Thermocouple Problem

Is the temperature varying intermittently? It sounds like perhaps you have an intermittent short circuit (or junction open circuit) on either the thermocouple cable or extension cable that is causing the junction point to move and provide you with a measurement of a cooler spot.

Try checking the cables for mechanical damage. Additionally check the thermocouple and extension cable plugs and sockets to ensure the wires have not twisted together creating artificial junctions.

Be very careful if performing high voltage (eg- 500/1000V) insulation tests as you may damage the thermocouple cable insulation (which is generally not rated for these voltages).

thanks.

there is one junction box in between which is located nearby the thermocouple;

on the JB , one side TC element cable is connected, other side extension wire is connected which is coming down for 100meter, used core size 0.22 sqmm,

IR tested using 100vdc,

reading is keep changing, going high to 200dec C and down to 40 Deg C this changes happen approx. 4 times in 1 hour interval;

1. 250 ohms sounds a bit much to me.

2. What's your ignition system? HV?

3. What kind of "transmitter"?

It isn't clear whether your measured temperature is 200° and the spikes are negative to 40°C or whether the measured temperature is 40°C and the spikes are positive up to 200°C. Which is it?

First thing I'd do is to remove the FF transmitter, substitute any device that reads thermocouple direct and determine whether the problem is the same (problem in the thermocouple and its wiring) or whether the temperature maintains a value (problem is in the transmitter).

If the problem is in the T-C or wiring, the thermocouple wire resistance seems reasonable. K wire resistance is slightly less than 2 ohms per meter for 20G wire.

JoAT's recommendation to check the wiring is valid. A shorted junction in the lead wire will show something close to ambient temperature. An open circuit will drive the xmtr's indication to whatever its fail-condition is, many times it is upscale, but there's always a choice of downscale.

If the problem appears to be in the transmitter, you might try to discover the maximum resistance that the transmitter can handle. I once discovered an analog temp transmitter that could only handle about 100 ohms of resistance on its input.

Any current carrying conductors in the vicinity of the TC cable run? AC or DC can both effect the reading.