Pressure Gauges: Made to Measure

Bourdon-tube designs

Since the invention of the Bourdon-tube gauge more than a century ago, pressure gauge manufacturers have been developing different types of gauges to meet specific needs without ever changing the basic principle of operation. Bourdon-tube gauges are now commonly available to measure a wide range of gauge, absolute, sealed, and differential pressures, plus vacuum. They are manufactured to an accuracy as high as 0.1% of span and in dial diameters from 1½ to 16 in. A variety of accessories can extend their performance and usefulness. Snubbers and gauge isolators can be installed to protect the sensitive internal workings from pressure spikes.

Gauges using C-shaped Bourdon tubes as the elastic chamber, as shown above left, are by far the most common. Pressurized fluid enters the stem at the bottom (which is sometimes entrancement instead) and passes into the Bourdon tube. The tube has a flattened cross section and is sealed at its tip. Any pressure in the tube in excess of the external pressure (usually atmospheric) causes the Bourdon tube to elastically change its shape to a more circular cross section.

This change in shape of the cross section tends to straighten the C-shape of the Bourdon tube. With the bottom stem end fixed, the straightening causes the tip at the opposite end to move a short distance — 1/16 to 1/2 in., depending on the size of the tube. A mechanical movement then transmits this tip motion to a gear train that rotates an indicating pointer over a graduated scale to display the applied pressure. Often, a movement is incorporated to provide mechanical advantage to multiply the relatively short movement of the tube tip.

Spiral and helical Bourdon tube designs

Bourdon tubes also may be made in the form of a spiral, Figure 1, or a helix, Figure 2. Each uses a long length of flattened tubing to provide increased tip travel. This does not change the operating principle of the Bourdon tube, but produces tip motion equal to the sum of the individual motions that would result from each part of the spiral or helix considered as a C-shape. Small-diameter spirals and helices can be manufactured to provide enough motion to drive the indicating pointer directly through an arc up to 270° without having to use a multiplying movement. Alternatively, they may be manufactured to be used in conjunction with a multiplying movement. In this case, the required motion is distributed over several turns, resulting in lower stress in the Bourdon material. This improves fatigue life when compared to a C-shaped Bourdon tube in the same pressure range.

Other designs

Low-pressure applications do not generate enough force in the Bourdon tube to operate the multiplying mechanism; therefore, Bourdon tube gauges are not generally used for pressure spans under 12 psi. For these ranges, some other form of elastic chamber must be used, a metallic bellows, for example. These bellows generally are made by forming thin-wall tubing. However, to obtain a reasonable fatigue life and motion that is more linear with pressure, a coil spring supplements the inherent spring rate of the bellows. These spring-loaded bellows gauges generally are used in pressure ranges having spans to 100 psi and to 1 in.-Hg.

Metallic diaphragms also are used as the elastic chamber in low pressure gauges. A diaphragm plate is formed from thin sheet metal into a shallow cup having concentric corrugations. To make an element with a low spring rate that generates substantial deflection from a small change in pressure, two plates can be soft soldered, brazed, or welded at their periphery to form a capsule, and additional capsules can be joined at their centers to form a stack.

Diaphragm elements may be used in an opposing arrangement. By evacuating one side of the assembly, the gauge can indicate absolute pressure. If a pressure is applied to one side of the assembly, and a second pressure is applied to the other side, then the differential pressure will be indicated. The differential pressure is limited with respect to the static pressure that can be applied. That is, the gauge may be suitable to indicate between 10 psi and 12 psi, but not be suitable to indicate between 100 psi and 102 psi. Also, the consequence of inadvertently applying full pressure to one side of the element and no pressure to the other side of the element must be considered.

Selection
Specifying a pressure gauge involves a number of considerations:

• connection size — nominal size of the port or fitting into which the gauge will be threaded, male or female, and thread size
• mounting configuration — bottom or back-center stem mounted or panel mounted
• dial size — large enough to be seen clearly from a distance but small enough to prevent taking up excessive space
• units of measure — determine whether the dial should be calibrated in psi, bar, kPa, etc. Many manufacturers offer gauges with dual-dimensioned scales
• materials of construction — gauges may have a glass or plastic crystal, metal or plastic case, and usually a brass connection. Ensure that materials are compatible with the environment and fluid
• dry or liquid filled — liquid-filled gauges generally contain glycerin to dampen effects of shock and vibration, and provide continuous lubrication of the movement to extend life, and
• pressure range — as a rule of thumb, select a gauge with a maximum pressure reading twice that of the anticipated measured pressure. This provides a safety margin to prevent temporary high-pressure pulsations or spikes from damaging the gauge.

Options and accessories
A variety of options and accessories are available to enhance life and operation of gauges. Digital readout is accomplished by mounting a strain gauge to the sensing element and using on-board electronics to convert the strain induced by pressure into digital readout on an LED or LCD panel. Digital gauges require a power source — generally a long life battery — and may use a switch so power is consumed only when a button is pushed to read pressure.

A gauge isolator, mounted between the gauge and circuit, prevents the gauge from being exposed to fluid pressure unless a button is pushed. In this manner, the gauge is not exposed to pressure spikes and pulsations unless they occur when pressure is being read.

Orifices or snubbers protect gauges by smoothing out pressure fluctuations seen by the gauge. Snubbers may cause gauges to respond sluggishly, but can extend life by damping rapid pressure fluctuations. To help protect the gauge from external physical shock, case protectors can be used, which encapsulate the gauge in rubber.

A wide variety of other useful options — such as an integral adjustable pressure switch — is available from manufacturers to make pressure gauges even more versatile.