2026-05-11
Specifying the correct mechanical pressure gauge requires precisely matching the internal sensing element to the process media and pressure profile. While the external housing dictates environmental protection, the internal elastic element—Bourdon tube, diaphragm, or capsule—determines the instrument's measurement range, overpressure tolerance, and compatibility with viscous or corrosive fluids. Selecting the wrong element type leads to premature fatigue failure, measurement drift, or catastrophic rupture. This technical reference details the operating principles, material limitations, and standard compliance criteria for the three primary mechanical pressure sensing technologies used in industrial applications.
The Bourdon tube is a radially formed tube with an elliptical cross-section. As internal process pressure increases, the cross-section tends toward a circular shape, causing the tube to uncoil. This microscopic motion is transferred via a mechanical movement mechanism to the pointer. Depending on the required pressure range, Bourdon tubes are manufactured in three distinct geometries: C-type (typically used for pressures up to 60 bar), spiral (for medium pressures), and helical (for extreme high pressures up to 6000 bar).
Standardized under EN 837-1, ASME B40.100, and GB/T 1226-2017, Bourdon gauges are the definitive industry standard for general industrial applications. Accuracy classes typically range from 1.0% to 1.6% of the full scale span, with precision test gauges achieving up to 0.1% accuracy. The primary advantage of the Bourdon tube is its high accuracy and excellent repeatability across a massive pressure spectrum.
However, Bourdon tubes possess inherent dead spaces at the sealed tip. They are strictly suitable for clean, non-crystallizing liquids and gases. They are entirely unsuitable for highly viscous, slurry, or crystallizing media. Such fluids will stagnate within the tube's dead space, leading to clogging, permanent deformation, or complete mechanical failure. Furthermore, Bourdon tubes are highly susceptible to fatigue from rapid pressure pulsations, necessitating the use of liquid-filled cases or internal restrictors in dynamic applications.
Diaphragm elements consist of a corrugated circular membrane clamped concentrically between two robust flanges. Process pressure applied to the lower side deflects the diaphragm upward against the movement mechanism. Governed by EN 837-2, diaphragm gauges are engineered specifically for low to medium pressure ranges, covering 0 to 40 bar. Standard accuracy classes are generally 1.6% or 2.5%, slightly lower than Bourdon tubes due to the non-linear deflection characteristics of the corrugated membrane.
The primary engineering advantage of the diaphragm element is its lack of dead spaces and its large surface area. This geometry makes it the definitive choice for viscous, crystallizing, or slurry media that would instantly obstruct a Bourdon tube. When paired with an open flange connection, the diaphragm sits flush with the process fluid, eliminating any cavities where media could accumulate.
Additionally, diaphragm elements offer superior overpressure protection. By machining the upper flange to match the exact contour of the corrugated diaphragm, the element can be fully supported when subjected to pressure spikes. This allows diaphragm gauges to withstand overpressures of 5 to 10 times their full scale value without permanent deformation—a capability impossible to replicate with a standard Bourdon tube.
Capsule elements are constructed by welding two corrugated diaphragms together at their periphery, forming a sealed, expandable internal cavity. Process pressure is admitted into the center of the capsule via a central port, causing both diaphragms to expand outward simultaneously. This dual-expansion generates a significantly larger mechanical stroke at very low pressures compared to a single diaphragm element.
Also categorized under EN 837-2, capsule gauges are strictly designed for micro-pressure measurement, covering ranges from 0 to 600 mbar. Typical accuracy classes range from 1.6% to 2.5%. These elements are highly sensitive and are commonly deployed in applications requiring precise draft measurement, such as burner systems, cleanroom HVAC monitoring, and medical gas distribution.
Because the internal cavity of the capsule is extremely narrow and impossible to drain or clean, these elements are exclusively suited for dry, clean gaseous media. Introducing liquids into a capsule gauge will result in capillary action retaining the fluid within the corrugations. This causes severe measurement drift due to the added mass of the liquid, and will eventually lead to internal corrosion or mechanical failure.
The wetted materials of the sensing element must be strictly matched to the process media to prevent corrosion-induced rupture. Standard Bourdon tubes are typically drawn from copper alloys (such as bronze) for non-corrosive environments, or 316L SS (stainless steel) for aggressive industrial applications. However, drawing Bourdon tubes from exotic alloys is metallurgically complex, prone to micro-cracking, and highly cost-prohibitive.
Diaphragm elements excel in highly corrosive applications because the flat membrane is easily stamped from sheet metal. This allows for the cost-effective utilization of specialized alloys. Typical wetted materials for diaphragm gauges include:
Furthermore, diaphragms can be lined with PTFE or tantalum foils to withstand severe acids that would destroy metallic elements. Capsule elements, due to their low-pressure requirements, are generally manufactured from 316L SS or specialized copper-beryllium alloys to maximize elasticity and hysteresis recovery at millibar ranges.
Plant engineers and B2B distributors can utilize the following technical matrix to specify the correct sensing element based on process parameters. Selecting the correct element ensures compliance with EN 837 standards and maximizes the instrument's operational lifespan.
| Parameter | Bourdon Tube (EN 837-1) | Diaphragm Element (EN 837-2) | Capsule Element (EN 837-2) |
|------------------------|-------------------------|------------------------------|----------------------------|
| Pressure Range | 0 to 6000 bar | 0 to 40 bar | 0 to 600 mbar |
| Typical Accuracy | 0.1% to 1.6% | 1.6% to 2.5% | 1.6% to 2.5% |
| Media State | Clean liquids and gases | Liquids, gases, slurries | Dry gases only |
| Viscous/Slurry Media | Unsuitable (clogging) | Excellent (open flange) | Unsuitable |
| Overpressure Safety | Low to Moderate | Very High (flange backed) | Low |
| Wetted Materials | Bronze, 316L SS, Monel | 316L SS, Hastelloy, Titanium | Bronze, 316L SS |Beyond media compatibility and pressure range, the operating environment heavily influences element selection. Bourdon tubes exposed to high-frequency vibration or rapid pressure pulsation will experience work hardening and eventual fatigue failure. In such environments, the gauge case must be filled with a dampening fluid (typically glycerine or silicone oil) to lubricate the movement and dampen the tube's oscillation.
Diaphragm and capsule elements are generally not liquid-filled. The mass of the fill fluid resting on the horizontal diaphragm or capsule can cause significant zero-point shifts, especially at low pressure ranges (below 2.5 bar). If a diaphragm gauge must be used in a high-vibration environment, engineers should specify a dry case with a dampened movement mechanism, or utilize a remote capillary line to isolate the gauge from the vibration source.