Thermocouple troubleshooting
Thermocouple troubleshooting
(OP)
There are three ungrounded, 1/8" OD type J thermocouples in a single pipe well operating at ~690 degF. One of them is reading ~15 degF lower than the other two. We're being asked to troubleshoot the coolest thermocouple to see if the temperature deviation is real or not. There is no indication the pipe well is compromised but we're not going to attempt replacement or removal of the thermocouples until the vessel is out of service due to high pressure.
We tested the mV reading from the thermocouple and it matches the temperature indicated to DCS. We simulated a mV reading to DCS and it read correctly. So far so good.
I also asked our techs to check insulation resistance between the lead wires and the sheath for all three thermocouples. All three read ~110 ohms. I expected a much, much higher resistance since they're ungrounded. These were checked with a battery power multi-meter rather than a megger. Do I need to megger test to get accurate insulation resistance?
Any other troubleshooting steps I can do?
We tested the mV reading from the thermocouple and it matches the temperature indicated to DCS. We simulated a mV reading to DCS and it read correctly. So far so good.
I also asked our techs to check insulation resistance between the lead wires and the sheath for all three thermocouples. All three read ~110 ohms. I expected a much, much higher resistance since they're ungrounded. These were checked with a battery power multi-meter rather than a megger. Do I need to megger test to get accurate insulation resistance?
Any other troubleshooting steps I can do?





RE: Thermocouple troubleshooting
TTFN

FAQ731-376: Eng-Tips.com Forum Policies
Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers
Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
There is a homework forum hosted by engineering.com: http://www.engineering.com/AskForum/aff/32.aspx
RE: Thermocouple troubleshooting
RE: Thermocouple troubleshooting
Apparently water vapor gets into the MgO (presumably through the transition) and lowers the MgO resistance. The lower MgO resistance better enables chemical pollution of the junction at high temperatures. Not that such applies to your case, but it does tell you that something's wrong with only 100 odd ohms between element and sheath, probably like the MI is soaking wet?
The resistance of a polluted thermocouple junction increases as the junction is less and less 'pure' alloy. A polluted junction no longer tracks the accepted thermocouple tables and associated polynomials that do temperature conversion. You get mV, but the mV aren't valid for the given temperatures for that type thermocouple. The thermocouple is a polluted 'bastard' thermocouple, with no standard table. The altered mV readings are 'drift' from the pure alloy tables.
Honeywell's temperature controllers and PAC T/C input boards measure the T/C loop resistance (probably with the current they use for burn-out detection, I'm speculating) and can alarm a 'warning' at 80 ohms resistance and alarm 'failure imminent' at about 160 ohms. A new, unpolluted isothermal T/C (both ends at the same temperature) will have junction resistance of less than 1 ohm. Extension wire has considerable resistance compared to copper wire, so if you use an ohmmeter, you need to take that into account.
There'll be an error when the T/C EMF bucks the ohmmeter current, too. But it looks like you've got a couple elements to compare against.
But I really suspect mechanical damage where the sheath is crushed and one of the legs is separated from the sheath by a couple grains of wet MgO.
RE: Thermocouple troubleshooting
If it's orientated vertically the moisture can flash off, cooling the tip condense higher up then run back down to do it over.
Since it's so hot if you vent the well for a few minutes it will drive any moisture out very quickly.