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Pressure Gauges Frequently Asked Questions


Pressure gauges are critical components in the majority of commercial and industrial processing systems. These instruments are used for visual monitoring of air and gas pressure in a wealth of applications ranging from oil & gas, fluid power, power gen and industrial automation to food & beverage and general manufacturing. The pressure gauge holds an essential role in the process serving to maintain smooth operation and identify potential safety threats such as dangerously high pressure, along with system failures like low pressure.

Below are answers to some of the most commonly asked questions about pressure gauges.

Q: What is the purpose of the ventable & non-ventable fill plug/relief plug?
A: A fill plug seals the fill hole in a pressure gauge case.  On liquid filled pressure gauges, a ventable fill plug is used to relieve internal case pressures that occur due to thermal expansion of the fill fluid.  In non-filled dry gauges, a non-ventable fill plug is used to occasionally drain the interior of the case from condensate or relieve internal case pressures.  Ventable fill plugs incorporate a vent pin to open and close a hole for relieving internal case pressures and do not have to be removed from the case hole like non-ventable fill plugs.

Q: What are the designed overpressure ratings for NOSHOK gauges?
A: Overpressure ratings are specific to the gauge type, pressure range and accuracy ratings of the gauge.  Normal overpressure protection can range from 1.1X to 1.3X depending on the gauge selected.  NOSHOK gauges comply to the EN-837-1 and ASME B40.100 standards in regards to overpressure protection. When selecting a pressure gauge, it is recommended that the normal system pressure be maintained around half of the full range of the gauge as to avoid overpressure conditions.

Q: How is the accuracy of a gauge affected by a Maximum Indicating Pointer?
A: A Maximum Indicating Pointer (MIP), also commonly referred to as a Tell Tale Pointer, adds an additional error to the pressure gauge due to the increase load on the bourdon tube. The lower the pressure range, the higher the error. Typically 1%. Consult factory.

Q: What is a Certified Calibration?
A: Certified Calibrations provide the user with a serial numbered gauge along with a calibration certificate that it has been certified in accordance to the pressure gauge standard with instruments that are traceable to NIST with accuracies of at least 4 to 1.

Q: What is a Certificate of Conformance? 
A: A Certificate of Conformance is a formal statement on company letterhead stating that an instrument complies with a particular standard and/or specification.  It contains the signatures of the required personnel. These Certificates are often needed to show industry inspectors that a system and its components are in compliance.

Q: How often does a gauge need to be calibrated?
A: NOSHOK pressure gauges require little or no calibration within the Warranty period. Some applications may be more aggressive than others, resulting in an increased frequency in the need for calibration. The environmental limitations for the pressure gauge series should be observed in all cases. Gauges used in situations outside these requirements may result in inaccuracies, premature wear and/or failure of the gauge and would require additional maintenance.  The frequency of calibration, therefore, is best left to the user to determine.

Q: When is it recommended to use an orifice? 
A: Orifices are a type of snubber.  On pressure systems that have rapidly increasing or decreasing pressure spikes, orifices lessen the effects of these energy pulses by blocking the wave energy using restricted flow.  They are recommended in dynamic pressure applications with mild pressure spikes.

Q: When is a diaphragm seal used, and when would you apply a diaphragm seal and capillary?
A: A diaphragm is used to isolate and protect the instrument from the process media. Damaging process media may include corrosives, particulates, temperatures, or any state that is not suitable for direct contact with the measuring element.  Diaphragms indirectly transmit system pressures by segregating the process pressure with a thin flexible membrane that in turn transfers the pressure through a fill fluid to the instrument.  Diaphragms are often used in conjunction with capillaries to further distance the instrument from the process media.  Capillary tubes transmit the diaphragm fill fluid to the instrument. Capillary tubes come in several lengths and provide the user a means to measure in a remote location and may also act as heat dissipaters in high temperature applications.

Q: What is the purpose of liquid filling a gauge, and in what applications would a liquid filled gauge be used? 
A: Primarily, in applications that have vibrations or pulsations, liquid filling enables reading the dial pointer by dampening the movement.  Liquid filling should be considered in any system that has high dynamic operating conditions.  In general, liquid filling helps extend the life of a gauge.  It reduces damaging resonance induced fracturing, reduces frictional wear, prevents aggressive ambient air from entering, prevents condensation formation, and improves reliability. 

Q: How does temperature affect the accuracy of a pressure gauge?
A: Temperature changes affect the stiffness of a bourdon tube. The stiffness change is produced by a combination of changes in the elastic (Young’s) modulus and a change in linear dimensions due to linear expansion and contraction.  The error caused by temperature change will follow the approximate formula:
± 0.04 x (t2 – t) % of the span.

Q: How do you size a pressure gauge relative to process pressures, normal operating pressures, and maximum pressures in the process? (Dynamic or static process pressures)
A: The pressure range of a gauge should be a minimum of 10% over the maximum working pressure in static conditions (no pressure fluctuations).  In dynamic conditions, the gauge range should be a minimum of 40% over the maximum working pressure.  Ideally, the pressure gauge range should be selected for a midscale reading during normal operating pressures.

Q: What does a gauge accuracy statement really mean? (Examples: 1% of span, 3-2-3 percent of span)
A: Accuracy is the difference between the true value and the gauge indication expressed as a percent of the gauge span.  It is determined by comparing a gauge indication to a known standard or certified true value and combines the effects of method, observer, apparatus, and environment. Accuracy error also includes hysteresis and repeatability errors. An ASME B40.1 class B gauge has three accuracies. For example, a 3-2-3 percent of span designation stands for 3% in the first quarter of the scale, 2% in the middle half of the scale and 3% in the upper quarter of the scale.

Q: What applications require the various lens materials, and to what maximum temperature can each be subjected?
A: Lens materials include Instrument Glass, Laminated Safety Glass, Tempered Glass, and plastic. Glass lenses are used for abrasion, chemical and wear resistant properties. Laminated safety glass reduces the possibility of shattering if the bourdon tube ruptures.  Tempered glass is 2 to 5 times stronger than plain glass.  Plastic lenses are used for impact, corrosion and chemical resistance.  Special attention should be paid to the temperature and corrosive environments.  Polycarbonate is selected for its superior impact resistance, acrylic for its clarity and scratch resistance and Homalite for its superior chemical resistance.  In general, gauges with plastic lenses should remain below 140° F.

Q: In what situation would a pigtail syphon be used?
A: Pigtail syphons should be used in steam applications and systems that contain superheated vapor.  The pigtail buffers the instrument from the damaging effects high temperature steam by holding system fluid in the coil to provide a steam trap for the fluid to condensate and dissipate the heat.

Q: What is the application for a gauge cleaned for O2 service?
A: Cleaning for Oxygen (O2) service is performed on gauges that are used on oxygen service or oxidizing media applications.  The cleaning removes all hydrocarbons (oil and grease are common hydrocarbons) that can react violently, resulting in explosions, fire, and injury to personnel and property.  Gauges cleaned for Oxygen service can be used in any application that requires the cleanliness level associated with oxygen cleaned gauge.  Glycerin filled gauges cannot be used on oxygen systems or on other oxidizing media.

Q: What fill fluids options are available, and in what applications would each be used?
A: Glycerin is the most common fill fluid.  Because of its unique fluid properties, Glycerin has become the standard for liquid filled gauges (see “What is the purpose of liquid filling a gauge?”). Glycerin’s clarity, viscosity, stability, cost, solubility, low toxicity make Glycerin an ideal fluid for many applications.  Mineral oils and silicone fluids are used when temperature extremes, chemical compatibility or viscosity fall outside of Glycerin use.  Halocarbon is an inert fluid that is compatible with chlorine, oxygen service some high temperature applications.  Keep in mind that Glycerin is not compatible with strong oxidizers such as oxygen, chlorine, hydrogen peroxide, or nitric acid. Glycerin & Silicone are explosive in contact with chlorine. Halocarbon is explosive in contact with aluminum and magnesium.

Q: What is the difference between the ASME B40.1 and EN 837-1 specification? 
A: The American National Standards Institute (ANSI) approves American National Standards which include the American Society of Mechanical Engineers (ASME) standard ASME B40.100. This Standard (B40.100) is confined to analog, dial-type gauges, which, utilizing elastic elements, mechanically sense pressure and indicate it by means of a pointer moving over a graduated scale. The European Committee for Standardization (CEN) is the officially recognized European standards body that develops European Standards (ENs) which include EN 837-1. The EN 837-1 includes mandatory dimensions, metrology, and testing requirement for sale in the European Union. ASME B40.100 includes similar requirements in a non-mandatory appendix.

Q: What is the purpose of throttle devices such as throttle plugs and screws?
A: Throttle devices limit the flow to the pressure instrument.  They are a type of snubber.

Q: What is the purpose of an over and under load stop in a pressure gauge? 
A: The tip motion of a bourdon tube is translated to rotary motion of a pointer by a linkage and sector gear acting on the pointer pinion gear.  Stop pins limit the movement of the bourdon tube, sector or pointer rotation in over and under pressure conditions that would otherwise move the pointer pinion off the sector gear which would damage the gauge.

Pressure Gauge System Detailed View of Gauge Movement


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