Seven steps to pressure gauge selection
April 28, 2004
Pressure gauges have been a reliable source of accurate pressure measurement for more than 100 years. The operation and design of pressure gauges dates back to the mid 1800s when Eugene Bourdon invented the Bourdon tube. This same principle is used today, albeit with enhancements to last longer and resist corrosion.
Many factors affect proper gauge selection, and if you follow the seven-step process, the task can become simpler and more exact.
Step 1 — Process media
While all the factors regarding proper pressure gauge selection are important, consideration for the actual process media that the gauge will be exposed to is vital. Since the pressure-sensing element most commonly used in a typical pressure gauge is a thin-wall Bourdon tube, materials must be selected to prevent corrosion.
Restrictions do exist in selecting Bourdon tube materials. The materials used must be limited to those that provide good spring properties; because the tube will be required to coil and uncoil. This function has to happen without the tube losing its preset memory.
Step 2 — Process pressure
It is important to know if the process pressure is static or dynamic in nature. If a pressure gauge sees a constant pressure continuously, then typically, that pressure gauge will last for years without degradation of performance. However, most pressure gauge applications are dynamic in nature, and will cause gauge failure unless appropriate measures are taken. The system operating pressure generally determines the selection of the proper pressure gauge range. The pressure gauge range should be twice the normal operating pressure.
Step 3 — Environment
We need to be aware of the conditions surrounding the pressure gauge such as vibration, ambient temperature, airborne dust and chemicals. Vibration can cause excessive movement wear, which results in pressure gauge failure. When vibration is present it becomes difficult to accurately read the pressure gauge, because the pointer is rapidly oscillating. When the ambient conditions are corrosive or dusty, hermetically sealed and/or liquid-filled pressure gauges should be specified.
Step 4 — Accuracy
Accuracy is the conformity of a pressure gauge reading to an accepted standard. Inaccuracy is the difference between the true value and the indication expressed as a percent of the span. Total accuracy error includes hysteresis and repeatability errors. It is important to know that accuracy is not a percentage of the gauge reading. Grades of accuracy are provided in ASME B40.1; they range from grade D (± 5.0 percent) to grade 4A, (±0.1percent) of full scale.
Step 5 — Size
Dial sizes of pressure gauges range from 1-1/2" to 16" diameter. The dial size of a pressure gauge is generally determined by readability requirements. If the gauge is remote from the operator, a larger, more legible dial is necessary. For gauges mounted on a panel close to an operator, smaller dial sizes are often acceptable. More accurate pressure gauges generally have larger dials, since more dial graduations are needed to read the higher degree of accuracy.
Step 6 — Connections
Generally in North America, 1/4 and 1/2 NPT tapered pipe threads are the most common connections used for pressure gauges. Proess pressure gauges with 4-1/2" dial sizes or larger are most often supplied with a 1/20 NPT connection. This is particularly true when these gauges are stem-mounted directly and liquid-filled, because the larger size gauge socket will support the pressure gauge better.
Step 7 — Mounting
Pressure gauges are mounted in directstem mount lower or back connect, remote wall/surface mount lower connect, panel surface mount back connect, panel hole U-clamp flush mount back connect, and panel hole front flange flush mount back connect. While the majority of pressure gauges are supplied with lower connections for stem mounting, it is still important to know how a pressure gauge will be mounted when installed.
For more information, contact Dresser Instruments at 800-328-8258 or visit www.ashcroft.com.