when thermocouple deep into the melting solder

[Ferry]

I work in a SMT assembly in Taiwan,and use (KIC)temperature profiler
datalogger, used K type thermocouple attach on PCB to measure
temperature profile from wave solder oven routine,but recently
something confused me, that the solder pot temperature be set up to
242C.,but sometime the results from the(KIC)temperature profiler
datalogger show the solder pot temperature only 183C.then I continue
use thermometer to measure temperature from solder pot, the reading
show 242C.

So the question is that when thermocouple deep into the melting solder
pot,mixed with different liquid metal whether the interference can be
occurred,
or this problem from the thermocouple or (KIC)temperature profiler
datalogger


Best regards

Ferry

[Filipe A.]

Ferry;

The way that thermocouples work is, by having two different metals connected together, a variation of temperature generates a voltage variation at the thermocouple. If you include different metals at that combination, you might change the characteristics the voltage varies with temperature, than the measurement will not be accurate anymore.
Hope this helps.
Filipe

[aasenjo]

I am afraid the welding or the introduction of a third metal will not be the problem, as long as both tips are at the same temperature. The following is taken out of the Industrial Heating Website:

http://www.industrialheating.com/CDA/ArticleInformation/features/BNP__Features__Item/0,2832,81512,00.html

"Although a mechanical joint such as a twist can provide the necessary contact between the two dissimilar wires for the thermocouple to operate, a more robust connection is generally needed for extended service life. The Law of Intermediate Temperatures states if two dissimilar homogeneous metals produce a thermal emf value, it will remain constant if a third material is introduced into the circuit, provided both ends of that material are at the same tem
perature. This permits welding of the hot junction."

So I would check the thermocouple connections and the temperature logger. This KIC logger is not a National Instruments product, we provide systems that will allow you to log data directly from your PC with accuracies of up to .1 Degree C with standard thermocouples. (using system calibration techniques)

I hope this helps,

Alejandro Asenjo
National Instruments
Application Engineer