Zeroing and Offset in Thermopile Power Sensors

Thermopile power sensors may give a reading even when there is no laser radiation pointed at them. Here’s how you can deal with it…

When should you use the zeroing function and when should use offset?

A sensor may give a reading even when there is no laser radiation pointed at it, for one of the following reasons:

1. A bias voltage that exists in the sensor or the display.
2. Background radiation in the sensor’s field of view, for example, from a hot metal subject just in front of the sensor
3. The sensor is still cooling down from the last measurement and is not in thermal equilibrium.

How to deal with each of these causes?

1. Zero the display with the sensor attached and covered or turned face down on the table
2. Press offset, only if the sensor is cold and in thermal equilibrium.
3. Do nothing. This will not affect the measurement. Once the laser hits the sensor, the reading will be determined by the flow of heat caused by the laser only.

Quick reminder: Zeroing is saved in the display memory and offset in not. After turning the display off and on again, the last zeroing action will still be in effect but the offset will not. This is because zeroing nulls unwanted bias from within the display or sensor, which is pretty stable, while offset nulls unwanted bias due to background radiation which may change during the day.

The following guidelines summarize it all up

1. Offset or zeroing must be done only when the sensor is cold and in equilibrium with the environment.
2. Zero the display with a sensor connected to it. Cover the sensor or lay it down on the table face down. This will zero the combination of display + sensor. Zeroing usually only has to be done once a month or if a significant offset reading is seen even if the sensor is in equilibrium and covered. This should also be done when the sensor is cold and in equilibrium with the environment.
3. The display and sensor are now zeroed. Before turning on the laser, a non-zero reading may appear due to background radiation. In that case the offset button can be used to null this reading. Offset should be done at the start of each measurement session when the sensor is cold and in equilibrium with the ambient.
4. In between measurements, the reading may not go back to zero for a long time. This is normal. Do not zero or offset the display.
5. If you have reason to believe that background radiation has changed considerably during the day, use offset again.