"Glossary" for Measurement

Accuracy, Percent (Meter)
The percent accuracy of a gas meter is the ratio of the volume registered by the meter divided by the volume registered by the standard, expressed as a percentage. Thus, a percentage of registration (accuracy) that is less than 100.0% indicates the meter is "slow" - i.e., the meter is indicating a smaller quantity than it should.

      % Accuracy = (Volume registered by the meter) x 100
                             (Volume registered by the standard)

Accuracy, Numerical

1. The degree of freedom from error; that is, the degree of conformity to truth. Accuracy is contrasted with precision where, for example, four-place numerals are less precise than six-place numerals.

2. A number or quantity which defines the limit of error under reference conditions Unless otherwise specified, accuracy is defined as that in effect under reference conditions. Accuracy usually includes the combined conformity, hysteresis, and repeatability errors.

The following are examples of accuracy.

1.Accuracy expressed in output units. For example, the accuracy is + 10F or + 1% of actual output units.

2.Accuracy expressed in percent of output span. For example, the accuracy is + 1/2% of output span.

3.Accuracy expressed in percent of full scale. For example, the accuracy is + 1/2% of full scale.

4.Accuracy expressed in percent scale length. For example, the accuracy is + 1/2% of scale length.

  Percent Accuracy = Indicated Value x 100
                                 True Value

Actual Cubic Feet/Hour (A.C.F.H.)
The actual volume of measurement chamber or volume under actual flowing conditions. Basically, the meter index reading or difference in meter index readings without P, T, or s corrections.

Adaptor Plate, Instrument
A specially designed plate mounted between a meter and an instrument to provide for a proper drive to the instrument.

Adiabatic
A term indicating that no heat is lost or gained by a material being subjected to a thermodynamic process.

Ambient Pressure
The pressure of the medium surrounding a device. Usually the local surrounding atmospheric pressure.

Ambient Temperature
The temperature of the air, atmosphere, or other fluid that completely surrounds the apparatus, equipment, or workplace.

Anemometer
An instrument for measuring the force or velocity of the wind or a wind gage. Any device for directly measuring velocities of air streams.

Atmospheric Pressure (See Figure 7 under "Pressure, Absolute")
The pressure of the weight of air and water vapor on the surface of the earth. The average atmospheric pressure at sea level for scientific purposes has been defined as 14.696 pounds per square inch absolute. A 0.15 psia change in actual atmospheric pressure will affect low pressure ("w.c.") metered volume by approximately 1%.

Auxiliary Devices
Devices used with a meter to provide an adjustment of the meter reading to permit obtaining special information, or to transmit information to a remote location.

Badge, Meter
A permanent plate, affixed in a conspicuous place on a meter, containing basic meter information such as meter maximum allowable operating pressure (MAOP) limits, meter type, meter capacity at 1/2" and 2" WC differential pressure, manufacturer name and logo, and meter serial number.

Balance Cam
The upper part of a bell prover which balances the main and secondary counterweights with the prover bell. Also referred to as the Cycloid arm.

Ball Valve
A shut-off or pressure regulator with an inter-valve ball of resilient material designed to seat on the inner port to provide a positive seal against flow.
Bar
A unit of pressure equal to 105 Newtons/meter2 (approximately 14.5 psi).

Barometer
An instrument used for measuring atmospheric pressure.

Base Conditions
The ANSI Z132 has established 600F and 14.73 psia as the base temperature and pressure to which all volumes are commonly referred.

Base Pressure
The pressure used as a standard in determining gas volume. Volumes are measured at operating pressures and then corrected to base pressure volume. Base pressure is normally defined in any gas measurement contract. The standard value for natural gas in the United States is 14.73 psia, established by the American National Standards Institute as standard Z-132.1 in 1969.

Base Pressure Index
A device which continuously and automatically compensates to correct gas volume at operating pressure to volume at base pressure, without regard for any correction for temperature.

Base Temperature
An arbitrary temperature to which measurements of a volume of gas is referred. The standard value in the United States is 600F (5200R) for natural gas. This was established be the American National Standards Institute as standard Z-132.1 in 1969.

Basic Air or Gas Time
The time required to pass one cubic foot of air or gas through a given orifice in a flow prover at stated base conditions. This time is stamped on the prover orifice in seconds.

Bell Prover (See Figure 1.)
A calibrated cylindrical bell in which a quantity of air is collected over an oil seal. The prover bell is connected to a meter under test and the quantity of air that passes through the meter as measured on the meter index is compared with the change in quantity of air entrapped within the bell to determine meter accuracy. In pressure proving, the prover air is at a positive pressure with respect to atmospheric and passes out of the bell through the meter. In vacuum proving, the prover pressure is below atmospheric pressure, and the air is drawn through the meter into the bell.

Bias
The amount of shift applied to the meter proving results from one proving room to achieve a given result at the receiving proving room. Usually expressed in tenths of a percent.

Bottle, Cubic Foot
A specially constructed device for calibrating bell provers. The bottle is designed to displace exactly one cubic foot of air when immersed in a tank containing light oil.

Bottling Method
The use of an immersion-type cubic foot bottle apparatus to determine the accuracy of a bell prover by transferring a cubic foot of air between the certified cubic foot bottle or portable standard and the prover. The bottling method is recognized and accepted by the gas industry as the basis for all calibrations for displacement gas meters. When properly conducted, and under ideal conditions, can yield accuracy within + 0.1%.

Bourdon Tube
An arc-shaped, spiral, or helical metal tube that is approximately elliptical in cross section and has one end attached to a pressure indicating, controlling, or recording device, while the other end is fixed. Pressure changes inside the tube affect its shape and actuate the device to which it is attached.

Boyle's Law for Gases
At a constant temperature, the volume of a given quantity an any gas varies inversely as the absolute pressure changes. 

 Volume at P2 = absolute pressure one
 Volume at P1   absolute pressure two

Or:	   P1 x V1 = P2 x V2

British Thermal Unit (BTU)
Quantity of heat necessary to raise the temperature of one pound-mass of water one degree Fahrenheit from 58.50F to 59.50F under a standard pressure of 30 inches of mercury at 320F. Natural gas averages 1000 BTU per cubic foot, so divide the BTU rating of an appliance by 1000 to obtain the cubic foot rating.

Butterfly Valve
A throttling valve, found on bell provers, made up of a disc that rotates on an axis within the valve body, thereby varying the cross-section that is open to fluid passage.

Calibrate

1. To ascertain, usually by comparison with a standard, the locations at which scale or chart graduations should be placed to correspond to a series of values of the quantity which the instrument is to measure, receive, or transmit.

2. To adjust the output of a device, to bring the desired value, within a specified tolerance for a particular value of the input.

3. To ascertain the error in the output of a device by checking it against a standard.

Calorimeter
An apparatus for measuring the heating value of a fuel.

Capacity Rate
The rate of flow (in cubic feet or cubic meters) of a liquified petroleum gas vapor-measuring device with a pressure drop across the meter of 0.5 inch WC or 2.0 inches WC depending on the rated capacity of the meter. (See Open Rate)

Certification (Prover)
A means to verify the accuracy of gas meter proving equipment. The bell prover is a secondary standard which has two accepted methods for determining its accuracy.

1. Bottling Method, immersion-type cubic foot bottle
2. Strapping Method, volume of provers by measurement

Charles' Law
The volumes assumed by a given mass of gas at different temperatures, the pressure remaining constant, are, within moderate ranges of temperature, directly proportional to the corresponding absolute temperatures. 

	Volume at temperature one = Absolute temperature one
	Volume at temperature two   Absolute temperature two

Or:			V1/T1 = V2/T2
Check Rate
A rate of flow that represents a certain percentage of the badged capacity rate of the meter. Utilities typically use 20% or 35% of meter capacity as a check rate.

Circle, Test
A graduated index circle with a hand for testing the meter.

City Gate Station
The point or measuring station at which a gas distribution utility receives gas from a pipeline or transmission company.

Compressibility
The property of a material which permits it to decrease in volume when subjected to an increase in pressure. In gas measurement, the compressibility factor "Z" is the deviation from the ideal gas law behavior. (See Supercompressibility.)

Condensable
A gas or vapor that can be converted to a liquid with changing conditions of temperature and/or pressure.

Condensate
The liquid resulting when a vapor is condensed by cooling or application of pressure.

Conformity
The closeness to which a curve approximates a specified functional curve.

Connections, Meter (See "Swivel, Meter")
The integral parts of a meter designed for attachment to piping or piping components.

Correction
The difference between the true value and the indication of the measured value. 

     Correction = True Value - Indication

Counterweight
The amount of weight necessary to balance a prover bell such that the pressure of the air within in the bell is constant, irrespective of the bell position during its travel.

Correlation
A factor, but not a bias, that serves as a means of aligning a production prover to a gas company reference standard.

Critical (Sonic) Flow
A limiting flow condition in nozzles. With a constant high pressure upstream, a further decrease in outlet pressure does not produce any increase in the mass rate of flow. The ratio of inlet to outlet absolute pressure at this limiting flow rate is termed the critical pressure ratio and is approximately 2:1. The condition for maximum mass flow rate is also the condition for maximum throat velocity and the speed of sound at throat conditions.

Critical Temperature and Pressure (See Table 1.)
That temperature above which a gas cannot be liquified by pressure alone. The pressure under which a substance may exist as a gas in equilibrium with the liquid at the critical temperature is the critical pressure.

Cubic Foot
The amount of gas required to fill a volume of one cubic foot under stated conditions of temperature, pressure and water vapor.

SCF = Standard Cubic Foot (One cubic foot of gas at standard conditions, i.e. 14.73 psia and 60oF without adjustments for water vapor)

DCF = 10 Cubic Feet (Multiply by 10)

CCF = One Hundred Cubic Feet (Multiply by 100)

MCF = One Thousand Cubic Feet (Multiply by 1,000)

DMCF = Ten Thousand Cubic Feet (Multiply by 10,000)

MMCF = One Million Cubic Feet (Multiply by 1,000,000)

Cubic-Foot Bottle
A specially constructed and calibrated metal bottle open at the lower end and so supported that it may be easily raised or lowered in a tank which contains a sealing medium. The bottle, when lowered, will displace exactly 1 cubic foot of air upon coming to rest on the bottom of the tank. The calibration of the bottle must be traceable to the National Institute of Standards and Technology (N.I.S.T.).

Cubic Foot, Metered
The quantity of gas that occupies one cubic foot under flowing conditions in the meter.

Dekatherm
A unit of heating value equal to 1,000,000 B.T.U.s.

Densitometer
An instrument used for measuring density.

Deviation, Percent 

		Q (measured - Q (standard) x 100 = Percent Deviation
                         Q (Standard)

Dew Point
The temperature at which a vapor begins to condense and deposit as a liquid.

Diaphragm
A flexible membrane separating two chambers.

Diaphragm Meter (See Figure 2.)
A gas meter in which passage of gas through two or more chambers moves flexible diaphragms that are geared to a volume indicating dial.

Differential Pressure
The pressure difference between two points.

Diluent
Usually a neutral fluid added to another fluid to reduce the concentration of the second fluid in a mixture.

Discharge Coefficient
The ratio of the actual rate of flow of a fluid through a meter to the rate computed by a theoretically derived equation or by empirical equation.

Drift
An undesired change in output over a period of time, which the change being unrelated to input, operating conditions or load. Drift is usually expressed as the change in output over a specified time with fixed input and operating conditions.

Enthalpy
A thermodynamic property of a substance defined as the sum of its internal energy plus the quantity Pv/J, where P = pressure of the substance, v = its volume, and J = the mechanical equivalent of heat. 

          H = Internal Energy + Pv/J
Entropy
The ratio of the heat added to a substance to the absolute temperature at which it is added. This is a measure of the unavailable energy in a thermodynamic system.

Equation of State
A mathematical relationship between pressure, volume, and the temperature of a fluid which permits the prediction of the real volumetric behavior. Since no single equation fits exactly, many functions have been developed to fit various P, V, and T conditions over limited ranges and special applications.

Error
The difference between the indication and the true value of the measured signal. 

          Error = Indicated Value - True Value
Exercise Volume
A meter proving term referring to a predetermined amount of air that must pass through a meter before testing can begin. This volume differs depending on meter size and can be set by a gas company based on past experience or as outlined by the governing state P.U.C.

Flash Point
The minimum temperature at which the vapors above liquid fuel can be ignited by an open flame.

Flowing Temperature
The temperature of the flowing gas at line conditions used in calculating the temperature correction factor.

Friction Error
The amount of change in the indication which occurs as a result of lightly tapping a gauge. It may be expressed as a percent of span.

Gas
The state of matter which has neither independent shape or volume. A gas expands to entirely fill its container.

Gauge Pressure (PSIG)
Pounds per square inch above atmospheric pressure.

Head
The differential pressure causing flow in a fluid system, usually measured in height of a liquid column when applied to liquids. Also the differential across a primary measuring device in feet of flowing fluid.

Heating Value
The amount of heat produced from the complete combustion of a unit quantity of fuel. There are two heating values: the gross (high) and the net (low) heating value. The gross value is that which is obtained when all of the products of combustion are cooled to the original temperature of the mixture before combustion, and the latent heat of the water vapor formed is reclaimed. The net value is that which is obtained when the products of combustion are not cooled and the latent heat of vaporization of the water is not included. The standard cubic foot of gas used in heating value measurements must also be specified on a wet, saturated or dry basis.

Hydrate
A compound formed by the union of molecules of water with other molecules or atoms. Under certain conditions, natural gas may unite chemically with free water to form hydrates, a solid compound resembling packed snow, at temperatures above the freezing point of water.

Hydrocarbon
A chemical compound composed solely of carbon and hydrogen. The compounds having a small number of carbon and hydrogen atoms in their molecule are usually gaseous; those with a larger number of atoms are liquid and the compounds with the largest number of atoms are solid.

Hydrostatic Test
A test of equipment where the item is filled with liquid, subjected to suitable pressure, and then shut in. If there is no loss of pressure during a stated period, the system is free of leaks. Also a test to determine if a container will hold a certain pressure, usually 1 1/2 times its rated maximum working pressure without deforming.

Hysteresis
The property of instrument readings or output which depend on approaching a point on the scale from the opposite direction during a full scale traverse in both directions after friction errors have been eliminated through tapping.

The hysteretic error is the difference between the upscale reading and downscale reading at any point on the scale obtained during full scale traverse, after friction errors have been eliminated through tapping. This also applies to regulators.

Ideal Gas Law
The ideal gas law is the combination of the volume, temperature, and pressure relationships of Boyle's and Charles's laws resulting in the relationship

PV = WRT

Real gases deviate from the ideal gas law by varying amounts.

Immersion Length
The length from the free end of a thermometer bulb or well to the point of immersion in the medium, the temperature of which is being measured.

Inch of Mercury
A pressure unit representing the pressure required to support a column of mercury one inch high at a specified temperature; 2.036 inches of mercury (at 320F and a standard gravity of 32.174 ft/sec.2) is equal to one pound per square inch (1 psi).

Inch of Water
A pressure unit representing the pressure required to support a vertical column of water one inch high. Usually reported as inches wc (water column) at a specified temperature; 27.707 inches of water (at 600F and standard gravity of 32.174 ft/sec2) is equal to one pound per square inch.

27.7" w.c. = 1.00 psi = 16 oz.
20.8" w.c. = 0.75 psi = 12 oz.
13.9" w.c. = 0.50 psi = 8 oz.
6.9" w.c. = 0.25 psi = 4 oz.

Index, Constant-Pressure Compensating
An index used to indicate a gas volume converted to a constant base pressure that is used in conjunction with a gas meter operating at a constant pressure which is other than the contract base pressure. Pressure compensating indexes will correct only for the specified pressure, temperature, and base pressure indicated. Any pressure above or below the specified pressure will result in measurement error.

Index, Meter
The mechanism which displays the volume of gas that has passed through the meter. Indexes consist of two general types: the circular dial type employs a pointer which sweeps through a circle marked with numerals indicating volume registered, while the direct reading (digital or counter type) employs a display marked with numerals indicating volume registered.

Index, Temperature Compensating
A meter index display used to correct volume under flowing gas conditions to a base temperature.

Indicator, Demand
A device that indicates on a scale, chart, or tape the maximum volume metered during a predetermined period of time.

Indicator, Volume
A component of an auxiliary device designed to indicate on a scale or chart, or both, the volume of gas passing through a meter in relation to time, temperature, pressure, or any combination thereof.

Induced Errors
Errors which stem from equipment quality, also factors needed to compensate for differences in reference standards.

Inert
Not acted upon chemically by the surrounding environment. Nitrogen and carbon dioxide are examples of inert constituents of natural gases; they dilute the gas but do not burn and thus add no heating value.

Inherit Errors
Errors due to natural variations in the process of measurement and will vary in sign (+/-) and magnitude each time they occur.

Integrated Circuit
An electronic-circuit element incorporating transistor, diode, and resistor in the same semi-conductor chip.

Integrating Device
A mechanism designed to automatically correct a gas-volume-related input to some predetermined base conditions.

Integrating Pressure Instrument (Base Pressure Instrument)
A mechanism that registers the total volume of gas that passed through the meter after correcting to standard cubic feet at a specified base pressure. The instrument is equipped with a second register that records the total volume at the flowing pressure. Each unit of volume flowing through the meter causes the integrating mechanism to make one cycle and apply the correct pressure multiplier for that unit. The summation of these products is registered on a counter index indicating the displaced volume at base pressure.

Integrating Pressure and Temperature Instrument (Base Volume Instrument)
A mechanism that registers, on a counter, the total quantity of gas passed through the meter corrected to standard cubic feet at a definite base pressure and base temperature. Each increment of volume is multiplied by a temperature factor corresponding to the flowing temperature and base temperature. This value is then multiplied by a pressure multiplier corresponding to the line pressure and base pressure. The product is totaled on a counter index. Another index reads the total quantity passed at flowing conditions.

Joule-Thomson Effect
The cooling which occurs when a compressed gas is allowed to expand in such a way that no external work is done. The effect is approximately 70F per 100 psi pressure drop for natural gas.

Laminar Flow
The type of fluid flow where all particles of the fluid move in paths parallel to each other and to the direction of flow, without cross motion from one path to another. In general, this type of flow occurs only at very low average velocities, or in fluids having a high viscosity index.

Life Test, Accelerated
A test under controlled conditions simulating long term operations designed to determine long term performance characteristics within a relatively short time.

Linearity
The closeness to which a curve approximates a straight line.

Line Pack
Inventory of gas in a pipeline or distribution system.

Magnehelic Gage
A gage used on manual and automated bell provers for measuring the air pressure throughout the proving cycle.

Manometer
A tube usually in the shape of a "U", partially filled with liquid of suitable density connected at each respective end to points of different pressure. The liquid is pushed up into the lower pressure side of the manometer and the difference in liquid level is an indication of pressure difference.

M.A.O.P.
Maximum Allowable Operating Pressure

Master Meter
The practice of proving a calibrated meter on a bell prover (reference standard), recording the proof data, and proving the same meter on other provers as a periodic check for correlation against the reference standard.

Maximum Working Pressure
The maximum actual operating pressure existing in a system during a normal annual operating cycle or the maximum pressure for safe operation of a system.

Meniscus
The curved interface between a liquid and a gas or between two immiscible liquids. Observed, generally, only in tubes of small diameter.

Mercaptans
Alkyl derivatives of hydrogen sulfide having the structure of a thioalcohol. Mercaptans are very weak acids, slightly soluble in water, and react with many metals to form insoluble mercaptides. Their overpowering odors make them useful as warning agents (odorants) for natural gas.

Meter Bar
A metal bar for mounting a gas meter, having fittings at the ends of the bar for connecting the inlet and outlet of the meter and to which, in turn, the gas line, service regulator, and house piping are connected.

Meter Class
A designation for a range of meter capacity. The class for a given type of meter is determined by its minimum capacity based on 0.6 specific gravity gas at 0.5 inches of water column differential pressure, under specified test conditions. The capacity so determined must equal or exceed the class designation, but be less than the next higher class.

Meter, Diaphragm (See Diaphragm Meter)

Meter, Positive Displacement
A meter that indicates the volume of gas passed through it by alternately filling and emptying compartments of known size and totaling the number of cycles accomplished.

Meter Factor
A numerical correction factor obtained by dividing the actual quantity of fluid delivered, measured by a standard, by the quantity indicated on the meter index at any given flow rate. 

      Meter Factor = Actual Quantity measured by Standard
                             Index Reading

Meter, Gas
An instrument for measuring and indicating, or recording the volume of gas that has passed through it.

Meter, Hard Case
A meter in which the case is made of an inflexible metal, such as iron or aluminum.

Meter, Inferential
A meter that operates by measuring another property of the gas flow, and then "inferring" the volume of gas by a known relationship between the measured property and the gas volume. The two most common types of inferential meters are orifice meters and turbine meters.

Meter, Rotary (See Figure 3.)
A meter that utilizes the principle of filling and emptying compartments of known size and totals the number of times the cycle is done, thereby indicating the volume of gas passing through the meter.

Meter Seal
A metal wire or tape seal attached to a gas meter or meter stop in such a way as to prevent its being opened by an unauthorized person.

Meter Stop
A shut-off valve located on the inlet side of the meter.

Meter, Temperature Compensated
A meter where the volume of gas is automatically corrected for variations in flowing gas temperature.

Meter, Tinned Case
A meter with the case made of tinned sheet metal and having joints sealed with tin solder.

Meter, Turbine (See Figure 4.)
A velocity measuring device in which the flow is parallel to the rotor axis and the speed of rotation is proportional to the rate of flow. The volume of gas measured is determined by the revolutions of the rotor and converting them to a continuously totalized volumetric reading.

Meter, Wet Test (See Figure 5.)
A positive displacement meter using a liquid, usually distilled water, as a sealant and as one side of the positive displacement chamber, to measure gas volume.

Methane
The first in the paraffin series of hydrocarbons (CH4). Methane is colorless, odorless, flammable, and forms the major portion of natural gas.

Mileage
Total accumulated index reading for a defined period of time.

Multiplex
To interweave or simultaneously transmit two or more messages on a single channel.

Multiplier, Combined Pressure and Temperature
Multiplication factor for correcting for the average combined pressure and temperature conditions back to base conditions. The Combined Pressure and Temperature Multiplier equals: 

      (460 + Tb)/(Tf + 460) x (Pg + Pa)/Pb

where:  Tb = Base Temperature, usually 600F
        Tf = Flowing Gas Temperature, 0F
        Pg = Meter Gauge Pressure, psig
        Pa = Atmospheric Pressure at the meter site, psia
        Pb = Base Pressure, psia

Natural Gas
A naturally occurring mixture of hydrocarbon and non-hydrocarbon gases found in porous geologic formations beneath the earth's surface, often in association with petroleum. The principal constituent is methane.

Associated - Free natural gas in immediate contact, but not in solution, with crude oil in the reservoir.

Dissolved - Natural gas in solution in crude oil in the reservoir.

Liquified Natural Gas (LNG) - A clear liquid, mostly methane, both tasteless and odorless. Its normal temperature at atmospheric pressure is -2590F. It remains liquid at -1160F and 673 psia. It occupies approximately 1/600 the volume of the same amount of natural gas in vapor state.

Non-Associated - Free natural gas not in contact with, nor dissolved in, crude oil in the reservoir.

Sour - Gas found in its natural state, containing such amounts of compounds of sulphur as to make it impractical to use, without purifying, because of its corrosive effect on piping and equipment.

Sweet - Gas found in its natural state, containing such small amounts of compounds of sulphur that it can be used without purifying, with no deleterious effect on piping and equipment (usually 1/4 grain/CCF or less of HSS).

Wet - Natural gas produced from strata containing condensable hydrocarbons or other liquids. The term is subject to varying legal definitions as specified by certain state statutes. (Usually maximum allowable is 7 lbs./MMCF for water content and 0.02 gals./MMCF for Natural Gasoline.)

Needle Valve
A valve that has a tapered needle-like plug which fits with precision on a seat. Accurate throttling of small flows is possible because the orifice formed between the tapered plug and its corresponding tapered seat can be varied in very narrow and precise increments.

Odorant
Substance giving a readily perceptible odor at low concentrations in the material into which it is mixed and used as a warning indication of the presence of the material, such as natural gas.

Offset
The difference between the mid-point of the adjustment range and 0% error, (100% Proof). This is done to allow for wear of the meter which can change the calibration. Offset is normally expressed in tenths of a percent.

Open Rate
The rate of slow (in cubic feet or cubic meters) that normally equals the badged capacity of a meter. This rate of flow should cause a pressure drop across the meter of 1/2 inch WC or 2-inch WC depending on the meter's rated capacity.

Optical Pick-Up
A sensing device used to detect the pointer on an index, or register, of a meter.

Orifice
A hole, generally round, in a plate, pipe or hollow body, designed to control or measure the flow of a fluid.

Orifice Meter (See Figure 6.)
A inferential meter for measuring flow in a pipeline and consisting of a Primary Element and Secondary Element. The Primary Elements include the orifice plate, orifice flanges or plate holder and the adjacent pipe sections. The Secondary Elements include equipment used to determine pressures, temperatures, and other variables which must be known in order to accurately measure gas. There is a known relationship between pressure drop and flow rate for the plate. Therefore a recording of flow rate versus time can be obtained and then integrated to establish volume.

Orifice Plate
A plate of corrosion resistant material which can be fastened between flanges or in a special fitting, perpendicular to the axis of a pipe, and having a hole in it with a diameter less than that of the pipe. This hole, in most cases, is round and concentric with the pipe.

Packing
Material used to seal a joint, such as in bell and spigot pipe, or to seal a gland through which a rotating shaft passes.

Paraffin
White waxy, odorless, tasteless substance obtained from petroleum and other oils.

Percentage Error, Meter
The percentage error of a gas meter is the difference between the meter's percentage registration and 100 percent, using the prover registration as the base.

Percent Error = (Vm - Vp)/VP x 100 
where:  Vm = metered volume 
        Vp = prover volume
Pilot Flow Rate
A minimum flow rate that a meter is required to register with a prescribed accuracy.

Pitot Tube
A device consisting of two small, concentric tubes, one inside the other, that can be inserted into a pipe carrying fluids to measure the flow of liquid or gas in the pipe. The tubes are so arranged that one is affected by the total pressure (velocity head + static pressure head) of the fluid and the other by the static pressure existing in the pipe. The outlet ends of the tubes are connected to either side of a manometer tube and the difference in pressure so measured is a function of the velocity of the fluid flowing in the pipe ahead of the tube.

Pour Point
Temperature below which a liquid, such as oil, ceases to flow.

Precision
The degree of discrimination with which a quantity is stated, i.e., a three digit numeral discriminates among 1,000 possibilities.

Pressure
Force applied to or distributed over a surface, measured as force per unit area. The pressure P due to a force F distributed over an area A: 

		Pressure = Force/Area

Pressure, Absolute (PSIA) (See Figure 7.)
Pressure in excess of a perfect vacuum. Absolute pressure is obtained by algebraically adding gauge pressure to atmospheric pressure. Absolute pressure must be used in equations of state and in all gas law calculations. Every 1% change from correct absolute pressure (psia) will create a 1% volume error. A 4" w.c. change in pressure from a specified pressure changes the indicated meter volume also by about 1%.

Pressure Drop
The drop in static pressure of the fluid (air, gas, or water) due to friction or obstruction as the fluid flows through pipe, valves, fittings, regulators, burners, and appliances.

Pressure Gauge
Instrument for measuring the pressure of a fluid.

Primary Element (Detector)
The first system element that responds quantitatively to the measured variable and performs the initial measurement function.

Proof, Meter (Percent)
The percent proof of a gas meter is the volume registered by the standard divided by the volume registered by the meter, expressed as a percentage. 

% Proof = Volume registered by the Standard x 100
          Volume registered by the meter

Propane
Gaseous member of the paraffin series of hydrocarbons (C3H8) that when liquified under pressure, is one of the components of liquified petroleum (LP) gas.

Prover
Apparatus used to prove or calibrate gas meters. A gas meter is tested for accuracy of registration by passing through it a volume of air from a prover and comparing the registration of the index of the meter with the volume indicated by the prover. Prover types include: Bell Provers, Transfer Provers, Low Pressure Flange Tap Prover, and Sonic Nozzle or Critical Flow Provers.

Prover, Critical-Flow or Sonic Nozzle (See Figure 8.)
A device using critical flow orifices or sonic nozzles that passes air through both meter and the orifice or nozzle at such pressure drops to insure critical flow or sonic velocity. The time for a given quantity of gas to pass through the meter compared to the orifice or nozzle standard time corrected for test conditions provides a measure of meter accuracy.

Prover, Flow
Apparatus used to determine the accuracy of meters by comparison with a calibrated orifice. Types of flow provers include critical flow prover and low pressure flange-tap and pipe-tap provers.

Prover, Low Pressure Flow (See Figure 9.)
An apparatus utilizing an orifice for testing meters at low pressures by passing gas or air through the orifice and meter and finally discharging it to atmosphere. The time for a given quantity of gas to pass through the meter compared to the orifice standard time corrected for test conditions provides a measurement of meter accuracy.

Prover, Transfer (See Figure 10.)
A device for determining the accuracy of a meter under test by comparing its reading against the reading obtained from a calibrated reference meter connected in series with the meter under test.

Proving Circle
A graduated circle provided with a movable pointer (proving hand) on the meter index used for testing the meter and for indicating gas flow.

Proving Dial - See Proving Circle.

Psychrometer
A hygrometer, or instrument for measuring the aqueous vapor in the atmosphere, consisting essentially of two similar thermometers, the bulb on one being kept wet. The wet bulb will be cooled as a result of evaporation and consequently will show a temperature lower than that of the dry bulb thermometer. The evaporation is less on a moist day and more on a dry day. This difference constitutes a measure of the humidity of the surrounding air.

Rangeability
Ratio of maximum operating capacity to minimum operating capacity within a specified tolerance and operating condition.

Rankine Scale of Temperature
The absolute Fahrenheit scale. 

	0F + 459.67 = 0R

Rate of Flow
The volume or units of a material passing a given point in a system per unit of time.

Ratio of Specific Heats
For gases, it is the ratio of the specific heat at constant pressure to the specific heat at constant volume. The ratio is important in thermodynamic equations and is given the symbol k where k = Cp/Cv. The ratio k lies between 1.2 and 1.4 for most gases.

Recording Instrument
An instrument in which the values of the measured quantity are recorded.

Reference Operating Conditions
The range of operating conditions of a device, within which operating influences are negligible. This range is usually narrow and gives the conditions under which reference performance is stated and the base from which the values of operating influences are determined.

Reference Errors
The accuracy of a measurement can be no greater than the accuracy of the reference standard. All standards have an uncertainty associated with certified values. For example, sonic nozzles for establishing reference flows can be certified by NIST to +/-0.25% with a confidence level of 99%.

Reference Standard
A device or means of providing a cubic foot reference on a repeatable basis. A bell prover is most often referred to as the reference standard in the gas industry.

Reference Standard Meter
A meter which has been selected for comparing other meters' accuracy or for determining variance between test equipment or meters.

Registration
The meter dial positions of a gas meter index. The difference between two successive registrations indicates the volume of gas that passed through the meter. Also, when used as a meter calibration term, registration is the indicated meter volume divided by the standard volume expressed as a percentage. 

      Registration = Index Reading - Previous Index Reading

      Percent Registration =  Volume indicated by the meter x 100
                              Volume registered by the standard

Relative Humidity
The ratio of the volume of water vapor in the atmosphere to the volume the atmosphere would hold if completely saturated at that temperature. Usually expressed as a percentage.

Repeatability
The closeness of agreement among a number of consecutive measurements of the output for the same value of the measured signal under the same operating conditions, approaching from the same direction, for full range traverses.

Reproducibility
The closeness of agreement among repeated measurements of the output for the same value of input made under the same operating conditions over a long period of time, approaching from either direction. Reproducibility includes hysteresis, drift, repeatability, and dead band. Operating conditions and input may vary between normal operating limits between measurements.

Note: Reproducibility is expressed as a maximum non-reproducibility in percent of span for a specified time. Normally this implies a long period of time, but under certain conditions, the period may be a short time during which drift may not be included.

Resolution
The smallest interval between two adjacent, discrete measured values which can be distinguished from each other under specified conditions.

Reynolds Number
The dimensionless ratio used to determine whether the flow in a certain system is turbulent or laminar. Reynolds number is as follows: 

      Rd = (Velocity x Diameter x Density)/Absolute viscosity

Round Robin Tests
A meter or group of meters used to determine prover variance and/or repeatability.

Scale
A graduated scale usually in cubic foot and/or fractional increments used on a prover bell to determine volume displacement of the prover.

Sensitivity
The ratio of a change in output to the change in input which causes the change in output after steady state has been reached. It is expressed as a numerical ratio with the units of measurement the same as the two quantities stated.

Span
The algebraic difference between the upper and lower range values.

Span Error
The difference between the actual span and the ideal span.

Specific Gravity
The ratio of the density of a substance to the density of a reference substance, both at specified physical conditions. As applied to gas, air is the reference substance. Two specific gravity definitions are recognized in gas measurement:

Real Specific Gravity - The density ratio between a gas and air determined by measurement at the same temperature and pressure.

Ideal Specific Gravity - The ratio of the molecular weight of a gas to the molecular weight of air. (Molecular weight of air = 28.96440)

Specific Heat
The heat required to raise a unit mass of a substance through a degree of temperature difference. Also the ratio of the thermal capacity of a substance to that of water. The specific heat of fluids varies with temperature and pressure.

Specific Weight
The weight of a unit volume, usually expressed as pounds weight per cubic foot.

Speed Dial
Dial on a meter index normally used in slow fire testing.

Standard Cubic Foot
The cubic foot volume at base conditions of temperature and pressure. (See Cubic Foot.)

Static Pressure
The force exerted per unit area by a gas or liquid, measured at right angles to the direction of flow, or the pressure when no fluid is flowing.

Strapping
A method of checking a bell prover by determining the relation between displaced volume and linear movement of a bell prover by means of measuring scale length, bell circumference, and sealing liquid.

Straightening Vanes (See Figure 11.)
Round, square, or other shape tubing installed axially inside the piping preceding an orifice or turbine meter to eliminate swirls and crosscurrents set up by the pipe fittings and valves. Proper installation of straightening vanes considerably reduces the amount of straight pipe required preceding the measuring element.

Supercompressibility
Boyle's law for gases states that the specific weight of a gas is directly proportional to the absolute pressure, the temperature remaining constant. All gases deviate from this law by varying amounts, and within the range of conditions ordinarily encountered in the natural gas industry, the actual specific weight under the higher pressure is usually greater than the ideal theoretical. The factor used to reflect this deviation from the ideal gas law in gas measurement with an orifice meter is call the supercompressibility flow factor, Fpv. For application to displacement and turbine meters, the supercompressibility ratio "s" is used where s = (Fpv)2.

In gas law calculations, the compressibility factor "Z" is used in the equation: 

			PV = WRTZ

     where:	P = absolute pressure in psia

    		V = volume in cubic feet

    		W = weight in pounds

    		R = specific gas constant = 1545.32/Mol. Wt.

    		T = absolute temperature in degrees Rankine

    		Z = Compressibility Factor

The relationship between the factors Fpv, s and Z is: 

			Fpv = s1/2 = (1/Z)1/2

In some publications, "s" is given the symbol "y" and is called "expansibility factor."

Based on 0.60 specific gravity gas and 500F flowing temperature, the following volumes are unaccounted-for if supercompressibility factors are ignored:

Table 2
Pressure (PSIG) Unaccounted-For (%)
15 0.25
30 0.50
45 0.76
60 1.03
75 1.28
90 1.52
105 1.79

Swivel, Meter (See Figure 12.)
The fitting that connects to the inlet and the outlet of a small gas meter.

Target Proof
The desired accuracy of the meter. A target proof of 100% would be a perfect meter. Usually expressed with tolerance limits, a target proof of 100% and a tolerance of +0.5% would allow meter proofs to fall within a range from 99.5% to 100.5%.

Telemetering
Use of an electrical apparatus, transmitting data to a distant point, for indicating, recording, or integrating the values of a variable quantity.

Temperature
Often defined as the condition of a body which determines the transfer of heat to or from other bodies. Particularly, it is a manifestation of the average translational kinetic energy of the molecules of a substance due to heat agitation. Also, the degree of hotness or coldness measured on a definite scale.

Temperature, Absolute (See Figure 13.)
Temperature above absolute zero. It has long been known from temperature-volume relationships that if the data were extrapolated to zero volume, the corresponding temperature would be about -2730C. If the sizes of the Celsius and Fahrenheit degrees are retained and the zero points shifted to -2730C, the Absolute Kelvin and Rankine scales are obtained. The exact SI (Systeme International d'Unites) values are K = 273.15 + 0C and 0R = 459.67 + 0F.

Temperature, Ambient
Environmental temperature unaffected by other heat sources, such as radiation from artificial objects.

Temperature, Base (See Base Temperature)

Temperature Compensated Meters
These meters measure volume at pipeline conditions, but contained within them is a device which will convert the volume at flowing temperature to a volume at base temperature. This device is a temperature-sensitive mechanism that continuously varies the diaphragm stroke to provide a temperature compensated volume output.

Temperature, Flowing
The temperature of the gas under metering conditions. A 50F change in flowing temperature to a 600F base temperature will change indicated meter volume by 1%.

Temperature Probe
A device placed into the gas stream that senses the flowing gas temperature.

Therm
A unit of heating value equal to 100,000 British Thermal Units (BTU).

Tinned Steel-Case Meter
A type of residential and small commercial gas meter in which the mechanism is housed in a case made of tinned steel plates soldered at the joints.

Tolerance
A fractional or percentage allowance for variation from a specified standard.

Torque
The force that causes rotation or twisting: the rotary force in a mechanism.

Transducer
An element or device which receives information in the form of one physical quantity and converts it to information in the form of the same or other physical quantity.

Transfer Proving
Any calibration of a meter that is based upon a comparison of its registration with that of another previously calibrated measurement device.

Transfer Standard Meter
The calibrated meter used in a transfer test. A transfer meter is traceable against accepted volumetric standards.

Transmitter
A transducer which responds to a measured variable by means of a sensing element, and converts it to a standardized transmission signal which is a function only of the measurement.

Unaccounted-For Gas
The difference between the quantity of gas measured into a system and the quantity of gas measured out of the same system. Unaccounted-for gas includes items as leakage, gas lost in operations, gas used but not measured, measurement errors, differences due to measurements made at different times, etc.

Vacuum (See Figure 7 under "Pressure, Absolute")
A pressure less than atmospheric pressure, measured from the base of zero pressure or from the base of atmospheric pressure.

Valve Seat
The stationary portion of the valve which, when in contact with the movable portion, stops flow completely.

Vapor
Any substance in its gaseous state.

Vapor Pressure
The pressure caused by vapor over the surface of a liquid, caused by the higher kinetic energy of the molecules of vapor.

Venturi Meter
A fluid-type meter in which the rate of fluid flow is determined by measuring the pressure drop caused by the flow of the fluid through a venturi tube. The pressure drop across the tube is a function of the fluid flow rate.

Viscosity
In a fluid in motion, the transfer of momentum from faster to slower layers results in a retarding effect or resistance to flow called viscosity. Viscosity is a shearing stress whose force varies with the rate at which the strain is changing. Thus, the unit of viscosity, the poise, is expressed in dyne-seconds per square centimeter. For gases, this important physical property increases with increasing pressure, temperature, and molecular weight.