Calibration

Pressure Calibration: Complete Guide for Industrial Users in Singapore

Pressure measurement underpins safety, process control, and product quality across Singapore's oil and gas, marine, pharmaceutical, and manufacturing sectors. This guide covers pressure types, calibration methods, traceability, and how to meet Singapore regulatory requirements for pressure instrument management.

Unitest Instruments·Updated 27 June 2026·11 min read

Pressure calibration is the process of comparing a pressure measuring instrument against a traceable reference standard to determine and document the errors and measurement uncertainty across the instrument's range. In Singapore's industrial landscape — from Jurong Island petrochemical plants and Sembcorp utilities facilities to marine engineering yards, pharmaceutical manufacturing, and compressed air systems in factories — accurate pressure measurement is critical to process safety, product quality, and regulatory compliance. A pressure gauge reading 5% high on a steam line or a compressed gas system can be the difference between safe operation and a dangerous overpressure event.

This guide explains pressure measurement fundamentals, calibration methods, traceability requirements, and the specific regulatory and industrial contexts that drive pressure calibration requirements in Singapore.

Types of Pressure Measurement

Before calibrating a pressure instrument, it is essential to understand which type of pressure the instrument measures, as this determines the reference standard needed and the calibration approach.

Gauge Pressure

Gauge pressure (psig, barg, kPag) is measured relative to local atmospheric pressure. A tyre pressure gauge reads gauge pressure — it reads zero when exposed to atmosphere. Most industrial pressure gauges measure gauge pressure. For calibration, the reference standard is connected to both the instrument under test and a pressure source, with atmospheric pressure as the common reference point.

Absolute Pressure

Absolute pressure (psia, bara, kPaa) is measured relative to a perfect vacuum (zero pressure). It is used in applications where atmospheric pressure variation would affect the measurement — vacuum systems, altitude-sensitive applications, and some meteorological and scientific measurements. Absolute pressure calibration requires a reference that has a known, stable zero reference relative to vacuum.

Differential Pressure

Differential pressure (DP) is the difference between two pressures measured at different points in a system. DP transmitters are widely used in flow measurement (using orifice plates, venturi tubes, or pitot tubes), liquid level measurement, and filter condition monitoring. Calibrating a DP instrument requires applying known, controlled pressures to both the high-pressure and low-pressure ports simultaneously and measuring the output at defined differential values.

Vacuum

Vacuum measurement covers pressures below atmospheric. Different technologies — Pirani gauges, capacitance manometers, ion gauges — are used for different vacuum ranges. Calibration of vacuum instruments requires specialised reference standards and is performed in a few specialist laboratories.

Pressure Calibration Reference Standards

The quality of a pressure calibration depends fundamentally on the reference standard used. In a calibration hierarchy, higher-accuracy standards are used to calibrate lower-accuracy working standards, which are in turn used for instrument calibration. The top of this hierarchy is the national metrology institute — in Singapore, A*STAR's National Metrology Centre maintains primary pressure standards traceable to SI units.

Dead-weight testers (pressure balances): The primary reference standard for most pressure calibration work is the dead-weight tester (also called a pressure balance or piston gauge). It generates a known pressure by balancing the force of calibrated weights on a precision piston of known effective area. Dead-weight testers can achieve very small measurement uncertainties (parts per million in the best laboratory-grade instruments) and are used as transfer standards in accredited calibration laboratories.

Digital pressure calibrators: High-accuracy digital pressure calibrators — such as those in the Fluke 700 series, available from Unitest Instruments through our Fluke product range — are used as working reference standards in calibration laboratories and for on-site calibration. They use quartz resonator or silicon resonant pressure sensors with high stability and are calibrated periodically against dead-weight testers or higher-accuracy references. Their portability makes them ideal for on-site calibration of installed pressure transmitters, gauges, and safety valves.

Pressure comparators: A pressure comparator is a device that transmits the same pressure from a reference standard to the instrument under calibration without generating its own pressure. It allows a precision reference (such as a digital calibrator) and the instrument under test to be connected to the same pressure source and their readings compared directly.

Pressure Calibration Methods

Direct Comparison Method

The most common method for industrial pressure calibration: the reference standard and the instrument under test are both connected to a common pressure source (a hand pump, pressure controller, or pressure comparator), and readings from both are recorded at defined pressure points across the operating range. The error at each point is the difference between the instrument's reading and the reference value. This method is straightforward, fast, and suitable for most gauge and absolute pressure instruments from vacuum up to several hundred bar.

Hydraulic and Pneumatic Reference Systems

For high-pressure calibration (above approximately 700 bar), hydraulic systems using oil are required; compressed air or nitrogen is used for pneumatic calibrations up to several hundred bar. For safety, gas pressure calibration above certain pressures requires specific precautions regarding pressure vessel standards and personnel safety — relevant to MOM Workplace Safety and Health Act requirements in Singapore.

On-Site Calibration of Installed Transmitters

Process pressure transmitters installed in pipelines and vessels are commonly calibrated in situ using a portable reference calibrator. The transmitter is isolated from the process using isolation valves, the process pressure is safely vented, and the reference calibrator is connected to the transmitter's process connection. The transmitter's output signal (4–20 mA, HART, or digital) is measured and compared to the applied reference pressure at multiple points. For instruments that cannot be fully isolated from the process, calibration may need to coincide with a planned process shutdown.

Unitest Instruments offers on-site pressure calibration services across Singapore. Contact our team to discuss on-site calibration scheduling for your facility.

Pressure Calibration in Singapore's Regulated Industries

Pressure calibration is required by numerous Singapore regulatory frameworks and industry standards.

Workplace Safety and Health (MOM): The WSH (Design) Regulations require that pressure vessels and pressure systems operating in Singapore workplaces be inspected and maintained to safe standards. While the WSH Act does not mandate calibration intervals explicitly, pressure gauges on safety-critical systems are expected to be in calibrated condition as part of the duty to maintain a safe workplace. Pressure relief valve settings must be verified against calibrated references.

Oil and gas and petrochemical (EMA): Plants on Jurong Island operating under EMA energy licences and process safety management (PSM) frameworks are required to maintain the integrity of their safety instrumented systems (SIS), which include pressure measurement as a key input. The IEC 61511 standard for safety instrumented systems requires that safety-critical pressure instruments be validated and calibrated at intervals consistent with their safety integrity level (SIL) requirements.

Marine engineering: Ships calling at Singapore ports and Singapore-flagged vessels must comply with class society requirements (e.g. Lloyd's Register, Bureau Veritas, ClassNK, DNV) that include periodic calibration of pressure measurement equipment in propulsion systems, boiler systems, cargo systems, and safety systems. The Port Marine Safety Code and class survey processes verify calibration records.

Pharmaceutical manufacturing (HSA GMP): Pressure is a critical process parameter in many pharmaceutical manufacturing operations — compressed air systems, steam sterilisation (autoclaves), bioreactor pressure, and clean room differential pressure. HSA GMP guidelines require that critical process parameters be measured with calibrated instruments and that calibration records be maintained.

Compressed air and gas systems (Singapore Standards): Singapore Standard SS 538 and related standards for compressed air quality in manufacturing and food processing require that monitoring instruments for pressure and flow be calibrated and maintained. For food-grade compressed air used in direct food contact applications, calibration records are part of the food safety management system documentation.

Common Pressure Calibration Pitfalls

Several errors frequently occur in pressure calibration programmes that undermine the value of the calibration exercise.

Incorrect pressure type: Calibrating a gauge pressure instrument as if it were an absolute pressure instrument (or vice versa) will produce incorrect results. Always confirm whether the instrument measures gauge, absolute, or differential pressure before selecting the reference standard and calibration method.

Inadequate warm-up time: Some pressure instruments — particularly electronic pressure transmitters — require a warm-up period after power-on before their readings stabilise. Calibrating immediately after power-on may produce readings that drift as the electronics warm up.

Pressure medium contamination: Using oil as the pressure medium for calibrating an instrument that will be used with gas (or vice versa) can cause errors due to contamination. Always match the calibration pressure medium to the process medium, or use a media separator.

Ignoring hysteresis: Mechanical pressure gauges exhibit hysteresis — the reading at a given pressure differs depending on whether the pressure is being increased or decreased. A complete calibration should include measurements at each calibration point with both increasing and decreasing pressure, and the hysteresis should be recorded and evaluated against the instrument's specification.

Thermal equilibration: Bringing an instrument from a cold environment into a warm laboratory (or calibrating on-site in hot conditions) without allowing time for thermal equilibration can affect the calibration result. Allow adequate time for the instrument to reach equilibrium with the calibration environment before taking readings.

Selecting a Pressure Calibration Service in Singapore

When selecting a pressure calibration laboratory, verify that the laboratory's SAC-SINGLAS scope of accreditation covers the pressure range and instrument type you need calibrated. Not all accredited laboratories cover the full range from vacuum through high pressure, and some may only hold accreditation for specific pressure types (gauge but not absolute, for example).

Unitest Instruments holds SAC-SINGLAS accreditation LA-2023-0845-C covering pressure calibration across its accredited ranges. With over 20 years of experience as Singapore's authorised Fluke distributor — Fluke pressure calibration tools are among the most widely used in Singapore industry — our team combines product expertise with calibration competence. Standard laboratory turnaround is 3–5 working days; on-site calibration is available for installed pressure transmitters and process gauges.

To discuss pressure calibration for your instruments, visit our calibration services page or contact us directly. For a broader understanding of calibration certificate interpretation, see our guide on how to read a calibration certificate.

Frequently asked questions

How often should industrial pressure gauges be calibrated?

The recommended calibration interval for industrial pressure gauges depends on their application, operating conditions, and the criticality of the measurement. For safety-critical applications (pressure relief valve setpoints, SIS inputs), calibration intervals of 3–6 months are common. For general process monitoring gauges, 12 months is a typical starting interval, which can be extended or shortened based on the gauge's calibration history. Gauges subject to frequent pressure fluctuations, vibration, or corrosive media should be calibrated more frequently. SAC-SINGLAS accredited calibration laboratories can advise on appropriate intervals for specific applications.

Can a pressure gauge that is reading incorrectly be adjusted during calibration?

Yes, many pressure gauges — particularly mechanical Bourdon tube gauges — have an adjustment mechanism (zero and span screws) that allows the technician to correct the reading. The calibration procedure verifies the instrument at multiple points, adjustments are made if required, and the post-adjustment readings are then documented on the calibration certificate. Electronic pressure transmitters can be spanned and zeroed digitally. It is important to note that after any adjustment, a full re-calibration (measuring at all calibration points) must be performed, as an adjustment at one end of the range can affect readings at other points.

What is the difference between a pressure gauge calibration and a pressure safety valve test?

Pressure gauge calibration verifies the accuracy of the gauge reading against a reference standard and produces a calibration certificate with measurement results and uncertainty. A pressure safety valve (PSV) test — also called a pressure relief valve (PRV) test — verifies that the valve opens at its set pressure and reseats correctly when pressure falls. These are two separate activities requiring different equipment and expertise. Both are required for compliant pressure management: the gauge calibration confirms the accuracy of the instrument monitoring the pressure, while the PSV test confirms that the safety protection device functions correctly.

What is the required accuracy for pressure instruments in pharmaceutical manufacturing?

HSA GMP guidelines and the Pharmaceutical Inspection Convention and Pharmaceutical Inspection Co-operation Scheme (PIC/S) GMP guide require that measuring equipment be of appropriate accuracy for its intended use, but do not specify universal accuracy requirements for pressure measurement. In practice, the required accuracy is determined by the process parameter tolerance — for example, an autoclave operating at 121°C (corresponding to approximately 2.05 bar absolute steam pressure) with a ±2°C temperature tolerance may require pressure measurement accurate to ±0.05 bar or better. The calibration uncertainty must be consistent with the measurement requirement, and GMP documentation must justify the chosen accuracy.

Is it safe to calibrate high-pressure instruments on-site in a working plant?

On-site calibration of high-pressure instruments in a working plant can be done safely when proper procedures are followed. Key precautions include: using isolation valves to safely isolate the instrument from the process before connecting calibration equipment; venting residual process pressure safely before breaking any connections; using calibration equipment rated for the pressure involved; ensuring all personnel are aware of the work and any associated hazards; and following the plant's permit-to-work system. Unitest Instruments' calibration technicians are experienced in on-site work in industrial environments and carry out pre-visit risk assessments to ensure safe working procedures.

Pressure Calibration for Industrial and Marine Applications

SAC-SINGLAS accredited pressure calibration with on-site service available across Singapore.