Best way to measure thermocouples?

[GEP]

I've had some issues with the top zone of one of my kilns recently. In one firing, the top zone underfired by a lot. In the last two firings, the top zone overfired a little. Not a lot, but enough to matter. This is not related to the way the kiln is loaded. These were normal loads, and in general I get very consistent temps from top to bottom.

I rarely measure the ohms of my TCs. I generally don't have problems with them, and they get replaced whenever I replace the elements. I tried to measure the ohms of the TCs today, and I'm wondering if I'm doing it right.

When measuring like this, the reading on the ohm meter jumps around a lot. I get a different answer every time I try it. Method #1.

 

When measuring like this, the reading on the ohm meter jumps around a lot, more than the first method. Method #2.

 

When measuring like this, the reading does not jump around. I get the same, repeatable result every time. Method #3.

Method #3 tells me that the top thermocouple is not in the same condition as the other two. The top one measures 0.4. The other two measure 1.0. I'm not sure if this means the top TC is in worse condition, or better condition. They have about 50 firings on them, which is too young to have two of them fail, I think. 

So I guess my question is, in Method #3 a reliable way to measure your thermocouples?

Edit to add: I'm planning to replace all three TCs anyways, I just want to make sure I'm not barking up the wrong tree. 

[neilestrick]

You actually need to test millivolts, not ohms, and compare that to a reference chart. That said, I have never once check the millivolt output of a thermocouple, because it could be working fine at room temp but not at high temp. There could be a hairline crack that's not a problem when cold, but opens up and as it gets hot, and causes a bad reading. At 50 firings it should not need replacing, but things happen and they do sometimes go bad prematurely. You should also open up the panel and remove the thermocouple and empty out the protection tube. If there's a bunch of crud in there it could be affecting the reading. Also make sure the thermocouple is touching the end of the tube when you put it back. If there's nothing obvious that could be the cause of the problem, replace it. They're cheap.

[GEP]

Thanks, @neilestrick! I’ll open up the panel tomorrow. 

[Magnolia Mud Research]

Have you cleaned the thermocouple tip?  A buildup of corrosion on the tip (which is the working part of the device) will lower the true thermocouple reading which is what the controller uses to decide if the target temperature has been achieved.  The corrosion insulates the thermocouple.  Cleaning will remove the insulation an the temperature reading will be closer to the temperature of the ware.  
LT

[GEP]

1 hour ago, Magnolia Mud Research said:

Have you cleaned the thermocouple tip?  A buildup of corrosion on the tip (which is the working part of the device) will lower the true thermocouple reading which is what the controller uses to decide if the target temperature has been achieved.  The corrosion insulates the thermocouple.  Cleaning will remove the insulation an the temperature reading will be closer to the temperature of the ware.  
LT

Thanks. I’ll open up the panel tomorrow and take out the tubes, and check on this. Too late to do it tonight, because the Ravens game starts in five minutes :-)

[GEP]

1 hour ago, Rockhopper said:

depending on what setting your meter is on, the 'jumping around' may just be a matter of the quality of the connection between the meter probes and the TC connectors, or connecting block.  In method one - the TC connector and meter probe are both hand-held, and could be moving around a bit.  In #2, you're using just the very point of the probe (minimal contact) - against a rivet that may not have a solid electrical connection to the TC.  In method 3, you're in direct contact with the TC connectors but, since they're still on the jumper block, they can't move around - making for a more steady connection.  (If you use method 3, but disconnect the wires from the opposite side of the block, instead of disconnecting the TC, you would have both a steady connection and an isolated component.)

Thanks, this all makes good sense.

[neilestrick]

1 hour ago, Rockhopper said:

@neilestrick  Not directly related to GEP's question - rather, curiosity sparked from reading the info at the top of the chart you linked to, which says "Maximum continuous temperature is around 1,100C".

According to my Orton chart,  the kiln will top 1100C somewhere between ^03 and ^01 - and be over 1200C at ^6.  I guess it depends on how they define "continuous" - but how does that 'maximum continuous temperature' not become a problem when a type K TC is used in a kiln ? 

Temperature rang for type K is 1260C, or about 2300F. They can be pushed beyond that, but accuracy suffers a little. For that reason, and for durability reasons, we recommend that people who fire to cone 10 regularly use a type S thermocouple. The vast majority of people who fire electric kilns will not go above cone 6, so manufacturers put type K in the kilns because they work well at that range, and they're relatively inexpensive. For someone who is firing cone 10 occasionally, the type K will usually be accurate enough, especially since most glazes have a broad enough firing range that being even 1/2 a cone off won't be a big deal. For people who use type K thermocouples in gas or wood kilns, the thermocouple is not used for accuracy purposes, but rather to show  approximate temps or rate of climb. Cones are used to measure actual heat work.

As for what they mean by continuous use, I'm sure they have number for that but I don't know what it is. I don't think that the kilns that we use fall into that category, though, because they are fired up and allowed to cool. They're probably referring to kilns and and such that are held at temperature indefinitely, like industrial tunnel kilns, etc.

[Bill Kielb]

8 hours ago, GEP said:

I've had some issues with the top zone of one of my kilns recently. In one firing, the top zone underfired by a lot. In the last two firings, the top zone overfired a little. Not a lot, but enough to matter. This is not related to the way the kiln is loaded. These were normal loads, and in general I get very consistent temps from top to bottom.

I rarely measure the ohms of my TCs. I generally don't have problems with them, and they get replaced whenever I replace the elements. I tried to measure the ohms of the TCs today, and I'm wondering if I'm doing it right.

When measuring like this, the reading on the ohm meter jumps around a lot. I get a different answer every time I try it. Method #1.

 

 

When measuring like this, the reading on the ohm meter jumps around a lot, more than the first method. Method #2.

 

 

When measuring like this, the reading does not jump around. I get the same, repeatable result every time. Method #3.

 

Method #3 tells me that the top thermocouple is not in the same condition as the other two. The top one measures 0.4. The other two measure 1.0. I'm not sure if this means the top TC is in worse condition, or better condition. They have about 50 firings on them, which is too young to have two of them fail, I think. 

So I guess my question is, in Method #3 a reliable way to measure your thermocouples?

Edit to add: I'm planning to replace all three TCs anyways, I just want to make sure I'm not barking up the wrong tree. 

Thermocouples generate voltage, very precise voltage for a given temperature actually, which makes them very accurate as long as there remains enough of a dissimilar metal  junction.

If they are all the same temperature they will generate the same voltage. I am more interested to see a close up picture of the front of the thermocouple. Measure thermocouples using your voltage scale (millivolts) and compare with chart at temperature.. you can measure across the thermocouple even while it is in circuit as long as you are using a reasonable (High impedance)  meter which almost all are now days.

In the chart below 72 degrees is exactly (for your purposes) 0.888 VDC.

To answer your question for best measurement measure - out of the circuit - and and make sure your probe tips have a really good connection. To answer your other question, it’s possible this is the cause  and nothing wrong with changing but it would also be just as likely not a cause. These things by design  tend to either work  or fail by opening, and most controllers will flag with the failure.

Chart below for your reference and an example thermocouple that presented as a minor crack in the  insulator but once uncovered, not a whole bunch of life remaining. 

 

[Bill Kielb]

4 hours ago, Rockhopper said:

@neilestrick  Not directly related to GEP's question - rather, curiosity sparked from reading the info at the top of the chart you linked to, which says "Maximum continuous temperature is around 1,100C".

According to my Orton chart,  the kiln will top 1100C somewhere between ^03 and ^01 - and be over 1200C at ^6.  I guess it depends on how they define "continuous" - but how does that 'maximum continuous temperature' not become a problem when a type K TC is used in a kiln ? 

Yeah, useful range for type K  -200C  to  1370 C under IEC 584-1 standards so thermocouples will easily measure that range. Different manufactures will limit the rating or guaranteed working range of the thermocouple they produce. To measure to that detail we need to start measuring microvolts  instead of millivolts which for kiln controllers and wiring is just not a thing. Pottery useful range (controller limited or Tc manufacture limited) is usually 32- 2400 F 

[Bill Kielb]

4 hours ago, Magnolia Mud Research said:

Have you cleaned the thermocouple tip?  A buildup of corrosion on the tip (which is the working part of the device) will lower the true thermocouple reading which is what the controller uses to decide if the target temperature has been achieved.  The corrosion insulates the thermocouple.  Cleaning will remove the insulation an the temperature reading will be closer to the temperature of the ware.  
LT

Lightly Cleaning
Not a bad idea for precision stuff but in a kiln these corrode downstream from the junction significantly. If you clean it and it falls apart, it’s bad. (See picture  above)  A layer of light oxide is not going to influence this much. They generate electric because of the junction of dissimilar metals and the temperature they are at. At high temperature a little oxide will not make a big difference in sensing rate.

Now clean your flame sensing rod once a year in your furnace, definitely a good thing to do with very fine polishing pad, no big scratches. Also clean any residue from the insulating surfaces, 3 micro amps  considered a strong signal. Easy to bleed off a microamp  or two through conductive contamination to ground.

[GEP]

I may have discovered the problem?

Upon removing the panel, I found that the red wire of the top thermocouple was not connected. The set screw was not all the way tight. I bet the wire was in the right place and somewhat connected, enough that I didn't get a TC FAIL error. But still faulty at times. Then the wire got pulled out when I opened the panel. 

 

All three thermocouples look about the same level of cruddy, appropriate for their age. The top thermocouple is on the left. I don't feel like I need to replace them, at the moment. I'm currently firing the kiln with shelves and cone packs only. If it doesn't fire evenly, I'll replace the TCs after all.

 

[neilestrick]

53 minutes ago, GEP said:

I may have discovered the problem?

Upon removing the panel, I found that the red wire of the top thermocouple was not connected. The set screw was not all the way tight. I bet the wire was in the right place and somewhat connected, enough that I didn't get a TC FAIL error. But still faulty at times. Then the wire got pulled out when I opened the panel. 

 

All three thermocouples look about the same level of cruddy, appropriate for their age. The top thermocouple is on the left. I don't feel like I need to replace them, at the moment. I'm currently firing the kiln with shelves and cone packs only. If it doesn't fire evenly, I'll replace the TCs after all.

 

 

The TC's don't look bad a t all. The loose wire is most likely the cause of the problem. Good job finding the problem!

[GEP]

Thanks @neilestrick. Now that the kiln is powered on, the millivolts for all three TCs are the same as each other. Though slightly different from your chart. Now reading 13 when the chart says it should be 15. Is this due to the age of the TCs, being about half way through their lifespan?

EDIT: I just remembered there is a 25° thermocouple lag offset.  Never mind!

[Bill Kielb]

6 hours ago, GEP said:

Thanks @neilestrick. Now that the kiln is powered on, the millivolts for all three TCs are the same as each other. Though slightly different from your chart. Now reading 13 when the chart says it should be 15. Is this due to the age of the TCs, being about half way through their lifespan?

EDIT: I just remembered there is a 25° thermocouple lag. Never mind!

Since you have protection tubes on each there will be offsets in the controller which is likely the difference between the chart and displayed temperature. That they  all read the same at the same temperature would mean they are likely all fine. Loose wire could  be the likely culprit, good catch. This is also consistent with your jumpy measurements earlier.

[GEP]

This morning’s result, left to right, top zone middle zone bottom zone. All is good here. Thanks for the help everybody! When this is your income, equipment snafus can be stressful.