Absolute viscosity:
A measure of resistance to shear or angular displacement. The units are dyne-second per square centimetre, or Poise, which is usually expressed as centi-Poise.

Accuracy:
The deviation from the absolute truth. This figure should include linearity, repeatability and calibration uncertainty.

Air eliminator:
A device for removing entrained gas from a fluid stream.

Anemometer:
A device for measuring wind speed.

Annubar:
A proprietary cantilevered Pitot tube.

Asymmetric flow:
This is when the flow across a the path of a liquid does not have a symmetrical velocity profile, this causes erroneous readings with most flowmeter types.

Axial flow meter:
A flowmeter that consists of a “propeller” in a tube.

Back pressure:
The measure of the pressure on the downstream side of a flowmeter.

Ballistic calibrator:
A type of flowmeter calibrator that uses the displacement of a piston of known dimensions to calibrate the meter on test.

B-ell prover:
A gas calibrator that immerses a “bell” of known dimensions into a bath of liquid thus dispelling the gas at a known velocity and pressure through a flowmeter on test.

Bernoulli:
His important formula basically states that the energy possessed at any point in a pipe is a constant regardless of cross sectional area.

Bubble flow meter:
Also called a bubble burette or soap bubble meter. This measures the volume displacement of a gas by introducing a soap film across a known bore and timing its passage.

Bypass flowmeter:
A flowmeter that meters only a small part of the flow and permits the majority to bypass the measuring element rather like a shunt on an Ampere meter.

Calibration:
The checking of a meters performance against a known standard.

Cavitation:
When the pressure in a system drops, part of the flowing liquid either turns to a vapour or releases gasses that were in solution, causing meter damage or erroneous readings.

Clamp-on:
This is generally a term referring to ultra-sonic flowmeters that may be positioned on the outside of a pipe to measure the flow within the pipe.

Coanda effect:
This is the term used when a stream of liquid attaches itself to a nearby solid surface.

Coefficient of discharge:
A mathematical function that that defines the ration between “true” flow and “indicated” flow used primarily with differential pressure devices.

Compressibility:
The change in volume or density due to a change in pressure.

Coriolis meter:
These meters are based on the Coriolis effect, meaning they use inertial forces within the fluid reacting against a moving flow tube to give a very accurate mass flow reading.

Corner tapping:
Type of “pressure tapping” used with differential pressure devices where the pressure points are in the corner of the adjacent flange.

Critical nozzle:
A type of restriction where the gas in the throat of the nozzle is sonic and the volumetric flow rate is readily calculated, therefore they are very useful as calibration devices.

Cross correlation:
A system of data processing that identifies the same pattern in two or more apparently random signals.

Cup anemometer:
A common type of wind speed indicator.

Dall tube:
A proprietary low loss differential pressure meter.

Density:
The ratio of mass to volume.

Differential pressure:
The difference between the measurements of pressure at two points in a system.

Discharge coefficient:
See coefficient of discharge.

Discrimination:
The smallest unit that the meter can distinguish.

Doppler Effect:
The apparent change in frequency of a moving body relative to a reverence point.

Eddy:
A swirl in a fluid.

Effective range:
The linear part of a meters performance curve.

Electrodes:
Measuring sensor in electromagnetic flowmeter.

Electromagnetic:
A type of flowmeter that uses the same principle as an electric generator, in that a voltage is induced when a conductor is moved in a magnetic field, the fluid is the conductor and meter body provides the magnetic field and the electrodes.

Flow conditioner:
A device for “normalising” asymmetric or swirling flow upstream of a flowmeter.

Fluidic flowmeter:
Device that uses fluidic properties such as the Coanda effect to measure the flow of a fluid.

Flumes:
A flow device for open channel.

Gas meter:
A generic term for any gas flow meter, typically a domestic unit.

Gear meter:
One of several types of flowmeter that use the meshing of circular or oval shaped gears to meter discrete volumes of a liquid.

Gravimetric calibration:
Measuring system that weighs a mass of fluid as a basis for calibration.

Head:
The height of a column of fluid equal to a pressure.

Hot wire:
Used as an anemometer or air velocity meter.

Inertia effects:
Errors caused by the inertia of the moving fluid or mechanical components associated with it.

Inferential meter:
A flowmeter who’s mass flow is calculated using the known composition of the fluid properties by measuring one or more of the critical parameters, e.g. velocity and density.

Insertion meter:
A term used for a flowmeter that is inserted to measure a point velocity in a fluid stream. The result of which is often used to calculate the flow in the whole conduit.

K factor:
The number of output pulses per unit volume from a flow meter, e.g pulses per litre

Kinematic viscosity:
This is the ratio of absolute viscosity to density, usually expressed as centistokes; cSt.

Laminar flow:
Flow where the fluid moves in a parallel manner along the conduit (not turbulent), typically at Reynolds numbers below 2000 and having a velocity profile across the conduit that is similar to a parabola.

Laser anemometry:
A high performance point fluid velocity device.

Linearity:
This is the deviation from the ideal over a specified flow range, a nominal ‘K’ factor normally expressed as a percentage.

Mach number:
The ratio of local flow velocity to acoustic velocity.

Manometer:
A gauge for measuring pressure using a column of fluid, typically water or mercury.

Mass flow:
Also called true mass flow, where the measurement is a direct reading of the mass of the fluid not just the volume or velocity.

Master meter:
A typically highly accurate flowmeter used as a reference for calibration purposes

Mean pipe velocity:
This is the average velocity across the whole pipe, not just a single point.

Meter error:
The deviation of the meter output relative to an absolute value.

Meter factor:
The number of pulses for per unit volume see ‘K’ factor.

Meter prover:
Similar to a ballistic prover but often portable.

Non-Newtonian fluids:
A fluid whose viscosity varies with the rate of shear strain.

Nozzles:
Various types of differential pressure devices.

Nutating disc:
A type of positive displacement meter that uses a disc with a slot in that “wobbles” rather than rotates on a spherical bearing in a specially shaped chamber.

Open channel meter:
One of many types of flowmeter that is specially designed to operate in a culvert, flume, stream or river.

Orifice plate:
A differential pressure device consisting of a plate that restricts the flow, causing a predictable pressure loss using Bernoulli’s equation and the principle of the conservation of energy, may be concentric, eccentric or chord.

Oval gear flow meter:
Type of positive displacement meter that uses two oval shaped lobes which have meshing teeth to form both the linkage and the drive. Also see Gear Meter.

Over-ranging:
operating flowmeter outside it’s designed flow range.

Peak flowmeter:
A meter used to measure the maximum flow rate, usually for measuring the lung efficiency of asthmatics.

Pelton wheel:
A type of radial flow turbine meter: turbine used for extracting energy from water.

Pick-up:
A type of sensor used on a flowmeter.

Pipe prover:
See ballistic prover

Pipe wall roughness:
The roughness of the pipe bore. This is very important when using theoretical analysis for the calculation of the fluid behaviour, e.g. with an orifice plate.

Pitot tube:
A differential pressure device that utilises the conversion of kinetic energy into pressure energy, ideal for point measurement, may be cantilever, or static.

Point velocity:
A fluid velocity at one point. This is not necessarily indicative of the whole flow in a conduit.

Positive displacement Flow Meter:
A flowmeter that uses a principle that carries discrete pockets of fluid from the inlet to the outlet with little or no leakage.

Pressure tapping:
The point at which pressure is read in a system, this position is critical with certain types of differential pressure device.

Pulsating flow:
A condition were the fluid is changing in velocity within the conduit.

Radial flowmeter:
A range of flowmeter that uses the kinetic energy from the fluid striking a rotating blade at ninety degrees to the flow direction, often called Pelton wheel meters.

Range:
The linear operating range of a flowmeter.

Rangeability:
Also called turndown, the ratio between the minimum and maximum flow rate for the linear flow range.

Radioactive tracer:
A method of calibrating that uses either the velocity or dilution rate of a short term radioactive material.

Repeatability:
The ability of a flowmeter to give the same result on repeated runs with the same operating conditions, not to be confused with accuracy or linearity, without excellent repeatability a flowmeter cannot have good performance.

Resolution:
The smallest unit that a meter can distinguish, more correctly referred to as discrimination.

Response time:
The time taken for an accurate reading to be attained.

Reynolds number:
A dimensionless ratio of dynamic and viscous forces, used to determine the behaviour of fluids in any condition.

Rotameter:
A type of variable area flowmeter that uses gravity to balance the force on a float in a tapered tube, the height of the float is indicative of the flow rate.

Sharp edge orifice plate:
A type of orifice plat flow meter.

Shunt meter:
A flowmeter where a small fraction of the main flow is metered to determine the flow for the whole conduit.

Sight flow:
flow indicator, not a flowmeter, which allows a visible indication of flow.

Sliding vane flow meter:
A very accurate type of positive displacement meter.

Slug flow:
A term used to describe a condition were the liquid in a pipe is interspersed with pockets of vapour that do not intermingle but travel in “slugs”.

Soap film burette:
See bubble flowmeter.

Specific gravity:
More correctly referred to as density.

Square root law:
This applies to most types of differential pressure devices, orifice plats, pitot tubes etc. where the flow is the square root of the differential pressure.

Static pressure:
A pressure measurement that is not effected by a dynamic change in the fluid flow and often used as a reference in determining differential pressure.

Target meter:
A meter that uses the force or displacement of a “target”, usually a disc, to determine the flow.

Temperature compensation:
The essential density and viscosity correction when accurate mass flow measurement is required with an inferential flow meter.

Thermal mass meter:
A type of mass flowmeter that uses the movement of heated fluid to give an accurate measurement of the mass flow in a pipe, not to be confused with a “hot wire” device that only measures a point velocity and not mass.

Three phase flow:
very undesirable condition when discreet “lumps” of solid liquid and gas are flowing in a pipe.

Time of flight:
Method used in several types of flowmeter, but usually refers to ultrasonic meters. A sound wave is sent both with and against the flow, one is accelerated and the other retarded half the difference is the velocity of the fluid stream.

Tracer flow meters:
Meters that use a “tag” to monitor the velocity of a fluid stream, may be radioactive, dye or heat.

Transfer standard:
A calibrated metering assembly that can be cross checked in multiple calibration laboratories.

Transit time:
A method of recording flow using the time taken for a reference to travel between pre-determined points. Usually ultrasonic but may be a tracer e.g. in river flow measurement.

Turbine flow meters:
Term used primarily for the “propeller in a tube” type of flowmeter but may also be used many other flow devices such as Pelton wheels.

Turndown ratio:
Also called rangeability, the ratio between the minimum and maximum flow rate for the linear flow range.

Turbulent flow:
Condition where the fluid as a whole is travelling in an apparently uniform stream but the individual particles within the fluid are travelling in a random manner in all directions.

Two-phase flow:
When two fluids are travelling in a pipe and not mixing.

Ultrasonic flow meters:
A type of flowmeter that uses high frequency sound waves to measure flow by either time of flight or Doppler methods.

Variable area meters:
A term for a large number of flowmeters that use the movement of one element relative to another in a specially shaped chamber to indicate flow rate.

Velocity meters:
A flowmeter that simply measures the velocity of the fluid usually at a single point e.g. a pitot tube.

Velocity profile:
A graphical or numeric representation of the various velocities across a conduit.

Vena contracta:
The point just after an orifice plate of flow tube where the flow forms its smallest diameter.

Venturi meter:
A type of differential pressure meter that controls the upstream and downstream pipe geometry to minimise the overall pressure loss.

Viscosity:
The resistance to shear at a known rate.

Viscometer:
A device for measuring a fluids viscosity.

Viscous flow:
A condition when the flow in a conduit is moving uniformly in the same direction, the velocity profile forms a parabola as the fluid nearer the walls travels more slowly due to viscous drag.

Vortex shedding:
When a bluff body is introduced into a fluid stream, it causes eddies to be shed from alternate sides at a frequency that is directly proportional to the fluid velocity, this phenomena is used in a vortex shedding flowmeter.

Water flow meter:
Generic term used for domestic flow totalising meters.