As found and as left: what each reading tells you

A calibration record that shows only the final readings answers one question: is the instrument good now? An auditor, and anyone running the process, also needs the answer to a second one: was it good while it was in service? That is why a calibration records two sets of readings.

As found: the instrument as it was working

As-found readings are taken before anything is touched. No zeroing, no trimming, no cleaning of the impulse line if you can avoid it. They describe the instrument exactly as it has been measuring the process since its last calibration.

If the as-found readings are inside tolerance, the measurements made with this instrument since its last calibration can be trusted. If they are outside, they cannot, and somebody has to ask what those measurements were used for: a control loop, a quality release, a safety trip.

As left: the instrument as it goes back

As-left readings are taken after any adjustment. They show the state the instrument returns to service in, and they are what the next calibration's as-found will be compared with.

When the as-found readings already pass with margin, no adjustment is needed and the as-left readings simply repeat them. Many workshops still record both, so that the record shows the decision not to adjust.

Reading them together

• Pass, then pass: the instrument held its calibration. A run of these is evidence for a longer interval.
• Fail, then pass: it drifted and was corrected. The as-found failure is the finding; check what the instrument was used for.
• Pass near the limit, then pass in the middle: it was adjusted before it failed. Watch the drift next time.
• Fail, then fail: it could not be brought into tolerance. Repair or replace it, and record why.

Errors in percent of span

Each reading is compared with what the instrument should show at that point. For a 0 to 10 bar transmitter with a 4 to 20 mA output, 5 bar should give 12.000 mA. A reading of 12.043 mA is 0.043 mA high, which is 0.27 % of the 16 mA span. With a tolerance of ±0.5 % of span, that point passes.

Taking the points going up and then coming down shows hysteresis as well: the difference between the two readings at the same applied value.

How Calibrator records them

Calibrator takes as found first, then offers two paths: no adjustment needed, or adjust and record as left. Both sets are kept, both appear on the certificate, and the chart shows each point against the tolerance band so the margin is visible at a glance.