A temperature expert explains! Dangerous radiation thermometer errors and correct countermeasures you should be aware of
radiation thermometer are convenient because they are non-contact, but using them incorrectly can lead to major measurement errors.
This article explains the three major factors -emissivity, ambient reflection, measurement distance and spot size -from a practical perspective, including formulas.
We have compiled some solutions that you can try right away.
For those who are having trouble selecting radiation thermometer and dealing with errors
"radiation thermometer are supposed to be convenient because they allow you to easily measure temperature without contact. But indicated value they give seem unstable..." "Compared to contact thermometers, the readings they display are completely different..."
If you use radiation thermometer for process management, quality control, or equipment safety maintenance at a manufacturing site, you may have had an experience like this at least once.
radiation thermometer are highly high performance and powerful measuring instruments that capture infrared energy emitted from an object and convert it into temperature.
However, because of this principle, if several "error factors" are not properly understood and used, it can lead to unexpected measurement errors, which can result in a decline in product quality or overlooking equipment malfunctions.
This article provides a systematic explanation of the three main causes of errors in radiation thermometer: (1) emissivity, (2) reflection from the surroundings, and (3) measurement distance and spot size, while also taking into account the meaning of the mathematical formulas.
In addition, we will also introduce in detail specific error prevention measures that can be put into practice in the field starting tomorrow.
1. Why do radiation thermometer produce errors? Understanding the basic principles
All objects emit infrared energy (electromagnetic waves) according to their temperature.
The higher the temperature, the more infrared radiation it emits.
radiation thermometer uses a lens to focus infrared rays from an object, then uses detecting element to convert that energy into an electrical signal, which is then converted into temperature unit (such as °C) and display.
This point of "measuring the infrared energy emitted by an object" is the most important point in understanding the error.
Contact thermometers measure the temperature of an object by utilizing its thermal conduction, while radiation thermometer measure its thermal radiation. This difference is the source of error.
2. Main cause of error ①: The most important but easily misunderstood "emissivity"
The most important and often misunderstood error factor in radiation thermometer is emissivity (ε: epsilon).
What is emissivity?
emissivity is an index that indicates how well an object can emit infrared rays.
When emissivity of an ideal object (blackbody) that radiates infrared rays completely is taken as 1.0, the amount of radiation that an actual object can emit is expressed as a value between 0 and 1.0.
- Objects with high emissivity (close to 1.0): Matte painted surfaces, human skin, water, wood, plastic, etc.
- Objects with low emissivity (close to 0): Polished metals (aluminum, stainless steel, gold, etc.).
radiation thermometer calculates the temperature by correcting the infrared energy with the set emissivity.
Therefore, if the actual emissivity of the target does not match the set value, a large error will occur.
Errors caused by incorrect emissivity settings (formula)
Many radiation thermometer are shipment set to emissivity 0.95.
If the energy radiated by an object is W obj, the true temperature is T obj, and emissivity is ε, then(
σ is constant).
The temperature display by the thermometer T easto find the temperature.
Substituting this into
[Specific examples]
When measuring metal with a true temperature of 300°C (573.15K) and emissivity of 0.2 at a setting of 0.95,This results in an extremely low display of approximately 115.5°C in Celsius.
3. Main cause of error ②: Reflections from the surroundings that are often overlooked
An object with low emissivity has high reflectivity (emissivity + reflectivity = 1).
A thermometer receives the sum of the infrared radiation emitted by the object and the infrared radiation coming from the surrounding area and reflected by the object.
How ambient heat sources cause errors
Total energy received.
emissivity ϵ The lower the value, the more dominant the influence of reflections. It will be.
[Specific examples]
If there is an 800°C furnace wall near a 300°C metal plate, a strong reflection will be picked up and display will be higher than the true value.
Conversely, it tends to be lower in cold environments.
4. Main Cause of Error 3: Unknown "Measurement Distance and Spot Size"
The D:S ratio is the ratio of the diameter S of a circle measured at a distance D. For example, if it is 12:1, the diameter is 1 cm at 12 cm, and the diameter is 10 cm at 120 cm.
If the target is smaller than the spot...
The background temperature is mixed in and averaged, causing the values to fluctuate or appear low.
Please note that many laser pointer models do not indicate the range itself.
5. Specific measures to reduce errors
Solution 1: Set emissivity correctly
- Check the emissivity chart (as it varies depending on the material and surface condition).
- Adjusted in accordance with the contact type (determines the practical emissivity specific to the object).
Countermeasure 2: Get closer to a "blackbody"
- Measured with blackbody tape attached at emissivity of 0.95.
- Use of blackbody paint (matte heat-resistant paint).
Countermeasure 3: Prevent ambient reflections
- The shield blocks light from the side.
- Adjust the measurement angle to avoid reflections.
Tip 4: Always be aware of distance and location
- Understand the D:S ratio (check specifications).
- Get as close as possible and measure at a distance that is at least twice the spot diameter. Alternatively, use focusable type radiation thermometer.
Summary: Understanding the errors makes radiation thermometer a powerful tool
- Main cause 1: emissivity: Adjust the settings/stabilize with blackbody tape or blackbody paint.
- Main cause ② Ambient reflection: Reduce reflections by using obstructions or adjusting the angle.
- Reason 3: Distance and Spot: proximity measurement based on the D:S ratio, or using focusable.
For more detailed information and help with product selection
We have prepared a list of emissivity of major materials and a "radiation thermometer Error Quick Reference Table (PDF)" that shows at a glance the error according to the ambient temperature.
FAQ
What are the tips for accurately measuring metal surfaces?
The most stable method is to apply blackbody tape (emissivity 0.95) and measure the surface.
How much margin should I leave for the D:S ratio?
Background contamination can be reduced by measuring at a distance where the target is at least twice the spot diameter.
