One point calibration is the simplest type of calibration. If your sensor output is already scaled to useful measurement units, a one point calibration can be used to correct for sensor offset errors in the following cases:
- Only one measurement point is needed. If you have an application that only requires accurate measurement of a single level, there is no need to worry about the rest of the measurement range. An example might be a temperature control system that needs to maintain the same temperature continuously.
- The sensor is known to be linear and have the correct slope over the desired measurement range. In this case, it is only necessary to calibrate one point in the measurement range and adjust the offset if necessary. Many temperature sensors are good candidates for one-point calibration.
A one point calibration can also be used as a "drift check" to detect changes in response and/or deterioration in sensor performance.
For example, thermocouples used at very high temperatures exhibit an 'aging' effect. This can be detected by performing periodic one point calibrations, and comparing the resulting offset with the previous calibration.
To perform a one point calibration:
- Take a measurement with your sensor.
- Compare that measurement with your reference standard.
- Subtract the sensor reading from the reference reading to get the offet.
- In your code, add the offset to every sensor reading to obtain the calibrated value.
Imagine that you have a competition robot that needs to position itself exactly 6" from a goal in preparation for scoring.
You have an ultrasonic rangefinder for your distance sensor. Since you only require maximum accuracy at one distance, a one point calibration is a simple and effictive solution.
Using a measuring tape as your reference standard, position the robot exactly 6" from the goal.
If you take a reading with your sensor and it says 6.3", then you have a -0.3" offset.
Now edit your code to subtract 0.3" from every reading. Since this is known to be a linear sensor it will likely be pretty accurate over most of its range. But you know with great confidence that it will be spot-on at the critical distance of 6".