The first thing to decide is what your calibration reference will be.
If it is important to get accurate readings in some standard units, you will need a Standard Reference to calibrate against. This can be:
A calibrated sensor - If you have a sensor or instrument that is known to be accurate. It can be used to make reference readings for comparison. Most laboratories will have instruments that have been calibrated against NIST standards. These will have documentation including the specfic reference against which they were calibrated, as well as any correction factors that need to be applied to the output.
A standard physical reference - Reasonably accurate physical standards can be used as standard references for some types of sensors
- Rulers and Meter sticks
- Temperature Sensors
- Boiling Water - 100°C at sea-level
- Ice-water Bath - The "Triple Point" of water is 0.01°C at sea-level
- Gravity is a constant 1G on the surface of the earth.
Thermocouple Characteristic Curves
CC0 Public Domain via Wikimedia Commons
Characteristic Curve – Each sensor will have a ‘characteristic curve’ that defines the sensor’s response to an input. The calibration process maps the sensor’s response to an ideal linear response. How to best accomplish that depends on the nature of the characteristic curve.
- Offset – An offset means that the sensor output is higher or lower than the ideal output. Offsets are easy to correct with a single-point calibration.
- Sensitivity or Slope – A difference in slope means that the sensor output changes at a different rate than the ideal. The Two-point calibration process can correct differences in slope.
- Linearity – Very few sensors have a completely linear characteristic curve. Some are linear enough over the measurement range that it is not a problem. But some sensors will require more complex calculations to linearize the output.
In the next few pages, we'll look at three different types of calibration:
- One Point Calibration
- Two Point Calibration
- Multi-Point Curve Fitting