Most common cause of poor pressure control at set points below atmospheric pressure is a malfunctioning vacuum sensor inside the EXHAUST port. The vacuum sensor is used to detect if vacuum is present at the EXHAUST because different control algorithms are used with and without vacuum.
Check that the vacuum sensor in the EXHAUST port is working with these instructions for a PPC4 pressure controller with the advanced color display. See PPC4 manual for menu path if have a PPC4 with the smaller green/black basic display.
[Settings], <More >, <Internal>, <Control Reference>. This should show the current state of the control reference sensor that is inside of the EXHAUST port. If you do not use a vacuum pump this should show “Atmosphere, Auto”. Auto is the default setting for “Setting”.
If you have a vacuum pump connected to the EXHAUST port: When this is set to Auto the state will change from Vacuum to Atmosphere when a vacuum pump to the EXHAUST port is turned off. You have to go Back from this screen and then back into it to update the State. If you have a vacuum pump connected can you check that the control reference sensor is working correctly? If you don’t have a vacuum pump please confirm that this screen shows “Atmosphere, Auto”.
It might also be worth opening the chassis to inspect the tubing that connects to the vacuum sensor as shown in the pics below. There have been some cases where it has popped off, ruptured, or the tubing has a crack in it.
Sensor replacement:
Vacuum sensor part number: 3137332
Description: 102603,XDUCER,110KPAA,PRESSURE SENSOR
*** To get pricing, an official quote sent, or place order can contact Fluke Customer Care at +1.800.752.5717 or can reach them through fluke-info@fluke.com
PPC4:
Another thing to try for fixing control issues is mild reset by this path; [Tools], <System>, <Reset>, <Settings>.
Sometimes updating the firmware can fix control problems. Instructions and a log of firmware revisions is here, PPC4 firmware
See more details in the PPC4 manual also, available at this link, PPC4 manual
If you have questions contact Fluke Calibration Pressure Technical Support at +1.877.355.3225 or contact us at the below link.