The WinPrompt manual describes Very Low Gauge Mode operation and notes that it is for pressures very close to atmospheric pressure or for negative gauge pressures. It does not quantify the range. The low point is -atm plus the lowest gauge mode pressure that you can float the piston and sleeve weight at. For example if your atmospheric pressure is 100 kPa absolute (14.5 psia) and the low point that you can float in gauge mode is 7 kPa (1 psi) then you should be able to do a -93 kPa (-13.5 psi) point in Very Low Gauge Mode. The positive range is less than 7 kPa (1 psi). If you enter positive pressures that are higher than valid for Very Low Gauge Mode, then WinPrompt will switch to Gauge and the mass loads will be much lower (vacuum pumps off, etc.)
From the WinPrompt manual...
Very Low Gauge mode is used to generate gauge pressures very close to atmospheric pressure or for negative gauge pressures. When operating in this mode, the deadweight gauge is physically operated in the absolute mode. The first point that is generated in the pressure table is a pressure point that is used to align the internal Auto-float’s barometer. This corrected barometric pressure reading is then subtracted from the deadweight gauge readings for the remainder of the pressure points used in the calibration. This initial point is designated as “R” in the sequence column of the Pressure-to-Mass and Calibration windows. To start the calibration, double click on the first sequence labeled as “R” in the Pressure-to-Mass window. Type the reading from the internal barometric reference sensor as the desired pressure in the Mass/Pressure Conversion pop up window and hit enter or select okay. This value should be obtained from the bottom, center section of the main WinPrompt screen. It should show the current barometric reading followed by Abs Ref. label. Select the trim icon to calculate the proper trim mass value and load the mass and trim mass onto the deadweight gauge. Place the bell jar onto the instrument base and Autofloat this pressure. Once a stable pressure has been obtained, double click on the first sequence labeled “R” in the Calibration window. The value for the internal barometer will automatically be shown in this “Accept“ window. Press the okay button, and then WinPrompt will calculate the other masses required in the remainder of the procedure.
Here is an example screen-view from WinPrompt