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PROBATUS

As-found / as-left calibration sheet calculator

Enter the instrument range, the tolerance and your readings. Each test point shows its error and a pass or fail as you type, and the finished sheet prints on one page.

Free, no login. What you enter stays in this browser.

New here? Load the example to see a finished job, then replace it with your own.

Instrument

The range is needed for percent of span.

Tolerance

±x% of (range max − range min)

Pass when the error is within tolerance. Uncertainty is reported only.

Test points

Leave as left empty if you have not adjusted yet.

NominalAs foundAs found errorAs leftAs left errorResultRemove
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Enter a tolerance, then a nominal value and at least an as-found reading for each point.

Next: Uncertainty budget

Work out the expanded uncertainty of the measurement.

Continue

How the sheet works out pass or fail

For each test point the sheet subtracts the nominal value from your reading. That difference is the error. It then compares the size of the error with the tolerance at that point:

  • Percent of span: tolerance × (range maximum − range minimum) ÷ 100. The limit is the same at every point.
  • Percent of reading: tolerance × nominal value ÷ 100. The limit grows with the reading.
  • Absolute: a fixed amount in the unit, such as ±0.2 °C.
  • Percent of reading plus floor: the percent-of-reading limit plus a fixed amount, as many multimeter specifications are written.

A point passes when the error is no larger than the limit. A 0 to 100 psi gauge with a tolerance of ±0.5 % of span has a limit of ±0.5 psi, so a reading of 50.9 psi at 50.0 psi fails and a reading of 50.1 psi passes.

Recording as-found and as-left readings

  1. Take the as-found readings before you touch any adjustment. They are the evidence of how the instrument performed in service.
  2. If every point is within tolerance and you make no adjustment, the as-left readings are the same. Use the copy button.
  3. If you adjust or repair, run the points again and enter them as left.

When a point fails as found but passes as left, the overall result is not a plain pass. The sheet reports it as out of tolerance as found, so the owner knows to check what the instrument measured since its last calibration.

Decision rules and measurement uncertainty

Simple acceptance judges the error alone. Guarded acceptance passes a point only when the error plus your expanded uncertainty is still inside the limit, which lowers the risk of passing an instrument that is really out of tolerance. The non-binary rule accepts like simple acceptance and adds a conditional pass or conditional fail where the uncertainty overlaps the limit. ISO/IEC 17025 asks you to agree the rule with your customer before the work.

Questions

What is the difference between as-found and as-left?
As-found readings are taken before anything is adjusted. They show the condition the instrument was in while it was in service. As-left readings are taken after adjustment or repair and show the condition it goes back into service in. When no adjustment is made, the as-left readings are the same as the as-found readings.
How is error as a percentage of span calculated?
Error is the reading minus the nominal (applied) value. Percent of span is that error divided by the range maximum minus the range minimum, times 100. A 0 to 100 psi gauge that reads 50.9 psi at 50.0 psi has an error of +0.9 psi, which is +0.9 % of span.
What does out of tolerance as found mean?
At least one reading was outside the tolerance before adjustment. It is recorded even when the instrument passes as left, because measurements made since its last good calibration may have been affected and the owner needs to assess that. This sheet reports it as "out of tolerance as found" instead of a plain pass.
Which tolerance types does the sheet support?
Percent of span, percent of reading, an absolute value in the unit, and percent of reading plus a fixed floor. You can also apply a decision rule: simple acceptance, guarded acceptance that subtracts the expanded uncertainty from the limit, or non-binary statements with conditional pass and conditional fail.

Other free calibration tools

Doing this for every instrument, every month?

Probatus keeps a sheet like this for every instrument: technicians fill it in on a phone with no signal, a supervisor approves it and the signed certificate is ready for the customer.

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