Page 189 - Mechanical Engineers' Handbook (Volume 2)

178 Temperature and Flow Transducers

Figure 27 Variable area flowmeter pressure distribution. (From Ref. 67, with permission.)

The float position then directly reads in percentage of full-scale flow provided the inlet
pressure is 25 psig, the inlet temperature is 70 F, and the specific gravity of the gas flowing
is 1.38.
For off-design operation of the variable-area meter it is necessary to correct for changes
in pressure, temperature, or specific gravity.
A sufficiently accurate equation for gas flow service can be derived from a force balance
on the float in combination with Eq. (25), thus resulting in the fundamental equation for
flow measurement

W W USE (27)
USE
calib

calib
where W USE mass flow rate under actual conditions of use, lb /s (kg/s)
m
W calib mass flow rate under calibration conditions for same float position, lb /s
m
(kg/s)
3
3
USE density of fluid flowing, lb /ft (kg/m )
m
3
3
calib density of calibration fluid, lb /ft (kg/m )
m
Example. Assume that the meter previously described is to be used in a new application
where the fluid is air (Specific gravity 1.00), the pressure is 100 psig, and the temperature
is 70 F. What would be the full-scale flow rate in SCFM (air)?
USE
W USE W calib
calib
P (100 14.7)(144)
USE USE
R AIR USE 53.3 (460 70)
T
0.585 lb /ft 3
m