How Torque Wrench Calibration Is Done

Calibration sets a torque wrench’s indicated value against a more accurate reference standard and reports the measurement uncertainty; verification is a quicker in-house check between calibrations. In Australia, choose NATA-accredited labs working to ISO/IEC 17025, with SI traceability (typically via the National Measurement Institute, NMI). The calibration method is defined in ISO 6789-2:2017; design/conformance requirements live in ISO 6789-1:2017.

Standards That Apply in Australia

• ISO 6789-1:2017 covers: design & quality conformance (Type I indicating, Type II setting tools). 

• ISO 6789-2:2017 sets: the calibration method and how to calculate measurement uncertainty (the lab’s certificate should reference this).

• Australia’s former AS 4115: was withdrawn (Oct 2016); calibration follows ISO 6789-2. 

• ISO/IEC 17025 via NATA: auditors expect NATA-endorsed certificates with traceability; NATA’s Metrological Traceability Policy explains how labs demonstrate SI links. You can search NATA’s directory for torque scopes.

Terminology tip: ISO 6789 uses “maximum permissible (relative) deviation” (MPD) instead of a loose “accuracy” label.

How Often to Calibrate

• Principle: set intervals by risk and usage (criticality, environment, transport, history). ISO 6789-2 itself suggests 12 months or 5,000 cycles (whichever first) if you don’t run your own control procedure; then adapt based on successive results. 

• Industry guidance (OEM): Norbar (AU) commonly advises every 12 months, with shorter intervals for heavy use/critical tasks; 5,000 cycles is widely cited as a default. Norbar Torque Tools+1

Decision Mini-table (Illustrative, not Prescriptive):

(Record the rationale in your QMS; ISO doesn’t mandate a single number.) Iteh Standards

Equipment Used

• Torque tester / transducer with known uncertainty (calibrated and traceable). Under ISO 6789, the measurement device uncertainty must be suitably small relative to the tool’s expected uncertainty (often expressed as ≤¼ of the tool’s expected uncertainty/MPD).

• Loader/arm & fixtures to apply torque horizontally and support the wrench at the handle load point; good systems minimise parasitic forces (e.g., floating supports/counter-balance).

• Adaptors to align square/hex drives; environmental control (temperature, etc.) and a data system to compute uncertainty per ISO 6789-2.

Step-by-step: How Torque Wrench Calibration is Done (ISO 6789-2)

The steps below reflect ISO 6789-2:2017 concepts used by accredited labs. Your certificate should list method, as-found/as-left, uncertainty, traceability, and equipment IDs. 

1. Pre-checks Identify tool type (Type I indicating vs Type II setting) and inspect ratchet/drive, scale and handle. Record tool ID.

2. Exercise the wrench Operate the wrench several times near the target value to settle components (per lab procedure).

3. Set-up Mount the wrench horizontally; align at the handle load point; use correct adaptors; minimise side loads; record ambient conditions.

4. Select test points Calibrate from the lowest marked value to the top of range; many labs test at minimum, ~60%, and 100% of the specified range, in each direction if applicable. (ISO 6789-2 requires coverage down to the lowest marked value.)

5. Apply load at the correct rate For Type II (setting) tools, increase smoothly to ~80%, then reach the target within a short, controlled window (commonly 0.5 to 4 s from 80% to target refer to the ISO tables by range). This avoids overshoot and improves repeatability.

6. Repeat readings Take repeated applications per point (per ISO class), capturing indicated vs reference values. 

7. Compute error & uncertainty ISO 6789-2 defines how to calculate relative measurement error and expanded uncertainty for the tool and to confirm the measurement device is suitable (its uncertainty interval ≤¼ of the tool’s expected uncertainty interval).

8. Adjust (Type II) & re-test If the tool is adjustable and out of tolerance, adjust and repeat the points to produce as-left results.

9. Issue certificate Include as-found/as-left, uncertainty, method = ISO 6789-2:2017, ambient conditions, equipment IDs, traceability (NMI/ILAC chain), technician sign-off, and next due date (your risk-based choice).

Pass/Fail Criteria & Accuracy

• MPD (maximum permissible relative deviation) is the ISO term; tools must meet the MPD for their type/class. (Manufacturers may specify tighter.)

• In practice, many hand wrenches work to ±4% or ±6% classes (depending on type/class and torque level). Use the tool datasheet and your quality procedure to select the rule.

Worked Example (Illustrative):

Target = 100 N·m; average indicated = 96.0 N·m; relative error = (96.0−100)/100 = −4.0%. Expanded uncertainty (k≈2) on the tool at this point = ±1.2%.Decision rule (per ISO/IEC 17025 QMS): if MPD = ±4%, this result just meets the limit at the point estimate; if your lab applies guard banding, uncertainty may influence the pass decision. (Your certificate should state the decision rule used.)

Compliance in Regulated Industries (Australia)

• Pharma (TGA / PIC/S GMP): Calibrated, qualified equipment with records is expected under the PIC/S Guide to GMP adopted by the TGA. (TGA currently references the PIC/S Guide; version updates are in progress with transition communications.)

• Food & beverage (FSANZ): Food safety standards require reliable measurements under documented controls; calibrated devices support HACCP and verification of critical fasteners on processing equipment. 

Maintenance Tips That Extend Calibration Stability

• Store at minimum load; avoid shock and over-range.

• Handle at the marked centre of the handle; don’t use extensions not accounted for.

• User verification between lab calibrations using a torque checker helps spot drift early (not a substitute for a full ISO 6789-2 calibration). 

• Transport in a padded case; record cycles to refine intervals.

Choosing a Provider (What to Look for)

• NATA accreditation for torque under ISO/IEC 17025 (check the Scope of Accreditation for ranges & CMCs).

• Certificates showing ISO 6789-2 method, uncertainty, and SI traceability (via NMI or an ILAC NMI). 

• Turnaround & logistics, on-site options, and digital record access.

CISCAL proof points: NATA Acc. No. 411; torque scope 1.25–1,500 N·m (CMC ±1.2%), multi-state presence, operating since 1963, and the SMART portal for real-time certificates and asset tracking.

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