Add support for calibration values on sensors

[kal]

This is a form of time series of values similar to the existing configuration value series, but should be associated with the specific variable that the calibration applies to. Calibration values may also be associated with a calibration method, although it is to be confirmed whether this could ever change for a given sensor type and variable combination or if the method could instead be associated with the variable, or the sensor type and variable.

[kal]

@holcoo - I've created this ticket for updating the model to support handling sensor calibration information. It would be good to get your thoughts on what elements affect the method used. Is it just the variable, or is it a combination of the variable and the sensor type, or is it something that could change over time even for the same combination of variable and sensor type?

[holcoo]

Hi Kal, assuming this 'method' is the calculation/equation used to convert raw data using the calibration coefficients, the calibration method might in theory differ between each sensor make/model.
Whilst many methods might be the same across sensor types, we could see a different method of calibration used between two different soil moisture sensors as they can use different technologies. The method(s) for a particular make/model might be applicable to only 1 (or more) variables for that sensor.

[kal]

Thanks @holcoo! Yes, this was referring to the method for applying calibration coefficients. So the model should support being able to specify a sensor type and one or more variables measured by the sensor along with the method to apply the coefficient.

[kal]

I've implemented this in the model now. Calibration coefficients are recorded as a value series (with start/end timestamps) against a sensor, and are scoped to the variable(s) to which the coefficent applies. The method for applying the coefficient is recorded against the sensor type and is also scoped to the variable(s) that use that method.

To support tracking the provenance of the coefficients, it is possible to link a calibration activity to the coefficient value series that it updates.

[holcoo]

Thanks @kal ! The calibration activity link sounds very useful too.
Linking in @simonstanley in case there are additional thoughts.