

Zener Barriers for Thermal Couples? I have not seen these before. Could you please let us know the part number for this style zener barrier? I am just curious how they would get pat the cold junction errors going through the barrier. Thanks

[TConnolly 13]

As long as all the terminals on the barrier are the same temperature the CJ errors will cancel each other out. Iso-thermal blocks are sometimes used to assure that the terminals are at the same temperature.

[DanW 41]

Ken, Why not RTDs? I whipped open my Pyromation catalog to confirm that conventional Pt DIN style go down below -250° quote from catalog: LOW RANGE WIRE WOUND (-200 to 204) ºC [-328 to 400] ºF 1/8" OD 3/16" OD 1/4" OD 3/8" OD sheaths R1T185L ± 0.1% 100 ? ? = 0.003 85 ºC etc. etc. The thin film only goes to -40°F, wire wound goes lower. It must be your analog input card ? ? ? Usually 3 wire RTDs don't use the same IS barrier as T/Cs do, either, it'd be an expensive changeover. Alaric, are the connection pads on your mold the S type T/C elements or the copper extension wire? Dan

[DanW 41]

It just occurred to me, maybe at cryogenic temps the self heating effect of an RTD is self-defeating? Pumping heat, albeit a small amount, into the process is not acceptable? Dan

[Ken Moore 23]

The barriers are P&F model Z955. Dan we are currently considering switch to something else. I was not aware that RTD's could handle the lower temperatures. The TC's are the bayonet style, so would need to find a RTD like that, so the existing thermowelds could be re-used. To retrofit from TC's to RTD's would require new I/O cards, and new barriers, and new conduit / wiring, cost is not the limiting factor, the lost production makes the cost almost insignificant. The limiting factor is the size of the existing NEMA 7 enclosure. Each TC requires only one barrier, but when using RTD's you need three barriers for every two RTD's, because of the third wire. I don't know if I have room for the extra barriers, I'll have to check next time we open the enclosure, looking at the drawings it appears that it will be very tight. There are 10 TC's so I would have to add 5 additional barriers. I'll give Pyromation a call today, if I could get RTD 4-20 transmitters, that would eliminate the need for the extra barrier space, since the transmitters would be two wire.

[DanW 41]

Ken, I'm really stumped why you'd get dropout on such a periodic basis. It's clearly a hazardous area, is it corrosive, too? You indicated you hadn't noticed anything in particular. Could copper wire to terminal screws really corrode over a 4 week period to the point you'd get drop-out? What about vibration? Are you running compressors close by to maintain the cryo? The drawback to changing out to RTDs is that the real problem is still an unknow, and RTDs aren't all that different than what you have: Type T T/Cs are copper alloy, not a heck of a lot different than copper wire from a transmitter. If you switch to RTDs you could have same problem - copper wire to terminal screws; all that cost and your boss mad at you. What else could cause this dropout? It's never the same circuit that goes out, right? Not like a broken component or bad solder joint in the barrier? Is the cold junction done by the AI card? cold junction would be common to all inputs. Could the cold junction be flakey? intermittent solder joint? By the time you wrestle the cover off, it fixes itself and you think you've done it by relanding all the copper connections? But if it's common to all inputs, all the inputs would flake out. What about an AI board swap for a couple weeks to see what happens? How hot does it get inside the enclosure? Can you monitor the cold junction to see? Could the temp be cycling somehow and affecting operation? One channel goes flakey when the temp goes up? Dan

[Ken Moore 23]

Had another open channel yesterday. One of the instrument techs and myself opened the enclosure, there was no visible sign of anything being out of order. We've had to do this type of thing so many times, that we made up a quick reference card and posted it inside the door. Slot-7 channel 1= barrier 43, slot-8 channel 8=barrier 57 etc... Once you have the door open, you can see the led flashing on the NT8, these things are so touchy that you can watch the led stop blinking, and think you've corrected the problem, then in about 5 seconds it starts blinking again, correcting one open channel, you can cause another to go open. So far we've tried new I/O modules, new barriers, and new wiring. The skid is outside underneath a shed roof, so it does get hot, but we have the same problems in the winter. These are piston pumps, so there is some vibration, but none of the other connections have the same problems, just the TC's. I think it's because of the very low voltage, around 0.0015 volts, for the temperature range we run at, that the slightest extra resistance makes it go wacky. I say it's an open channel, but the LED flashes for open channel, or out of range, so the increased resistance could be causing an out of range condition. I've already run replacing all the TC's with RTD's and 4-20 transmitters by management, cost will be about 20-25k and 12 hours downtime, plus a minor update to the PLC logic, which can be done offline ahead of the shutdown. I have a verbal okay, will get a hard budget together, submit an AR and start planning the replacement.

[Beuwolf_1 0]

Just had to ask if it was a feed back problem with grounding. We've had Type J TCs give strange readings due to stray voltage differences between grounds. Just an idea that I though would be worth mentioning.