5 Service and Maintenance
36
Series 275 Convectron Gauge Bridge Amp PC Board
Instruction Manual - 275331
5.3.2 Convectron Gauge Calibration
An atmospheric pressure calibration is performed on the Convectron Gauge using N
2
at the factory
before the gauge is shipped. The factory calibration sets the atmospheric calibration point for N
2
to
760 Torr.
Periodic resets of the vacuum chamber calibration point improve the accuracy and repeatability of
the Convectron Gauge. Regardless of the process gas being used, always use N
2
or air to calibrate
the Convectron Gauge at vacuum chamber pressure.
1.
Using N
2
or air, pump down the vacuum chamber pressure to a pressure that is lower than
10
–4
Torr. When the pressure indication in the 1 x 10
–4
Torr range has stabilized, a Convectron
Gauge calibration at vacuum chamber pressure may be performed.
2.
Use a high-precision, high-input impedance (Zin > 1 MW) DVM to measure the voltage across
the analog output connections (pins #11 & 12 of the interconnect cable - see Figure 3-4).
3.
Use a flat-head instrument screwdriver to adjust the ADJUST VACUUM potentiometer to a
reading of 0.0 Vdc. The vacuum adjust potentiometer is # R201, shown on Figure 2-2 and
Figure 3-5.
5.3.3 Convectron Gauge Test Procedure
The small diameter sensor wire inside the gauge tube can be damaged by even small voltages. Do
not perform electrical continuity tests with instruments applying in excess of 0.1 volt when the
gauge is at vacuum, or 2 volts when at atmospheric pressure.
1.
Vent the vacuum chamber to atmosphere and allow the gauge to cool to ambient temerature.
2.
Disconnect the cable fromthe gauge.
3.
Use a
low-voltage (maximum 0.1 V) ohmmeter
to check resistance values across the pins on
the gauge. See Figure 5-1.
The
g
auge should show the resistances values listed in Figure 5-1. Pin numbers are embossed on the
blue plastic pin feed-through of the gauge. Measure the resistance across the pins with the gauge at
atmospheric pressure, using an ohmmeter that will not apply more than 10 mA.
4.
Replace the gauge tube if required.
5.
Connect the cable and check the gauge for proper operation.
Figure 5-1
Convectron Gauge Connector and Resistance in Ohms
• Pins 1 to 2: 19 to 22 ohms
• Pins 2 to 3: 50 to 60 ohms
• Pins 1 to 5: 180 to 185 ohms
If the resistance from pins 1 to 2 reads about 800 ohms,
the sensor wire in the gauge is broken. Replace the
gauge tube.
If the resistance values shown here are correct,
but you still think the gauge is not reading
correctly, the gold plating on the sensor wire
may be eroded and the gauge will have to be
replaced.