Answer / carl_ellis

Calibrating a point level switch

Contact level switches detect the presence or absence of
the medium, whether solid or liquid, by direct contact with
the medium. The switch output for most devices is an
on/off binary signal that reflects the presence or absence
of the medium. Some level switches use a 4-20mA output
that does not modulate, the current signal is either 4mA or
20mA, depending its detection state.

Most level switches offer some form of sensitivity
adjustment (depends on the technology) and some have an on-
delay or off-delay adjustment of the output relay trip,
used to avoid nuisance trips.

Calibration consists of
- testing whether the switch output changes state when the
presence or absence of the medium is changed and adjusting
the sensitivity setting for the specific medium used with
the switch.

- whether the on-delay and off-delay settings comply with
the requirements of the installed application

Depending on the technology, there might be sensitivity
adjustments or on-delay or off-delay adjustments, which are
specific to the installed application, and a calibration
would cover.

A non-contact level switch, like Siemen ultrasonic ULS200,
can be tested in place by comparing the reading with a
physical distance measurement (tape measurement) or out-of-
process by shooting against a solid target and measuring
the distance to check the trip and reset setpoints.

Carl Ellis
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