"Just how hard is it to accurately measure the temperature of the OA vapor
as it exits the discharge tube"

Depends on what you mean by accurate.  Within +/-20 degrees C is hard.  Within +/- one degree C would be very hard.  Like days or even weeks of effort in the lab with a few thousand $ worth of equipment, then days of data treatment.  You are dealing with a transient state so must make the measurement with a very low thermal mass sensor and must make the measurement very fast. You have to account for the thermal conductivity and thermal mass of your sensor to calculate out thermal lag,  Then you probably need to correct for radiation effects.  You have two types of radiation issues you need to check.  One is radiation gains from the vaporizer to the sensor and the other is radiation heat loss of the sensor itself  if the sensor is positioned so it can "see" the end of the tube the sublimate exits from.  You should do those kinds of radiation calculations before you go to the lab as they might be so important they swamp your measured signal.  I dealt with a similar but simpler situation in a gas phase chemical reaction 50 years ago and the only way we could get gas phase temps was via total calculation based on wall temperatures measured by looking at radiation with a thermocouple.  But, in our case the reaction was not transient it was a continuous steady state which makes calculations a lot easier.

Dick

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