Commissioning torque calibration services: why a torque tool is calibrated in both directions and across its range rather than at one setting, what the certificate has to state to be usable, and how the interval should follow the tool's own history

Specifying the calibration

1. Ask for several points across the working range. Tools are not linear across their range and error at the low end of a wrench's scale is common. Points near the bottom, middle and top of the range, rather than a single check at the setting you happen to use, is what characterises the tool.
2. Require both directions where the tool is used both ways. A tool used clockwise and anticlockwise needs calibrating in both, because the mechanism is frequently not symmetrical. A certificate covering one direction describes half the tool.
3. Insist on as-found and as-left values. The as-found reading is what tells you whether work done since the last calibration was within tolerance. A provider that adjusts before recording has removed the only evidence that answers that question, and that is the single most important line on the certificate.
4. Check the accreditation scope covers your range. Accreditation is granted for specific measurements over specific ranges. A provider accredited for torque up to a modest value is not accredited for your large wrench, and the scope document is where that is confirmed.
5. Set the interval from the tool's own drift history. Annual is a convention. A tool used constantly, dropped, or historically out of tolerance belongs on a shorter interval, and a tool that never moves can be justified onto a longer one with the evidence to support it.

Handling and storage between calibrations

A click wrench left wound to its maximum setting drifts, and one that has been dropped should be treated as out of calibration until checked. Wind tools down to the low end of the scale for storage and record any drop.

Do not use a torque wrench to loosen fasteners. It is the commonest way a calibrated tool goes out of tolerance and it is entirely avoidable with a breaker bar.

Running it as a programme

Number every tool, hold the interval and the last and next dates in a list somebody looks at, and label the tool with its due date. A tool whose status nobody can tell is a tool that will be used past its date.

Keep the as-found history per tool. After two cycles it tells you whether the interval is right, which is the only evidence-based way to set one.

When the tool fails its calibration

An out of tolerance result triggers a review of work done since the last calibration, and that review is much easier if the tool's use was recorded. Decide in advance what the review process is rather than inventing one under pressure.

Repair and recalibrate, or retire. A tool that has failed twice is telling you something, and replacing it is usually cheaper than the next review.

Common questions

What should a torque calibration certificate state?

The points tested across the range, the directions, as-found and as-left readings, the reference standards and their traceability, the environmental conditions, the measurement uncertainty and the acceptance criteria applied.

Why do as-found readings matter so much?

Because they tell you whether work done since the last calibration was within tolerance. If a provider adjusts the tool before recording, that evidence is gone and every assembly since the last visit is unverifiable.

How often do torque tools need calibrating?

At an interval justified by the tool's use and its own drift history, reviewed rather than assumed. Heavy use, a drop, or a previous out of tolerance result are all reasons to shorten it regardless of convention.

Do torque screwdrivers need the same treatment?

Yes, and low range tools are frequently the ones drifting furthest in relative terms. The same requirements apply: points across the range, both directions where used, as-found values and a stated uncertainty.

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