Quick Answer
Pressure gauge calibration involves comparing the gauge reading against a NABL-calibrated reference standard (dead weight tester or digital reference gauge) at typically 5 test points (0%, 25%, 50%, 75%, 100% of range) in ascending and descending order. The calibration records as-found and as-left error and measurement uncertainty per ISO/IEC 17025.
Key Takeaways
- Pressure gauges are calibrated using dead weight testers (primary standard) or calibrated reference digital gauges.
- Calibration covers both ascending and descending readings to quantify hysteresis.
- ASME B40.100 specifies accuracy classes: Grade A (±1%), Grade 2A (±0.5%), Grade 4A (±0.1%).
- Prism calibrates pressure gauges from vacuum (–1 bar) to +700 bar gauge range.
- Safety-critical gauges (boiler, pressure vessel, fire suppression) require 6-month calibration intervals.
Types of Pressure Gauges We Calibrate
Bourdon Tube Gauge
Most common. C-type, spiral, or helical Bourdon tube. Range: 0–1 bar to 0–700 bar. Calibrated with dead weight tester.
Diaphragm Gauge
For low pressure (0–100 mbar) and corrosive/viscous media. Calibrated with low-pressure controller.
Capsule Gauge
For very low pressure measurement (0–600 mbar). Used in gas supply systems and HVAC.
Differential Pressure Gauge
Measures pressure difference. Used for filter monitoring, level measurement, orifice plate flow. Calibrated with DP controller.
Compound Gauge
Reads both vacuum (negative) and positive pressure. Range: -1 bar to +35 bar typically.
Digital Pressure Gauge
Electronic sensing element with digital display. Calibrated same as analog but with higher resolution measurement.
Dead Weight Tester — The Primary Pressure Standard
A dead weight tester (DWT) or piston gauge generates pressure by loading calibrated masses onto a precision piston of known area: P = F/A (pressure = force / area). It is the primary pressure standard used by NABL labs because it generates pressure directly from fundamental SI units (mass, length, time) with uncertainties as low as ±0.01% of reading. Prism operates DWTs covering 0.1 bar to 700 bar range.
5-Step Pressure Gauge Calibration Procedure
Pre-check
Inspect the gauge for damage, check pointer at zero with no pressure applied. Note zero error.
Ascending Series
Apply pressure in steps: 0, 25%, 50%, 75%, 100% of range. Record indicated pressure vs. applied pressure at each step.
Descending Series
Release pressure in reverse order: 100%, 75%, 50%, 25%, 0%. Record hysteresis (difference between ascending and descending readings).
As-Found Assessment
Compare measured error at each point against gauge's accuracy class tolerance. Record as-found error for certificate.
As-Left and Certificate
If within tolerance, issue NABL certificate as-is. If out of tolerance, adjust (trim) if possible, verify, and record as-left data.
Frequently Asked Questions
How accurate should a pressure gauge be for industrial use?
Most industrial process gauges are ASME Grade A (±1% full scale). Precision test gauges are Grade 2A (±0.5%) or Grade 4A (±0.1%). For critical applications (boiler, safety relief valve), Grade A minimum is required.
How often should pressure gauges be calibrated?
Annual calibration is standard. Boiler drum pressure gauges (IBR requirement), safety valve inlet gauges, and pharmaceutical steam system gauges should be calibrated every 6 months.
Can Prism calibrate pressure gauges in psi, bar, and kPa?
Yes. Prism calibrates gauges in all pressure units: bar, psi, kPa, MPa, kgf/cm², mbar, mmHg, and inH₂O. The certificate states results in the gauge's native unit and SI units.
What is hysteresis in pressure gauges?
Hysteresis is the difference between the gauge reading when pressure is being increased versus when it is being decreased at the same applied pressure. Excessive hysteresis indicates mechanical wear or damage and means the gauge must be repaired or replaced.
Can Prism calibrate high-pressure gauges above 400 bar?
Yes. Prism calibrates pressure gauges up to 700 bar using a high-pressure dead weight tester with oil as the pressuring medium.
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