The below content is from the 3990 Manual Pressure Control Pack Operator's Manual that is available at this link, Fluke Calibration Product Manuals
The content is specific to the 3990 Pressure Control Pack but also applies to older Pressure Control Packs. The 3990 Pressure Control Pack has valves and a schematic on it that make switching modes easier, and without breaking and switching tubing connections.
Also see the slides from this Webinar, Expanding Use of the PG webinar.pdf
Specifically, see slides 12 and 13, and slides 22 to 26 for negative gauge mode with a 2465 piston gauge and 3990 manual pressure control pack.
Vacuum (Negative Gauge) Mode
If the bottom of the piston is exposed to ambient barometric pressure, the top of the piston is used as test pressure. The test pressure must be lower than ambient pressure, and is referred to as a negative gauge or vacuum mode pressure. In practice, the bottom of the 3990-801 DWG piston is connected to the reference port of the device under test, and both the 3990-801 and the device under test are totally isolated from ambient pressure. Thus, the test port pressure is not truly a negative gauge mode pressure. Rather, it is a negative differential pressure referenced to a pressure that is very close to ambient, typically within about 0.25 kPa (0.036 psi) of current barometric pressure. Since both the DWG and the device under test are referenced to the same near-ambient pressure, the device under test responds only to differential pressure across the DWG piston.
Important Setup info/Further details on how it works
If look at the schematic on top of the 3990 you will see the Pressure Adjuster (also referred to as as a Variable Volume or fine adjustment) will be connected to what is now the reference side under the piston via the Test Port connection as well as to the Reference port going to the DUT.
Some DUTs might not have a Reference connection on them so the fine adjust feature will not be available. A potential work around is to use the metering valve on the 2465 bell jar to provide some adjustment, but it would only be positive adjustment up to ATM. If the Reference connection is not made the Pressure Adjuster will not do anything to effect the test pressure as it would be open to ATM. If the Reference connection is capped the Pressure Adjuster would only change the pressure to the reference side of the piston which would cause a measurement error as only the bottom of the piston would see the reference pressure change and not the DUT.
When the connections are made in the normal configuration to the Reference port as shown above, it initially traps a pressure that is the current ATM pressure under the piston as well as on the reference port of the DUT and the reference side becomes a sealed system rather than open to ATM changes. This assumes the DUT is not by other means venting to ATM by another open connection as there are some sensor designs that do this through say another "vent" port and thus would not allow using this method. You can then use the Pressure Adjuster to make small positive or negative pressure adjustments that are applied to both the bottom of the piston and reference port of the DUT to help float the piston with fine pressure control. This is really more of a Differential mode, but it does mimic what a Negative Gauge mode test would be too.
Their are alternative connections of the Reference port regardless if it is available or not on the DUT that might be preferred. If the Reference port is not connected and the Reference Vent is left open it would be a more of a traditional Negative Gauge mode test as both the 2465 and DUT reference would be open to ATM, but would be limited to floating the piston with just the coarse control only as previously mentioned. It is typically better to connect the DUT to the Reference port if it has this connection, but could then just leave the Reference Vent valve open and thus both are now referencing the same ATM pressure to help eliminate any small reference location pressure difference. These alternatives are mentioned as some might prefer this as there is perhaps some risk on some devices that if unintentionally developed too much pressure change on the reference side you could damage the device or not be within typical ATM range. For example, some DUTs incorporate a barometer sensor tied into their reference port so if somehow got over normal barometric range could shift them or perhaps pop off a very low pressure tube internally.
Vacuum Mode (Negative Gauge) Operation
Please refer to Figures 1, 2, or 3, as appropriate.
- Verify that the mode valve is set to Vacuum (Negative Gauge).
- Verify that the bell jar is mounted on the deadweight gauge.
- Close the Test Port Vent valve and carefully open the Vacuum Supply valve to decrease pressure in the test port and in the DWG bell jar.
a. Adjust the Vacuum Supply valve to achieve the desired rate of change in pressure.
b. Close the Vacuum Supply valve once the pressure is close to the target value or the piston begins to float.
c. Close the Reference Vent valve.
d. Push to close the Equalize valve and rotate the Pressure Adjuster knob to adjust the final pressure and float position. - If the Pressure Adjuster must be turned too many rotations to establish a floating pressure:
a. Pull to open the Equalize valve.
b. Open the Reference Vent valve, then use the Vacuum Supply or Test Port Vent valve to bring the pressure closer to the target value.
c. Close the Reference Vent valve, push to close the Equalize knob and rotate the Pressure Adjuster knob to adjust the final pressure and float position. - If the Pressure Adjuster knob will not rotate, it is probably at the end of its stroke. In this case:
a. Pull to open the Equalize valve.
b. Open the Reference Vent valve, then rotate the Pressure Adjuster knob in the opposite direction about 10 to 20 turns.
c. Close the Reference Vent valve, push to close the Equalize valve and rotate the Pressure Adjuster knob to adjust the final pressure and float position.