Non-Contact Temperature Sensor Temperature Compensation

This article describes about temperature compensation of non-contact temperature sensor during quick change in the object temperature. Below discussed slope compensation method can reduce the errors occurred due to the temperature calculation formula of thermopile sensor* at scenario, where object temperature changes quickly than the ambient temperature.

*Thermopile sensor is an IR based non-contact temperature sensor.

Thermopile Sensor

Thermopile sensor works on the principle of thermocouple, i.e. measures object temperature from the voltage difference between hot and cold junction. In IR type temperature sensor, hot junction is created by IR rays falling on IR sensor and cold junction is the ambient temperature. IR sensor uses the IR rays which is emitted from the objects as the IR intensity is depending on its temperature.

Here, the object has the characteristic of changing its temperature upto 1500°C from 0°C in few milliseconds and vice versa,. Temperature of object affects the ambient temperature of thermopile sensor in a slow response. So the ambient temperature reaches its maximum in few seconds whereas the object in few milliseconds.
Temperature calculation

There are different types of thermopile sensor available, outputs may be voltage, amplified voltage or current. For our illustration, amplified voltage type has been discussed below. Most of the thermopile sensors have internal reference temperature sensor; if it is not available, external sensor which is placed much closer to thermopile sensor can be used for temperature compensation.

Temperature calculation formula will be suggested by sensor manufacturer, formula may change from manufacturer to manufacturer. In general it contains the factors of emissivity of object, reference temperature, and internal gain as shown below.

Object Temperature = function of (Temperatureobject , Temperaturereference)

Temperatureobject may be derived from sensor’s Object voltage output, emissivity and some constants. Similarly Temperaturereference may be derived from sensor’s reference voltage output and gain.

Problem Faced

The derived formula may work well in normal situations where, the object temperature variation has less impact on sensor’s ambient temperature, gradual increase of object temperature etc.,

If the object temperature varies rapidly (order of 1000°C in few milliseconds) and reference temperature changes gradually, it doesn’t work. Overshoot and undershoot issues may arise.

Referring to the Fig 1, when the object is cooled quickly undershoot happens and it reaches -20°C.

As the Thermocouple measures the temperature difference between the hot and cold junctions, overshoot occurs due to sudden increase in object temperature and gradual increase of reference temperature. Similarly undershoot occurs due to sudden reduction in object temperature (hot junction) and gradual reduction in reference temperature (cold junction). When cold junction value is more than the hot junction, it results negative values.

Solution

To overcome the overshoot and undershoot temperature issues, the following “Temperature Compensation” method can be used as explained below,

By measuring the slope of reference temperature dynamically and including the slope factor in object temperature measurement, the overshoot and undershoot error can be eliminated as shown in Fig 2.
As the error is added in the sensor output itself, sensor’s object temperature output needs to be compensated before applying it on the temperature calculation formula.

For example,

Sensor’s object temperature output can be compensated by addition or subtraction with slope factor.

Slope is the ratio of, rate of change of y-axis between 2 points (say y2 and y1) to the rate of change of x-axis between 2 points (say x2 and x1), in our case y-axis is reference temperature voltage output and x-axis is time.

Conclusion

By using slope compensation method, output variation due to quick change of object temperature can be reduced and object temperature can be measured accurately.