FLUID POWER HANDBOOK
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GAUGES
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FLUID POWER WORLD
7 • 2020
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PRESSURE
gauges measure a fluid’s intensity. They ensure reliable operation and reduce the risks of pressure spikes or changes that could cause damage to the system. In addition, they prevent leaks by alerting personnel of unusual changes in system pressure.
Hydraulic pressure gauges are available to measure up to 10,000 psi, though typical hydraulic systems operate in the 3,000 to 5,000 psi range. Hydraulic gauges are often installed at or near the pump’s pressure port for indication of system pressure, but can be installed anywhere on the machine where pressure needs to be monitored — especially if sub-circuits operate at a pressure rate different from pump pressure, such as after a reducing valve. Often, pressure-reducing valves have a gauge port to tap into, allowing you to directly monitor its downstream pressure setting. Pressure gauges are now more routinely designed with hydraulic friendly pressure connections (such as SAE/Metric straight threads) to prevent system leaks. Analog gauges with custom scales are more common and digital pressure gauges with customizable firmware allow process measurement of pressure-based measurement of leaks or other parameters like torque, load, force and hardness. Pressure is measured in many locations throughout pneumatic and compressed air systems. It is measured at receiver(s), as well as every system FRL or stand-alone regulator and sometimes at pneumatic actuators. These gauges can be rated up to 300 psi. Pressure is measured in three ways—absolute, gauge and vacuum. Absolute pressure is a measure of actual pressure including ambient air, which is zero-referenced with a perfect vacuum, but can be as high as 14.7 psi at sea level. Absolute pressure readings are considered in applications interacting with ambient air, such as the compression ratio calculation for flow (cfm) requirements. Gauge pressure is zeroreferenced against ambient pressure and is used in most applications operating in, but not with, ambient air, such as in fluid power systems. Disconnected from equipment, gauge pressure will read zero. Finally vacuum “pressure” is expressed in Torr, or referenced against ambient pressure, as with “in.-Hg” (inches of mercury) units, which measures pressure below ambient. The pressure range at which a hydraulic gauge will be working is a primary selection factor for the type of material 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. The most common gauges are Bourdon tubes and bellow gauges. Bourdon tubes take pressure and convert it into mechanical energy. This energy moves a dial in the gauge, displaying the pressure in the system. Bourdon tube gauges 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
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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. Bellow gauges function similarly to Bourdon tubes, but 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. 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 with just one gauge. The testpoint fitting attaches to the gauge, which can be screwed onto the test points throughout the circuit, allowing you to connect under pressure to measure throughout the system. Most gauges are 21⁄2 in. in diameter, and can be top-mount or panel-mount styles. Common threats to gauge reliability are vibration, pulsation and pressure spikes. Therefore, it’s best to look for gauges designed specifically for hydraulic applications to reduce costly downtime. A forged brass case prevents resonant frequencies from destroying internal components; a liquid-filled case protects the gauge from vibration and extreme pressure cycles; and a restrictor prevents damage from pressure spikes. When choosing between a dry, water- or glycerinfilled gauge, it is also important to consider temperature range, needle response time required, changes in pressure and expected vibration. Gauge accessories, such as specialized restrictors, piston snubbers or diaphragm seals, may be used to help prevent premature gauge failure.
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