getting different temps out of my thermocouple -

[Burchtree]

did some heat-treating last night and it was obviously too hot. I was using the Omega non-grounded hand probe stuck into the side of the forge as seen at the bottom. Since the extech has dual sensors, I went ahead and stuck the bead-wire sensor in from the top and located it about an inch away from the tip of the Omega probe. On the average, the probe was reading 500 degrees lower than the bead-wire and I have a feeling the bead wire is right. So . . . is the probe off, or is how I've got it arranged effecting its temp problems?

Thanks all once again --

 

[redpygmy]

Hi Michael.
You may have a cold junction in the probe sensor. The probe is probably a welded bead thermocouple in a protective sheath. It's highly unlikely that your oven has such a wide range. The connector on any TC is screwed together and you can check those connectors really easy. If you are reading a lower than expected temperature it's sure to be caused by a cold or secondary junction where the wires are touching each other.I worked in a calibration lab a long time and thermocouples are good, cheap sensors but the bead and connectors can be the source for cold junctions which always (usually) read less.
Larry

 

[JCaswell]

I use type K thermocouples in my salt pots in conjunction with digital process controllers. The TCs that are used for the 'hot side' (about 1,500F) fail from time to time, the result being a much lower reading. Sometimes if I jiggle the wire as it goes into the thermocouple it will flash briefly to the right temp. I've had much less trouble with this since switching to 3/8 inch thick thermocouples that are much longer than needed. This allows me to bend them 90 degrees in the middle (half the probe goes vertically into the salt pot) and run the wired end horizontally well away from the heat source. TCs are pretty cheap and, according to the boys I talked with at Watlow, should be considered consumable parts in these high-temp situations.

 

[Burchtree]

Thanks for the input -- I'll see about taking the probe apart. One thing I need to do (but forgot this morning) is to check to see if they read the same temp at room temperature. . .

 

[howiesatwork]

Take both the probes and insert them in boiling water.
Should read 100°C on both.
If the sheathed probe reads low, you have your culprit.
If both read correct, it might the the unshielded one is reacting to the flame temp instead of the chamber temp, as the heavier probe will react slower and average out the temp.