Instrutech CVM-211 User Manual

Instruction Manual InstruTech CVM-211 "Stinger" Convection Vacuum Gauge Module
InstruTech, Inc page 11
A
A
p
p
p
p
e
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n
n
d
d
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x
x
B
B
:
:
U
U
s
s
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i
n
n
g
g
t
t
h
h
e
e
r
r
m
m
a
a
l
l
v
v
a
a
c
c
u
u
u
u
m
m
g
g
a
a
u
u
g
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s
s
w
w
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t
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Principal of Operation
Thermal conductivity gauges measure pressure indirectly, by sensing the loss of heat from a sensor to the
surrounding gases. The higher the pressure of the surrounding gas, the more heat is conducted away from the
sensor. Pirani thermal conductivity gauges maintain a sensor (usually a wire) at some constant temperature, and
measure the current or power required to maintain that temperature. A standard Pirani gauge has a useful
measuring range of about 10
-4
Torr to 10 Torr. By taking advantage of convection currents that are generated
above 1 Torr, convection-enhanced Pirani gauges increase the measuring range to just above atmosphere.
Output Signals and Displays
A vacuum gauge sensor, by itself, does not generate any output signal or readout. Electronic signal conditioning
is required to convert heat loss into a useable signal and/or readout of pressure. Note that the output signal
voltage is not the same as the displayed pressure readings. Table 3 below shows the displayed readings at
various pressures for selected gases.
Non-Linear Output
Granville-Phillips' first Convectron
®
gauge controllers
produced a non-linear output signal of 0.375 to 5.659 VDC
for 0 to 1000 Torr of N
2
, roughly in the shape of an "S" curve,
as shown at right. GP adopted the same output curve for
most of their Mini-Convectron
®
modules and controllers with
non-linear output (though in recent years, some GP
controllers produced a different S-curve).
The non-linear output from InstruTech convection gauges,
modules, and controllers duplicates exactly the original S-
curve of 0.375 to 5.659 VDC for 0 to 1000 Torr.
Table 1 below contains the lookup data for converting the
non-linear output voltage into pressure values.
Log-Linear Output
Many InstruTech modules and controllers also provide a log-
linear output signal, as an alternative to the non-linear signal
described above. This output, shown at right, is a 1 Volt per
decade signal that may be easier to use for data logging or
control.
Table 2 below contains the lookup data and provides the
formulas for converting the log-linear output voltage into
pressure values.
The above discussion of output signal assume the gas being
Measured is Nitrogen. The following discussion illustrates how
Other gases affect both readings and outputs from thermal
Vacuum gauge.
Non-Linear Analog Output
10
+3
10
-3
10
-2
10
-1
10
0
10
+1
10
+2
1
2
3
4
5
6
Pressure - Torr
Output - Volts
Non-Linear Analog Output
10
+3
10
-3
10
-2
10
-1
10
0
10
+1
10
+2
1
2
3
4
5
6
Pressure - Torr
Output - Volts
10
+3
10
-3
10
-2
10
-1
10
0
10
+1
10
+2
1
2
3
4
5
6
Pressure - Torr
Output - Volts
10
+3
10
-3
10
-2
10
-1
10
0
10
+1
10
+2
2
3
4
5
6
7
8
Pressure - Torr
Output - Volts
Log-Linear Analog Output
1
10
+3
10
-3
10
-2
10
-1
10
0
10
+1
10
+2
2
3
4
5
6
7
8
Pressure - Torr
Output - Volts
Log-Linear Analog Output
1

Instrutech CVM-211 Specifications

General

Brand	Instrutech
Model	CVM-211
Category	Control Unit
Language	English