Ashcroft Pressure Gauges: Bourdon Tube Advantages

How Mechanical Pressure Gauges Convert Pressure Into a Reading

A mechanical pressure gauge converts process pressure into visible pointer movement through the elastic deformation of a sensing element. In many industrial gauges, that element is a Bourdon tube: a hollow metal tube with a flattened, oval cross-section that has been bent into a defined shape.

When pressure enters the tube, the internal force tends to make its oval cross-section more circular. This small change in shape causes the curved tube to straighten slightly. The free end of the tube moves as pressure rises, while the fixed end remains connected to the gauge socket and process connection.

That movement is small, but the gauge mechanism amplifies it. A linkage connects the free end of the Bourdon tube to a movement mechanism, commonly involving a lever, sector gear, and pinion. The pinion rotates the pointer across the dial. As pressure changes, the elastic motion of the tube is translated into a proportional indication on the gauge face.

This principle is simple, but gauge performance depends on much more than the basic mechanism. Important design variables include:

Bourdon tube material
Tube shape and wall thickness
Manufacturing method
Pressure range
Case and window construction
Socket and wetted-part materials
Movement design
Dial size and readability
Protection against vibration, pulsation, corrosion, or overpressure

A gauge suitable for dry compressed air may not be suitable for corrosive chemical media, high-temperature steam, oxygen service, or a hydraulic system with severe pressure spikes. The pressure-sensing element, the pressure boundary, and the gauge installation all need to match the actual service conditions.

Why Ashcroft Is Associated With Bourdon Tube Pressure Gauges

Ashcroft has a long historical association with the Bourdon tube gauge. The company traces its pressure-gauge heritage to the nineteenth century, when it obtained U.S. rights to the Bourdon pressure-sensing design in 1852. This helped establish Ashcroft as an early American manufacturer of mechanical pressure gauges based on the Bourdon principle.

The timing was significant. Steam power was central to industrial development, transportation, and marine engineering. Boilers, steam engines, locomotives, and ships required operators to monitor pressure reliably. Mechanical gauges gave crews a direct visual indication of steam-system conditions and became an important part of routine operation and safety practice.

Bourdon tube gauges remain widely used because they can measure a broad range of pressures without requiring electrical power at the point of indication. They are especially common in applications where a robust local pressure indication is needed, including compressed-air systems, pumps, hydraulic equipment, process piping, boilers, and utility systems.

However, historical association should not be confused with a universal technical advantage for every application. Modern Bourdon tube gauges are offered by many suppliers, and designs vary by model, material, manufacturing method, safety features, pressure range, and applicable standard. When evaluating Ashcroft pressure gauges advantages, the useful question is not whether a gauge uses a Bourdon tube alone, but whether its particular construction and documented ratings fit the duty.

Bourdon Tube Styles and Their Pressure-Range Roles

Bourdon tubes are produced in several shapes. The most common are C-shaped, spiral, and helical forms. The tube geometry affects the amount of motion available, the pressure range that can be accommodated, and the mechanical design of the gauge.

A C-shaped Bourdon tube is curved into an arc resembling the letter “C.” As pressure enters the tube, the arc tends to open slightly. The tip movement is transferred through the linkage to the pointer. C-tube elements are commonly used in lower-to-medium pressure mechanical gauge ranges, although the exact practical range depends on the gauge design, material, diameter, wall thickness, and accuracy requirement.

Helical Bourdon tubes are wound into multiple turns. Their geometry can produce greater tip movement and is commonly used for higher-pressure gauge applications. The increased motion can support readable indication while allowing the sensing element to be designed for higher internal pressure.

For the highest pressure ranges, some gauge constructions use helical elements made from high-strength, corrosion-resistant alloys such as Inconel. Whether an Inconel helical element is necessary depends on the stated pressure range, media, temperature, corrosion risk, and the manufacturer’s documented configuration. Material selection should never be based on pressure alone.

The practical distinction is therefore not that one tube style is inherently “better.” Instead:

C-shaped tubes are often appropriate where moderate pressure ranges and compact gauge construction are needed.
Helical tubes are commonly selected where higher ranges require a sensing element that can provide useful mechanical motion.
High-strength alloy helical tubes may be used where high pressure and demanding service requirements justify that construction.

Bourdon Tube Construction Methods

The way a Bourdon tube is made can affect its dimensions, material continuity, wall-thickness control, and suitability for a particular pressure and process duty. Selection should consider pressure range, fluid compatibility, expected pressure cycling, temperature, vibration, pulsation, and the consequences of a pressure-boundary failure.

One traditional approach uses seamless tubing. In general, seamless tube production begins with billet material that is heated and processed by extrusion or piercing to create a hollow form. The tube may then undergo repeated drawing and annealing operations to reach the required dimensions and metallurgical condition. Drawing reduces dimensions and can improve dimensional control, while annealing restores ductility between forming stages.

A second approach is welded-and-drawn tubing. This process begins with strip material formed into a tubular shape. The seam is welded, after which the tube is drawn and annealed as needed to achieve its final dimensions and properties. The finished tube is then shaped into the required Bourdon configuration.

Ashcroft also describes a patented as-welded Bourdon tube construction. In this approach, the tube is formed with the weld positioned in a lower-stress area of the Bourdon element, while wall thickness is controlled during production. According to Ashcroft’s description, the final Bourdon tube is formed without the additional drawing and annealing stages associated with welded-and-drawn tubing.

These construction methods should not be treated as interchangeable labels or broad indicators of safety. A welded-and-drawn tube and an as-welded tube can have different production routes, but application suitability must be assessed from the specific gauge data sheet, materials, pressure rating, certification requirements, and service conditions. No general conclusion about defect rates, cycle life, or failure performance should be made without model-specific evidence.

Factors That Determine Whether a Mechanical Gauge Fits the Application

A mechanical gauge should be selected as part of the pressure system, not as an isolated dial instrument. The most important selection factor is the pressure range, but several related conditions determine whether the gauge will provide dependable service.

Pressure range and normal operating level

The selected full-scale range should allow the normal operating pressure to sit comfortably below full scale. Running a gauge continuously near its maximum indication can increase stress on the sensing element and leave little room to observe upward changes in process pressure.

A common engineering principle is to distinguish between:

Normal operating pressure: the pressure expected during ordinary operation.
Maximum operating pressure: the highest sustained pressure expected during intended operation.
Transient pressure: a short-duration rise caused by pump starts, valve closure, compressor cycling, or other system events.
Overpressure: pressure above the gauge’s full-scale range.
Proof pressure: a defined test pressure that the gauge can withstand without a specified change in accuracy.
Rupture pressure: pressure at which the pressure-containing assembly may no longer retain pressure.

A proof-pressure rating is not an instruction to operate the gauge at that pressure. It is a performance limit under stated test conditions.

Media compatibility

The pressure medium must be compatible with the gauge’s wetted materials. Media can include air, water, hydraulic oil, steam, refrigerants, oxygen, fuel gas, chemicals, slurry, or corrosive process fluids. Incompatibility may lead to corrosion, embrittlement, contamination, leakage, or inaccurate readings.

For corrosive, viscous, crystallizing, sanitary, or high-temperature media, a diaphragm seal, isolator, capillary system, or other protective arrangement may be required. Such accessories introduce their own temperature and response considerations, so they should be evaluated as part of the total measurement assembly.

Temperature

Temperature affects both the process connection and the sensing element. Elevated media temperature can transfer heat into the gauge and alter its performance or damage internal components. Low temperatures can affect fill fluids, seals, and mechanical movement. Ambient temperature also matters, particularly for outdoor installations or enclosed equipment where heat can build up.

Vibration and pulsation

Vibration can cause pointer flutter, loosen mechanical components over time, and make readings difficult. Pulsation creates repeated pressure cycling that can fatigue the sensing element or produce unstable indication.

Possible mitigation methods include liquid-filled cases, pulsation dampeners, restrictors, snubbers, or a different installation point. These measures must be chosen carefully because excessive restriction can slow the gauge response and conceal fast pressure changes.

Installation environment

The installation environment can expose the gauge to weather, dust, washdown, impact, corrosive atmosphere, electromagnetic equipment, or limited visibility. A gauge that is technically pressure-rated for the system may still be unsuitable if its enclosure, mounting arrangement, window material, or dial visibility does not fit the location.

Rupture, Proof Pressure, and Overpressure in Ashcroft Bourdon Tubes

Pressure-gauge performance involves two related but separate concerns: maintaining measurement accuracy and retaining pressure safely. Both are important when a gauge is exposed to vibration, pulsation, pressure cycling, or abnormal system events.

Overpressure is pressure applied above the gauge’s full-scale range. It may be temporary or sustained, and it can affect accuracy, pointer zero position, tube elasticity, or the pressure boundary.

Proof pressure is generally defined as the highest pressure that may be applied without a specified change in accuracy. The exact test conditions, duration, allowable accuracy shift, and configuration depend on the applicable standard or manufacturer documentation.

Rupture pressure, sometimes called burst pressure, refers to the pressure at which a pressure-containing assembly may no longer retain pressure. This is a pressure-boundary concept, not a normal operating rating. Rupture-pressure information is relevant to hazard assessment, but it does not eliminate the need for relief devices, proper range selection, system controls, and safe installation practices.

Understanding Ashcroft Proof-Pressure Ratings

Ashcroft proof-pressure ratings vary with gauge design. Relevant factors can include dial size, pressure range, tube construction, movement design, and whether an overload stop is included.

For specified Ashcroft process gauges with 4.5-inch and larger dials, manufacturer documentation has cited proof pressure of 130% of span for ranges up to 1,000 psi. For cited configurations above 1,000 psi through 15,000 psi, the stated proof pressure is 110% of span, with an additional allowance in configurations using an overload stop.

Specified industrial-gauge models may use different proof-pressure percentages above and below a 600 psi range threshold. These figures must be checked against the current data sheet for the exact model, dial size, range, and configuration. A percentage from one gauge family should not be transferred automatically to another.

Where the applicable specification confirms it, Ashcroft guidance states that maximum continuous pressure is limited to 75% of gauge span. This reinforces the key distinction between a continuous operating condition and a one-time or short-duration proof-pressure test.

ASME and EN requirements can differ in terminology, test methods, design expectations, and applicability. The correct reference depends on the gauge model, project specification, region, and certification requirement. Users should confirm the applicable standard rather than assuming that all gauges are qualified to the same criteria.

How to Use Ashcroft Rupture-Pressure Data

Ashcroft rupture-pressure data should be obtained from the current manufacturer figures or model-specific data sheets. The referenced documentation distinguishes between figures for specified industrial gauges in 2.5-inch, 3.5-inch, 63 mm, and 100 mm sizes and figures for process gauges with 4.5-inch and larger dials.

Those figures are useful for pressure-boundary evaluation, particularly where the consequences of a gauge failure must be considered. However, rupture-pressure values are configuration-specific. Dial size, range, construction, case design, and gauge family can affect the listed information.

Rupture data should therefore be used to support, not replace, sound engineering controls. Proper selection still requires:

Choosing an appropriate full-scale range
Accounting for pressure spikes and pulsation
Verifying compatibility with the process medium
Installing the gauge in a suitable location
Following the manufacturer’s mounting and safety guidance
Maintaining appropriate upstream pressure-control and relief measures

Key Advantages to Evaluate in an Ashcroft Pressure Gauge

The main Ashcroft pressure gauges advantages are best understood as design and documentation considerations rather than blanket claims of superiority.

A potential advantage is the company’s long association with Bourdon tube pressure measurement and the availability of gauges using different Bourdon tube geometries for different pressure duties. C-shaped elements, helical designs, and higher-strength alloy constructions can serve different application needs when selected according to documented limits.

Another consideration is construction choice. Seamless, welded-and-drawn, and Ashcroft’s described as-welded Bourdon tube approaches reflect different manufacturing methods. The practical benefit depends on the specific gauge construction and the application’s pressure, media, cycle-life, and failure-risk requirements.

Documented pressure-performance information is also important. Proof-pressure and rupture-pressure data, when available for the exact gauge configuration, can help engineers evaluate pressure-boundary margins and expected behavior during abnormal pressure events. These values must not be confused with continuous operating ratings.

Ultimately, the relevant advantage is application-specific. A suitable mechanical pressure gauge is one that matches the pressure range, media, temperature, vibration level, pulsation severity, installation environment, readability needs, and required safety margin. Before selecting a gauge, confirm the current manufacturer data sheet, the applicable ASME or EN requirements, and the actual conditions at the installation point.