# Thermocouple Errors in Temperature Controller

1. Aug 2, 2006

### ColdFusion85

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?

2. Aug 2, 2006

### ZapperZ

Staff Emeritus
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. Aug 2, 2006

### Andrew Mason

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. Aug 2, 2006

### FredGarvin

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://www.temperatures.com/tctables.html

Last edited: Aug 2, 2006
5. Aug 2, 2006

### Andrew Mason

The OP did not post his voltage readings, so how do you know this?

AM

6. Aug 2, 2006

### ColdFusion85

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. Aug 2, 2006

### FredGarvin

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. Aug 2, 2006

### FredGarvin

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. Aug 2, 2006

### Andrew Mason

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. Aug 2, 2006

### ColdFusion85

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. Aug 2, 2006

### Andrew Mason

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. Aug 2, 2006

### ColdFusion85

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. Aug 2, 2006

### FredGarvin

What is the calibration standard that is telling you what the temp of the N2 actually is?

14. Aug 2, 2006

### Andrew Mason

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