Bronkhorst EX-FLOW Mass Flow - Manual (Page 9)

BRONKHORST HIGH-TECH B.V.
1.5 Gas conversion factors
The general formula for determining the relationship between signal and mass flow is:
V K c K c
signal p m p v
= ⋅ ⋅ ⋅ = ⋅ ⋅ ⋅ ρ ρ Φ Φ
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 reads:
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 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 Appendix 1.
Example:
Meter calibrated on N
2
(200 ml
n
/min) .
Gasflow passing the meter is CO
2
Output signal reads 80.0%.
Actual
CO
2
-flow = 80.0 0.74
1.000
= 59.2%
or 59.2
100
⋅ 200 = 118.4 ml / min
n
n means normal conditions,
at normal conditions volumes are converted to a temperature of 0°C and pressure of
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 or the gas to be metered, however, it will introduce inaccuracies.
The approximate accuracy of the conversion factors listed is typical for conversion factors;
> 1 2% x factor
< 1 2% / factor
9.17.028 page 9