Robin2:
{A} there is no value in updating the PID calcs more often than new input data are collected
Worse than "no value". PID has history (the integral). Feeding it stale data poisons the history (causes wind-up).
All the systems I touched would automatically switch a PID loop to manual and generate an alarm if the input was stale at the time the loop was updated.
{B} the PID calcs should not be done at more often than half the update rate of the sensor
That's what I remember.
Do you mean that if I sample a temperature every 6 seconds I should only update the PID calcs every 12 seconds?
Correct.
And (a separate but related question) if the heating rate is such that the temperature will never increase by 1°C in less than 6 seconds are you saying that I should test the temperature every 3 seconds and call the PID function every 6 seconds? (Of course I am assuming that 1°C steps are sufficient)
"Change" instead of "increase" (controlling temperature is bidirectional).
Yup.