Pressure Gauge Selection Guide: Bourdon Tube, Digital & Differential Gauges Explained

Bourdon Tube Gauges: The Industrial Standard

The Bourdon tube gauge is the most widely used mechanical pressure gauge in industrial applications. Invented by Eugène Bourdon in 1849, the design has remained essentially unchanged: a curved, hollow tube (C-type Bourdon tube) is fixed at one end (the process connection) and free at the other. Pressure applied inside the tube causes it to straighten slightly. This mechanical deflection is amplified by a rack-and-pinion mechanism and drives a pointer across a calibrated dial.

Bourdon tube gauges require no electrical power, are inherently robust, and can measure gauge pressures from vacuum (negative gauge pressure) to over 1000 bar. They are standardised under EN 837-1 (flanged, diaphragm, and capsule gauges are covered by EN 837-2 and EN 837-3).

Accuracy Classes Under EN 837

EN 837-1 defines accuracy classes as a percentage of the full-scale span:

• Class 0.6: ±0.6% FS. High-accuracy test gauges for calibration use
• Class 1.0: ±1.0% FS, precision industrial gauges
• Class 1.6: ±1.6% FS. Standard industrial gauges (most common)
• Class 2.5: ±2.5% FS. Economy gauges for general indication
• Class 4.0: ±4.0% FS. Very rough indication only

A Class 1.6 gauge with a range of 0–10 bar can read ±0.16 bar in error at any point on the scale. At 2 bar working pressure on a 10 bar scale, this is ±8% of reading. Very poor if the process requires tight pressure control. This is why gauges should be selected so that the working pressure falls in the upper half of the scale: at 8 bar on a 10 bar gauge, ±0.16 bar is ±2% of reading.

The 4:1 test accuracy ratio (TAR) principle states that the reference gauge used for calibration should have accuracy at least 4× better than the gauge being calibrated. A Class 1.6 gauge requires a reference with ≤0.4% FS accuracy, a Class 0.1 digital pressure calibrator or deadweight tester.

Dial Size and Legibility

Standard dial sizes for industrial gauges are 63mm, 100mm, and 160mm. Larger dials provide better pointer resolution (finer scale divisions) and better legibility at distance. For panel mounting where operators read from 2–3 metres away, 100mm minimum is recommended. For high-pressure header monitoring in plant rooms where gauges are overhead or at distance, 160mm is standard. For tight-space instrument installations on local gauge panels adjacent to equipment, 63mm is acceptable.

Liquid-Filled (Glycerine-Filled) Gauges

For applications with pulsating pressure (pump discharge, compressor outlet, pneumatic systems), vibration (rotating machinery), or pressure spikes, a liquid-filled gauge is strongly recommended. The glycerine (or silicone oil) fill dampens pointer movement, protects the Bourdon tube and movement from fatigue caused by cycling stress, and lubricates the gear mechanism.

Glycerine-filled gauges are the default for most process plant applications. Silicon oil-filled gauges are used where glycerine compatibility is a concern (pharmaceutical, food contact, oxygen service. Where glycerine's carbon content is a fire risk). Dry gauges are acceptable for stable, non-pulsating gas or clean liquid measurements.

Wetted Materials Selection

The wetted parts of a pressure gauge (the Bourdon tube, socket, and any diaphragm), must be chemically compatible with the process fluid:

• 316 Stainless Steel (SS316): suitable for most clean aqueous fluids, steam, air, nitrogen, and non-corrosive hydrocarbons. This is the default for most process plant applications.
• Hastelloy C: for hydrochloric acid, seawater, chlorinated compounds, and other highly corrosive media where SS316 fails by pitting corrosion or stress corrosion cracking.
• Monel 400: for hydrofluoric acid (HF) service. Monel is one of few materials resistant to HF. Required in alkylation unit pressure gauges.
• PVDF (Kynar): for strong oxidising acids (nitric acid, sulphuric acid), chlorine, and bleach. Wetted parts are wetted plastic rather than metal.
• Tantalum: for the most aggressive media, fuming sulphuric acid, chlorosulphonic acid.
• Gold-plated diaphragm: for oxygen service, where hydrocarbon contamination from standard gauge lubricants is a fire and explosion risk.

Mounting Orientation and Vibration Isolation

How a Bourdon tube gauge is mounted has a measurable effect on both its reading and its service life, and this is one of the more commonly overlooked aspects of gauge selection during specification. Direct-mounted gauges (threaded straight into the process connection) are simple and low-cost, but they place the full weight of the gauge, and any pipe vibration, directly on the Bourdon tube movement, which accelerates wear on the gear mechanism over time. For applications with any meaningful vibration, pump-adjacent piping, compressor headers, rotating equipment, a remote-mounted gauge connected via flexible capillary tubing, or a panel-mounted gauge isolated from the vibrating line by a short length of tubing, materially extends service life and reduces pointer flutter that makes the gauge difficult to read accurately.

Orientation also affects long-term accuracy for liquid-filled gauges. Most Bourdon tube gauges are calibrated and rated for vertical (dial facing the operator) mounting, and mounting a gauge on its side or at a steep angle can introduce a small zero offset due to the fill liquid's own weight acting differently on the internal mechanism. For precision applications where this matters, specify the intended mounting orientation to the calibration provider so the gauge is verified in the orientation it will actually be installed in, not simply on a bench in the laboratory's default position.

Common Failure Modes and What They Indicate

A gauge showing visible signs of distress is telling you something specific about the process it is connected to, and recognising these patterns helps a maintenance team decide whether to simply replace the gauge or investigate further. A pointer that has visibly bent or a dial with a permanently offset zero after the gauge is removed from pressure almost always indicates an overpressure event exceeded the Bourdon tube's elastic limit, and the entire gauge should be replaced rather than merely re-zeroed, since the tube's spring characteristic has been permanently altered and its accuracy across the rest of the scale can no longer be trusted even if the zero is manually reset.

Condensation or fogging inside the gauge window on a liquid-filled gauge indicates a seal failure, allowing moisture ingress that will progressively corrode the internal movement and eventually seize the mechanism; this gauge should be taken out of service promptly rather than left in place until it fails completely. A pointer that flutters or oscillates continuously rather than settling to a steady reading usually indicates either genuine process pulsation that needs a snubber, or a worn movement with excessive gear backlash that calibration alone cannot fix. Distinguishing between these two causes (a process problem versus an instrument problem) is exactly the kind of diagnostic judgement an experienced calibration technician brings during a routine service visit, beyond simply confirming the gauge reads within tolerance at the test points.

Vacuum and Compound Gauges

Not every application involves positive gauge pressure. Vacuum gauges measure pressure below atmospheric, typically scaled from 0 to -1 bar (or 0 to 760 mmHg), and are used on applications like vacuum filtration, degassing systems, and suction-side pump monitoring. Compound gauges combine both ranges on a single dial, reading from a vacuum range through zero (atmospheric) into a positive pressure range, which is common on refrigeration service gauges and on pump suction lines that can experience either condition depending on operating state.

The same Bourdon tube principle applies in vacuum service, though the tube deflects in the opposite direction under vacuum compared to positive pressure. Accuracy classes and calibration principles are identical to positive-pressure Bourdon gauges, but the reference standard used for calibration must be capable of generating a controlled vacuum, typically a vacuum pump combined with a calibrated vacuum reference, rather than the deadweight testers used for positive-pressure calibration. When specifying a vacuum or compound gauge, confirm the calibration provider's accredited scope explicitly covers negative gauge pressure, since not every pressure-accredited laboratory holds scope for vacuum measurement.

Digital Pressure Gauges

Digital pressure gauges use a piezoresistive or strain-gauge pressure sensor to measure pressure electronically and display the result on a digital readout. Key advantages over Bourdon tube gauges:

• Higher accuracy: typically 0.1–0.25% FS, versus 1.6–2.5% for Bourdon gauges
• MAX/MIN memory: records peak and lowest pressures for testing and commissioning
• Multiple pressure units: toggle between bar, PSI, kPa, MPa, mbar from the keypad
• Alarms: programmable high/low pressure alarms
• Data logging: some models store up to several thousand readings with timestamps
• Battery powered: no loop power required; typical battery life 1000–2000 hours

Popular digital pressure gauges: WIKA DG-10 and CPG500 (excellent Singapore support), Druck PACE5000/6000 series (high accuracy, reference-grade), GE Druck DPI 104 (compact, rugged).

The limitation of digital gauges is battery dependence and cost. For permanent process monitoring where a visual indication is sufficient and power is not available, the Bourdon tube gauge remains the preferred choice.

Differential Pressure Gauges

A differential pressure (DP) gauge measures the pressure difference between two process connections. A high-pressure (HP) port and a low-pressure (LP) port. It does not measure absolute or gauge pressure; it measures ΔP = P_HP − P_LP.

Key applications for differential pressure gauges in Singapore:

• Filter differential pressure monitoring: as a filter cartridge, strainer, or bag filter clogs, pressure drop across the filter increases. A DP gauge on the inlet and outlet indicates when the filter requires cleaning or replacement. Alert setpoints typically 0.5–1.0 bar ΔP for process filters.
• Clean room differential pressure: pharmaceutical clean rooms and operating theatres require positive pressure differentials (typically +12.5 Pa to +25 Pa relative to adjacent lower-grade areas) to prevent contamination ingress. Low-range DP gauges (0–100 Pa or 0–250 Pa range) monitor these critical room pressure differences.
• Flow measurement: differential pressure generated across an orifice plate, venturi tube, or flow nozzle is proportional to the square of the flow rate. DP gauges can serve as flow indicators when combined with a calibrated primary element.
• Heat exchanger fouling: rising ΔP across a heat exchanger bundle indicates tube fouling.

DP gauge types: diaphragm type (most common, good for low ΔP with high line pressure), piston type (for higher ΔP), and capsule type (for very low ΔP, 0–25 mbar range). Governed by EN 837-3.

Overpressure Protection

Bourdon tube gauges must be protected against overpressure events that could damage or rupture the tube:

• Rated overpressure: EN 837-1 requires that gauges survive at least 1.3× their full-scale range without permanent damage to zero setting or span.
• Short-term overpressure (burst pressure): typically rated at 2× full scale for safety.
• Snubber/restrictor: a flow-restricting orifice (typically 0.3–0.8mm) installed in the gauge connection dampens pressure spikes and pulsations, protecting the Bourdon tube from fatigue.
• Pressure relief valve (blow-out back): a back plate fitted with a burst disc or pressure-relief vent. If the Bourdon tube fails catastrophically, the back plate vents pressure away from the operator. Required for hazardous fluid services (flammable gas, toxic fluids, steam).
• Syphon: for steam service, a pigtail or U-type syphon fills with condensate, protecting the Bourdon tube from high-temperature steam contact. The condensate layer transmits pressure while keeping steam out of the gauge.

Ambient Temperature Effects on Pressure Gauge Accuracy

Bourdon tube gauges are specified for accuracy at a reference temperature, typically 20°C, and their stated accuracy class assumes operation reasonably close to that reference. The metal of the Bourdon tube itself expands and contracts with temperature, and the spring constant that governs how far the tube deflects under a given pressure also shifts slightly with temperature, meaning a gauge's actual error can grow beyond its rated accuracy class when operated well outside the temperature range its calibration assumed. Most industrial gauges specify an additional temperature error coefficient, often in the region of 0.4% of span per 10°C deviation from the reference temperature, which is a meaningful figure for a gauge mounted directly on a hot process line or exposed to direct sun on an outdoor installation in Singapore's climate.

For gauges mounted directly on high-temperature process connections (steam lines, hot oil systems), a syphon or capillary extension that keeps the gauge movement itself away from the hot process fluid also indirectly protects accuracy, not just the mechanical components, by keeping the Bourdon tube closer to its rated reference temperature range. When specifying gauges for outdoor or high-ambient-temperature installations, checking the manufacturer's stated temperature error coefficient against the actual expected operating temperature range is a step worth taking before assuming the printed accuracy class will hold in service.

Calibration of Pressure Gauges

Pressure gauges are measurement instruments and must be calibrated regularly against a traceable reference standard. See our guide to pressure calibration instruments and equipment for the methods and standards involved.

Primary reference standard: the deadweight tester (piston-gauge) is the primary standard for pressure calibration, generating known pressures by loading calibrated masses onto a piston of known cross-sectional area. Deadweight testers achieve uncertainties of 0.005–0.02% of reading.

For field calibration: portable digital pressure calibrators (Fluke 700 series, Druck DPI 610/612/615) with internal or external reference transducers. Accuracy: 0.025–0.1% FS. For SAC-SINGLAS traceable calibration, the calibrator itself must have a valid SINGLAS calibration certificate.

Calibration procedure: 5-point calibration at 0%, 25%, 50%, 75%, and 100% of gauge range in both increasing and decreasing directions; comparison of gauge reading against reference; assessment against EN 837 tolerance; adjustment if required.

Calibration interval: 12 months for process gauges; 6 months for safety-critical applications, pressure safety valve (PSV) test gauges, and SIS-associated gauges. After any overpressure event, recalibrate before returning to service.

A well-run gauge calibration programme in Singapore also benefits from tracking as-found data across successive calibrations rather than treating each service visit in isolation. A Bourdon tube gauge that consistently returns within a fraction of its accuracy class limit across several years is a reasonable candidate for extending its interval with documented justification, while a gauge that shows progressive drift, even if each individual result still technically passes, is signalling mechanical wear that will eventually produce an out-of-tolerance finding if the interval is not shortened first. This same principle applies across the full range of instruments discussed here: Bourdon tube gauges, digital gauges, and differential pressure gauges all benefit from a calibration provider that records and reports as-found data clearly enough for a plant engineer to spot these trends, rather than a certificate that states only a final pass or fail.

Gauge Type Comparison