Hi
I have a CR1000 with 8 thermocouples TypeT. Thermocouples are homemade twisted and then isolated thermocouple cable. The cables hang down from a buoy and are used to measure watertemperature down to 8m depth. This has been working pretty good for 5 years now.
But from time to time individual thermocouple-channels show weird data. For example jumping randomly between the actual value that is for example 6 degC and much higher vales e.g. 40 deg C.
Sometimes I replaced the particular thermocouple with a new one to solve the problem but I recognized several times that values got back to normal without any action been taken.
I am wondering if that can be a weakness in the wiring panel. I alway checked that the screws at the wiring panel are tightened firmly.
Any ideas?
Best regards
Uta
Norwegian institute for water research
* Last updated by: uta on 9/25/2015 @ 2:48 AM *
If it is the case of a loose connection, you could detect it with a voltage range that checks for open inputs.(ex. mV2_5C) If there was a loose connection at the wiring panel, the measurement would return NAN. ShortCut uses voltage ranges with this option.
If you are using a voltage range with open circuit detection, then the bad reading are probably caused by failure of the isolation on the thermocouple end.
I assume when you say it is a twisted thermocouple, the sensing end is either welded or soldered as well. I have seen some installations where the sensing end was just twisted together. Those sensors worked at the beginning but then due to vibration or surface corrosion, the connections became intermittent.
Hi everyone,
The topic is not recent but I'd like to come back to the problem :
I have 38 thermocouples wired via two AM416 muxes, powered by a CR23X (on which are wired light sensors).
These thermocouples are fixed to leaves by adhesive tape for half, or measuring air temperature for the other half.
The end of the thermocouple is twisted very tight on about 0,5cm, so I am pretty sure there is at least one contact point between the two wires. Plus, the installation has been done a few days ago and is supposed to last two months, so there will be no time for degradation of the set up.
For 10 of the probes, both free or fix on a surface, I have intermittent values or no values at all (-INF), and I can't find the reason why. Could it be some logger channels failing ? The twisting ? Bending the wire too much ?
Many thanks for your help and ideas about that !
Ada
Edit : FYI the thermocouples are T type from Omega, I have no soldering iron powerful enough to melt it, and brazing is not an option either...
Some quick things to check:
1. If you have connectors in between your junction and the mux, check that the wires did not break when you installed the connectors (at the screw terminals).
2. If the TCs are of a very fine gauge it is possible to have broken the wire within the insulation; the break may not be visible but can cause intermittent opens when the wire moves (in the wind, for instance).
3. Check that your wires are well seated within the mux terminal block. Sometimes fine gauge wires can be loose even when the terminal block screw is tight. Something as simple as a strand of wire from a previous installation can leave enough of a gap when using fine gauge TCs.
Good luck.
I am still suspicious of the contact of the two twisted wires. It doesn’t take much movement or vibration of the wire to cause an intermittent connection. Also, depending on the environment oxidation may occur quickly resulting in a poor bond between the two wires again causing an intermittent connection.
There are low melting point paste solders that can be melted with a simple butane fire starter or cigarette lighter. I have seen that for sale on Amazon and places that cater to jewelry makers. Indium is one company which makes solder that melts as low at 47 degrees C.
Remember, you don't need to melt the thermocoupe wire itself, just the solder that bonds the wires. Almost any soldering iron will get hot enough for that.
You really need a good electrical contact. I wouldn't try anything else until you solder the junctions.
I don't know if they are available in your area, but you can get butane powered soldering irons. You can't take them on planes, but they work well in the field.
With a twisted junction, there is very little contact surface between the metals. The current output of the thermocouple will be exceedingly small and could easily be overcome by noise.