I Thought I Knew Thermocouples...But

I'd go with a "no contact" sensor myself

I agree with Fredski. A no contact temperature sensor would likely be best here.

I'm not sure what the OP wants here?

I want to know how a TC can shift or change to a higher value. I understand how it can read low. Most of the time in this application they just open up. Have not seen this before.

As far as a non contact measurement, what can you use to peer through a 2" hole in refractory? That sounds interesting.

I like Tony's answer below as a plausible reason for a thermocouple to read high. You also have to remember that a thermocouple produces only a very small voltage. A thermocouple does have a very low source impedance so interference does not easily get induced but any that does get on these wires can produce a very high error.

There is a large spectrum of non-contact measuring instruments. Here is one of the most well known.

Thermocouple effect appears on each and every junction between disimilar metals or alloys according to that paticular junction or connection termperature. The total voltage you get is the algebric sum of those voltages and not necesarily the wanted couple voltage. To eliminate the errors from this issue, the connections of any extension cable MUST be of the same metal as each TC metal, OR both connections be of absolutely same metals AND temperatures. S.M.

Just a thought. Have you considered a type N t/c? They will work happily at 1000°C and are less expensive than type R. OK you will have to change the comp cable, but if your indicator/controller is modern it may accept different t/c inputs. Also, if you use a mineral insulated t/c of say 6 mm diameter which should fit into your ceramic well, then you've got further protection.

As I said, just a thought.........

Thanks for the input folks. No solution as of yet but I can tell you what we have looked into and some of the results.

We tried the noncontact method with several of the handheld IR meters but they only worked up to 500 C. The cone sensing (target gets larger as distance increases) made it very difficult to get measurements 24" into the well without averaging in cold refractory along the edges. The thermal imager worked the best once it was finely focused but it gave us no electrical feedback.

THe extension wire was of the same type but we tried a section from another roll just to be sure. No change in readings.

There were several ideas on chemical contamination of the wire or junction. These elements are bare thermocouple wire that is ran through a 6mm ceramic tube with two tiny holes that the individual wires travel. At the tip is the exposed junction. There is no mineral insulation or stainless material on the element. At the termination end which is outside the furnace the ceramic tube is swaged into a SS tube and may have some sort of MI at that point to the exit wires ~3 -4 inches.

The millivolt range for this TC is ~ 0 to 21. At 900c the rated output is 9.2 mV or about 40% of the output.

We should have our new TC elements here this week so we'll get to do some more tests and hopefully get something working properly.

Your style thermocouple assemblies do not have MgO insulation, but the tip and wires are exposed to whatever the atmosphere is inside the ceramic protection tube. I suspect that your elements have become chemically polluted because of the symptoms you describe.

Thermocouples that are chemically polluted (and drift) increase in resistance compared to a new thermocouple of the same type.

Why not test and see?

The best way to measure resistance is when the thermocouple is isothermal, that is, it has no temperature gradient across it because when a thermocouple has no temperature gradient it outputs zero millivolts. Zero millivolts will not alter the measurement taken by an ohmmeter, which always applies a voltage to the circuit to make the measurement. A thermocouple is near to isothermal when has been sitting on a bench for a period and both ends of the T/C are at the same temperature.

I'd suggest that when you get your new thermocouples, you put the news and the old ones on a bench, let them sit for period and equilibrate to whatever the ambient temperature is and then take a resistance reading with an ohm meter. Swap the meter leads and take a 2nd resistance reading with the opposite polarity. Write the results down.

Do the same (at the same time) for your used thermocouples. I suspect you'll see significant resistance differences between the new thermocouples and the old thermocouples, indicating chemical pollution.

I only have base metal thermcouples (type K) to work with (no noble metal, like Type R or S). New, unused base metal thermocouples are less than a couple ohms. Base metal thermocouples drift at around 80 ohms and I've seen them go as high as 200 ohms.

Base metal extension wire has a fairly high resistance compared to copper wire, and I'm not talking about measuring extension wire resistance as part of this measurement, this is about the resistance of the thermocouple itself.

Please keep us posted as to your findings.

The results are in. Iris and tonykuphaldt were correct. There was a combination of wire contamination and MgO insulation contamination. The new thermocouples measured ~1.2 ohms. the defective one was 200k.

We separated the ceramic element at the point the SS started at the termination end. The TC wire to the junction side read 1.2 ohms. The TC wire on the SS side to the + output lead read 70k. The other TC wire to the - output was 1 ohm. Somewhere inside the SS section there was a junction where the output leads were connected to the TC leads and this area had what appeared to be this MgO insulation. From the positive lead to the SS surface there was a potential of 215 mV and on the negative side there was a potential of 112mV.

Thanks again for the help and the insights.

Cheers