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9.
Reading Pressure
Pressure display and ranging are automatic in the 921. Most readings will take place between zero
pressure and the full scale of the 921. For pressure lower than the minimum capability of the 921,
the display will show
-LO
. For pressure greater than the maximum full scale of the 921, the dis-
play will show
HI
.
C. Analog Output
The analog output is calculated from the value of the digital display. The output is logarithmic,
0.5 volt/decade; the source impedance for the output is 1 K ohm. The output voltage is calculated
from:
V=0.50*(log
10
(10
10
*Pressure))
where V is the Analog Output in volts; P is the pressure in Torr or mbar.
Some examples follow; because of normal tolerances in the electronics, there may be minor dif-
ferences in the values you observe compared to those shown:
displayed pressure
Analog Output - volts
OFF
or
OP
0.00
1.0 x 10
-10
Torr (mBar) or less
0.00
1.0 x 10
-9
Torr (or mBar)
0.50
3.0 x 10
-9
0.74
1.0 x 10
-8
1.00
1.0 x 10
-7
1.50
1.0 x 10
-6
2.00
1.0 x 10
-5
2.50
1.0 x 10
-4
3.00
1.0 x 10
-3
3.50
1.0 x 10
-2
4.00
The pressure as a function of the Analog Output voltage is:
P=10
-10
*log
-1
(2V)or
P=10
-10
*10
(2V)
Torr (or mbar)
where P is pressure in mTorr or µbar; V is the Analog Output in volts.
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