Q10 What should you note when measuring the surface temperature?

There are largely the following three points.

(1)Sensor and the heat capacity of a measurement object

A temperature sensor basically receives the heat from a measurement object, makes the temperature of a temperature sensing part equal to that of the measurement object and displays the temperature of the temperature sensing part. If the measurement object with too low a heat capacity is measured, however, measurement itself may become an error factor.
For example, suppose there are large, medium-size and small measurement objects with the same temperature. If the sensor with the same heat capacity is applied to them to measure the temperature, the situation differs from one object to another.

See the figure above. The left shows when the large measurement object is measured, the middle shows when the medium-size one is measured, and the right shows when the small one is measured.
When the small measurement object is measured, the heat of the measurement object is overwhelmingly cooled down by applying the sensor, compared with the large or medium-size one. Eventually, the sensor measures the temperature of the cooled-down measurement object, which is not its intended purpose.
You may think, “Why don’t you measure any measurement objects with a low-heat capacity sensor, then?” But care should be taken for the low-heat capacity sensor because it is not sturdy enough and worsens the contact by slightly tilting to apply it, displaying a lower temperature value.
When measuring a general-size metal, however, such a problem does not happen easily. When measuring small substances such as a semiconductor element or resin-made ones, these precautions are required.

(2)Heat conductivity (heat capacity) and volume

A substance has heat conductivity. Simply speaking, it is a degree of easiness to transfer the heat in the substance.

Heat Conductivity Near Normal Temperature (W/m*K) (Reference Values)

The table above lists the heat conductivity of some typical substances.
Heat conductivity tends to be higher for metals and lower for resins. It also tends to be higher for the substances with higher electrical conductivity. It may be troublesome when measuring the heat conductivity and temperature

For example, the figure below shows a temperature distribution chart when measuring aluminum (heat conductivity = 210) and Teflon (heat conductivity = 0.25), using a surface thermometer.
The top is aluminum and the bottom is Teflon.

In the case of aluminum, the internal temperature of the substance becomes irregular momentarily when applying the sensor, but the entire temperature immediately settles to become consistent due to high heat conductivity. In the case of Teflon, the entire temperature settles very slowly due to low heat conductivity, eventually saturating with the temperature still distributed. This occurs because a heat radiation rate from the sensor is higher than a heat transfer rate of Teflon.
(* Aluminum also has similar heat radiation, but the temperature becomes consistent due to high heat conductivity.)
Based on the above, care should be taken when measuring a substance with low heat conductivity such as Teflon, even if its volume is big.
Possible measures to cope with this is to use a low-heat capacity sensor (small one or with resin guard) which hardly has an effect on the measurement object, or to examine in advance how low the sensor measures the temperature and make a correction to use because this phenomenon is reproduced to some extent.

(3)Proper contact

Because a thermometer has the nature of “bringing the hot junction (temperature sensing part) into contact with the measurement object to receive the heat therefrom and making the temperature of the hot juncture (temperature sensing part) equal to that of the measurement object to measure the temperature of the hot juncture (temperature sensing part),” it is significant to have proper contact between the temperature sensing part and the measurement object.
Major factors to worsen the contact include “measurement with the tilted sensor,” “dust or adhered substance on the surface of the measurement object,” “deformed or damaged temperature sensing part of the sensor,” and so on. They have a bad effect on the durability of the sensor as well as measurement accuracy.

Precautions for Measuring with the Surface Temperature Sensor

・ Do not use a high-heat capacity sensor to measure a low-heat capacity object.
・ Check the surface of the measurement object for adhered dust, etc.
・ Check the temperature sensing part of the sensor for deformation.