MKS 247D - Scaling Controls

Scaling Controls Cha
p
ter Three: Overview
46
Scaling Controls
There are four
Scaling Control Potentiometers
on the rear panel of the 247 unit; one for each
channel (refer to Figure 6, page 43). These 10-turn potentiometers are used to adjust the full
scale voltage signals from the MFCs, which correspond to the flow rate, to a level that enables
the digital panel meter (DPM) to display the flow rate and set point directly, in sccm or slm.
The 247 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 MFCs. Therefore, the +5 VDC output voltage from each 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 MFCs is set with the
Scaling Control Potentiometers
.
Note
It is
critical
for proper system operation that the Scaling Control Factors
are calculated properly and that the Scaling Control Potentiometers are
set correctly.