Instrument calibration in a testing lab: NABL rules and records

Why do you need to calibrate instruments in a testing lab?

You calibrate an instrument so that the results it produces can be traced back to SI units with a known uncertainty. Without that chain, a reported value of 186.9 mg/L or 86 µg/m³ has no defensible link to a reference.

NABL's quality manual guide, NABL 160A (Issue 01, 2 January 2026), says measuring equipment is calibrated when its accuracy or measurement uncertainty affects the validity of results, or when calibration is needed to establish metrological traceability. This mirrors ISO/IEC 17025:2017 clause 6.4.

So not every piece of equipment needs an external calibration. A magnetic stirrer whose speed does not affect the result does not. An analytical balance, a thermometer used to control an incubator, or the flow meter on an air sampler does.

What does NABL require for metrological traceability?

NABL's traceability rules are in NABL 142, "Policy on Metrological Traceability of Measurement Results" (Issue 07, 11 January 2021). It says measuring equipment, including standards and reference materials, shall be calibrated by one of these:

1. The National Physical Laboratory (NPL, India's national metrology institute) or another national metrology institute whose service is covered by the CIPM MRA.
2. An accredited calibration laboratory whose scope of accreditation specifically covers the calibration, accredited by a body covered by the ILAC arrangements (such as NABL).
3. Only where the first two cannot be met, NPL or another national metrology institute whose service is suitable, with evidence that NABL will assess.

NABL 142 adds that an accredited calibration lab shows its service is covered by using the NABL symbol or the NABL Accredited CAB Combined ILAC MRA Mark on the certificate.

When you receive a NABL calibration certificate, check four things:

• The certificate carries the NABL symbol or the combined ILAC MRA mark.
• The calibration lab's current scope covers that parameter and range. Check it on NABL's website.
• The results, corrections and measurement uncertainty are stated.
• The results meet your own acceptance criteria before the instrument goes back into use.

NABL 142 also allows that if a calibration contributes insignificantly to a test's uncertainty, the lab may show this with quantitative evidence instead of demonstrating traceability.

How often should lab instruments be calibrated?

If you are looking for NABL guidelines for calibration frequency, the short answer is that there is no single NABL frequency for every instrument. The lab sets each interval, writes it into a calibration programme, and adjusts it from experience. NABL 160A says the calibration programme "is reviewed and adjusted depending on the usage of the equipment and to gain confidence in the calibration status".

Do not expect the calibration lab to decide for you. NABL 160A notes that a calibration certificate and label carry no recommendation on the calibration interval unless the customer asks for one.

For guidance on choosing intervals, labs use ILAC-G24:2022 / OIML D 10:2022, "Guidelines for the determination of recalibration intervals of measuring equipment". It says the first interval should come from a risk assessment and lists the factors to weigh, including:

• the measurement uncertainty the lab needs;
• the risk of exceeding maximum permissible error in use;
• the manufacturer's recommendations;
• tendency to wear and drift;
• extent and severity of use, and the environment;
• frequency and results of intermediate checks;
• transport of the equipment, staff training and legal requirements.

The guideline then describes ways to adjust intervals: a "staircase" method that lengthens or shortens the interval after each calibration, control charts, "in-use" time, and in-service checks. It warns against setting very long intervals, because a late finding can mean withdrawing many reported results.

Record your reasoning, because NABL guidelines for calibration frequency leave the decision with you. An assessor who asks why a balance is on a twelve-month interval and an oven thermometer on six should find the answer in your programme.

What are intermediate checks, and how often are they done?

Intermediate checks are quick checks between calibrations that show the instrument is still within its calibration status. ILAC-G24 says a lab should have an appropriate system of intermediate checks, and NABL 160A asks for them to follow a written procedure. The lab sets their frequency, usually by risk and use.

Write the acceptance limit for each check before you start. A check without a limit tells you nothing.

What should an SOP for calibration of instruments include?

An SOP for calibration of instruments can be a single procedure covering all equipment, with instrument-specific details in the register. It should state:

1. Which equipment needs calibration, and which needs only verification or maintenance.
2. How calibration providers are chosen and how their accreditation scope is checked.
3. How intervals are set and reviewed, and who approves changes.
4. How certificates are reviewed against acceptance criteria, and how corrections are applied.
5. How intermediate checks are done, how often, and what to do when one fails.
6. How equipment is labelled.
7. How out-of-calibration equipment is taken out of use and its past results evaluated.
8. Which records are kept, and for how long.

What should an equipment register contain?

NABL 160A lists the records a lab keeps for each measuring instrument: identity including software and firmware version; manufacturer, type and serial number; evidence of verification; current location; calibration dates, results, adjustments, acceptance criteria and the next due date or interval; reference material details; the maintenance plan and work done; and any damage, malfunction, modification or repair.

Here is a register layout that covers those points, with an example entry:

The "Status" and "Next due" fields are the ones an assessor checks first against the label on the instrument. If they disagree, it becomes a finding. See common NABL nonconformities for others.

How should calibrated instruments be labelled?

NABL 160A says each measuring instrument carries a calibration label with the instrument ID, the date of calibration and the due date, so the user knows its status. Where a label cannot be stuck on the instrument itself, it goes on its box or container.

Equipment that is out of use gets an "out of service" label, and stays labelled until it is shown to work correctly again.

What happens when an instrument is out of calibration?

Stop using it, then work out what it affected. NABL 160A says equipment giving questionable results, or not meeting requirements, is taken out of service and labelled, and the effect on results is evaluated under the lab's procedure for nonconforming work.

1. Take the instrument out of use and label it "out of service".
2. Find the last good calibration or intermediate check.
3. List every test result produced with it since that point.
4. Judge whether the error could change any reported result or conformity statement.
5. If it could, follow your nonconforming work procedure: notify the customer and, where needed, amend or withdraw reports.
6. Repair, recalibrate and verify before returning the instrument to use, and record everything in the register.
7. Review the interval. An instrument found out of tolerance at calibration is a sign the interval may be too long.

Step 3 is much easier if each worksheet records which instrument was used. If it does not, you may have to treat every result from that period as suspect.