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172-082010_400 I Series Software Page 10 of 35
The series will converge when the resultant R is equal to the original SL value divided by the
gas conversion factor GCF. At this point the guess value G is equal to normalized sensor value
S.
From this the sensor voltage can be recovered by multiplying by the sensor full scale voltage
and adding the sensor offset voltage.
This is the value that should be reported back by the sensor when the flow reaches the desired
value. However the sensor voltage is offset from circuit common by 2.00 volts. Therefore the
voltage that should be generated by the pulse width modulator circuit (PWM) is the sensor
voltage plus 2.00 volts.
The PWM circuit operates on counts with 0 counts being 5 volts and 64000 counts being equal
to 0 volts. To convert to counts, divide the PWM by 5 volts and multiply by 64000. Subtract
this number from 64000 to get the counts to actually transmit to the PWM circuit.
This number should be transmitted to the PWM and then sufficient time should be allowed for
the PID loop to open the valve if necessary and to come to a stable final value. Small
adjustments may be made after this time if the final value is not precisely the desired value.
1
G
GCF
SL
=
()
(
)
[
]
1
5
1
3
11
)( RGCGBGA =
′
+
′
+
211
)( GR
GCF
SL
G =−+
()
(
)
[
]
2
5
2
3
22
)( RGCGBGA =
′
+
′
+
ADADSFSS
=
+
⋅
0
PWMAD
=
+
00.2
PWM
PWM
=
⎟
⎠
⎞
⎜
⎝
⎛
−
5
164000