GLOSSARY OF TERMS: Gas Flow Meas.

ACCURACY: The degree of error; the difference between the measured value and the actual value of the measured quantity. The error is usually expressed as a percentage (i.e., the difference is divided by the actual flowrate).

CORIOLIS: Coriolis flowmeters work when the fluid flowing through a vibrating flow tube causes a deflection of the tube proportional to mass flowrate. Coriolis flowmeters can be used to measure mass flowrate of liquids, slurries and gases. Measurement of fluid density or concentration is also provided by Coriolis technology.

CONDITIONING: Flow conditioning is used to improve the flow profile of a gas flowing in a pipe and to remove non-uniform velocities in the flow stream.

DIAPHRAGM METER: A common type of flowmeter is the positive displacement "diaphragm" meter. This meter measures the volumetric flowrate of a liquid or gas by separating the flow stream into discreet known volumes and counting them over time. Vanes, gears, pistons, or diaphragms are used to separate the fluid.

DELTA P METER: Delta P devices, such as orifice plates, Venturis and sonic nozzles, can be used to measure the volumetric flowrate of most liquids and gases. Delta P (DP) devices have an inherent relationship between the pressure drop across the device and the volumetric flowrate that the device is measuring. A pressure transducer or manometer records the difference between these two pressures.

FLOW PROFILE: A three-dimensional representation of the velocity distribution of a fluid in a pipe.

FLOW SENSOR: Typically, a pair of Resistance Temperature Detectors (RTDs) is used as a set in a thermal mass flowmeter to determine gas mass flow through thermal dispersion.

INLINE METER: A flowmeter that is comprised of a flow sensor and the section of a pipe through which the entire gas flow passes.

INSERTION METER: A flowmeter that is inserted into the existing pipe to measure gas velocity at a single point. The flowrate can be calculated when the velocity and the area of the pipe are known.

LINEARITY: A constant relation between the change in the output and input in a metering system.

MASS FLOW: Mass flow describes the mass of a medium that moves through a cross section (such as a pipe or a duct) in a certain time interval.

MASS VELOCITY: Represented by ρV, where ρ (rho) is the Fluid Density, and V is the Fluid Velocity. Also referred to as Bulk Velocity.

PITOT TUBE: A Pitot is used to measure air and gas velocities. It consists of a tube pointing directly into the gas flow. As this tube contains the gas, a pressure can be measured; the moving fluid is brought to rest (stagnates) as there is no outlet to allow flow to continue. The modern application is a pitot-static probe, which measures both the stagnation pressure, with a hole in the front, and the static pressure in the moving stream, with holes on the sides. A pressure transducer or manometer records the difference between these two pressures.

PRESSURE LOSS: The drop in pressure as measured at the inlet of a device minus the pressure as measured at the outlet of that same device.

REPEATABILITY: The repeatability (or precision) of a measurement entails the ability to reproduce the same value (e.g., gas flowrate) with multiple measurements of the same parameter under the same conditions.

RTD: A Resistance Temperature Detector (RTD) is a device with a temperature coefficient that varies consistently with temperature. It is used as a temperature measurement device, usually by passing a low-level current through it and measuring the voltage drop.

SENSITIVITY: The degree to which an output can be resolved. It is directly related to the slope of the raw calibration curve.

SKIN RESISTANCE: The total thermal resistance of the heated element in a thermal mass flowmeter as seen in the radial direction.

THERMAL MFM: Thermal Mass Flowmeters (TMFMs) are based on the following principle — the mass flowrate of gases can be inferred by measuring the temperature rise of the media (“heat gain”) or the temperature drop of a heated sensor (“heat loss”). Thermal flowmeters infer mass flow rather than volumetric flowrate.

TURBINE: The air passing through a turbine flowmeter spins a rotor. The rotational speed of the rotor is related to the velocity of the fluid. The velocity multiplied by the cross-sectional area of the turbine provides the volumetric flowrate.

TURNDOWN RATIO: The turndown ratio refers to the flowrate range over which a meter will inherently maintain a certain accuracy and repeatability.

UNCERTAINTY: The range in which an actual measured value of a meter can be expected to remain within a specified confidence level.

ULTRASONIC: Ultrasonic flowmeters are based on the transit-time sound velocity or Doppler frequency shift of the mean velocity of a fluid. In practice, ultrasonic flowmeters have sonic transducers mounted on the outside of the pipe.

VORTEX SHEDDING: Vortex-shedding flowmeters are based on the following principle. The frequency of vortices shed from a bluff body placed in the flow stream is in direct proportion to the velocity of the fluid. Knowing the velocity and the area, the volumetric flowrate can be calculated.

VOLUMETRIC FLOW: The volumetric flow rate in fluid dynamics is the volume of fluid that passes through a given surface (such as a pipe or duct) per unit time. It is usually represented by the symbol Q.

This glossary of terms and definitions was compiled by Gary Russell, flow engineer, and Bob Steinberg, president of Sage Metering Inc. (www.sagemetering.com). Mr. Steinberg can be reached at [email protected].