ASNT Level III Visual Testing, VT

When the rated temperature is reached, the stick leaves a liquid
streak instead of a dry mark and welding can begin.
To ensure accurate temperature indication with no override,
two or more indicators can be used to alert the operator
that the test object is approaching the correct temperature.
When a range of recommended preheat temperatures is
given, use of several indicators might be appropriate. For
example, carbon-molybdenum steel should he preheated to
between 95 and 205 °C (between 200 and 400 °F). A bundle
of indicators with ratings at 95, 120, 150, 175 and 205 °C
(200, 250, 300, 350 and 400 °F) might be useful for
determining how much of the test object is within the preheat
temperature range.
Charlie Chong/ Fion Zhang