CAPSENSE Water Sensor Output Changing With Temperature

[Patfahy]

Hi,

I have a board that has 13 CAPSENSE sensor pads similar to the CY8CKIT-022 but instead of a flex PCB it is 1.6mm FR4. It is used to measure water level in a plastic tank.

I am having problems trying to detect water as the temperature of the water changes from 20C - 70C. As the water changes temperature, the raw sensor outputs fluctuate with temperature. Hotter temperatures increase the raw output, colder temperatures decrease them. 

Is this normal behaviour ? In the test project, a lower limit is calibrated with no water so that any sensor that is significantly above the limit has water beside it. My question is how does this work when the temperature increases, meaning the raw count does, which puts it above the calibration point.

If the tank is full of hot water and loses it all. All of the raw sensor outputs are still significantly above the calibrated low point, even though there is no water, as the board is still hot. So it would still flag as full. 

Do I have to implement a temperature curve into the lower limit. Is something wrong? should temperature affect the outputs so drastically ?

I understand how to implement a solution to get around this. I am just wondering should I have to? Or have I calibrated something wrong?

All raw counts lower after temperature decrease. Room temperature to ~14C

[PandaS · Accepted solution]

Hi @Patfahy ,

The raw count variation with temperature is expected and is normal behavior. CapSense system doesn't have a mechanism to compensate for temperature.
So, you need to have a reference sensor (an auxiliary sensor that doesn't come in contact with the liquid) to detect temperature variations or some other software mechanism for temperature compensation.

Thankyou

Sobhit