I have just managed to pick up a spare UMA 3-310-50 which is widely used in GA as a vacuum gauge
On the back are two ports
and a close inspection reveals that one of them is just a dummy hole which is blocked off.
Testing it with a vacuum pump (with its own gauge) confirms that.
Why do they bother?
On pre-GT TBs they show the gauge (item 30) with just one inlet
whereas on the GTs (=“mod 151”) they show two connections
of which the RH one, which goes to the T-piece of the back of the AI (coupling ref 290), is just a dead end.
Then they can also be used as pressure indicators
I don’t understand. Why have a fake “2-port” instrument, on which the pipe nipples screw in (so why screw in the dead one) and have a dummy pipe going to the dead port in the actual aircraft?
They use the same housing for all their instruments.
When you connected only one port it operates as an absolute pressure indicator (on your pre-GT image)
Using two port system you can will have a differential pressure indicator (your GT image)
With the absolute pressure you will measure the vacuum. The pump will do more than 5" so part 130 will regulate so that the suction is 5". To do this it opens the valve to the filter (120). This extra opening will enable to suck in more air (to lower the suction to 5")
The differential pressure indicator will measure between the vacuum inlet filter (150) and the vacuum regulator. When the inlet filter (150) get clogged the their is low flow. The differential pressure indicator senses this with a lower reading. While the absolute indicator would stay on 5".
The differential system is therefore better.
Most aircraft use a vacuum system, although there are aircraft which use a pressure system instead of a vacuum system. The same indicator differential indicator can then be used to indicated pressure, using reversed connections. The absolute indicator, is not reverseable.
Looking at the TB20 IPC, I see only one vac gauge P/N for all aircraft – despite that first drawing showing just one pipe going to the vac gauge.
I also found some writing from a TB20 non GT owner saying that his gauge had two pipes on the back, not one. The 3-310-50 I have seems to have one of the two ports blocked off. There is no hole there. Also I found the 3-310-50 is suitable for the GT too.
So maybe Socata drew up the pipework for a differential vacuum indicator, installed the two pipes, but never got around to installing or even specifying one?
