Thermocouple Type-K: 3 Wires & Ceramic Shield?

[Artyman]

I have a type-K thermo-couple. In all the thermocouple, I have seen so far there is just two wires and a joint. But in the one I got, there is third wire which is connected to the joint and is twisted around the ceramic shield of the thermocouple. I do not know what does it do?
Any idea ...Thank you

 

[berkeman]

Can you post a picture? What is the product model number that you bought? Can you post a datasheet link?

 

[Artyman]

Can you post a picture? What is the product model number that you bought? Can you post a datasheet link?

thank you for the response. I do not have a datasheet. I just found it in a very old furnace.

 

[berkeman]

Don @dlgoff gave a great answer in a recent thermocouple thread. Maybe he can comment on this one...

 

[dlgoff]

I'm fairly sure that this extra wire is for grounding via a thermowell (the ceramic wire separaters tells me the thermocouple is intended to be slide down a well.) i.e. the extra wire makes a connection with the inside of the thermowell which gets attached to some grounded equipment. Here's a thermowell that gets threaded into the equipment. They can also be permanently welded into the equipment.

From this Omega http://www.omega.com/technical-learning/preventing-vibration-damage-to-thermocouples-and-rtd-sensors.html:

Thermocouple junctions may be grounded or ungrounded. They are often covered with protective metal but may be left exposed to improve response time. Grounding is frequently required to prevent buildup of static charge, which may negatively impact accuracy.

 

[Artyman]

I'm fairly sure that this extra wire is for grounding via a thermowell (the ceramic wire separaters tells me the thermocouple is intended to be slide down a well.) i.e. the extra wire makes a connection with the inside of the thermowell which gets attached to some grounded equipment. Here's a thermowell that gets threaded into the equipment. They can also be permanently welded into the equipment.

View attachment 210176

From this Omega http://www.omega.com/technical-learning/preventing-vibration-damage-to-thermocouples-and-rtd-sensors.html:

Thanks dlgoff. My first guess was the grounding issue, but the confusing part is "why this shield grounding wire is welded to joint?"...

 

[Asymptotic]

My first guess was the grounding issue, but the confusing part is "why this shield grounding wire is welded to joint?

Variations on the theme exist, but thermocouples come in grounded or ungrounded, and shielded or exposed junction types. On this one the junction is exposed, and equipped with a grounding wire.

If it were a shielded type (rather than being contained within a thermowell like the one @dlgoff described) it would typically be contained within a closed stainless steel or Inconel tube packed with magnesium oxide (to insulate the connection wires), and the junction would either be welded to the tip (grounded type), or offset back a bit to allow headspace for MgO insulation (ungrounded). In a shielded thermocouple, the grounded type has slightly faster response due to the direct connection between junction and sheath. An exposed junction TC has even faster response.

When used in the furnace, did it measure and control the temperature of the air within, or was it positioned directly in the flame as part of a proven ignition safety circuit? An ungrounded thermocouple can't form ground loops, and is generally preferred from a measurement perspective. I've never seen this sort of TC either, and unless it is part of a gas burner safety circuit the reason behind the intentional grounding wire is throwing me.

 

[NTL2009]

This got me reading up on thermocouples, and I found some interesting stuff. I knew they were used to power Voyager spacecraft (radioactive decay was the heat source). But a kerosene powered radio?

http://www.aqpl43.dsl.pipex.com/MUSEUM/POWER/thermoelectric/thermoelectric.htm#rl

 

[jim hardy]

They power the gas valve in home appliances and gas log fireplaces.
http://www.homedepot.com/p/Rheem-PR...r-Natural-Gas-Water-Heaters-SP20824/206523600

 

[Averagesupernova]

They power the gas valve in home appliances and gas log fireplaces.
http://www.homedepot.com/p/Rheem-PR...r-Natural-Gas-Water-Heaters-SP20824/206523600

But not type K. I believe those are type J. I always had a hard time believing that a thermocouple could power a solenoid valve or relay, but it is most certainly the case. Here are some I am familiar with:
https://www.google.com/search?q=bas...gB&biw=1280&bih=589#imgrc=_&spf=1504402005010
-
I recall reading in the encyclopedia many years ago about a candle powered single transistor radio. It used a thermocouple of some sort (most likely called a thermopile) and if memory serves me it developed about a half a volt. I never attempted to build one, but it fascinated me. I have tried googling for it but not much turns up.

 

[NTL2009]

... I always had a hard time believing that a thermocouple could power a solenoid valve or relay, but it is most certainly the case. ...

But it doesn't really "power a valve" as we normally think. Recall that with these gas valves, you press the valve in and hold it until the thermocouple gets hot and provides a holding current. At that point, the current can maintain the holding force, but there is no way it could pull it in on its own. It's at a very delicate balancing point, and there is just enough electro-magnetic force to hold it.

Just like a magnet that has attached it self to a metal plate holds hard, but move it a short distance away, and there is very little force. It's a damn clever arrangement.

 

[Averagesupernova]

A thermocouple relay certainly does 'pull in'. A valve may not but a relay does.

 

[NTL2009]

A thermocouple relay certainly does 'pull in'. A valve may not but a relay does.

OK, I'm sure there are designs that do pull in. I was thinking in terms of the ones I'm familiar with - the thermocouples on the older style North American residential gas water heaters and furnaces - the kind Jim Hardy was referring to. Those only hold, they do not pull in. I recall wondering how my old gas water heater could work, since it didn't need an AC power connection. And that was it - I provided the 'pull in" force when I lit the pilot, it only had to supply the hold-in force with that tiny power from the thermocouple ( µwatts? mV and mA I think).

 

[jim hardy]

the kind Jim Hardy was referring to. Those only hold, they do not pull in.

Actually on the gas fireplaces I've encountered there's a low millivolt thermocouple, about twelve millivolts for the hold in valve that operates as you say, with a pushbutton for safety

and a thermopile about 500 millivolts for the on-off valve that's controlled by a wall switch...or sometimes by a thermostat(yikes! that's outlawed in a lot of places) ..

https://customer.honeywell.com/resources/techlit/TechLitDocuments/69-0000s/69-1219.pdf

 

[Artyman]

Variations on the theme exist, but thermocouples come in grounded or ungrounded, and shielded or exposed junction types. On this one the junction is exposed, and equipped with a grounding wire.

If it were a shielded type (rather than being contained within a thermowell like the one @dlgoff described) it would typically be contained within a closed stainless steel or Inconel tube packed with magnesium oxide (to insulate the connection wires), and the junction would either be welded to the tip (grounded type), or offset back a bit to allow headspace for MgO insulation (ungrounded). In a shielded thermocouple, the grounded type has slightly faster response due to the direct connection between junction and sheath. An exposed junction TC has even faster response.

When used in the furnace, did it measure and control the temperature of the air within, or was it positioned directly in the flame as part of a proven ignition safety circuit? An ungrounded thermocouple can't form ground loops, and is generally preferred from a measurement perspective. I've never seen this sort of TC either, and unless it is part of a gas burner safety circuit the reason behind the intentional grounding wire is throwing me.

Thanks. The furnace does not work. I should find appropriate thermocouple and temp-controller to warm up the furnace. In the manual of this furnace they just said the typeK TC, thermal controller and heating element. there is well for the thermocouple with ceramic shield. Please find more captured photos for this furnace 's thermocouple.

 

[jim hardy]

The furnace does not work.

You're troubleshooting a gas furnace ? I hope it's for a kiln or something, not heating an occupied building.

Read its resistance both ways with a DMM you should get very few ohms.
then
Connect a meter to the two wires at signal end and heat it gently with a flame - barbecue lighter or something even a candle.. You should see roughly two millivolts for every hundred degreesF.

Do not ever try to fix a gas valve in a heating system - just replace it. .

If this is for heating an occupied building - call your furnace serviceman there's huge liability and PF can't risk being party to amateur repairs. .Nor can I.old jim

 

[berkeman]

Thread closed temporarily for Moderation...

 

[berkeman]

Do not ever try to fix a gas valve in a heating system - just replace it. .

If this is for heating an occupied building - call your furnace serviceman there's huge liability and PF can't risk being party to amateur repairs. .Nor can I.

Good advice. Thread will remain closed.

 

[berkeman]

After a PM conversation with the OP, this thread can be re-opened. It does not look like there is a CO risk in his project.

this tube is an electric tube furnace, not a gas one. Also, this won't use for building with people, this is very small and we use it just for mm size ceramic samples.

 

[jim hardy]

Whew ! Thanks, guys !

 

[Artyman]

After a PM conversation with the OP, this thread can be re-opened. It does not look like there is a CO risk in his project.

Thank you for consideration. Yes it is an electric tube furnace for lab work (mm size ceramic fabrication). I hope every one could get it from the right point. I am not a technician.

 

[Asymptotic]

Thanks. The furnace does not work. I should find appropriate thermocouple and temp-controller to warm up the furnace. In the manual of this furnace they just said the typeK TC, thermal controller and heating element. there is well for the thermocouple with ceramic shield.

... it is an electric tube furnace for lab work (mm size ceramic fabrication). I hope every one could get it from the right point. I am not a technician.

What is the manufacturer and model number of the furnace?

• What temperature is displayed by the temperature controller?
• Does the controller have a lamp/LED/something else? that turns on to indicate when heating output is commanded on?
• What is used for heater power control? A solid-state relay, electromechanical relay, or something else?

There is a good possibility the root cause of this 'does not work' problem is not, or at least not exclusively, a thermocouple issue.

I'm betting those brass (or perhaps copper) terminals were originally parts of the broken ceramic device the thermocouple extension wire runs through. What lies underneath looks like a thermally insulated pass-thru hole to route TC extension wire out of the oven chamber, and to the controller.

Wires from the TC itself, and TC extension wire are in contact with each another under the screw of each brass block. A thermocouple is formed any time two different metals come into contact with one another, and (if the wires weren't in contact with each other) each of these connections would exhibit thermoelectric behavior.

Generally speaking, the possibility of short circuits between wires is greater when thermocouple (+) and (-) and (in this case) ground wires are poorly insulated and in close proximity to one another (shown circled in red). This ought to manifest as a displayed temperature remaining suspiciously lower than expected - the measured temperature signal is an average of the intended thermocouple junction and 'false' thermocouple created where the wires have shorted together.

 

[NTL2009]

You're troubleshooting a gas furnace ? I hope it's for a kiln or something, not heating an occupied building. ...

Do not ever try to fix a gas valve in a heating system - just replace it. ...

old jim

Agreed, and I'm glad the thread was re-opened. Although a little off-topic from the OP's application, sharing some safety info is always a good thing I think. So I will share this:

About 30 years ago, I had the thermocouple sensor in my gas furnace valve go bad. I knew enough to troubleshoot it down to that component (thermocouple was good), but the local repair outlet would not sell me the replacement part. I found a "thermocouple tester" in the hardware store, which consisted of this sensor and an indicator light, so I bought the "tester" and was going to use it to replace the bad sensor in the gas valve. If memory serves, there just wasn't any good way to get the valve apart to replace that component, so I think I ended up with a whole new gas valve anyway. Or maybe I was able to fix it, I just don't remember.

But it happened again a few years ago in my newer home on a ~ 15 year old furnace. This time, it was clear that there was absolutely no way you could take that valve apart to replace any component. In one way, I was mad about that - it seems a shame to throw away an entire valve because one little $5 component was defective. But after thinking about it some more, I realized that if that component could be replaced, it could also be by-passed. And if it can be done, some one will do it. So I resigned myself to the idea that safety for all was more important than my being able to do a repair.

In industrial settings, where only licensed repair techs will be doing work, I believe these systems may be more modular which means you could replace a component, but also bypass that component. And hopefully a trained tech would not do such a thing... but I wonder?

 

[jim hardy]

Ahh an electric kiln of sorts.

A thermocouple in proximity to sizeable electric heaters - grounding it seems a good idea.
I never encountered one like that, mine were in thermowells like the one dlgoff posted.

If memory serves, there just wasn't any good way to get the valve apart to replace that component, so I think I ended up with a whole new gas valve anyway. Or maybe I was able to fix it, I just don't remember.

Some of them look like a copper tube going into the valve
My local hardware store carries them in several lengths
https://www.robertshaw.com/ProductDetail.aspx?id=2147492934&cat=2147484248

Replacing the thermocouple is about all you can do to them .

 

[Artyman]

Agreed, and I'm glad the thread was re-opened. Although a little off-topic from the OP's application, sharing some safety info is always a good thing I think. So I will share this:

About 30 years ago, I had the thermocouple sensor in my gas furnace valve go bad. I knew enough to troubleshoot it down to that component (thermocouple was good), but the local repair outlet would not sell me the replacement part. I found a "thermocouple tester" in the hardware store, which consisted of this sensor and an indicator light, so I bought the "tester" and was going to use it to replace the bad sensor in the gas valve. If memory serves, there just wasn't any good way to get the valve apart to replace that component, so I think I ended up with a whole new gas valve anyway. Or maybe I was able to fix it, I just don't remember.

But it happened again a few years ago in my newer home on a ~ 15 year old furnace. This time, it was clear that there was absolutely no way you could take that valve apart to replace any component. In one way, I was mad about that - it seems a shame to throw away an entire valve because one little $5 component was defective. But after thinking about it some more, I realized that if that component could be replaced, it could also be by-passed. And if it can be done, some one will do it. So I resigned myself to the idea that safety for all was more important than my being able to do a repair.

In industrial settings, where only licensed repair techs will be doing work, I believe these systems may be more modular which means you could replace a component, but also bypass that component. And hopefully a trained tech would not do such a thing... but I wonder?

Agreed, and I'm glad the thread was re-opened. Although a little off-topic from the OP's application, sharing some safety info is always a good thing I think. So I will share this:

About 30 years ago, I had the thermocouple sensor in my gas furnace valve go bad. I knew enough to troubleshoot it down to that component (thermocouple was good), but the local repair outlet would not sell me the replacement part. I found a "thermocouple tester" in the hardware store, which consisted of this sensor and an indicator light, so I bought the "tester" and was going to use it to replace the bad sensor in the gas valve. If memory serves, there just wasn't any good way to get the valve apart to replace that component, so I think I ended up with a whole new gas valve anyway. Or maybe I was able to fix it, I just don't remember.

But it happened again a few years ago in my newer home on a ~ 15 year old furnace. This time, it was clear that there was absolutely no way you could take that valve apart to replace any component. In one way, I was mad about that - it seems a shame to throw away an entire valve because one little $5 component was defective. But after thinking about it some more, I realized that if that component could be replaced, it could also be by-passed. And if it can be done, some one will do it. So I resigned myself to the idea that safety for all was more important than my being able to do a repair.

In industrial settings, where only licensed repair techs will be doing work, I believe these systems may be more modular which means you could replace a component, but also bypass that component. And hopefully a trained tech would not do such a thing... but I wonder?

Thank you for sharing the safety notes. But please do not go again in Gas Furnace topic. It is not a gas furnace at all. I like to use the information related to electric tube furnace and do not like to see this topic is closed again! again thank you for sharing your information.

 

[jim hardy]

Yes i was the one who first mentioned gas . I thought that's what it was.
Being from Florida i haven't seen a lot of 'Furnaces' - to me that word means an appendage to central air conditioning unit, usually gas fired .

Definition of furnace

1. 
   : an enclosed structure in which heat is produced (as for heating a house or for reducing ore)

Sorry for the derailment. But i do hope it helps NTL with his thermocouple .

Presumably your thermocouple connects to some sort of electronic temperature controller ?

 

[Artyman]

Yes i was the one who first mentioned gas . I thought that's what it was.
Being from Florida i haven't seen a lot of 'Furnaces' - to me that word means an appendage to central air conditioning unit, usually gas fired .
Sorry for the derailment. But i do hope it helps NTL with his thermocouple .

Presumably your thermocouple connects to some sort of electronic temperature controller ?

Yes, I think it was my bad that from the beginning I did not mentioned the details. but it is an experience for me any question must be in details.
yes it will be connected to a temperature controller. The problem is here. The out put of thermocouple to a temperature control unit must be two. But as you can see in the photos there are 3. I still do not know what is the reason for that.

 

[Asymptotic]

The two wires going to the brass terminals are (+) and (-) thermocouple output. The third wire is unexplained, but may have been intended to ground the thermocouple.

 

[jim hardy]

I agree with @Asymp

it would make sense to ground a thermocouple that's in proximity to high powered electric heaters.
Especially if it's not in a thermowell .

But machinery is full of surprises...
If we knew the nature of the device that reads the millivolts we could venture better guesses.

old jim

 

[Averagesupernova]

My guess the third wire is to help with noise imunity while preventing the whole thermocouple from floating. I would think there would be better noise immunity when this third wire is grounded back at the amplifier/transmitter than just grounding it at the weld. I would think of it as a shielded twisted pair microphone cable.

 

[Asymptotic]

My guess the third wire is to help with noise imunity while preventing the whole thermocouple from floating. I would think there would be better noise immunity when this third wire is grounded back at the amplifier/transmitter than just grounding it at the weld. I would think of it as a shielded twisted pair microphone cable.

A propensity for noise issues depends on controller instrumentation amp design as well as TC construction and wiring. I ran into this when replacing an ancient Barber-Coleman MACO V control system with a (then, only recently introduced) Allen-Bradley SLC500 PLC equipped with a half dozen 1747-NT4, 4 channel thermocouple input modules. Temperature measurements drifted by a degree or so, some more than others, and apparently at random, but closer investigation revealed it depended on how many zones were simultaneously calling for heat. Turned out the -NT4 module was more sensitive to common mode noise than the MACO V (several years later A-B introduced an -INT4 module with better isolation), and a quick fix was to swap out the original grounded thermocouple probes for ungrounded ones. Another option was to replace all the TC extension wire runs with shielded TC extension cable, ground the shield at the TC plug negative connection (in the process introducing an ongoing maintenance issue), and bond the negative TC extension wire at the module's analog common.

TC input wiring

It was a long walk to get here, but most other manufacturers also recommend bonding a grounded thermocouple's cable shield to the source (TC) rather than at the instrument side. The cable shield for an ungrounded TC is grounded at the input module.

Ungrounded thermocouples aren't a panacea, and don't necessarily help with noise. Many TC instrumentation amps connect the inverting input to ground through a high resistance. It they don't, any charge that builds up on the wires through op amp reverse bias leakage, static, etc. has nowhere to go, and affects measurements.

Figure 6 was borrowed from an excellent Analog Devices technical article, "Two Ways to Measure Temperature Using Thermocouples Feature Simplicity, Accuracy, and Flexibility" regarding their TC amplifiers.

In the OP's situation I'd bet there is a screw within reach of the TC ground wire with witness marks indicating prior connection.