5 Operation
25
Series 903 Cold Cathode Vacuum Sensor
Instruction Manual - #109030026
magnetron to reduce these problems by using separate feedthroughs for the anode high voltage and
the cathode current. This geometry uses a cylindrical cathode, a central wire anode, and external
cylindrical magnet which provides an axial field. The cathode is insulated from the grounded metal
housing.
The inverted magnetron geometry has a characteristic electrical conductance vs. pressure curve
which is more reproducible than other arrangements, and also works well to low pressures without
risk of the discharge going out. This gives the 903 Sensor a measuring range from 5 x 10
-3
to
3 x 10
-10
Torr.
5.3 Starting the 903 Sensor
The 903 Sensor starts quickly in low vacuum. In the UHV pressure range, starting may be delayed
by several minutes. The SensaVac IgniTorr™ Cold Cathode Starting Device may be used to reduce
starting time (see Section 6.6, Accessories and Spare Parts).
5.3.1 Reading Pressure
To measure nitrogen gas pressure with the 903 Sensor, turn ON the power and start the high
voltage. Refer to the graph on the next page, showing pressure as a function of output voltage for
nitrogen. To independently convert voltage to pressure, use the equation in the next section.
Cold cathode ionization systems are gas dependent and the Series 903 Sensor's output is defined
for air/nitrogen. See Section 5.6, Using the Series 903 IMT AP with Other Gases, to determine the
true pressure of another gas.
Figure 5-2: Analog Output Voltage Chart
When using the graph, remember that the
pressure scale is logarithmic, and the voltage
scale is linear. Equal increments of distance
along the pressure scale do not correspond
to equal pressure changes.
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