I posted a message about this particular problem and received several
helpful suggestions. I am still, however, confused so let me set out my
observations:
1. The thermocouple reads the correct temperature when immersed in an ice
bath and in liquid nitrogen as well as in the open air. That is, under these
circumstances, the temperature indicated on the BVT 1000 is the expected
temperature.
2. The tip of the thermocouple, when fully inserted into the probe, extends
into the sample chamber approximately 5mm. I have tried measuring the
temperature with the thermocouple at differing positions in the probe.
Always the same result. The BVT 1000 says 280K and the methanol says 265K
(approximately).
3. A second thermocouple of exactly the same length gives very similar
results to those of observation 2.
4. A new methanol sample gives exactly the same results as in 2. That is,
BTV 1000 says 280K ... methanol says 265K.
5. At room temperature, the BVT 1000 reads 294K and the methanol says
282K.
The confusion that arises in my mind is how can the thermocouple read
the temperature correctly when out of the probe but not in the probe? I
have been told that the position of the tip of the thermocouple in the
sample chamber has a major effect on the temperature reading but this has
not been my oberservation. It seems to stay constant for large variations
in positioning of the thermocouple. Observation 5 is the real puzzler. If
the probe has equilibrated at room temperature for several days how can
the methanol indicate such a low temperature? It is certainly not that
cold in our lab! If the BVT 1000 is not calibrated correctly how can
observation 1 be made?
Keith Brown
University of Saskatchewan
Saskatoon, Saskatchewan
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