The technology used in pressure gauges has been around for more than a century, and the pressure gauge is still one of the most common methods of measuring pressure today. The majority of pressure gauges still incorporate the Bourdon tube, socket and geared movement, along with a pointer and dial to indicate process pressure.
Pressure gauges can be used for monitoring liquids and gas pressure in applications including hydraulics and pneumatics, medical, pumps and compressors, refrigeration controls, utilities and gas monitoring. In addition to visual pressure indication, some pressure gauges are configured to provide electrical output and monitor other variables such as temperature.
Gauge selection considerations
Several factors should be considered when selecting a pressure gauge for an application.
The first points to consider are the engineering and system design requirements, including cost and safety factors, as well as the level of accuracy needed. Pressure gauges are available with various ranges of accuracy from ±.25, ±.5, ±1, ±1.5 and 2 to 2.5 percent. Different scale options are available for many single and dual scale ranges. The level of accuracy required for newly designed systems may be different from that for replacement components.
Engineering requirements also come into play regarding pressure gauge accuracy. Higher accuracy dictates a larger dial size for the display of small and readable pressure scale increments, so space may be a consideration. In addition, the user or operator’s ability to read the scale from a distance may be a factor depending on the physical circumstances of the system or application. Pressure gauge range selection should also permit the desired readable scale increments from a required viewing distance.
To select the correct pressure range for a gauge, where there is dynamic pressure with potential of pulses and surges, choose a full-scale pressure where the operating pressure occurs in the middle half (50 to 75 percent) of the scale to allow for pressure swings because of pulsation and pressure spikes in the system.
Other engineering considerations include overpressure conditions and loss of gauge accuracy. Process media generally flow through a piping system at relatively high pressure, and gauges rated to that pressure are often installed to monitor the process. A problem can occur when pumps are switched on or off, or valves are opened or closed, and the media surges through the pipe and causes a spike on the pressure gauge, which can damage the instrument. To avoid this, a pressure gauge should be applied to account for pressure surges and spikes in the system. Overpressure protectors such as snubbers or valves can be applied to address conditions where spikes are most likely to occur.
Gauge accuracy can also be affected by temperature changes. In general, for every 18°F (10°C) shift in temperature from which the gauge is calibrated, the user can experience up to a ±0.4 percent additional error. The cause is the change in the elasticity or spring rate of the Bourdon tube element with temperature. While it is difficult to circumvent the influence of ambient temperature, the influence of process temperature can be addressed. In steam service, the common practice is to install coil syphons or pigtail syphons to dissipate process heat. Another common practice is to install a diaphragm seal with capillary to separate the gauge from the high heat source. Many options are available with fill fluid in the seal and capillary system to withstand temperatures up to 600°F. In severely cold ambient conditions, many users elect to heat trace instrumentation via electric or steam trace. Process and ambient temperature is an important consideration when selecting and applying pressure gauges. To address temperature extremes, mounting options may be considered depending on the operating temperature range; Diaphragm seals, capillaries, cooling elements and pigtail steam syphons can be used to dissipate heat and protect instruments.
Another possible cause of gauge failure is corrosion. The gauge’s Bourdon tube can be weakened by corrosive chemicals in media or environment. This may occur as a pinhole leakage or early fatigue failure because of stress cracking. Therefore, it is critical to be aware of the chemical compatibility of the media with a gauge’s case and construction materials. Diaphragm seals can also be employed to protect the gauge from the media.
Vibration can have a significant effect on gauge performance. Vibration due to pumps, motors and other rotating equipment can cause excess wear and possible premature failure of the internal working parts of a pressure gauge, which include the Bourdon tube and the movement or gear mechanism. Vibration also causes difficulty in accurate reading of the gauge because of pointer oscillation. One of the most common causes of pressure gauge failure is exposure to continuous vibration. The most widely accepted remedy is to utilize a liquid-filled pressure gauge. The fill fluid of choice is either glycerin or silicone. Liquid-filled gauges address not only pointer oscillation, but also serve to protect and lubricate the internal geared movement.
Pressure gauges are available in a variety of process connections, sizes and case mounting styles. The proper process connection must be specified depending on system requirements and application.
A wide variety of options and accessories are available for consideration when selecting a pressure gauge. Piston- and sintered-type snubbers can be used to increase gauge readability by smoothing out pressure surges, pulsations and spikes. Swivel adaptors can adjust the line of sight for better reading positioning. Maximum indicating pointers are invaluable tools for identifying pressure spikes in a system. They can be extremely helpful during system startup and troubleshooting, but they can add an additional error to the gauge because of the increased load on the Bourdon tube.
Other accessories include set-pointers, which are used to identify an operating minimum or maximum pressure or vacuum value, and rubber case protectors which help protect gauges that are subjected to direct physical shock and vibration.
While the pressure gauge may be a seemingly simple device, it is important to consider these factors before selecting the right type and accessories to fit an application.
Stanley Wright is national sales manager and Sheryl Pritt is marketing manager for NOSHOK.