Glossary

A


304 Stainless Steel
Grade 304 is the standard "18/8" stainless; it is the most versatile and most widely used stainless steel, available in a wide range of products.
316 Stainless Steel
Grade 316 is the standard molybdenum-bearing grade. The molybdenum gives 316 better overall corrosion resistant properties than Grade 304, particularly higher resistance to pitting and crevice corrosion in chloride environments.
3A
A symbol of the 3A Sanitary Standards Symbol Administrative Council approving the construction and use of food processing equipment
Absolute pressure
Pressure measurement scale used in physics and engineering calculations referenced to zero (0) at full vacuum, where zero represents a total absence of pressure. Usually denoted on a pressure measurement scale by the suffix “a.” (See “psia.”)
Accuracy
The maximum difference allowed between the true value and the instrument indication, usually expressed as a percentage of the full measurement span of the instrument.
Accuracy Grades
Accuracy is the difference (error) between the true value and the indication expressed as percent of the span. ASME B40.1 and DIN accuracy grades are: Grade 4A (0.1% Full Scale) Grade 3A (0.25% Full Scale) Grade 2A (0.5% Full Scale) Grade 1A (1% Full Scale) Grade A (1% middle half, 2% first and last quarters) Grade B (2% middle half, 3% first and last quarters) Grade C (3% middle half, 4% first and last quarters) Grade D (5% Full Scale) With regard to measurement accuracy (% of full scale), in cases where the accuracy differs between middle span and the first and last quarters of the scale, the largest % error is reported.
ASME B40.1
A voluntary industry standard developed by the American Society of Mechanical Engineers covering the construction and use of pressure gauges
ASME B40.2
A voluntary industry developed by the American Society of Mechanical Engineers covering the construction and use of diaphragm seals.
Atm
Abbreviation for atmosphere: a unit of pressure measurement.
Atmospheric pressure
The ambient air pressure produced by the weight of the earth’s atmosphere: 14.696 psia (29.90” Hg) at sea level with standard weather conditions.

B


Ball valve
A ¼ turn valve that incorporates a ball-shaped rotating valve plug
Bar
A unit of pressure measurement
Bimetal
Two strips of different metals bonded together and held at one end. When heated, the two strips expand at different rates, resulting in a bending effect that is used to measure the temperature change. Blowout Back – already part of glossary page.
Bleed screw
A hole for venting air from a hydraulic system during startup, closed by a screw plug
Blow-out back
A design feature on safety pressure gauges in conjunction with sold front for improved user safety; if the instrument fails, the process media should exit through the rear of the instrument case, protecting the operator from harmful chemicals or high exit velocities.
Bore Sizes
Inside diameter size of thermowell
Bourdon tube
The sensitive element common to most pressure gauges, it is made from a partially flattened tube, curled and closed at one end. The final form is most often a partial turn or “C” shape. Other forms are multi-turn into a helix or spiral shape. As pressure enters the open end, the tube begins to straighten and expand. This motion is transmitted to the dial indicator through a mechanical movement.
BSPT
British Standard Pipe Taper: a British thread standard.
Buna N Seals
Synthetic rubber seals used as a sealing gasket in diaphragm seals, pressure switches and pressure gauges
Butt Weld Connections
Connections that are made when two similar metals are welded together. Buttwelds are achieved when two pieces of metal are positioned coplanar and touching on one edge. The two pieces are fused at the touching edge.

C


Capillary tube bleeder
A tube for venting air from a hydraulic system during startup, closed by a screw plug. (See “bleed screw.”)
Case
Cases are available in a wide variety of materials and configurations. The combination of material and configuration is generally determined by the demands of the application. The more demanding the environment the more rugged the pressure gauge case construction needs to be. For example, polypropylene or stainless steel cased pressure gauges are used for industrial applications and aluminum or steel for construction and commercial applications. Gauge mounting or retrofitting needs may affect case selection (i.e., a gauge to be panel mounted will require either a front flange or u-clamp style case). For safety considerations, a “solid-front” case style may be required. Each application will have a unique set of requirements which will help guide in selecting the appropriate case style.
Celsius
A temperature measurement scale calibrated to 0°C at the freezing point of water and 100°C at the boiling point of water at sea level.
Clamped Diaphragm
Diaphragm seal assembly that is constructed of a upper & lower housing, two o-rings, and a diaphragm clamped together. The design allows for the diaphragm to be replaced while the pressure instument remains intact. Case style used to permanently seal liquid filled & weatherproof gauges
Clean-out
A design feature on diaphragm seals allowing removal of the instrument housing form the process housing. This feature facilitates the cleaning of the internal parts of a diaphragm seal.
cm H2O
Abbreviation for centimeters of water: a unit of pressure measurement.
cm Hg
Abbreviation for centimeters of mercury: a unit of pressure measurement.
Coil syphon
A coiled length of pipe that acts as a water trap to disperse excess heat; a coil syphon is usually used on a dead-ended steam line to protect pressure instruments from exposure to excessive heat transfer
Connection
The socket connection provides an entrance port for the process medium as well as a means to mount the gauge to a pipeline or vessel. Male connections can be provided with NPT, BSPT, or other thread styles, in sizes from 1/8" through 1/2".  A high-pressure, 9/16" LHT Autoclave connection is also available. The mounting location indicates where the connection protrudes from the case.
Crimped Ring
Case style used to permanently seal liquid filled & weatherproof gauges

D


Degree
A single unit of measure on a temperature scale.
Dial-face
The instrument component that contains a printed, graduated scale on which a pointer will indicate a measurement reading.
Diaphragm
A thin, non-porous membrane that is capable of flexing in response to pressure or force.
Diaphragm capsule
A thin walled, convoluted, elastic pressure sensing element that will change length with pressure changes; this motion is transmitted to the dial indicator through a mechanical movement.
Diaphragm seal
A protective device used to isolate a pressure sensing instrument from the process fluid being measured.
Differential Pressure
The difference between two pressure measurements. In general, a measurement of fluid force per unit area (measured in units such as pounds per square in.) subtracted from a higher measurement of fluid force per unit area.
Duplex Pressure
A duplex pressure gauge is designed with two independent pressure systems with a special movement that drives one red pointer and one black pointer to display two pressure readings on the dial.

E


Electric contact
The working mechanical components of an electrical switch, operated by the pointer of an instrument
Environmental Conditions (Pressure Gauge)
Pressure gauge selection will be influenced by the environmental conditions under which the gauge is expected to perform. Condensation or waterproofing concerns can be addressed through the use of hermetically sealed or weatherproofed gauges. Case construction should be considered when selecting a gauge to be used in conditions of extreme or sustained heat or cold, or where the atmospheric environment may contain corrosive elements.
Escapements
The device within the movement that converts continuous rotational motion into an oscillating or back and forth motion of the pressure gauge needle.

F


Fahrenheit
A temperature measurement scale measuring 32°F at the freezing point of water and 212°F at the boiling point of water at sea level.
Field Refillable
Liquid Filled pressure gauges that can be refilled in the field if needed
Figure interval
The number of units measure between adjacent numeric graduations on a dial-face.
Fill port
A connection on the instrument housing of a diaphragm seal providing access to the area above the diaphragm.
Flushing connection
An optional port on the process housing of some diaphragm seals designed for back-flushing of contaminants or solids from the process system.
FMB
An instrument mounting style: front flanged, panel mounted case, with back connection.
FML
An instrument mounting style: front flanged, panel mounted case, with lower connection.
Friction pointer
A pointer style that allows for zero adjustment by manually rotating the pointer on the pointer shaft of the instrument
FSB
An instrument mounting style: flangeless, stem mounted case, with back connection.
FSL
An instrument mounting style: flangeless, stem mounted case, with lower connection.
Ft H2O
Abbreviation for feet of water: a unit of pressure measurement.

G


Gauge cock
A ¼ turn plug valve used for isolating a pressure instrument form the process in non-hazardous, non-critical applications.
Gauge pressure
Pressure measurement scale most commonly used in industry referenced to zero (0) at ambient atmospheric pressure. Usually denoted on a pressure measurement scale by the suffix “g.” (See “psig”)
Graduations
The individual indication markings on a dial-face or measuring scale.

H


Hg
Symbol for the element mercury on the Periodic Table of Elements
HRB
An instrument mounting style: hinged ring, panel mounted case with back connection.
HRL
An instrument mounting style: hinged ring, panel mounted case with lower connection.
Hydraulic
Force that is applied at one point is transmitted to another point using an incompressible fluid.

I


Impulse dampener
A device that slows the rate of transmission of a pressure surge through a flow path
in
Abbreviation for inches: a unit of linear measurement.
in H2O
Abbreviation for inches of water: a unit of pressure measurement.
in Hg
Abbreviation for inches of mercury: a unit of pressure measurement.
Index pointer
An adjustable indicator on the instrument window to identify a predetermined reading limit. (See “red set hand.”)
Instrument housing
The non-wetted, upper portion of a diaphragm seal to which the pressure instrument is attached. Sometimes known as “upper housing” or “instrument flange.”

K


Kg/cm^2
Abbreviation for kilograms pre square centimeter: a unit of pressure measurement.
kPa
Abbreviation for kilopascals: a unit of pressure measurement

L


Liquid fill
A fluid used to fill the case of pressure gauge or the internals of a diaphragm seal and pressure gauge assembly.
Liquid Fillable
Dry pressure gauges that can be filled with liquid (usually Glycerine or Silicone). Liquid filling absorbs vibration and pressure spikes, lessening the shock effects of the application. In addition, the liquid lubricates internal moving elements dramatically reducing wear on the movement.
Low pressure gauge
A style of pressure gauge for measuring small pressure changes. Usually constructed with a diaphragm capsule or bellows as the sensitive element.

M


mA
Abbreviation for milliamperes: a unit of electrical flow measurement.
Maximum registering pointer
An indicator on the instrument window that follows the measuring indicator in one direction only, remaining at the highest reading. Also may be used as a minimum registering pointer.
Mbar
Abbreviation for millibars: a unit of pressure measurement
m H2O
abbreviation for meters of water: a unit of pressure measurement
Micrometer pointer
A friction pointer using gears as the means of adjustment
Minor division
The number of units of measure between adjacent graduation markings on a dial-face
mm
Abbreviation for millimeters: a unit of linear measurement.
mm H2O
Abbreviation for millimeters of water: a unit of pressure measurement
mm Hg
Abbreviation for millimeters of mercury: a unit of pressure measurement
Movement
The instrument component that mechanically translates the motion of the sensitive element into the rotary motion of the pointer
MPa
Abbreviation for MegaPascals: a unit of pressure measurement

N


Needle valve
A type of valve that maintains a desired flow by incorporating a tapered rising valve plug. Usually used to isolate a pressure instrument
Non-cleanout
A type of diaphragm seal that cannot be disassembled without being refilled and recalibrated. (See “clean-out”.)
Non-wetted part
A component not in contact with the process media
NPT
National Pipe Taper: n American thread standard

O


OEM
Original equipment manufacturer
O.D.
Outside Diameter
Organic Blue Spirt Fill
Standard fill fluid for Trerice Industrial Thermometers
Output signal
The resulting voltage, amperage or pressure from a measurement or control device, often used to operate another instrument or control valve.
Overload stop
An optional pressure gauge feature intended to prevent loss of calibration in the event of minor gauge over-pressuring
oz/in^2
Abbreviation for ounces per square inch: a unit of pressure measurement

P


Pneumatic
Worked by or filled with compressed air or other similar gases
Pointer
Three of the most common types of pressure gauge pointers are: a micro-adjustable pointer (providing high quality and accuracy); a friction-type pointer (providing adjustability and durability); and a plain, non-adjustable pointer (providing economic reliability).
Pointer jack
A tool for removing the pointer from an instrument without causing damage to the pointer or instrument
Powerflex Movement
Ashcroft’s Patented Pressure movement. The suspended construction isolates the gauge movement from the socket, extending the gauge life
Pressure
A measure of force distributed over a surface area
Pressure gauge
A mechanical, dial type instrument for measuring pressure, usually incorporating a bourdon tube or diaphragm capsule as the sensitive element
Pressure relief plug
A plug in a hole in an instrument case designed to relieve slowly developed, low internal case pressure preventing further pressure buildup
Pressure snubber
A device that slows the rate of transmission of a pressure surge through a flow path
Pressure transmitter
A pressure measurement device designed to convert a measured value to an electric, pneumatic or mechanical signal. Primarily used with a remote indicator or pressure controller.
Pressure Transducers
An instrument that converts pressure into an analog electrical signal. Applied pressure produces a deflection of the diaphragm which introduces strain to the gages. The strain will produce an electrical resistance change proportional to the pressure.
Process housing
The wetted lower portion of a diaphragm seal by which the seal is mounted onto the process. Sometimes known as “lower housing” or “process flange.”
Process Temperature Control Systems
A process management system designed to maintain a process temperature at a desired set point. Once a set point has been established, the system works to maintain the process at the desired temperature utilizing a pressure sensor, temperature controller, and a control valve.
psi
Abbreviation for pounds per square inch: a unit or pressure measurement
psia
Abbreviation for pounds per square inch of gauge pressure: a unit of pressure measurement

R


Range
The low and high limits of a measurement scale
Reaumur
A temperature measurement scale measuring 0°R at the freezing point of water and 80°R at the boiling point of water at sea level.
Recalibratable
Pressure gauges that can be repaired, and recalibrated after being put into service.
Red set hand
An indicator permanently affixed to the instrument dial-face to identify a predetermined reading limit. (see “index pointer.”)
Restrictor
A component that is pressed or threaded into the pressure gauge socket that slows the rate of transmission of pressure through the flow path by utilizing a narrow orifice
Ring
The exterior component on a gauge that attaches the window to the case

S


Safety pressure gauge
A pressure gauge type designed to minimize or prevent injury to an operator in the event of failure of the internal pressure-containing components. (See “solid front.”)
SMB
An instrument mounting style: surface mounted, back flanged case, with back connection.
SML
An instrument mounting style: surface mounted, back flanged case, with lower connection.
Snubber screw
A threaded component screwed into the pressure gauge socket that slows the rate of transmission of pressure through the flow path
Socket
The component of a pressure gauge consisting of the connecting threads, and attached internally to the bourdon tube
Socket Weld
A thermowell that is welded between its outside diameter and the fitting end of the pipe or pressure vessel.
Solid front
A design feature of a safety pressure gauge which incorporates a separating barrier between the dial-face and bourdon tube. In the event of failure of the pressure-containing components, the barrier should direct the force of any process media away from the operator. (See “safety pressure gauge.”)
Span
The difference between the high and low limits of the measuring range of an instrument
SS
Abbreviation for Stainless Steel

T


Test plug
A re-sealable port in a process line through which test probes may be inserted for temporary measurements
Thermowell
Acts as a barrier between the process medium and the sensing element of a temperature measuring device. It protects the temperature sensor from corrosive, high pressure, and high velocity processes.
Torque
Measures how hard something is rotated
TRB
An instrument mounting style: turret style, surface mounted, with back connection
TRL
An instrument mounting style: turret style, surface mounted, with lower connection
True Zero Indication
Ashcroft commercial gauge feature that helps assure safety and a quality pressure reading by not utilizing a stop pin on the dial.
Tube
See “bourdon tube.”

U


U-Clamp
A “U” shaped component and attachment hardware for mounting a gauge case into a panel cutout without using bolt holes in the panel.
UCB
An instrument mounting style: U-clamp, panel mounted case, with back connection.
UL Listed
Tested and listed by the Underwriters Laboratories, Inc.
Unit of measure
Smallest integer on a measurement scale for which the scale is named
USDA
United States Department of Agriculture: a government organization responsible for developing the inspection standards of agricultural products.

V


Vacuum
Pressures below ambient atmospheric pressure
Vapor Actuated
Vapor Actuated Dial Thermometers are noted for their economical cost and excellent speed of response. The physical principles of vapor actuation require that the dialface beprinted with a nonlinear, progressively graduated temperature scale. These instruments are available for direct mounting, or for remote mounting with capillary lengths up to 100 feet. Vapor Actuated Thermometers are available in temperature ranges up to 450°F (232°C).
Vdc
Abbreviation for volts direct current: a unit of electromotive force measurement

W


Weatherproofed case
A sealed, environmentally resistant instrument case
Wetted part
Under normal operating conditions, only the tube and socket assembly (Wetted Parts) of a pressure gauge will come into contact with the fluid being measured (Pressure Medium). The selection of the assembly will be determined by the composition of the medium. Air, gas, steam, water and other noncorrosive media are usually satisfied by a bronze or brass bourdon tube and brass socket assembly. Stainless steel or Monel wetted parts are used when the medium contains corrosive elements or when high operating pressures or temperatures will be encountered. A diaphragm seal is recommended for highly corrosive media or that which may solidify or deposit solids within the tube and socket assembly of the gauge.
Window
The window provides a means for viewing the measurement, as well as protection of the dial face and pointer of the instrument, and is normally held in place by a ring, which is screwed or snapped to the case of the gauge. Plastic, clear glass and laminated glass are common window materials. Ring styles include threaded, bayonet, friction, crimped and hinged, depending upon the case type. 

Z


Zero adjustment
The act of realigning the pointer with the dial-face graduations, an aspect of the calibration procedure
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