Q9 What should you note when measuring the internal 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, when measuring the temperature of a liquid in a measuring cylinder, the situation differs among the cases of inserting a slim, medium-thickness and thick sensors.


See the figure above. The left shows when the slim sensor is inserted, the middle shows when the medium-thickness one is inserted, and the right shows when the thick one is inserted.
When the thick sensor is inserted, the heat of the measurement object is overwhelmingly cooled down, compared with the slim or medium-thickness one. Eventually, the sensor measures the temperature of the cooled-down liquid, which is not its intended purpose.
You may think, “Why don’t you measure anything with the slim sensor, then?” But the slim sensor tends to break easily because it is not sturdy enough.” To the contrary, the thick one is sturdy and hardly breaks. Since its internal thermocouple wire can be made thick, it is available at high temperature as well.
Anritsu Meter recommends you to choose the right sensor for the right purpose.

(2)Insertion length

An insertion length is important when measuring the internal temperature. (It is preferably 15 to 20 times the sensor’s outer diameter.)
If the internal length is short, the sensor fails to properly receives the heat from the measurement object, causes a higher effect of heat radiation, being likely to result in lower temperature measurement than actual temperature.


See the figure above. The left shows the deeply inserted sensor, the middle shows the slightly shallowly inserted one, and the right shows the most shallowly inserted one. The sensor’s temperature sensing part is attached to the nose. If it is shallowly inserted, accordingly, the temperature sensing part is cooled down by an effect of heat radiation generated outside a hole.
If it is inserted deeply, the temperature sensing part is kept away from a heat radiation area, being protected against an effect of heat radiation generated outside the hole.
The depth almost free from the effect of heat radiation is 15 to 20 times the outer diameter.

(3)Clearance with a hole

A clearance with a hole diameter is also significant.
A big clearance worsens the contact of the sensor and allows in a wind through the clearance, being likely to slow down the sensor’s response speed and result in lower temperature measurement than actual temperature.


If the insertion length is deep enough, the temperature can be measured without problem by making the actual hole diameter about 0.2 to 0.3 mm larger than the sensor’s outer diameter; approx. ø3.3 for the sensor’s outer diameter = ø3, and ø5.3 for ø5.

Precautions for Measuring with the Internal Temperature Sensor

* Do not use a thick sensor to measure a low-heat capacity object.
* Check the insertion length (15 to 20 times the outer diameter).
* Check a sensor insertion hole for an excessive clearance (outer diameter + 0.2 to 0.3).