Bronkhorst IN-FLOW - Manual (Page 17)

BRONKHORST HIGH-TECH B.V.
9.17.022 page 17
1.6 Conversion factors
1.6.1 Gas conversion factors (by-pass measurement)
The general formula for determining the relationship between signal and mass flow is:
V K c K c
signal p m p v
= ⋅ ⋅ = ⋅ ⋅ ⋅ Φ Φ ρ
in which:
V
signal
= output signal
K = constant
ρ = density
c
p
= specific heat
Φ
m
= mass flow
Φ
v
= volume flow
As soon as the c
p
value and density of the gas to be metered change, the signal must be corrected. The
conversion factor C is:
C
c
c
p
p
=
⋅
⋅
1
2
1
2
ρ
ρ
in which:
c
p
= specific heat
ρ
n
= density at normal conditions
(1) gas calibrated
(2) gas to be measured
Note:
The c
p
value used for the calculation of the conversion factor must be taken at a temperature approx. 50°C.
higher than the required temperature.
This factor is called c
p
cal.
The conversion factors for commonly used gases related to N
2
at normal conditions are stated in the Gas
Conversion Table in the appendix 1.
Example:
Meter calibrated on N
2
(200 ml
n
/min).
Gas flow passing the meter is CO
2
.
Output signal reads 80.0%.
Actual CO
2
flow = 80.0 ⋅ 0.74
1.00
= 59.2%
so
59 2
100
. ⋅
200 = 118.4 ml
n
/min
* n means normal conditions
At normal conditions volumes are converted to a temperature of 0°C and pressure of 1 atm
or 1013,25 mbar. (760 Torr)
Note:
Best accuracy is always achieved by performing calibration under operating conditions. Should this not be
possible or practical, then the use of a theoretical conversion factor is a means to determine the flow rate of
the instrument on the gas to be metered, however, it will introduce inaccuracies.