When selecting a pressure gauge, it is important to consider the following factors
to insure safety and accuracy.
Pressure Gauge Operating Principle
Gauge Types
Gauge Size
Gauge Mounting & Connection
Pressure Range
Pressure Fluid Composition
Gauge Case & Housing
Front Ring & Window
Dial/Pointer
Temperature/Ambient Conditions
Accuracy
Liquid Filled Option
Shock, Vibration, Pulsation
Chemical Diaphragm Seals/Gauge Protection
Chemical Compatibility Table
Pressure Conversion Chart
Pressure Gauge Operating Principle
Most standard dial type pressure gauges use a bourdon tube-sensing element generally made of a copper alloy (brass) or stainless steel for measuring pressures 15 PSI and above. Bourdon tube gauges are widely used in all branches of industry to measure pressure and vacuum. The construction is simple yet rugged and operation does not require any additional power source. The C-shaped or spirally wound bourdon tube flexes when pressure is applied producing a rotational movement, which in turn causes the pointer to indicate the measured pressure. These gauges are generally suitable for all clean and non-clogging liquids and gaseous media. Low pressure gauges typically use an extremely sensitive and highly accurate capsule design for measuring gaseous media from as low as 15 INWC to 240 INWC (10 PSI). Digital gauges use an electronic pressure sensor to measure the pressure and then transmit it to a digital display readout.
Gauge Types
Industrial Gauges: Industrial quality heavy-duty sensing elements and case designs, typically used in more demanding applications. These gauges usually have greater accuracy requirements than general purpose gauges. The industrial stainless steel type gauge is built for extended life and designed for harsh corrosive environments.
Commercial Gauges: Often referred to as general purpose or equipment gauges. Though commercial gauges may have to be ruggedly constructed, the service conditions are not expected to be severe. Designed for low cost without refinements, they are frequently referred to as “throw-away” gauges since it is not economical to attempt repairs on this type of gauge. Most commercial gauges have a black ABS or painted steel case with brass internal parts and connection.
Process Gauges: Process gauges combine the heavy-duty operating and construction requirements of industrial gauges with the more exacting accuracy and service life demands of many process applications. Designed for the chemical and petroleum processing industries, the cases (typically 4-1/2”) are made of corrosion resistant thermoplastic and offer a solid front/blow out back safety case design.
Digital Gauges: Industrial quality gauges with a digital display of the pressure or vacuum being measured. Digital readings are generally easier to read, faster, and more accurate than standard dial type pressure gauges. Digital gauges are typically battery powered.
Precision & Test Gauges: Dial type test gauges are cost effective, extremely sensitive and highly accurate. These gauges are excellent for instrument shops, gauge repair and calibration shops, testing laboratories and other applications demanding precision and consistent results. Test gauges feature adjustable knife-edge pointers and mirror bands on the dial to assure precise readings.
Low Pressure Gauges: Low pressure gauges utilize a capsule sensing element design and accurately measure pressures less than 15 PSIG. Typical measuring units are inches of water, mm of water, oz/sq inch, and pounds per square inch.
Specialty Gauges: Special gauges designed for specific applications.
Gauge Size
The gauge size is typically referring to the diameter of the dial (ANSI) or case (DIN) for bourdon tube or capsule pressure gauges. The dial is the viewing area of the gauge that shows the pressure scale and needle pointer. Gauges are available in a wide range of sizes from 1-1/2" up to 10". Generally, the more accurate gauges are larger and more costly.
Gauge Mounting & Connection
Lower mount (LM), center back mount (CBM), and/or lower back mount (LBM) are available for most gauge types. Most of our standard gauges have NPT threaded connections. Many gauges can be panel mounted using either a U-clamp bracket or a front flange option. Pressure gauges should be mounted in the upright position if possible.
Pressure Range
A gauge range of twice the normal working pressure is generally selected. The maximum operating pressure in all cases should be limited to 75% of the gauge range. Where alternating pressure and pulsation are encountered, working pressure should be limited to 2/3 of the gauge range.
Pressure Fluid Composition
Copper alloy (brass) or stainless steel is the standard wetted parts (materials in contact with the pressure media) available on most pressure gauges. Since the sensing element of a pressure gauge may be exposed directly to the measured medium, consider the characteristics of this media. It may be corrosive, solidify at various temperatures, or contain solids that may leave deposits inside the sensing element. For pressure fluids that will not solidify under normal conditions or leave deposits, a standard bourdon tube gauge may be acceptable. Otherwise, a sealgauge or gauge with a chemical seal/gauge protector should be used.
Gauge Case & Housing
Stainless steel, black ABS, painted steel, brass and fiberglass reinforced thermoplastic are standard case materials, Select a material suitable for your application. Stainless, brass, or ABS cases are recommended for outdoor service. Most standard gauges have a dry case (no liquid fill), while industrial and process gauges have a glycerine fill option available.
Front Ring & Window
The front ring can be made of acrylic, stainless steel, brass, steel, or thermoplastic. The viewing window is held in place by the front ring. Some rings are a permanent crimp design, while others can be removed providing dial/pointer access for recalibration or repair. Window materials are typically clear acrylic, polycarbonate, flat glass or safety glass.
Dial/Pointer
The pressure scale is printed on the face of the dial. The pointer typically moves clockwise to measure pressure and counter clockwise to measure vacuum. Some gauges offer adjustable pointers for recalibration. Test gauges offer a mirrored band on the dial and a knife-edge pointer making it easy to take very accurate readings.
Temperature/Ambient Conditions
The normal ambient temperature range for most standard dry or silicone filled gauges is –40 to +140 degrees F. The normal ambient temperature range for glycerine filled gauges is –4 to +140 degrees F. The error caused by temperature change is +/-0.3% per 18 degrees F rise or fall, respectively. The correction is for the temperature of the gauge, and not the temperature of the measured medium. Steam and other hot media may raise the safe working limits of the gauge. In these cases, syphons, cooling towers, or chemical diaphragm seals may be required. Moisture and weather effects must also be considered. Liquid filled gauges prevent condensation build up. Stainless steel, brass or plastic cased gauges are recommended for outdoor use.
Accuracy
Gauges are available with accuracies from +/-3/2/3% to 0.1% of span (ASME grade B to grade 4A) Gauges with +/-3/2/3% of span accuracy offer a +/-2% of span accuracy over the middle third of the scale and +/-3% of span on the balance of the scale. Generally, bourdon tube gauges with better accuracies are larger and more expensive.
Liquid Filled Option
Liquid filled gauges help absorb vibration and pressure spikes, increasing the service life of the gauge. The dampening action of the liquid reduces pointer flutter and lubricates the internal parts, dramatically reducing wear in the movement. Glycerine is the standard fill fluid and provides the best performance in most applications. Silicone has a low viscosity and works well when extreme temperature fluctuation or icing is expected. The fill liquid must be compatible with the gauge application.
Shock, Vibration, Pulsation
Consider liquid filled gauges to enhance the reliability and integrity of the measuring system when operating for long periods under extreme operating conditions. In addition, optional pressure snubbers will reduce pressure spikes and pulsation, pointer flutter and extend the life of the gauge.
Chemical Diaphragm Seals/Gauge Protection
Diaphragm seals are used to isolate pressure gauges and transmitters from clogging and/or corrosive media. Measurement of higher media temperatures can be obtained by adding a seal.
Diaphragm seal systems can extend the life of the pressure instruments and reduce the cost of installation and maintenance. Standard diaphragm seal bodies and diaphragms are made of stainless steel (other materials available).
Chemical Compatibility Table
Use this Chemical Compatibility Table to make sure the gauge wetted parts (parts in contact with the pressure media) are compatible with the pressure media for your application.
Pressure Conversion Chart
Use this Pressure Units Conversion Chart to convert pressure units from one standard to another.
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