This is a mode of operation for the PG7601 that allows a pressure to be defined as either a "negative gauge" or a pressure which is lower than the minimum mass load of the PG7601. Defining these types of pressure with a piston gauge can be attractive because it provides a stability and uncertainty that is better than many of the industrialized calibrators specifically designed for this application.
Document 9940TN02 is an application note with an overview of how to do differential mode (negative gauge mode also) measurements with a PG7601 and an uncertainty analysis which addresses the specifics of operating in differential mode. The linked copy (below) has been revised to include updated uncertainty for the PG7601, and simplified connection diagrams.
The RPM3 is no longer available and has been replaced by the RPM4 A100K. The 3990 manual pressure controller can be used instead of the PK-7600-DIF differential mode controller.
For "How to Use" instruction, please see this article: PG7601 Differential Mode / Negative Gauge mode -- How to Use
The document "Expanding Use of the PG Webinar.pdf" is the presentation from a Webinar about this topic. Speaker notes are available in the upper left hand corner of each slide.
A spreadsheet is provided that gives typical uncertainties for using the PG7601 in Differential Mode.
For more information, please contact the Pressure Technical Support Team at Pressure Support, or call Fluke Calibration at +1.877.355.3225