5 min read
A calibration certificate has two jobs. The obvious one is to say the tool is accurate now. The useful one is to say what the tool was doing before it was touched, because that is the only statement on the document that tells you anything about the work the tool has already done.
As-found and as-left
As-found readings are taken before any adjustment. As-left readings are taken after. A certificate carrying only as-left data is telling you the tool is fine today and saying absolutely nothing about the twelve months you have just spent using it.
| What you see | What it means |
|---|---|
| As-found inside tolerance at every point | The interval that just ended was safe. The work done is not in question. |
| As-found marginal at one or two points | The tool was drifting. Shorten the interval; consider which joints were affected. |
| As-found outside tolerance | Work done with this tool since the last certificate needs review. This is a finding, not a formality. |
| As-left only, no as-found | The certificate cannot answer the question that matters. Ask the provider why. |
Test points across the range
A tool tested at one point has been tested at one point. Mechanisms do not drift uniformly, and it is entirely normal for a wrench to be accurate at mid-range and out at the top, which is exactly where heavy work uses it. A certificate should show readings at several points spanning the usable range, typically including something near the bottom, the middle and the top.
Traceability
Traceability is the chain that connects the reading on your certificate to a recognised standard. The certificate should name the reference the measurement is traceable to and identify the equipment used. Without it, "accurate" means accurate according to an instrument of unknown accuracy, which is not a measurement, it is an opinion.
The identity fields nobody checks
- The asset number, not just the make and model. Two identical wrenches in one store is how certificates end up attached to the wrong tool.
- The date and the recalibration date, which must agree with the sticker on the tool.
- The technician and the equipment used, which turn a result into something auditable.
- The tolerance applied, because "pass" means nothing without knowing what it was judged against.
Common questions
What is the difference between as-found and as-left?
As-found is measured before any adjustment and describes how the tool was behaving during the period you have just used it. As-left is measured after adjustment and describes how it will behave going forward.
Why does a certificate need more than one test point?
Because tools do not drift uniformly. A wrench can be accurate mid-range and out at the top of its range, which is where the heaviest and most consequential work happens.
How long should calibration records be kept?
Longer than you expect to need them. Audits ask for history, not current status, and a tool’s certificate trail is also how you set a defensible interval.