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K Type Thermocouple

09/20/2021 5:03 AM

Can a K Type Thermocouple be used for measuring temperature upto 1300'c

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#1

Re: K Type Thermocouple

09/20/2021 5:31 AM

The limit is 1100C for type K.

Type B is good up to 1700C

A calibrated Spectrometer can be used for very high temperatures,but it requires a hot temperature body for calibration.

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#2

Re: K Type Thermocouple

09/20/2021 6:36 AM

Try this.

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#3

Re: K Type Thermocouple

09/21/2021 10:03 AM

It's a judgement call.

I find an analog input on a single loop controller (and by extension, probably a PLC/DCS/temperature transmitter) that will range up to 1316°C for Type K.

The Type K mV vs temperature table goes up to 1372°C.

But the first thermocouple vendor I pulled up states a maximum range of 1260°C Type K. I'm sure you can find vendor to claim 1300°C performance by shopping around. The 3rd column states the maximum range.

But to control at any given setpoint, one usually needs some range of temperature above the control setpoint and 1300°C is pushing the upper limit of a Type K.

Does the application warrant using a sensor that is working beyond the 'useable' range of that sensor by some sensor manufacturers?

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#4
In reply to #3

Re: K Type Thermocouple

09/22/2021 7:03 AM

Our use of K type thermocouple is to control a furnace of Max.attainable temperature of 1300'c for a short while and 1250'c is the max. working temp. Control is by PID programmable temperature controller working in conjunction with SSR.

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#7
In reply to #4

Re: K Type Thermocouple

09/22/2021 7:39 AM

Here is a chart that will hep you decide which is best for your application.

https://assets.omega.com/pdf/tables_and_graphs/thermocouple-type-b-celsius.pdf

Type "B" seems to me to be the best candidate,but I do not know your specific conditions,atmosphere,pressure,etc.

Good luck!

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#6
In reply to #3

Re: K Type Thermocouple

09/22/2021 7:22 AM

<...an analog input on a single loop controller...that will range up to 1316°C for Type K....>

Just because the input is suitable doesn't mean that the thermocouple itself is suitable.

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#8
In reply to #3

Re: K Type Thermocouple

09/22/2021 3:13 PM

Yes, one can use this thermal probe in this upper limit. But like you, Carl_E, clearly explain the accuracy of the measurement will always be in question.

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#5

Re: K Type Thermocouple

09/22/2021 7:08 AM

Possibly, though only at the user's risk, as it is beyond the manufacturer's top range for this equipment.

How would the user establish that <...1300oC...> has been accomplished, so as to validate the use of it, though? One would need something else that is suitable for this temperature to do that, which means that the something else could have been used instead and would therefore be a better choice.

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#9

Re: K Type Thermocouple

09/23/2021 11:06 AM

A 1250°C operating setpoint provides some margin to the top end of a Type K.

But the hazard faced in using at Type K at this temperature is that the controller is likely to report a false temperature value, one whose value has drifted because the junction has been polluted and the thermocouple is no longer a thermocouple junction of chromel and alumel alloys, but is now a junction of unknown alloys, for which the international tables of mV vs temperature are no longer valid. The junction pollution by the movement of metallic ions is greater the higher the temperatures.

The problem is that a drifted thermocouple has for all practical purposes failed, but it has not broken or failed open which all controllers have been to detect for decades. A drifted thermcouple continues to generate a mV signal, but the mV signal cannot be correctly converted to a temperature value because the junction is no longer a junction of pure alloys, it is a junction of polluted alloys for which there is no published reference data.

Drift is a serious factor addressed by AMS2750, the US pyrometry standard for many heat treat industry groups. AMS2750 has time-at-temperature restrictions which requires monitoring how many exposures a base metal (Type K) thermcouple can have before it has to be replaced, because of the probability of drift beyond the stated number of exposures. That number of exposures has decreased in succeeding revisions of AMS2750.

There is one US vendor who offers a proprietary insulating compound, 'MI-dry' (MI is Mineral Insulation, the white powder inside of thermocouple sheath), used in place of MgO, that maintains a higher resistance over time and temperature, reducing ion migration therefore reducing the tendency to drift.

US DOE researchers published data in the mid '90's showing that a T/C junction resistance increases as the pollution factor increases. Based on that research, some PID controllers, like the Honeywell UDC's, offer a "thermocouple health" feature that reads the thermocouple resistance on a periodic basis. A new, unused junction has a very low resistance, on the order of 1-2 Ohms. As pollution occurs and the junction resistance increases, the Honeywell can alarm at one of two preset alarm points - a 'warning' and a 'failure' level.

Taking some precautions to guard against drift might provide suitable service from a base metal Type K at the operating temperature. Otherwise, it's time for noble metal T/C's.

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Commentator

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#10

Re: K Type Thermocouple

09/23/2021 12:00 PM

The Thermocouple is placed within a ceramic sheath . Does that help ?

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#13
In reply to #10

Re: K Type Thermocouple

09/24/2021 4:18 AM

All it will do is slow down the response of the thermocouple, which could be an issue as far as the control loop and its tuning is concerned. The Ziegler-Nichols method gives a number of approaches to loop tuning and it might be found that introducing a <...ceramic sheath...> into the mix will render the previous tuning for the loop inappropriate, rendering re-tuning a necessity.

If in doubt, consult a qualified Controls Engineer.

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#11

Re: K Type Thermocouple

09/23/2021 6:30 PM

Wouid a ceramic protection help? I can't answer that. I just realized I've only ever encountered ceramic protection tubes in use with noble metal T/C's. I suspect ceramic tubes are even more porous than the alloy protection tubes, which would expose the junction to hydrogen, Mn, or other deleterious elements.

Maybe this is why (vendor recommendation):

"such conditions usually require a noble metal thermocouple such as Pt or Pt alloys"

The best summary of K/N thermocouple issues, noting both drift and the Green Rot phenomenon is here:

https://instrumentationtools.com/thermocouples-green-rot-effect/

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#12

Re: K Type Thermocouple

09/24/2021 3:08 AM

Thanks to everybody. This discussion opened up many aspects of K Type thermocouple worth knowing and has much academic value if not anything else .

Regards

A. Mukerji

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