A study was conducted to measure changes in gas temperature along the length of a Method 5 sampling train due to variations in stack gas temperature, sampling rate, filter box temperature and method for controlling the probe heating element. For each run condition, temperatures were measured in the stack, at ambient and at four internal and external positions along the sampling train at one minute intervals. Measurements were continued until the system was observed to reach a state of thermal equilibrium. The results from these experiments, presented herein, demonstrate that gas temperatures cannot be predicted or controlled on the basis of externally measured temperatures. The use of an internal thermocouple, having its reference junction at the back of the probe, to proportionally control the probe heater element is shown to provide a predictable gas temperature and a flat thermal profile along the system. This procedure for controlling gas temperature is recommended as a modification to Method 5.

June 1979.