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made out of. Because of this, the gauge style and the material make up two of the most important selection criteria for gauges. There are many types of gauge styles, the most common being Bourdon tubes and bellow gauges. Bourdon tubes function by taking the pressure and converting it into mechanical energy. This energy moves a dial in the gauge, displaying the current amount of pressure in the system. Bourdon tube gauges are currently some of the most common gauges and have different configurations such as curved, helical and spiral. The different style of tubing, the size of the tube and the material it is made out of all vary based on the pressure range. One important characteristic to note is the cross section of the tubing changes with increasing pressure. Generally, as the working pressure of the gauge increases, the shape of the cross section of the tube’s design will gradually change from an oval shape to a circular shape. Bourdon tube operation is simple. They consist of a semicircular and flat tube of metal, fixed at one end and attached to a sensitive lever mechanism at the other. As pressure increases inside the tube, the force of the fluid attempts to straighten out the curved tube. The tube then pulls away from the lever, which being connected to the needle on the display, shows the pressure at the fluid port. While bellow gauges function similarly to Bourdon tubes, they differ in the fact that they use a spring to judge the amount of energy to push the dial. The spring is expanded and compressed by the pressure in the tubes and the energy created by that movement is transferred into gears that move the pressure dial. The pressure range at which the gauge will be working is a primary selection factor for the type of material
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used to make the gauge. Gauges operating at higher pressures generally tend to be made of materials such as steel; when operating at lower pressures, they tend to be made of bronze. Most pressure gauges in North America come with a 1⁄4-in. NPT male, but SAE thread is gaining popularity. The use of test-point adapters at various locations on the hydraulic system allows for measurement during troubleshooting without having to purchase dozens of pressure gauges. The test-point fitting attaches to the gauge, which can be screwed onto the test points throughout the circuit, allowing you to connect under pressure to measure at various points in the system. Most gauges are 21⁄2 in. in diameter, and can be either top-mount or panel-mount styles, but gauges are available in every size, material and construction imaginable. Whether used for testing equipment or operating machinery, the right pressure gauge helps reduce costly downtime. In mechanical gauge applications for hydraulic systems, the common threats to gauge reliability are vibration, pulsation and pressure spikes. Therefore, it’s best to look for gauges designed specifically for hydraulic applications. These features include: a forged brass case to prevent resonant frequencies from destroying internal components; a liquid-filled case to protect the gauge from vibration and extreme pressure cycles; and a restrictor to prevent damage to the gauge from pressure spikes. Although the liquid used in the gauge varies from application to application, glycerin is commonly used and performs well in many conditions. The higher the viscosity of the liquid, the more it dampens the vibrations. When choosing between a dry, water- or glycerin-filled gauge, it is also important to consider the following: temperature range, needle response time required, changes in pressure and the amount of vibration expected from the application. Finally, depending on the demands of the application, gauge accessories, such as specialized restrictors, piston snubbers or even diaphragm seals, may be needed to prevent premature A LOOK at the internal gauge failure. functions of a bourdon tube pressure guage. PHOTO COURTESY OF NOSHOK
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