GLOSSARY OF TERMS: Flow Measurement

ACCURACY: The deviation from the absolute truth; this figure should include linearity, repeatability, and calibration uncertainty.

ASYMMETRIC FLOW: When the flow across a path of a liquid does not have a symmetrical profile. This causes erroneous readings with most flowmeter types.

BACK PRESSURE: Measure of pressure on downstream side of a flowmeter.

BERNOULLI: Daniel Bernoulli’s formula basically states that the energy possessed at any point in a pipe is the same regardless of cross section.

CALIBRATION: The checking of a meter’s performance against known standard.

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

DENSITY: Ratio of mass to volume.

DOPPLER EFFECT: The apparent change in frequency of a moving body relative to a reference point.

ELECTROMAGNETIC: 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 the meter body provides the magnetic field and the electrodes.

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 flowmeter.

MASS FLOW: True mass flow where the measurement is a direct reading of the mass of the fluid, not just the volume or velocity.

MASTER METER: Typically a highly accurate flowmeter used as a reference for calibration purposes.

NUTATING DISC: 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.

ORIFICE PLATE: 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.

PITOT TUBE: Differential-pressure device that utilizes the conversion of kinetic energy into pressure energy. Ideal for point measurement.

RANGEABILITY: Also called turndown. The ratio between the minimum and maximum flowrate for the linear flow range.

SLUG FLOW: Term used to describe a condition were the liquid in a pipe is interspersed with pockets of vapor that do not intermingle but travel in “slugs."

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.

The terms and definitions for this issue’s Word Search come from a glossary of terms compiled by Titan Enterprises Ltd. (www.flowmeters.co.uk), a United Kingdom-based manufacturer of flowmetering solutions.