Second guessing the temperature indicator

There have been incidents where kiln operators have altered, or disregarded, the readings of instruments to conform to their expected behaviour of a particular glass.

This can be called the “I expect my glass to do something at a particular temperature and if the temperature indicator reads different; then the instrument is wrong” syndrome.
This attitude is not all that rare. For example, the American Kiln builder Skutt, who claims to be advised by leading glass artists, includes instructions in their literature for offsetting of instrument readings "if (the operator believes) the readings are predictably and consistently incorrect".

A digital temperature indicator senses a voltage from a thermocouple and translates that to a temperature value. 
The most modern indicators do this with precision; with accuracy generally within ±2° C. right across the scale. They are designed to eliminate the earlier problem of an instrument reading correctly at room temperature and significantly in error at higher readings.

Widely used by potters and commercial ceramists, Pyrometric cones are clay shapes of precise dimensions and composition, designed to slump and “kiss the batt” at a specified temperature when heated at a specified rate of temperature rise.
Varying that rate of rise will change the temperature at which they respond.

• Too slow a rate will allow more time for the heat to penetrate and soften the inside; so they drop at a lower temperature.

• Too fast a rate will allow less time; so they drop at a higher temperature.

To keep this short, here’s some points of guidance;

• Varying the rate of temperature change can vary the temperature at which a particular change takes place in ceramic (pottery and glass) bodies.

• Varying the thickness of the body can have a similar effect.

• Varying the position of the body in a kiln, or changing kilns, can also have similar effect.

• There can be differing results from  a temperature indicator and pyrometric cone when monitoring the same firing; even when they are both functioning correctly. To be expected as their response times differ and they (cone and thermocouple) are in different locations
  

• Temperature indicators show the temperature only at the end of the probe, nowhere else.

• If there is reason to suspect the temperature reading of an indicator, have the devise checked against a known standard or sub-standard, not against the behaviour of a piece of glass.