Page 191 - Mechanical Engineers' Handbook (Volume 2)

180 Temperature and Flow Transducers

Figure 28 Laminar flowmeter pressure distribution. (From Ref. 67, with permission.)

The SCFM flow rate can be taken directly from the calibration curve if the meter is
being used at 29.92 in. Hg and 70 F. If the pressure is 2 atm, however, then the density, the
mass flow, and the number of SCFM corresponding to the same pressure drop signal would
be doubled. Tables of pressure corrections, often provided with each meter, simplify this
calculation.
If the temperature of the metered flow is higher than 70 F, the density is less than
standard density and the viscosity of the flow is higher. Temperature correction factors, often
provided in tabular form, account for both effects. The temperature correction factor for air
flow is roughly proportional to absolute temperature raised to the 1.7 power, and it accounts
for both the decrease in density and the increase in viscosity.
The equation for a laminar flowmeter in terms of the calibration chart is
SCFM (chart CFM)(CP)(CT)
or
SCFM (chart CFM) 530 1.7
P
act
29.92 460 T act (28)
Laminar flowmeters respond linearly to flow; hence they can be used to directly measure
the time average of a fluctuating flow. Commercial meters will usually average properly up
to about 100 Hz.

9.6 Instability Meters
Two modern flow-metering systems derive their signal from the frequency of occurrence of
an unstable fluid-dynamic phenomenon. One uses a swirling flow in a divergent passage to
generate a precessing stall region. The other introduces a prismatic bluff body into a pipe
and generates a periodic vortex trail. In each case, the frequency of the unstable event is
related to the volume flow rate through the meter. Each claims 100 1 usable range, and each
produces a pulse train whose repetition rate is linearly related to flow.
The Swirlmeter (registered trademark of the Fischer and Porter Co.) is shown schemat-
ically in Fig. 29. The meter has no moving parts. Stationary blades impart a swirling motion