Torque is a specification, not a feeling. You can't tell whether a fastener is at 20 Nm by how it sounds or how it feels, not with any useful accuracy. A tool that says it's applying 20 Nm may be applying 16 or 24, depending on when it was last calibrated, how it's been used, and what condition it's in.

Calibration is the process of checking that the tool is actually delivering what it says it's delivering, and adjusting it if it isn't.

Why torque matters

Fasteners fail in two ways: they loosen (undertightened) or they break (overtightened). Both are problems, but in safety-critical joints, both can be catastrophic. Automotive safety systems, aircraft components, pressure vessels, lifting equipment, all require fasteners tightened to specification and verifiable as such.

Even in non-critical assembly, incorrect torque causes production problems: stripped threads, cracked housings, components that rattle loose in service, warranty claims. Calibrated tools reduce these failures and the costs they generate.

What calibration involves

Calibration compares the output of the tool under test against a reference standard. The reference standard is a torque transducer, a precision sensor that converts applied torque to an electrical signal. The transducer is itself calibrated, and its calibration traces back through a chain of increasingly accurate measurements to national measurement standards, in the UK, held by the National Physical Laboratory (NPL).

For a click torque wrench, calibration means applying a known torque to the wrench at several points in its range and recording what value the wrench clicks at. For a DC electric nutrunner, it means running the tool and measuring peak torque, rundown torque and repeatability. The readings are compared against the tool's specification, and the results are recorded on a calibration certificate.

The five key parameters for DC electric tools

For DC nutrunners, screwdrivers and similar controlled assembly tools, calibration covers:

  • Peak torque, the maximum torque before shutoff. If it's too low, the joint won't reach preload. Too high, and you risk overtightening.
  • First peak torque, for pulse tools, the initial torque before run-down. Controls the initial preload in the joint.
  • Rundown torque, the stabilised torque during tightening. The value the joint 'sees' after the initial impact.
  • Torque accuracy, how closely the actual torque delivered matches the tool's indicated setting.
  • Torque repeatability, how consistent the results are when the same setting is applied multiple times. A tool with poor repeatability cannot maintain consistent quality.

A calibration report documents all of these. Without it, you cannot demonstrate to an auditor that the tool was delivering the specified torque during the period it was in use.

What UKAS ISO-17025 means

ISO-17025 is the international standard for the competence of calibration laboratories. UKAS (United Kingdom Accreditation Service) is the national body that assesses laboratories against this standard and grants accreditation to those that meet it.

A certificate from a UKAS ISO-17025 accredited laboratory means: the calibration was performed by a competent laboratory, using equipment that can be traced to national standards, following a defined procedure. It is the evidence that satisfies ISO 9001, IATF 16949 and AS9100 requirements for calibrated measuring equipment. A certificate from a non-accredited source, including many in-house calibration programmes, does not carry the same weight in an audit.

How often should tools be calibrated?

The answer depends on:

  • The tool's application, safety-critical fastening requires more frequent verification than non-critical assembly
  • How much the tool is used, high-cycle tools accumulate wear faster
  • The manufacturer's recommendation, most specify calibration intervals by operating hours
  • Your quality management system, ISO 9001 and equivalent standards require calibration at defined intervals, documented in the quality plan

A pragmatic starting point for most tools in active production use is annual calibration. For safety-critical applications or high-cycle tools, consider six-monthly. For tools used occasionally, annual may be more than sufficient, though the tool must be calibrated before use if any significant time has elapsed.

As-found readings on the calibration certificate tell you whether the interval was right. If a tool is consistently within tolerance when it arrives for calibration, your interval may be longer than necessary. If it's consistently out, it may be too long.

What to look for on a calibration certificate

A compliant calibration certificate should include:

  • Date of calibration
  • Tool identification, make, model, serial number
  • As-found readings (before adjustment, if any)
  • As-left readings (after adjustment)
  • Statement of measurement uncertainty
  • Traceability statement linking the reference equipment to national standards
  • Name and signature of the calibration technician
  • Next due date or calibration interval

If any of these are absent, particularly measurement uncertainty and traceability, the certificate is incomplete and may not satisfy an auditor.

We can help

Total Airtool Services operates a UKAS ISO-17025 accredited calibration laboratory from our Kenilworth workshop. We calibrate all makes and types of torque wrenches, electric screwdrivers, nutrunners and pneumatic tools. Typical turnaround is 48–72 hours. On-site calibration is also available for large volumes or production-critical tools.


Find out more about our calibration service or call 01926 857193 to discuss your requirement.