Trouble with Thermocouples

D

dustfurn

Joined
Oct 2, 2007
Messages
72
I recently finished a vertical, temperature controlled heat-treat forge, and I've noticed a few strange things with my temperature readings:

-I have two different K thermocouples & two different thermocouple thermometers (a simple type K thermometer and a JLD612 PID controller set to use type K thermocouples).

-The thermometers are kept in the same ambient temperature environment (~60 degrees F).

-The thermocouple probes are within < 0.125" of each other in the forge muffle.

-The two thermometers show different readings by about a 120 degree F at around 1400 degrees!.

-Both of the temperature readings are off significantly (in the worst case by up to 200 degrees ) compared to readings taken by an infrared thermometer with appropriate emissivity settings.

-Switching out the thermocouples between the same thermometer also gives very different temp readings (but not the same differences as described above).

-They both read boiling water as 204 and 206 degrees, respectively (at ~900feet, which seems weird to me).


I have two questions, really: What is going on, and how can people possibly claim to accurately control temperatures if there are such wild differences between two thermometers and two thermocouples of the same type!?

a 100 degree difference is pretty significant!

Thanks for any insight,
Dustin
 
Last edited:
One thing you need to remember is that in industry, where these probes are generally deployed, they can make corrections to tune for each probe. Each thermocouple will be accurate to +- a few degrees at a given temperature, but baseline tuning may be necessary to get truly accurate readings.

I know of one product called the Stoker that is a controller for charcoal fired smokers. The manual describes configuration of a "correction factor" for tuning thermocouples where you place the probe into boiling water and adjust until you hit 212F.

There are also other factors that can affect the readings. For example, what type and gauge of wire do you use for your probe leads? It is what the manufacturer specified? Are your connections as described by the manufacturer, etc?

Personally, I use the TC in my forge to let me know "about" where I am, but for stuff like HT, I'm also using things like my observation of decalesence/recalesence to decide my proper temps to quench.

-d
 
Thermocouple meters can benefit from calibration. They are also sensitive to the type of lead wire and the connections between the meter and the probe. A single crimp connector, such as I have seen on some thermocouple probes, can negatively influence the accuracy. Battery operated T/C meters can become seriously inaccurate when the battery becomes low. I have had good luck with Omega both for probes and meter/controllers (usually from EBay). -Doug
 
DougSeward said:
Thermocouple meters can benefit from calibration. They are also sensitive to the type of lead wire and the connections between the meter and the probe. A single crimp connector, such as I have seen on some thermocouple probes, can negatively influence the accuracy. Battery operated T/C meters can become seriously inaccurate when the battery becomes low. I have had good luck with Omega both for probes and meter/controllers (usually from EBay). -Doug
Click to expand...

One of the thermocouples/thermometers I purchased straight from omega. The battery is brand new. Wire may be crimped on one of the thermocouples, but the other is brand new.
 
The wire crimped to one of the thermocouples may be part of the problem. Unless the wire and all connecting materials are exactly the same type of metal as the thermocouple lead, that creates another junction of dis-similar metals, another thermocouple in the circuit.

When I was a young engineer, I once spent a week trying to adjust what was supposed to be a very precise furnace controller that showed a 5 degree C oscillation in temperature from the thermocouple we had in the furnace. It was not until a more experienced engineer pointed to the joint between the thermocouple leads and the connector into the recording meter and explained that the oscillation might be that joint heating and cooling with the cycles of the room's HVAC that the mystery was solved.
 
Thermocouple wire should be used from the controller to the thermocouple itself. (Type K will be yellow).

In heat treat and forging, repeatability is more important than accuracy. For instance, you state that the two thermocouples indicate differently at boiling water. Do they alway indicate the same temperature? Lets look at this another way. this is only what, 6-8 degrees? Over a range of 0-2000 degrees, that isn't really that bad,now is it...20 degrees would be 1 percent, which would be pretty darn accurate. (Thermocouples, by the time we go through the wires and the terminals, they may be up to 4-8 percent inaccurate!) That is why repeatability is much more important. In other words, I want it read "this" at a certain temperature, each and every time.

One other thing, beware of the infrared pyrometers to check the temperatures. They are "close".

What I would be concerned about is the 200 degree differences. Here is another possibility. When you get the thermocouples heated up, it may be possible that they are sagging and may form another TC junction inside the thermowell or jacket.

Type K thermocouples are not that expensive. I would recommend getting a couple more and checking them.

I have seen these particular problems working on controls for furnaces and heat treat ovens. I have access to a calibrator so many of the problems that you state, I don't tend to have in my shop. I am able to prove my controller and thermocouples.

Craig
 
You say it is in a vertical furnace? I think if you have one measuring low down, and one near the top, then of course you will get different readings, the rising heat makes the top hotter. try running it horizontal.
 
Sam Salvati said:
You say it is in a vertical furnace? I think if you have one measuring low down, and one near the top, then of course you will get different readings, the rising heat makes the top hotter. try running it horizontal.
Click to expand...

If the probes are in close proximity, this shouldn't be an issue. From my original message:

The thermocouple probes are within < 0.125" of each other in the forge muffle.
Click to expand...
 
C L Wilkins said:
Thermocouple wire should be used from the controller to the thermocouple itself. (Type K will be yellow).

In heat treat and forging, repeatability is more important than accuracy. For instance, you state that the two thermocouples indicate differently at boiling water. Do they alway indicate the same temperature? Lets look at this another way. this is only what, 6-8 degrees? Over a range of 0-2000 degrees, that isn't really that bad,now is it...20 degrees would be 1 percent, which would be pretty darn accurate. (Thermocouples, by the time we go through the wires and the terminals, they may be up to 4-8 percent inaccurate!) That is why repeatability is much more important. In other words, I want it read "this" at a certain temperature, each and every time.

One other thing, beware of the infrared pyrometers to check the temperatures. They are "close".

What I would be concerned about is the 200 degree differences. Here is another possibility. When you get the thermocouples heated up, it may be possible that they are sagging and may form another TC junction inside the thermowell or jacket.

Type K thermocouples are not that expensive. I would recommend getting a couple more and checking them.

I have seen these particular problems working on controls for furnaces and heat treat ovens. I have access to a calibrator so many of the problems that you state, I don't tend to have in my shop. I am able to prove my controller and thermocouples.

Craig
Click to expand...


Thanks for the reply.

One additional observation that i noticed last night when fooling around with this stuff was that the PID controller temperature was reading within 1 - 2% of the infrared thermometer until the set point temp on the PID controller was just about to be reached. This triggered the PID controller to turn off the current to the solenoid valve. *Immediately* the temperature reading dropped by ~160 degrees F! When the PID controller switched the current back on, however, the temperature reading stayed at this lower level. Makes me think there's something defective with the controller...

Also, you mentioned that you have access to a calibrator. Where would one go to check calibration on a thermocouple + controller??

Thanks,
Dustin
 
Dustin,
How close does your thermocouple wire run to the solenoid control wire? I would try to keep these wires separated as thermocouples operate at very low voltage (millivolt) and induced current from 120V can affect accuracy.

I also have a calibrating meter I bought from Sper Instruments. The way it works is that it generates a precise voltage which can be used to verify the correct reading on the Thermocouple meter / Controller.

If you have access to thermocouple hookup wire, you can make a quick and dirty thermocouple by twisting the wires together. I use this type of thermocouple for non-critical work (monitoring BBQ). If you have a Oxy-Acetylene torch, you can weld the wires together to form a Bead thermocouple. I bought some wire at auction and have a good supply. I could send you some if you think it will help. -Doug
 
dustfurn said:
I recently finished a vertical, temperature controlled heat-treat forge, and I've noticed a few strange things with my temperature readings:

-I have two different K thermocouples & two different thermocouple thermometers (a simple type K thermometer and a JLD612 PID controller set to use type K thermocouples).

-The thermometers are kept in the same ambient temperature environment (~60 degrees F).

-The thermocouple probes are within < 0.125" of each other in the forge muffle.

-The two thermometers show different readings by about a 120 degree F at around 1400 degrees!.

-Both of the temperature readings are off significantly (in the worst case by up to 200 degrees ) compared to readings taken by an infrared thermometer with appropriate emissivity settings.

-Switching out the thermocouples between the same thermometer also gives very different temp readings (but not the same differences as described above).

-They both read boiling water as 204 and 206 degrees, respectively (at ~900feet, which seems weird to me).


I have two questions, really: What is going on, and how can people possibly claim to accurately control temperatures if there are such wild differences between two thermometers and two thermocouples of the same type!?

a 100 degree difference is pretty significant!

Thanks for any insight,
Dustin
Click to expand...

Guys, just a few basics on thermocouples. Any two different metals in good electrical contact with each other constitute a thermocouple. Thermocouples produce a voltage based on the junction temperature of the two dissimilar metals. This voltage is in the millivolt range, the hotter the thermocouple junction the more voltage it puts out.

A type K thermocouple (TC) is made up of a positive lead made of nickel chrome alloy and sheathed in a yellow insulation. The negative lead is made of a nickel aluminum alloy and the insulation is red. The two wires are fused together at the tip which forms the thermocouple. The connectors used with thermocouples have the pins made from the same alloys as the wire. The extension wires are also made from the same alloys. This eliminates the creation of a TC junction at any connection point between the TC and the unit being used to monitor the voltage (the meter) and display it as the temperature of the junction. The connection points on the meter are not usually the same material as the TC since most meters can be used with a variety of different thermocouples. Since this connection point creates two new TCs, one on the negative lead connection point and one on the positive lead connection point, the meter needs to be calibrated to remove the false readings being generated at the connection points. This is usually done using a known temperature item to place the thermocouple against then tweaking the calibration screw until the meter reads the same temperature as the item being tested. Boiling water is usually a good point to start, the melting temperature of pure metals is also a given. Now for some suggestions, make sure all connection points are clean and secure, make sure the wire colors are yellow and red, keep you meter at room temperature if possible and out of drafts, check the batteries, if applicable. I hope this helps.

Jim Arbuckle
ABS JS (also an electronic eng)
 
Just an update on this, I ordered a new thermocouple from omega, and everything seems fine now. Apparently both of my thermocouples were bad, including one which was brand new.
 
Since this came back up, I thought I would add a few thermocouple and forge temp thoughts.

Lets assume a vertical forge with a simple on/off PID control and a type K thermocouple. The TC is in the center of the top, projecting down 3". The PID is set at 1500F.

OK, When the PID is on the burner runs at full blast. It heats the refractory up and also heats the TC with the flame,too. The PID reading shows the temperature of the TC junction, so it reads a higher reading than the walls of the forge are. Once the TC reaches the set point ( 1500), the burner shuts off. Now the TC is reading the radiant temperature of the forge lining. This is probably several hundred degrees lower than the set point,and thus the reading very quickly drops to 1300. The PID now cycles the burner back on, heating the TC with the flame again. This on-off continues and each time the radiant heat of the refractory lining will increase a bit. Eventually, the lining will reach the set point and the readings will stay the same, dropping slowly as the forge cools, and then rising as the burner comes on, heating it up again.

The point I'm making is that no readings of any reliability can be made until the forge is fully soaked and the refractory is up to the set point. Running a forge for ten to fifteen minutes is needed to bring thing up to even. A thermocouple sheath also helps shield the TC from the flames.
On cast refractory forge linings, this heat soaking time is a requirement.
Stacy
 
You must log in or register to reply here.
Back
Top