MKS 246C - Gauge Factor Scaling Control

Gauge Factor Scaling Control Cha
p
ter Three: Overview
44
Gauge Factor Scaling Control
There is one Gauge Factor Scaling Control potentiometer on the rear panel of the 246 unit (refer
to Figure 8, page 41). This 10-turn potentiometer is used to adjust the full scale voltage signal
from the MFC, which corresponds to the flow rate, to a level that enables the digital panel meter
(DPM) to display the flow rate directly, in sccm or slm.
The 246 unit uses a digital panel meter that reads 1 VDC as 1000 counts full scale (FS).
Although the DPM can accommodate a
maximum
of 2 VDC and can read up to 1999 counts, it
cannot be adjusted to the 5000 counts needed to accommodate the +5 VDC full scale output
signals from the MFC. Therefore, the +5 VDC output voltage from the MFC must be scaled
down so that the full scale counts (1000) read on the meter represent the full scale voltage from
the MFCs. When the adjustment is properly made, flow can be read directly from the meter.
The amount by which the +5 VDC output signal is scaled down, the
Scaling Control Factor,
is
application dependent and must be calculated for each MFC in use. The Scaling Control Factor
for the MFC is set with the Scaling Control potentiometer.
Note
It is
critical
for proper system operation that the scaling control factor is
calculated properly and that the potentiometer is set correctly.