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Another method to measure temperature is to use indicators and labels that are not reusable. These indicators are placed on the object whose temperature is being measured, and they will indicate the highest temperature the object was subjected to. Fig. 1 shows one example of these types of temperature sensors. The simplest sensor is a single dot of material that changes color when the specified temperature is exceeded. More complex indicators have four or more dots that will each change color at a different temperature. For example, the dots may change color at 5°F increments beginning at 100°F through 130°F. If the temperature has reached 110°F, the first three dots would change color to indicate this is the highest temperature. Most of these indicators must be discarded after they are used. These indicators use material that changes color at specific temperatures, and don't change back to the original color once the temperature is lowered.
Above: Fig. 1: Example of nonreversible temperature
indicators that are used one time and discarded (crayon or marker variety).
Other examples of these types of temperature sensors include crayons (Fig. 1) and pellets that will melt when their specified temperature is exceeded. These types of indicators are placed on the surface where the temperature is to be measured.
When the temperature reaches the specified value, the crayon or pellet will melt. It's not possible to determine how much the temperature has exceeded the melting point of the material, so a series of pellets or crayons are usually used so that one or more remain unmelted. This provides a means to determine that the temperature is in a range of not more than the temperature of the unmelted pellet but more than the range of the melted pellet.
