## UNDERSTANDING FLOW MEASUREMENT AND RESTRICTION

A Brief Glossary of Common Terms

**Accuracy: **The extent to which a measurement agrees with a reference. A qualitative assessment; not the same as uncertainty (a quantitative value).

**Beta Ratio (Beta): ** For differential producers with circular cross-sections the ratio the bore I.D.-to-pipe I.D. Typically ranging from 0.20 to 0.75, depending on meter type.

**Cavitation:** The implosion of vapor pockets occurring when the local pressure of a fluid rises above the vapor pressure of the line fluid.

**Critical Flow (Choked Flow, Sonic Flow):** The condition at which the velocity of a flowing fluid, typically a gas or vapor, reaches the velocity of sound at local conditions.

**Differential Pressure:** The pressure difference between high- and low-pressure taps. This is the flow signal generated by a differential pressure producing primary flow element.

**Discharge Coefficient (CD):** The value of the ratio of the actual flow rate to the theoretical flow rate, where the theoretical flow rate is calculated for an ideal fluid and no energy loss.

**Expansibility Factor:** The coefficient that accounts for the compressibility of a fluid. Y(ε) is equal to one for ideal, incompressible fluids.

**Flow Profile:** A graphic representation of the velocity distribution of the flow as it approaches a primary flow element. Flow through a straight, smooth, pipe of sufficient length should develop a blunt flow profile. Valves and pipe fittings introduce non-uniform velocity distributions.

**Meter:** Wyatt uses this term to refer to primary flow elements used for measurement, as opposed to restriction. Not to be confused with the transmitter, or secondary instrumentation.

**Pressure Ratio: ** In gas or vapor flow, the ratio of the absolute pressure at the low-pressure tap to that at the high-pressure tap. Ideally, this value should be greater than 0.8.

**Primary Element:** Device (venturi, orifice, flow nozzle, etc.) that generates a signal to determine the flow rate.

**Reliability:** A qualitative assessment of the degree to which empirical evidence agrees with and substantiates the theoretical understanding of a phenomenon.

**Transmitter: ** The instrument that converts the differential pressure signal to (typically) an analog or digital signal that can be recorded or processed.

**Uncertainty:** The estimate of an error band within which the true value falls with stated probability.