Nothing says the adjustable altimeter must use
inches of mercury. (However, installation of a replacement instrument needs the right paperwork.) The Pacer was type certificated under CAR 3 so there's no requirement for pitot heat and it has no published limitation against IFR flight.
The numbers indicate that this barometer can measure between 29 and 31
inches of mercury. While you can read many barometers to within 2/100 (.02) inch of mercury, readings to 1/10 (.1) inch is normally good enough.
Pulling a wide range, such as 25-28
inches of mercury, will be detrimental to achieving a reliable reading.
If you're cleaning liquid waste only--sludge for example--it requires a relatively small cfm number but high pressure as measured in
inches of mercury, say 25 to 26 inches.
The committee picked a standard calling for the devices to reduce the vacuum to less than 4.5
inches of mercury within three seconds.