Quick Answer
The instrument calibration process at a NABL lab involves: (1) instrument receipt and identification, (2) pre-calibration inspection, (3) environmental conditioning, (4) comparison with reference standard, (5) as-found data recording, (6) adjustment (if needed), (7) as-left data recording, (8) uncertainty calculation, and (9) NABL certificate issue.
Key Takeaways
- Calibration compares your instrument's output against a traceable reference standard of known accuracy.
- The calibration result is expressed as 'error' and 'expanded uncertainty' — not just pass/fail.
- NABL-accredited calibration includes a complete uncertainty budget per ISO/IEC 17025:2017.
- Adjustment (correction) is optional — calibration and adjustment are two distinct steps.
- Calibration certificates must state traceability chain, environmental conditions, and measurement uncertainty.
What Is Instrument Calibration?
Calibration is the process of comparing an instrument's measurement output against a reference standard of known accuracy under defined environmental conditions. It quantifies the instrument's measurement error and uncertainty. Calibration does NOT automatically adjust the instrument — adjustment is a separate step performed only if the error exceeds the acceptable limit.
“Calibration is not just about certificates — it is about having confidence in your measurements, which ultimately determines product quality and process safety.”
— Er. Parthiv Kinariwala, MD, Prism Calibration Centre
The 9-Step NABL Calibration Process at Prism
Receipt & Identification
Instrument is received, labelled with a unique Job ID, and entered into the Lab Information Management System (LIMS). Serial number, make, model, and range are verified.
Pre-Calibration Inspection
Visual check for physical damage, contamination, or malfunctions. Instruments with visible defects are flagged for repair before calibration.
Environmental Conditioning
Instrument is allowed to stabilize in the calibration lab environment (typically 23°C ± 2°C, 50% ± 10% RH) for a minimum soak time (30 min to 4 hrs depending on instrument type).
Reference Standard Setup
Calibrated reference standard (traceable to NPL India or BIPM) is connected. Reference standards have uncertainty at least 3× better than the instrument under test (rule of 4:1 TUR).
As-Found Measurement
Measurements taken at 5–10 calibration points covering the instrument's full range, without any adjustment. This captures the instrument's 'as-found' condition before any intervention.
Adjustment (If Required)
If as-found error exceeds the instrument's accuracy specification or client's acceptance criteria, the instrument is adjusted (trimmed) to bring it within tolerance. Not all instruments are adjustable.
As-Left Measurement
After adjustment, measurements are retaken to confirm the instrument now meets specifications. This is the 'as-left' condition recorded on the certificate.
Uncertainty Calculation
Measurement uncertainty is calculated per EURACHEM/CITAC CG4, GUM (ISO/IEC Guide 98-3), and NABL guidelines. Expanded uncertainty (k=2, 95% confidence) is stated on the certificate.
Certificate Issue
NABL-accredited calibration certificate is issued with: instrument details, calibration date, environmental conditions, as-found and as-left data, expanded uncertainty, traceability statement, and NABL logo.
Calibration vs. Verification vs. Adjustment — Key Differences
| Activity | What It Does | Who Can Do It |
|---|---|---|
| Calibration | Quantifies error and uncertainty against traceable reference | NABL-accredited lab |
| Adjustment | Corrects the instrument output to reduce error | Trained technician or NABL lab |
| Verification | Confirms instrument meets specifications (pass/fail only) | Any trained person |
| Validation | Confirms a process or system meets its intended purpose | Qualified person per procedure |
Measurement Traceability — What It Means
Traceability means the calibration result can be related to national or international measurement standards through an unbroken chain of comparisons, each with stated uncertainty. Prism's traceability chain: SI Units → BIPM (international) → NPL India (national) → Prism Reference Standards → Your instrument. This is what makes our NABL certificates accepted worldwide.
Frequently Asked Questions
What is the difference between calibration and adjustment?
Calibration measures the error of an instrument against a reference standard. Adjustment corrects the instrument to reduce that error. Calibration is always done first; adjustment is performed only if the error is outside acceptable limits.
How long does instrument calibration take?
Simple instruments (thermometer, pressure gauge) take 1–2 hours. Complex instruments (HART transmitters, oscilloscopes, balances) take 4–8 hours. Prism delivers most calibrations within 24–48 hours.
What is measurement uncertainty in calibration?
Measurement uncertainty is a quantified range within which the true value of the measurement is expected to lie. For example, if a thermometer reads 100°C with ±0.5°C uncertainty, the true temperature is between 99.5°C and 100.5°C. NABL certificates always state expanded uncertainty (k=2, 95% confidence).
What is TUR (Test Uncertainty Ratio) in calibration?
TUR (or TAR) is the ratio of the instrument's accuracy to the calibration reference's uncertainty. A TUR of 4:1 means the reference is 4× more accurate than the instrument. ISO/IEC 17025 requires a minimum 3:1 TUR (4:1 preferred) for valid calibration.
Does Prism provide calibration certificates in digital format?
Yes. Prism provides digital PDF calibration certificates with a unique certificate number that can be verified. Physical hard copies are also provided on request.
Written by
Er. Parthiv Kinariwala
Managing Director · Prism Calibration Centre · NABL CC-2480 · Ahmedabad
Er. Parthiv Kinariwala founded Prism Calibration Centre in 2004 and has over 20 years of hands-on experience in calibration engineering, NABL accreditation, and industrial compliance. His team performs 10,000+ calibrations annually from the Vatva GIDC laboratory, serving 5000+ industries across Gujarat.
Prism Calibration Centre — Vatva GIDC, Ahmedabad
Prism Calibration Centre
F-101, Rudraksh Complex 2, Phase 3, GIDC Vatva, Near Jasoda Nagar Cross Road, Ahmedabad — 382445, Gujarat, India
Phone: +91 98245 26444
Email: info@prismcalibration.com
NABL: CC-2480 · ISO/IEC 17025:2017
Hours: Mon–Sat, 9:00 AM – 7:00 PM