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Instruction Manual
Linear Output
Some InstruTech controllers & modules offer a user-
programmable linear analog output. In this case, the user
chooses the two end points, and the signal is a linear output
voltage between those two points.
A couple of examples are shown at right:
Example-A: A full scale of 10 Volts was chosen to equal 1
Torr, and 10 mVolts was chosen to equal "zero" pressure
(considered here to be 10
default settings, which also happen to be the same as the
Granville-Phillips linear output.]
Example-B: In this case, a full scale of 5 Volts equals 100
mTorr and 1 Volt equals 10 mTorr.
Because you can choose virtually any zero and span settings,
there are an infinite number of possible linear signal
configurations. If needed, you must generate your own table
of signal vs pressure values for your particular setup.
Some considerations in choosing the best analog signal to work
1. The non-linear S-curve is convenient if you are replacing a Convectron
want to change anything in the control system. For new systems, however, the formulas needed to interpret the
non-linear signal are complex and difficult to use. The linear or log-linear signal is generally easier to interface
with today's data acquisition or control systems.
2. The linear signal is good when you want to monitor pressure over a limited range of 1 or 2 decades. If you try
to use the linear signal over too wide a range, the signal at the low end of pressures will be too small to be
useable by most electronic systems. For example: If you tried to setup the linear signal to cover the range from
-3
1000 to 10
Torr, the signal at 10
lost in the noise and you could never see it. For this reason, and to distinguish a valid low pressure reading from
a power-loss/off/fail condition, the minimum linear signal cannot be set lower than 10 milliVolts.
3. If you need to monitor pressure over a wider range, you should use the log-linear signal. This output covers
the entire measuring range with a high level 1 to 8 Volt signal. The signal changes by 1 Volt for each decade,
and, even at the very low end, the smallest signal is still milliVolts, not microVolts. Also, the formulas defining the
relationship between Voltage and pressure are simple and can be readily programmed into most computers /
data acquisition systems.
InstruTech, Inc
-4
Torr). [Note: These are the factory
-3
Torr would be 1 microVolt. For most systems, that level of signal would be
InstruTech VGC-301 Convection Vacuum Gauge Controller
with:
®
10
10
Output - Volts
Output - Volts
8
8
Linear
Linear
Analog
Analog
6
6
Output
Output
4
4
example A
example A
2
2
Pressure - Torr
Pressure - Torr
0
0
0.1
0.1
0.5
0.5
5
5
Output - Volts
Output - Volts
4
4
Linear
Linear
Analog
Analog
3
3
Output
Output
2
2
example B
example B
1
1
Pressure - milliTorr
Pressure - milliTorr
0
0
10
10
50
50
which has that output, and you don't
1
1
100
100
page 17
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