MKS 247D - Scaling Control Factor Calculation

How To Setup the System Cha
p
ter Four: O
p
eration
56
6. Calculate the SCALING CONTROL FACTOR for
each
MFC in use.
The Scaling Control Factor specifies how much the +5 VDC output signal from the MFC
will be scaled down so that the flow rate can be read directly from the Digital Panel
Meter, which reads 1 VDC as 1000 counts full scale.
Note
It is
critical
for
p
ro
p
er s
y
stem o
p
eration that the Scalin
g
Control Factors
are calculated properly. Refer to
Scaling Controls
, page 46, for more
information.
a. The
Scaling Control Factor
is the product of the Gauge Factor for the MFC in use
and the Gas Correction Factor for the gas in use:
S
CALING
C
ONTROL
F
ACTOR
=G
AUGE
F
ACTOR
x G
AS
C
ORRECTION
F
ACTOR
b. The
Gauge Factor
is a factory set value which scales the 5 VDC signal to the
appropriate full scale range for the MFC, so that the digital panel meter reads 1000
counts. The gauge factors for various flow ranges are listed in Table 10, page 47.
c. The
Gas Correction Factor (GCF)
is used to indicate the ratio of flow rates of
different gases which will produce the same output voltage from a mass flow
controller. The GCF is a function of specific heat, density, and the molecular
structure of the gases. Since flow controllers are usually calibrated with nitrogen,
nitrogen is used as the baseline gas (GCF = 1). Refer to Table 17, page 83, for a list
of GCFs for commonly used pure gases.
If the pure gas you are using is not listed in Table 17, page 83, or you are using a gas
mixture, you must calculate its GCF. Refer to
Gas Correction Factor (GCF)
, page
49, for more information.
Example:
To calculate the Scaling Control Factor for a 20 sccm MFC, which is flowing pure argon
gas, multiply the Gauge Factor for argon (200), listed in Table 10, page 47, times the
Gas Correction Factor for argon (1.44), listed in Table 17, page 83:
S
CALING
C
ONTROL
F
ACTOR
=G
AUGE
F
ACTOR
x G
AS
C
ORRECTION
F
ACTOR
= 200 x 1.44
= 288
Since the full scale setting of the Scaling Control Potentiometer is 1000, the Scaling
Control Factor of 288 is 28.8% of FS.