Answer / carl_ellis

The key to 'calibrating' a temperature switch is it is a
switch, it is not a continuous analog sensor. So there's
no need to mess around with a zero and span value, like you
would on temperature transmitter.

All that matters is setting the switch's trip point, the
setpoint, and in some cases, the reset
(deadband/hysteresis) point. There is no need to mess
around anything but the trip point, it's time consuming
enough to get a precise setpoint on switch.

You need to know
- the setpoint
- in some cases, the reset (deadband/hysteresis) point
values
- whether the setpoint (SP) is for a rising or falling
temperature. Because they're mechanical switches, a SP for
falling is NOT same as a SP for rising.
- switch functionality - Does the switch close or open on
rise or fall. There are applications for all the
variations: close on rise, close on fall, open on rise,
open on fall.

Check how to change the setpoint on the switch. Some
switches have a setpoint lock (nut). Some are blind
adjustments, some are adjustments with indicated
temperature values. Some have a separate
deadband/hysteresis adjustment. The latest temperature
switches are digital, and the setpoint and hysteresis reset
point are entered as values.

Many procedures require an actual test with a calibrating
heat/cold (dry block calibrator or oil bath) source and a
temperature "standard" is needed for a reference
temperature, to prove the mechanical setting is correct.
Sometimes the standard is a certified dry block with a
readout, sometimes it's a mercury thermometer, sometimes
its certified RTD with a certified readout. Older cal
equipment has stand-alone electrical equipment, like an
indicator lamp or an ohmmeter that is used to indicate
switch closure on the temp switch, more modern cal
equipment has built-in connections and an indicator for
that.

Make the setpoint adjustment, run the temperature source,
heat or cool, and note the trip and reset points. If
needed, make adjustments and re-run the test until the
setpoint (and reset point) are within the spec needed.

Record the results, put a cal sticker or tag on the beast,
and you're on to the next one.

Tell 'em Carl sent you.

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