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Name: Mike F.
Status: educator
Age: 20s
Location: N/A
Country: N/A
Date: 4/22/2003


Question:
I am conducting dry ice experiments and I have seen a curious anomaly. I am using a type T thermocouple, measuring the surface temperature between two dry ice blocks, and expecting temperatures around -78.5C (+/- 1C). However, I get temperatures readings down to -86C. Is this a red flag for my instrument calibration or is it possible that the reading is real?


Replies:
This is a "red flag" that something is going on. Your emf reading is too high. This means that the apparent temperature differential between the two junctions is too large. Some things to check, or to alter:

1. Check the thermocouple ice bath. Was the ice slurry good?

2. Check the thermocouple for stresses. One way to do this is to put both junctions in the ice bath. The emf reading should be 0 millivolts.

3. Check the voltmeter to see if it is reading correctly. One way to do this is to create a "fake" emf with a battery and resistor so that the voltage drop across the resistor (calculated by Ohm's Law) is about what you would expect for the emf from the CO2.

4. Check the solder joints of the junctions. Easiest thing may be to just re-solder the joints to make sure the electrical connection is secure at both ends. Alternatively, just use a different thermocouple, either a type T or even another type. The thermocouple tables can be found in most chemistry, physics, engineering handbooks.

5. Two blocks of solid dry ice is not a very good experimental configuration. You will get better thermal contact using a dry ice / acetone bath (or some other liquid medium). This reduces "heat leak" into the junctions which can cause all sorts of strange signals because the emf not only depends upon the temperature at the junctions but the gradient of the temperature across the junction. Be careful if you use acetone -- very flammable, even at low temperature.

6. If you have liquid nitrogen available, check the emf at that temperature (77kelvins = ~ -196 C). Being a liquid L-N2 makes things a lot simpler.

Vince Calder



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