Thermocouple Errors in Temperature Controller

In summary, the OP made a Type K thermocouple and tested it at various temperatures using a multimeter. The readings were correct at room temperature and in ice water, but not in liquid nitrogen where the reading was -6.5 mV instead of the expected -5.8 mV. The possible source of error could be the non-linear calibration curve of the thermocouple and the incorrect connection of the leads. It is suggested to use a scope or VTVM instead of a multimeter for more accurate readings.
  • #1
ColdFusion85
142
0
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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  • #2
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.
 
  • #3
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
 
  • #4
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
[tex]E = C_0+C_1T+C_2T^2+C_3T^3+...[/tex] 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:
  • #5
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
 
  • #6
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
 
  • #7
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.
 
  • #8
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?
 
  • #9
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...:uhh:
 
  • #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...:uhh:
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
 

1. What is a thermocouple error in a temperature controller?

A thermocouple error in a temperature controller occurs when there is a discrepancy between the actual temperature and the temperature reading displayed on the controller. This can be caused by various factors such as damaged or faulty thermocouples, improper calibration, or electrical interference.

2. How can I identify a thermocouple error in my temperature controller?

One way to identify a thermocouple error is to compare the temperature reading on the controller with a known accurate measurement, such as a thermometer. If there is a significant difference between the two readings, it is likely that there is a thermocouple error.

3. What are the common causes of thermocouple errors in temperature controllers?

Some common causes of thermocouple errors include physical damage to the thermocouple wires, improper installation or connection of the thermocouple, and electrical interference from nearby equipment or wiring. In some cases, the temperature controller itself may be faulty.

4. How can I troubleshoot and fix a thermocouple error in my temperature controller?

If you suspect a thermocouple error, start by checking the physical condition of the thermocouple wires. If they are damaged, they may need to be replaced. If the wires appear to be intact, try recalibrating the temperature controller according to the manufacturer's instructions. If the error persists, it may be necessary to replace the thermocouple or the temperature controller itself.

5. How can I prevent thermocouple errors in my temperature controller?

To prevent thermocouple errors, it is important to regularly inspect and maintain the thermocouple and its connections. Make sure to follow proper installation procedures and keep the thermocouple wires away from potential sources of electrical interference. It is also recommended to periodically recalibrate the temperature controller to ensure accurate readings.

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