17
Appendix B
Standard Flow Rate vs. Volumetric Flow Rate
Since thermal flow sensors are sensitive to changes in air density and
air velocity, all thermal flowmeters indicate flow rates with reference to
a set of standard conditions. For TSI instruments, standard conditions
are defined as 21.1° C (70° F) and 101.3 kPa (14.7 psia). Other
manufacturers may use different values.
Standard flow rate is the flow rate the air would be moving if the
temperature and pressure were at standard conditions. It is usually
the most useful measure of airflow because it defines the heat-
carrying capacity of the air.
Volumetric flow rate is the true volume flow of the gas exiting the
flowmeter.
In some instances, volumetric flow rate rather than standard flow rate
may be of interest. To display volumetric flow rate, the Models
4040/4043/4045 will multiply the standard flow measurement by the
following density correction factor:
m
m
P
3
.
101
11
.
21
15
.
273
T
15
.
273
)
Flow
Standard
(
Flow
Volumetric
Where
Tm = Gas temperature measured in flow tube in units of degrees
Celsius
Pm = Absolute pressure measured in flow tube in units of kPa
To demonstrate the difference in output, consider the following.
Measured flow displays 100 Std L/min at 15C and 117kPa. The
volumetric flow as calculated by the Models 4040/4043/4045 would
be as follows.
min
/
L
78
.
84
117
3
.
101
11
.
21
15
.
273
15
15
.
273
)
100
(
Flow
Volumetric
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