Pressure Gauge Selection

 

Process Medium The process medium to which the gauge will be exposed is especially important when using a thin walled Bourdon tube because, if the wrong materials are selected, corrosion may occur which could lead to catastrophic failure. Materials which display the essential combination of properties (good spring memory, easy to form, easy to join, reasonably priced) are phosphor bronze, 316 stainless steel and Monel. Where these materials cannot satisfy the application, a diaphragm seal (gauge isolator) can be added to prevent the process media from contacting the Bourdon tube. This protects the gauge from corrosion attack, and also prevents viscous or dirty media from clogging the small bore Bourdon tube. The only limitation in using a diaphragm seal is that it typically degrades the accuracy of the pressure gauge by an additional 0.5% of the full scale deviation. Pressure Gauge Range It is important to select a pressure range which accommodates all anticipated pressure swings, and which prevents excessive pointer movement. It is recommended to confine normal operating pressure to 25% - 75% of scale. With fluctuating pressures (e.g. pulsation caused by a pump or compressor), the maximum operating pressure should be lower (50% of the full range). Gauges intended for severe service conditions may be liquid filled and restricted to reduce Bourdon tube stresses. To minimise sensing element stress and to extend the life of the gauge use internal restricton screws, pulsation dampeners, pressure snubbers, gauge savers or diaphragm seals. The Environment Temperature changes cause the elastic modulus of the Bourdon tube to indicate higher pressure than actual as temperature increases, (lower as temperature decreases), except if made with expensive constant modulus materials. All Rototherm pressure gauges are calibrated at an ambient temperature of 20ºC ±3ºC. Where ambient conditions are corrosive or contain a large number of particles, specify hermetically sealed and / or liquid filled pressure gauges to prevent foreign elements from entering the case. Vibration can cause wear to the gears of the rotary movement and can make it difficult to accurately read pressure with an oscillating pointer. Filling a gauge with dampening fluid, such as glycerine, helps prevent these problems. Accuracy Accuracy is the conformity of a pressure gauge reading to an accepted standard (e.g. deadweight tester). Inaccuracy is the difference (error) between the true value and the indication, expressed as a percent of the span. It includes the combined errors of method, observer, apparatus and environment. Total accuracy error includes hysteresis and repeatability errors. Accuracy is not a percentage of the gauge reading - for mechanical pressure gauges, accuracy is a percentage of the full range, full scale or span of the gauge. Guidelines are: • Test Gauges (0.25%) • Critical Processes (0.5%) • General Industry Processes (1.0%) • Less Critical Commercial Uses (2.0%) Dial Size Sizes range from 63mm (2.5”) to 300mm (12”) diameters, with the 63mm (2.5”), 100mm (4”) and 150mm (6”) being the most popular. The dial size is generally determined by the readability requirements (larger for remote reading and smaller where the gauge is close to the operator). More accurate pressure gauges generally have larger dials as more dial graduations are needed to read the higher degree of accuracy. Connections Factors to consider include gauge pressures, size and weight, space limitations, leak integrity and past experience. 150mm (6”) and 100mm (4”) process gauges usually have 1/2" BSP or NPT connections, especially when direct stem mounted and liquid filled. Smaller dial sizes generally have 1/4" or 1/8" connections. High pressure and cone seal type connections can also be supplied. Mountings Pressure gauges may be: • Direct stem mount bottom connection • Wall -surface mount bottom connection • Panel surface mount back connection • Panel hole U clamp (yoke) flush mount back connection • Panel hole front flange flush mount back connection

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