Chapter 1
Why do radiation thermometer have a specific type of error?
radiation thermometer detects the infrared energy emitted from an object and determines its temperature from its intensity.
Unlike contact-type temperature sensors, which detect temperature thermally by bringing the sensor into contact with the object, this method uses a fundamentally different measurement principle.
| Method | What to use | Main points to note |
|---|---|---|
| Contact type | Heat transfer between the object and the sensor | Contact condition, insertion depth, heat dissipation, responsiveness |
| IR Thermometers | Infrared rays from the target | emissivity, reflectance, measurement distance, optical path, target surface |
Therefore, to use radiation thermometer correctly, you need to consider not only the "accuracy of the thermometer," but also what kind of infrared radiation is reaching the thermometer from the object.
If you want to learn more about the principles of radiation thermometer, emissivity, D:S ratio, and how to select the right model, please refer to our guide which covers the basics.
Cause 1
Reason 1: emissivity setting does not match the object.
When considering the error of radiation thermometer, emissivity is particularly important.
emissivity is a value that represents how much thermal radiation compared to blackbody at the same temperature. It is expressed on a scale from 0 to 1.
emissivity is not uniquely determined by the material alone. It changes depending on surface roughness, oxidation, coating, wavelength, temperature, and other factors.
A surface with relatively high emissivity
Painted surfaces and oxidized surfaces may exhibit relatively high emissivity.
Surfaces that tend to have low emissivity
Shiny metal surfaces tend to have low emissivity and are therefore more susceptible to reflections from the surrounding environment.
The values listed in emissivity table are approximate. Even for the same material, the values can vary depending on the surface condition and measurement conditions, so simply using the values in the table does not guarantee accurate measurements.
Why does a mistake emissivity lead to a large error?
radiation thermometer determines the temperature of an object from the detected infrared energy and a set emissivity.
Therefore, if the actual radiation characteristic of the object and the thermometer settings differ significantly, a large discrepancy may occur in display temperature.
Especially with low-emissivity objects such as shiny metals, it's necessary to consider not only emissivity setting but also the infrared radiation reflected from the surroundings.
Cause 2
Cause 2: The surrounding heat source is reflected in the object.
The infrared radiation received by radiation thermometer is not necessarily limited to that emitted from the object itself.
In particular, with objects that do not transmit infrared radiation, in addition to the radiation emitted by the object itself, infrared radiation reflected from the object's surface from the surroundings may reach the thermometer.
This reflection effect is particularly pronounced on low-emissivity surfaces such as glossy metals, so caution is advised.
High-temperature furnace/heater nearby
When infrared radiation from a high-temperature object is reflected by the surface of the object, it may be measured as having a higher temperature than the actual temperature.
The surrounding area is cold
Depending on the measurement conditions, display value may be lower due to the influence of a low-temperature ambient environment.
If display value changes significantly when the measurement angle is slightly altered, this can be a clue to whether reflections from the surroundings are affecting the reading.
Cause 3
Reason 3: The measurement distance is too far, and objects other than the target are also being measured.
radiation thermometer do not necessarily measure the temperature at only one point where the laser is shining.
In reality, infrared radiation is received from within the measurement field of field of view defined for each model.
The D:S ratio is used as a guide to understand this relationship. D represents the measurement distance, and S represents the measurement diameter at that distance.
For example, if D:S = 12:1, a simple guideline would be that a distance of 120mm corresponds to a measurement diameter of approximately 10mm. However, actual field of view characteristic vary depending on the model, so please check the measurement diameter and measurement distance diagram in the product specifications.
When the object is smaller than the measurement field of view
If the background is included in the measurement field of view of view, infrared radiation from sources other than the target object will also be detected, which may result in a temperature reading that differs from the actual temperature of the object being measured.
Particular care is needed when measuring small parts, narrow pipes, or locations at a distance.
Rather than following a uniform rule such as "the object must be several times the spot diameter," please check whether the object is large enough for the measurement field of view specifications of radiation thermometer you are using.
Other factors
Error factors other than emissivity, reflectance, and distance
The reasons for discrepancies in measurements are not limited to just the three main factors.
| Check items | Possible events |
|---|---|
| Measurement angle | The apparent radiation characteristic and reflection effects may change depending on the angle and surface condition. |
| Windows and glass | Ordinary glass and similar materials may not adequately transmit infrared light within the wavelength range being used. |
| Smoke, steam, dust | Infrared rays can be absorption or scattered by materials in the optical path, which may affect the measurement. |
| Changes in the target surface | emissivity can change due to oxidation, dirt, paint, wetting, etc. |
| Environment surrounding the thermometer | Measurements may be affected by factors such as operating temperature ranges or rapid changes in ambient temperature. |
| Location of measurement | Since radiation thermometer primarily measure surface temperature, the measured temperature may not coincide with the internal temperature or the measurement location of contact-type sensors. |
Troubleshooting
Isolating the cause from the symptoms
When the measurement value is low
- Is emissivity setting appropriate for the target surface?
- Is the object being measured sufficiently larger than the measurement field of view?
- Is the low-temperature background included in the measurement field of view?
- Are you measuring through window materials or anything like that?
- Are we really comparing the same surface locations?
When the measurement value is high
- Check if there are any high-temperature objects nearby, such as furnace walls, heaters, or lights.
- Is the surrounding high-temperature material reflecting off the glossy metal surface?
- Does the value change when the measurement angle is changed?
- Has the condition of the target surface changed from what was expected?
When the value fluctuates
- Is the object smaller than the measurement field of view?
- Check if the object or thermometer is moving.
- Is the measurement location changing each time?
- Has the reflective surrounding environment changed?
- Check if smoke, steam, dust, etc. are crossing the light path.
When the reading does not match that of a contact thermometer.
First, make sure both thermometers are actually measuring the same temperature.
radiation thermometer measure surface temperature. On the other hand, with contact temperature sensors, the measurement value is affected by the sensor's mounting position, contact condition, insertion depth, and heat dissipation.
Therefore, the mere fact that "the two thermometers display" is not enough to determine which one is correct.
Countermeasures
Measures to reduce the error of radiation thermometer
1. Check emissivity
Don't judge solely by the material name; check the surface condition and measurement conditions.
2. Utilize high emissivity surfaces
If conditions permit, one method to stabilize measurement conditions is to use tapes or painted surfaces with known radiation characteristic.
3. Check ambient reflections.
If reflections from high-temperature objects are suspected, check the effect on display value by changing the measurement angle and surrounding environment.
4. Check the measurement field of view.
To ensure the object fully fills the measurement field of view, check the distance, measurement diameter, and the optical specifications of the instrument.
5. Fix the measurement conditions.
By keeping the measurement position, distance, angle, and target surface as consistent as possible, it becomes easier to improve repeatability of the measurements.
6. Check for proofreading if necessary.
If doubts remain about the values even after checking the measurement conditions, or if reliability is required for quality assurance, we will also check the calibration status.
Calibration in non-contact temperature measurement is explained in detail in the following article.
The Importance of "Non-Contact Temperature Measurement and Calibration"
Download
Check emissivity and error quick reference table.
We have prepared a "radiation thermometer Error Quick Reference Chart" that can be used to check emissivity of major materials and the error of radiation thermometer.
radiation thermometer Error Quick Reference Chart
This can be used as reference material when checking emissivity and measurement errors on-site.
FAQ
Frequently Asked Questions about the Error of radiation thermometer
Q. The readings from radiation thermometer and contact thermometer are different. Which one is correct?
display value alone is insufficient for making a judgment. radiation thermometer primarily measure surface temperature, while contact thermometers are affected by the sensor's contact condition and installation method. It is necessary to standardize the measurement location, target, and installation conditions to isolate the cause.
Q. Why do radiation thermometer give inaccurate readings when measuring metals?
This is because shiny metals have low emissivity and easily reflect infrared radiation from their surroundings. In addition to the emissivity setting, please also check the surrounding high-temperature objects and the measurement angle.
Q. Can I measure accurately by using blackbody tape?
Using tape with known radiation characteristic is one way to stabilize measurement conditions. However, factors such as the tape's heat resistance temperature, adhesion, temperature difference with the target object, and wavelength used must also be considered.
Q. Can measurement be taken if the laser pointer is pointing at the target?
Not necessarily. Lasers are merely a guide for aiming, and some models do not indicate the actual measurement field of view. Please check the measurement diameter and distance in the product specifications.
Q. Can you measure the temperature through glass?
This depends on the measurement wavelength of radiation thermometer used and the transmission characteristic of the window material. If typical glass does not transmit infrared rays of the measurement wavelength, the temperature of the object cannot be accurately measured through the glass.
related
Learn more about radiation thermometer
Foundation and Selection
Are you having trouble with measurement values or selecting radiation thermometer?
By considering factors such as the object being measured, temperature range, material, surface condition, measurement distance, and surrounding environment, you can identify potential error factors and determine the most suitable measurement method.
I want to ask about radiation thermometer.
