45
Series 340 Vacuum Gauge Controller
Instruction Manual - 340005 - Rev. D
Chapter 5
5
Electrometer Module
5.1 Introduction to the Electrometer Module
The Ion Gauge (IG) Electrometer Module provides ion gauge pressure readout from less than
1 x 10
-9
Torr (1.3 x 10
-9
mbar or 1.3 x 10
-7
pascal) to 1 x 10
-1
Torr, air equivalent, depending on the
gauge and emission current used.
Adjustments are provided for gauge sensitivity and emission current. Adjustment and an internal
switch allow change to mbar or pascal pressure units. A user selectable
slow update
feature triggers
measurement averaging, resulting in a display update frequency of about once every three seconds.
The overpressure shutdown threshold is adjustable on the Electometer Module PC board. See Figure
5-1.
Internal failure-indicator LED’s, Figure 5-1, aid diagnosis of problems by indicating certain
out-of-bounds electronic conditions.
5.2 Units of Measure
The Series 340 Vacuum Gauge Controller (VGC) is shipped from the factory preset to display the
unit of measure — Torr, millibar, or pascal — that you requested. To change the units, proceed as
follows:
1.
Turn OFF power to the control unit.
2.
Remove the top cover. Locate the ion gauge (IG) electrometer module.
3.
Locate the unit of measure control switch.
4.
Set the switch to the desired position: OFF = Torr/mbar units; ON = pascal units.
NOTE: If a Convectron gauge will be connected to the 340 VGC, see Section
6.2 on page 51 for the Convectron gauge to set the Units of Measure to be
consistent with the Electrometer module setting.
5.
Slip the label card out of the top of the front panel and insert the appropriate pressure units
label.
Selection between Torr and mbar units is accomplished by adjusting the IG tube sensitivity for the
appropriate units. For example, a typical Bayard-Alpert gauge has a sensitivity of 10/Torr or
7.5/mbar. Thus, for this gauge, adjusting the sensitivity for a display reading of 1.0+1 will result in
display of pressure in Torr. Adjusting to 7.5+0 will result in display in mbar. See Section 5.3.4 for
adjustment procedures.
