Basics of Radiation Thermometers
This section introduces the basics of radiation thermometers, how to select them, and key points in using them.
What are radiation thermometers?
Digital Infrared Temperature Sensor FT Series
Your cheek feels warm when you move your palm close to it because the cheek skin senses the infrared rays being emitted from your palm.
All objects emit infrared rays, and the higher the temperature of an object, the stronger the infrared rays it emits.
Radiation thermometers use these infrared rays to measure the temperature.
What are infrared rays?
Infrared rays are a type of the same light as the visible light that people generally see with their eyes and are also called IR (InfraRed).
However, people cannot see them with the naked eye because their wavelength is longer (frequency is lower) than that of visible light.
The wavelength is approximately 0.7 to 400 μm.
Infrared rays were discovered by British astronomer Sir Frederick William Herschel in 1800.
Features of radiation thermometers
Use of radiation thermometers offers the following two advantages.
- The temperature can be measured at high speed.
- The temperature can be measured in a non-contact manner.
Radiation thermometers are effective for measuring the temperature of moving/rotating objects and objects whose surface temperature changes when coming into contact with a sensor (objects with small heat capacity).
On the other hand, they have some disadvantages such as the inability to measure the temperature of gases or the inside of an object. It is also necessary to set the emissivity of a radiation thermometer according to the target object.
Principle of radiation thermometers
Infrared rays radiated from an object are collected by a lens into a sensing element called a “thermopile.”
A thermopile is a sensing element that generates electric signals according to the temperature after absorbing infrared rays emitted from an object and being warmed by the absorption.
These signals are amplified and the emissivity is corrected to display the temperature.
- A
- Thermocouple cold junction
- B
- Thermocouple hot junction
- C
- Infrared absorbing film (right: view from above)
As shown above, a radiation thermometer is configured with many thermocouples connected in series.
The hot junctions of the thermocouples are collected in the center, and the cold junctions are collected on the periphery.
Because the infrared rays collected by the lens only hit the hot junctions, only the hot junctions are heated.
The Seebeck effect generates a voltage difference between hot and cold junctions, thereby enabling temperature measurement.
(A radiation thermometer has a built-in thermistor to measure the temperature of cold junctions.)
What is the emissivity?
The amount of infrared rays emitted from objects differs, even if they have the same temperature, depending on their materials and surface conditions.
When measuring the temperature using a radiation thermometer, you need to correct the ratio of this emission according to the target object.
This ratio is called “emissivity.”
The “emissivity” is a constant that depends on each object and a perfect blackbody has an emissivity of “1,” while an object that completely reflects or is permeable to infrared rays (such as air), opposite to a blackbody, has an emissivity of “0.”
This means that the emissivity of all objects falls between 0 and 1.
When light is incident on an object surface, the energy is absorbed by the object, is reflected by the surface, or penetrates the object.
Assuming that the incident energy is “1,” the following formula is established.
1 = absorptivity + reflectance + permeability
Also, according to Kirchhoff’s Law, the absorbed energy is equal to the energy to be radiated from the object. Hence the following formula is established.
Absorptivity = Emissivity
From the above formula, you can see that the higher absorptivity (or the lesser reflectance or permeability) an object has for incident energy, the higher the emissivity.
What is a blackbody?
When it comes to the emissivity, you need to understand what a “blackbody” is.
A “blackbody” absorbs all the light, irrespective of the wavelength, incident on the surface, with no reflection or permeation. Hence, a blackbody is an ideal object for a radiation thermometer.
Since its reflectance and permeability are both “0,” its absorptivity is “1,” and thus the emissivity is also “1.”
How to determine the emissivity
When the emissivity is known
If the emissivity of an object is described as a physical constant in any reference or other material, use the value as it is.
Determine the emissivity by taking into account the conditions where the emissivity has been measured (such as the surface condition of the object).
When the emissivity is not known
Actually measure the temperature of an object and use the value displayed on the radiation thermometer.
- Method using a contact-type thermometer
Measure the temperature of an object using two contact-type thermometers, such as a radiation thermometer and a thermocouple, and set the emissivity so that the values displayed on them will match. - Method using blackbody spray (tape)
This spray is used to obtain the emissivity of an object.
- Step 1
- Apply blackbody spray to part of the object.
- Step 2
- Using a radiation thermometer set at the emissivity of the blackbody, measure the temperature of the part applied with blackbody spray.
- Step 3
- Measure the temperature of the part not applied with blackbody spray and set the emissivity so that it will correspond to the value displayed in step 2.
- Step 4
- Determine the emissivity set in step 3 as the emissivity of this object.
How to select radiation thermometers
Selection based on the usage method
Radiation thermometers are generally classified into the following two types.
Hand-held type
A radiation thermometer in which a detector and a converter are not separated but integrated. Thanks to its compact size and light weight, you can carry it and measure the temperature with it in your hand.
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Installation type
A radiation thermometer in which a detector and a converter are structured separately and are coupled electrically with a connection cable.
The thermometer is fixed in place while the temperature is measured.
You can see the “surface temperature” with non-contact measurement! Digital Infrared Temperature Sensor FT Series
Download the catalog for more details.
Selection based on object size and measuring distance
A radiation thermometer has a specific measurable range (called the spot diameter) and measuring distance.
To measure the temperature accurately, it is necessary to use the designated spot diameter and measuring distance.
The above figure shows the relationship between spot diameter and measuring distance of a radiation thermometer.
When selecting a radiation thermometer, make sure that the spot diameter is smaller than the target object.
Key points in using radiation thermometers
Emissivity setting
Measurement errors occur when the setting of the emissivity is different from the emissivity specific to the target object.
Since the relationship between the emissivity and the object temperature is not linear, temperatures measured under different conditions cannot be modified or corrected at later times.
(A 1% difference in the emissivity setting does not necessarily generate a 1% difference in the temperature.)
Relationship between emissivity setting errors and temperature measurement errors (typical examples)
| Object temperature | Emissivity setting error (°C/°F) | ||
|---|---|---|---|
| 1% | 5% | 10% | |
| 0°C (32°F) | 0.5°C (0.9°F) | 1.5°C (2.7°F) | 2.5°C (4.5°F) |
| 100°C (212°F) | 0.6°C (1.08°F) | 3.0°C (5.4°F) | 6.0°C (10.8°F) |
| 200°C (392°F) | 1.5°C (2.7°F) | 6.5°C (1.7°F) | 12.0°C (21.6°F) |
| 300°C (572°F) | 2.0°C (3.6°F) | 9.5°C (17.1°F) | 18.0°C (32.4°F) |
Spot diameter and target object
To stably measure the temperature of an object, make sure that approximately 1.5 times the spot diameter fits inside the object.
During high-temperature measurement
When measuring a hot object, the infrared rays emitted from the object heat the radiation thermometer body, which not only prevents the accurate display of the temperature but may also damage the thermometer in the worst scenario. In such a case, shield the infrared rays unnecessary for measurement as shown below.
Wiring to an instrument (recorder)
Measurement of 4 to 20 mA output
Measurement method using an instrument equipped with a 4 to 20 mA input
Satisfy the relationship of “maximum load resistance of the 4 to 20 mA output > load resistance of the 4 to 20 mA input.”
If this relationship is not satisfied, a measurement error will occur.
Measurement method using a shunt resistor to convert current to voltage
The current that flows through a shunt resistor is converted to voltage through Ohm’s Law (E = I × R).
The converted voltage can be measured with an instrument that has a voltage input range.
Satisfy the relationship of “maximum load resistance of the 4 to 20 mA output > resistance value of the shunt resistor.”
If this relationship is not satisfied, a measurement error will occur.
Method using a signal converter
By using a signal converter, you can measure 4 to 20 mA output with an instrument that has a voltage input range.
Is it possible to wire a 4 to 20 mA output in parallel?
Yes, it is possible.
Measurement method using voltage input
When the 4 to 20 mA target output device is connected to another 4 to 20 mA input device, measurement can directly be performed using an instrument that has a voltage range.
The voltage converted from current through the load resistance of the other 4 to 20 mA input device is measured.
Method using an instrument equipped with a 4 to 20 mA input
Simultaneous measurement is possible through wiring in series.
It is necessary to satisfy the relationship of “maximum load resistance of the 4 to 20 mA output > total load resistance of the two 4 to 20 mA inputs.” Also note that a potential difference occurs between the − terminals of each input because load resistances are connected in series. Even if there is a potential difference, make sure that there are no problems in the circuit.
Measurement of analog voltage output
Measurement is possible through direct connection.
Adjust the input range according to the output voltage.
