Thermocouple Errors in Temperature Controller

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Discussion Overview

The discussion revolves around the performance of a Type K thermocouple in measuring low temperatures, specifically in the context of a temperature controller. Participants explore potential sources of error in readings taken at various temperatures, including room temperature, ice water, and liquid nitrogen.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant reports correct readings at room temperature and ice water but an incorrect reading of -156°C at liquid nitrogen, prompting inquiries into possible sources of error.
  • Another participant suggests checking the polarity of the thermocouple connections, as incorrect connections can lead to erroneous readings.
  • A participant raises the issue of thermoelectric voltage from contact points if different metals are used in connections, which could affect measurements.
  • Discussion includes a reference to a governing equation for EMF vs. temperature for K-type thermocouples, noting that the calibration curve is non-linear, which may explain discrepancies in expected results.
  • Participants discuss the importance of voltage readings at specific temperatures, with one participant calculating expected mV values for the thermocouple at liquid nitrogen temperatures.
  • There is a suggestion to use a scope or VTVM instead of a multimeter to avoid current draw affecting the readings.
  • One participant expresses confusion over the significant discrepancy between expected and actual readings at liquid nitrogen temperatures, questioning the calibration standard used for the temperature of nitrogen.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the exact cause of the discrepancies in temperature readings. Multiple competing views and hypotheses about potential errors remain present throughout the discussion.

Contextual Notes

Participants reference specific voltage readings and expected values based on thermocouple calibration, but there are unresolved questions about the accuracy of the temperature controller and the calibration standards used.

ColdFusion85
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I recently made a Type K thermocouple (Chromel-Alumenel type) and am trying to see if it is working correctly. I do not have the necessary equipment or time to use cold junction compensation, so I was told by my professor to just test the temperatures and voltage readings at room temperature (20 degrees C), in ice water (0 degrees C), and in liquid nitrogen (should be about -196 C). I got correct readings for room temp and ice water but for liquid N2 i got -156 C. What are the possible sources of error that could cause this TC to work correctly at room and ice temps, but not at much lower temps such as N2?
 
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Have you checked the polarity of your connection? Type K thermocouple requires you to know which side is which, and it can go into your controller only in one particular way.

Zz.
 
ColdFusion85 said:
I recently made a Type K thermocouple (Chromel-Alumenel type) and am trying to see if it is working correctly. I do not have the necessary equipment or time to use cold junction compensation, so I was told by my professor to just test the temperatures and voltage readings at room temperature (20 degrees C), in ice water (0 degrees C), and in liquid nitrogen (should be about -196 C). I got correct readings for room temp and ice water but for liquid N2 i got -156 C. What are the possible sources of error that could cause this TC to work correctly at room and ice temps, but not at much lower temps such as N2?
How are you physically connecting to the thermocouple to measure the output voltage? You might be getting some thermoelectric voltage from the contact points if they are different metals.

AM
 
In the manual on the use of thermocouples (ASTM), there is a chart of EMF vs. temp for K-type TCs. The governing equation for the table's values is
E = C_0+C_1T+C_2T^2+C_3T^3+... The coefficients have values based on temperature range. The ranges are -270°C to 0°C and 0°C to 1372°C. The second range also has a modifying exponential factor included with the first two coefficients. Your results matched pretty well with the second set of values, but not the first. So essentially, you are expecting a linear result from a non linear calibration curve.

EDIT: Usually, whenever I have switched the yellow and red leads, the result is simply a sign error.

http://srdata.nist.gov/its90/download/download.html

http://www.temperatures.com/tctables.html
 
Last edited:
FredGarvin said:
Your results matched pretty well with the second set of values, but not the first. So essentially, you are expecting a linear result from a non linear calibration curve.
The OP did not post his voltage readings, so how do you know this?

AM
 
ZapperZ, the connection is correct, no doubt there.

AM, I am using a multimeter, and I tried both placing the probes to the leads, as well as just wrapping the wire around the probes themselves. Neither changed the value I was getting in liquid N2 (-156 degrees C). Regardless, I should not be getting an error as large as I am.

FredGarvin, could you please elaborate on your response? How would you fix the problem?


Cheers
 
Andrew Mason said:
The OP did not post his voltage readings, so how do you know this?

AM
The OP stated that he got correct readings for ice water and room temperature. The problem was for the extreme cold temps in the LN2.
 
ColdFusion85 said:
FredGarvin, could you please elaborate on your response? How would you fix the problem?
I am not familiar with the equipment you are using. Can you elaborate on the equipment you are using to give you the temperature reading?
 
ColdFusion85 said:
ZapperZ, the connection is correct, no doubt there.

AM, I am using a multimeter, and I tried both placing the probes to the leads, as well as just wrapping the wire around the probes themselves. Neither changed the value I was getting in liquid N2 (-156 degrees C). Regardless, I should not be getting an error as large as I am.
Perhaps you could tell us what readings you are getting.

Using the voltage reading at 0C (273K) for a reference of 0, you should be getting -5.8 mV at -196C (77K). Since you say you are getting a temperature of -156C I gather you are getting a reading of about -5.1 mV (ie. 5.1 mV less than the reading at 0C.). Is this correct?

I would suggest that you use a scope or VTVM rather than a multimeter as these will not draw any current.

AM
 
  • #10
I am using a temperature controller made by Omega. It is an older model one, I can't recall off the top of my head. AM, you are correct about what readings I should be getting for the liquid N2. However, my reading for the N2 was -6.5 mv. I should be getting something closer to -5.7 or 8. The negative is because the TC input on the outside of the flange is at room temperature.
 
  • #11
ColdFusion85 said:
I am using a temperature controller made by Omega. It is an older model one, I can't recall off the top of my head. AM, you are correct about what readings I should be getting for the liquid N2. However, my reading for the N2 was -6.5 mv. I should be getting something closer to -5.7 or 8. The negative is because the TC input on the outside of the flange is at room temperature.
What is the reading at 0? At room temperature? What is the difference between the reading in N2 and the reading at 0C? It should be about -5.8 mV.

AM
 
  • #12
Andrew Mason said:
What is the reading at 0? At room temperature? What is the difference between the reading in N2 and the reading at 0C? It should be about -5.8 mV.

AM

Reading at 0 C is -0.8 mV, which is correct because the room temperature was roughly 20 C, which corresponds to 0.798 mV for a K TC. At room temperature the reading was 0 mV because the junction was at the same temp as the controller ports. At liquid N2 the reading was -6.5 mV, so that corresponds to roughly -5.7 mV...??

If this is correct, then the controller should not be 40 C off...this is what is baffling me...:rolleyes:
 
  • #13
What is the calibration standard that is telling you what the temp of the N2 actually is?
 
  • #14
ColdFusion85 said:
Reading at 0 C is -0.8 mV, which is correct because the room temperature was roughly 20 C, which corresponds to 0.798 mV for a K TC. At room temperature the reading was 0 mV because the junction was at the same temp as the controller ports. At liquid N2 the reading was -6.5 mV, so that corresponds to roughly -5.7 mV...??

If this is correct, then the controller should not be 40 C off...this is what is baffling me...:rolleyes:
Why do you think it is 40C off? -6.5 mV is -5.7 mV below the reading at 0C, which is very close to what it should be at -196C.

AM
 

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