Page 185 - Mechanical Engineers' Handbook (Volume 2)

174 Temperature and Flow Transducers

The paints are cheap and easy to apply. They provide a good ‘‘first-look’’ capability.
The principal disadvantages of the paints and crayons are that they require visual inter-
pretation and they are one-shot, irreversible indicators. They read peak temperature during
the test cycle, yet they cannot record whether the peak was reached during steady state or
during soak-back.

9 FLOW RATE
The objective of flow rate measurement is to determine the quantity of matter flowing. In
some instances a flowmeter returns this information directly, but in most cases the flowmeter
signal is derived from some property of the flow: volume, heat transfer rate, momentum flux,
and so on. In most cases the flowmeter signal requires correction for pressure, temperature,
or viscosity before the flow rate is known.
Interest in mass flow rate stems from a basic principle of engineering: The creation rate
of mass is zero. If a flow is measured in mass flow units, then unless the pipe leaks or more
fluid is added, the mass flow rate is the same everywhere along the pipe, regardless of
changes in the density of the fluid flowing. This is not so for a volume-based measurement.
The volume flow rate represented by a fixed mass flow rate of gas depends upon the mo-
lecular weight, the temperature, and the pressure of the gas. Hence, as either T or P change,
the volume flow measure would change, even though the mass flow measure remained con-
stant.
The term SCFM (standard cubic feet per minute) is frequently used in gas flow metering.
The number of SCFM corresponding to a particular flow rate is a measure of the volume
flow rate that would have been observed had the same mass flow rate of the same gas
occurred at standard conditions of temperature and pressure. In other words, it is the volume
flow rate that the actual mass flow rate would have produced had it been delivered at standard
temperature and pressure.

9.1 Nomenclature
Mass. Quantity of matter. When weighed under standard gravitational conditions (i.e.,
on the average surface of the earth), one pound mass weighs one pound (exerts a
force of one pound) and 1 kg exerts a force of 9.8 N. Symbol: lb or kg.
m
Mass Flow Rate. The rate at which matter passes the measuring location. Pounds mass
(or kg) per unit time. Symbol: W or ˙m depending on the author.
Volume Flow Rate. The volume occupied by the mass passing the measuring location.
Cubic feet (or cubic meters) per unit time. Symbol: Q.
Standard Conditions. An arbitrarily chosen pair of values (different organizations may
use different values) for temperature and pressure used to describe a standard state
for measurement of the density of a gas. (e.g., 70 F and 14.7 psia or 20 C and 760
mm Hg).
Standard Density (...of a gas). The density (lb per cubic foot or kg/m ) of the gas
3
m
in question when its temperature and pressure are those of standard conditions.
The following symbols and terms are commonly used in flow-metering situations. In
the rate statements that follow, the time base is taken as minutes or seconds.