'Tempilstik' is a trade name for temperature indicating crayons. The crayons can indicate temperature in several ways:
- If a certain temperature, or temperature within a certain range is required, several marks from crayons indicating a range of temperatures can be applied to the material on the reverse side to the heat source. When the crayon indicating the desired temperature has melted, but the next crayon mark has not, the desired temperature has been reached.
- If a certain minimum temperature is required, the crayon mark will melt when the minimum temperature is reached.
The range of temperatures that can be shown by the use of temperature indicating crayons is very large, from 38°C (100°F) to approximately 1370°C (2500°F), a different crayon for each temperature. They are good for a quick evaluation of temperature, and are used to check preheat, postheat and other heat treatments. See FAQ: Why is preheat used when arc welding steel, and how is it applied?
It is not possible to control temperature using temperature crayons, as they only indicate when a certain temperature has been reached or exceeded. To control and correct, temperature crayons should be used in conjunction with thermocouples or contact thermometers to ensure the minimum temperature does not drop below the specified value, or exceed any maximum requirement, after checking with a temperature crayon.
There are many other temperature indicating products that can be used in the same way as a crayon, for example, paints, pellets and indication cards that indicate temperature by circles of temperature sensitive paper. These are all single-use items, and give no indication of temperature drop or rise after application, so further applications of the temperature indicating product to the workpiece are needed to check, for example, the interpass temperature, and whether it is below that specified, and above the preheat temperature.
The composition of the temperature indicating products is very carefully controlled in order not to contain any elements, as far as possible, which will affect the workpiece in a detrimental fashion. For example the sticks have been developed over time to have a minimal level of lead, bismuth, antimony, tin, or other low melting point metals to avoid formation of low-melting point eutectic alloys at the surface of the workpiece.
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