61
937A Multi-Sensor System
Logarithmic Output
Logarithmic output is scaled for 0.6 V per decade of pressure. Only one
curve is needed to determine the pressure from the logarithmic output for
any sensor (see Appendix C). For example, at 1.0 x 10
-3
Torr, a Pirani
sensor will have the same logarithmic voltage output value, 5.4 V, as a
cold cathode sensor.
The pressure
P as a function of voltage V
is calculated by the equation,
Inversely,
where,
P is in Torr
V is in volts.
For example, if
V = 5 V, then P = 10
(8.33-12)
= 10
-3.67
= 2.0 x 10
-4
Torr.
A data acquisition system with a 12-bit A/D converter and an input range of
0 to 10 V will have enough resolution to resolve the equivalent of 1% of
reading in the logarithmic output.
If Logarithmic Output Is
0.6 to 9.6 V
0.2 V
9.8 V
10 V
10V
10V
10V
0 V
Then
Normal range
Sensor exposed to a pressure less than in its
measurement range (LO displayed)
Sensor exposed to a pressure higher than in its
measurement range (HI displayed)
Thermal conductivity sensor not connected
properly or filament is broke
Cold cathode high voltage disabled
No sensor on a channel
Immediately after the Series 937A Controller is
switched on
When the Controller is not powered, but never
when power is on
P =10
( )
V
0.6
-12
V = 0.6 [log
10
(
P) + 12]
V (volts) P(Torr) V(volts) P(Torr)
0.6
1.0x10
-11
5.4
1.0x10
-3
1.2
1.0x10
-10
6.0
1.0x10
-2
1.8
1.0x10
-9
6.6
1.0x10
-1
2.4
1.0x10
-8
7.2
1.0x10
0
3.0
1.0x10
-7
7.8
1.0x10
+1
3.6
1.0x10
-6
8.4
1.0x10
+2
4.2
1.0x10
-5
9.0
1.0x10
+3
4.2
1.0x10
-4
9.6
1.0x10
+4