Reference
Principle
We can divide temperature measurement in to two types: contact and non-contact. Contact measurement can only accurately measure temperature when the testing object and the sensor reach thermal equilibrium. This can mean longer response times and reading inaccuracies offset by ambient temperature. By contrast, non-contact measurement uses infra-red radiation to measure the temperature and does not require a direct touch. Additionally, this method of measurement can be read quickly and accurately.
Our latest infrared temperature measurement module is the MLX90614. This module measures the surface temperature by detecting infrared radiation energy and wavelength distribution. The IR temperature probe consists of an optical system, photoelectric detector, amplifier, signal processing and output module. The optical system collects the infrared radiation in its field of view, whose area is decided by optical components and position of the thermometer, and the infrared radiation energy is converted in to corresponding electrical signals when converging on the photoelectric detector. After being processed by the amplifier and signal processing circuit, calibrated according to the algorithm and target emissivity, the signal is converted in to temperature value of the target. The MLX90614 is self calibrating and has a low noise amplifier integrated in to the signal processing chip. The chip itself is a 17 bit ADC and DSP device, giving accurate and reliable results.
Before using the infrared temperature measurement module it is important to understand the concept of "field of view" (FOV). FOV is determined when a thermopile receives 50% of the radiation signal, and also related to the main axis of the sensor. As shown in the figure below, the size of the FOV is indicated on the horizontal axis. This measured temperature is actually the weighted average temperature of the object in the FOV and the measurement accuracy can only be ensured when the testing object totally covers the FOV of the infrared sensor. This means that the distance between the measurement terminal point and the bus bar must be ensured to meet the demands to guarantee the temperature measurement accuracy.
SEN0206 module has a FOV of 35° so tan35° = the radius of the testing object ÷ the distance between the infrared sensor and testing object. e.g.: the radius of the testing object is 5cm, so the measuring distance is 7cm (that means the measurement result is most accurate within this scope).
The FOV diagram of the SEN0206 sensor is shown below:
All measured points must be fully within the FOV to ensure accurate temperature readings.All measured points must be fully within the FOV to ensure accurate temperature readings.
SEN0263 module has a FOV of 5° so tan5° = the radius of the testing object ÷ the distance between the infrared sensor and testing object. e.g.: the radius of the testing object is 5cm, so the measuring distance is 57cm (that means the measurement result is most accurate within this scope).
The FOV diagram of the SEN0263 sensor is shown below:
All measured points must be fully within the FOV to ensure accurate temperature readings.All measured points must be fully within the FOV to ensure accurate temperature readings.
MLX90614 Emissivity Compensation Method
How can the emissivity of the MLX90614 be compensated? When does the emissivity need to be adjusted?
Emissivity is a coefficient that describes how efficiently an object emits infrared (IR) radiation compared to an ideal blackbody emitter. This coefficient is used by the MLX90614 to calculate the object temperature.
During manufacturing, the MLX90614 is calibrated using a blackbody with an emissivity of 99.9%, which is considered as E = 1.
Different materials have different emissivity values. Therefore, to measure temperature accurately, the emissivity must be taken into account by uploading a new emissivity coefficient into the MLX90614 EEPROM.
As a general rule of thumb, emissivity compensation is only necessary when the IR radiation received by the sensor is reduced in some way, such as:
- Due to a low emissivity of the target object
- When an IR-transparent material (with a transmittance less than 1) is placed in front of the MLX90614
How to Determine the New Emissivity (E)
To determine the emissivity coefficient that should be written to the EEPROM of the MLX90614, the following procedures can be used. Before starting, make sure the emissivity stored in the device is set to E = 1.
Case 1: Transparent Material Placed Between the Sensor and the Object
The MLX90614 can be used to determine the original temperature of the object (assuming E = 1) before placing a transparent material between the object and the sensor.
-
Heat the object to a temperature different from the ambient temperature.
Assume To = 60 °C. The temperature must remain stable, and the temperature difference between the object and the sensor should be at least 30 °C. -
Measure the temperature using the MLX90614 and record the readings:
Treal = 60 °C, Ta_real = 25 °C (example values). -
Place the transparent material in front of the MLX90614.
The transparent material must be at the same temperature as the ambient temperature measured by the MLX90614. Otherwise, the sensor will detect the temperature of the material itself, introducing measurement errors.
- Measure again and record the readings:
Tnew = 50 °C, Ta_new = 25 °C.
Case 2: Object with Emissivity Different from 1
To calculate the new emissivity value, the real temperature of the object must be known. There are two common methods to determine the real temperature:
- Measure the temperature using a precise contact thermometer
- Paint part of the object with a coating that has an emissivity close to 1, then measure this area using the MLX90614
(Note: the painted area must be larger than the FOV of the MLX90614)
Procedure:
-
Heat the object to a temperature different from the ambient temperature.
Assume To = 60 °C and keep it stable. -
Determine the real temperature using one of the methods above and record the ambient temperature measured by the MLX90614:
Treal = 60 °C, Ta_real = 25 °C (example values). -
Measure the temperature of the uncoated surface using the MLX90614 and record the readings:
Tnew = 40 °C, Ta_new = 25 °C. -
Calculate the new emissivity coefficient
All temperatures must be converted to Kelvin for the calculation.
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