5 Electrometer Module
47
Series 340 Vacuum Gauge Controller
Instruction Manual - 340005 - Rev. D
5.3.2 Emission Range Switch
The Emission Range Switch (Figure 5-2) selects between three emission ranges; 0.1 milliampere, 1.0
milliampere, or 10.0 milliamperes. Adjustment within each range is achieved with the emission
adjustment potentiometer. See Section 5.3.3 below.
In general, higher emissions are used at lower pressures. For example, if you are using a broad
range Ion gauge and want to measure to its upper pressure limit, the 0.1 mA range is recommended.
If you are measuring very low pressures or have a low-sensitivity ion gauge, the 10.0 mA range is
better. In general, lower emissions will increase gauge life.
NOTE: Changing the emission range by one decade will also change the
overpressure shutdown point by one decade. Refer to Section 5.4.3 for details of
the overpressure shutdown adjustment. Adjustment of emission within a range
(see below) will not affect the overpressure shutdown point.
5.3.3 Emission Adjustment
The Emission Adjustment Potentiometer (Figure 5-2) provides control of the emission within the
decade value selected by the emission range switch. The Calibration Switch must be set with the IG
tube turned on to view emission during adjustment. The span of adjustment is from approximately
10% to 120% of the range value.
Theoretically, varying the emission current will not affect the pressure reading since the
electrometer is actually calculating:
NOTE: There can be slight differences depending
on the gauge cleanliness and gauge pumping.
5.3.4 Sensitivity Adjustment
The Sensitivity Adjustment Potentiometer (Figure 5-2) is used to match pressure outputs to ion
gauges of different sensitivities. The Calibration Switch must be set to the left or to the right with the
ion gauge OFF to view the sensitivity during the adjustment.
The 340 VGC is shipped from the factory set for a gauge sensitivity of 10/Torr, which is typical for
glass Bayard-Alpert gauges. UHV nude gauges, such as the Granville-Phillips 274022, 274023 and
274028, typically have a sensitivity of about 25/Torr for N
2
. If an ion gauge of this type is being
used, the sensitivity potentiometer should be adjusted for the proper sensitivity (25/Torr); otherwise,
a measurement error will result. The approximate range of the adjustment is 3 to 50/Torr.
Do not leave the calibration switch set to the left or right after you are done viewing
sensitivity or emission; otherwise, the displayed reading may be mistaken for an
actual pressure reading.
P =
I +
S (I-)
OR
PRESSURE =
ION CURRENT
SENSITIVITY (EMISSION CURRENT)
