User's Manual

174-082010_Digital AVC Page 15 of 17
3.6 Gauge Tube Operating Principle
Operation of the Hastings gauge tube is based on a low voltage AC bridge that heats a noble metal
thermopile. A change in pressure in the gauge tube changes the molecular collision rate and therefore the
thermal conduction of the gas or gas mixture surrounding the thermopile. This results in a temperature shift
in the AC heated thermocouples A and B (Fig. 6). The resultant temperature shift causes a change in the
DC output from couples A and B inversely with pressure changes. The DC thermocouple C (when installed)
is in series with the circuit load. Thermocouple C provides compensation for transient changes in ambient
temperature.
3.7 Calibration Procedure
NOTE: ONCE CALIBRATION IS COMPLETE THE CALIBRATION DATA IS PERMANENTLY STORED
IN NON-VOLATILE MEMORY. A LOSS OF POWER WILL NOT ERASE THE CALIBRATION DATA. TO
ERASE THE CALIBRATION DATA, REFER TO STEP 2.
3.7.1 Preparation
1. The following procedures can be carried out on a unit installed in a vacuum system as long as a
calibrated reference meter is installed in the same system in close proximity to the unit being
calibrated.
2. The following procedure assumes that the appropriate DV4, DV5 or DV6 gauge tube (See the
bottom of the instrument to determine the appropriate tube) corresponding to the set up of the DAVC
is connected to the unit.
3. Power the DAVC with a DC supply capable of providing from 12 to 30 Volts DC and at least 0.5
Amps.
4. Using a pointed object, such as a ballpoint pen, press and hold the “ATM” push button located on
the top cover until the two LEDS are on continuously (approximately 3 seconds). This deletes any
previously stored data.
3.7.2 Set High End
NOTE: TUBE MUST BE AT ATMOSPHERE TO PROPERLY SET THE HIGH END.
While at one atmosphere, press the ATM button and release to set the atmosphere tube output for this
individual tube. The LEDS will flash while the button is pressed. Push the button only as long as it takes
the LED’s to blink one or twice. Holding the button longer than 3 seconds will reset the adjustment back to
the default value. The high-end setting is now set.
3.7.3 Set Low End W/Vacuum System
NOTE: The LOW END can be adjusted by either bringing the system to a known vacuum or by using a
HASTINGS REFERENCE TUBE (see below).
A. Set system to known vacuum.
B. Turn the HTR potentiometer on the top panel until the either the voltage measured between pins 6,
Analog Signal Common (violet) and 7, Pressure Signal Output (yellow) reads the voltage
corresponding to the pressure as read on the Pressure Vs. Voltage chart for the tube being
measured or, if using serial communication, until the proper pressure is read while using the
streaming P1 command.
C. The low end is now adjusted.
3.7.4 Set Low End W/Reference Tube.
A. Connect the THI reference tube.