# What does the suction gauge actually show?

If I recall correctly correctly the vacuum suction gauge in the TB10 has a scale from 3 to 6, with the green range being 4 to 5 (I might slightly off on those numbers though).

It's clearly marked as showing inHg, but unlike the manifold gauge it's on the lower stop when the engine isn't running (rather than the upper stop if, as the manifold gauge, trying to reach ambient pressure). I haven't really paid attention to it's behavior in the aircraft (only checking that it's in the green), and I just looked at it in Xplane right now which might be wrong of course.

Is it showing the negative difference to ambient pressure? Or the difference in pressure before and after the vacuum pump (which may or may not be the same thing)? Or something else completely? :)

According to the diagram below from a TB200 pilots manual the gauge shows the difference between vacuum line and the ambient pressure:

The pressure difference between the ambient and the vacuum line drives the airflow which spins the gyros in you instruments. In the Socata design, there is an implicit assumption that the air filter is not blocked. If the filter would be blocked, the suction gauge would probably exceed the green area unless the vacuum relief valve opens before the red is reached.

If your vacuum pump stops working, the dial will fall to its lower stop. If the air filter is blocked, and both the vacuum pump and the vacuum relief valve working properly, the dial should exceed 5 inHg.

Other designs are a bit more fault tolerant in the sense that they do not make any assumptions on the working of the filter or the relief valve. In the examples below (thanks to voretaq7) the suction gauge indicates the pressure difference over the instrument itself, which is a more robust presentation of how the system is performing.

• How does the diagram tell you it's the difference to ambient pressure? Feb 9, 2015 at 13:09
• @falstro There is no other reference pressure around the Vacuum Gage than the ambient pressure, so the only difference it can measure is from ambient pressure. If it would measure the difference over the vacuum pump, it would need access to the the vent line. Feb 9, 2015 at 13:12
• The manifold pressure gauge shows absolute pressure, couldn't this one be similar? Feb 9, 2015 at 13:14
• @falstro It could be designed that way, but that would be useless. What you are interested in is the difference between ambient pressure and the pressure in the vacuum line since that is what drives your instruments. Displaying the absolute pressure value in the vacuum line has little use, unless you know what the ambient pressure is and then you have to do the math yourself which is pretty error prone. Feb 9, 2015 at 13:20
• The suction (or pressure) gauge is always a differential pressure (from the "low" side of the instruments to the "high" side) because what spins the gyro is the air rushing from the low pressure side to the high pressure side. They can be plumbed in different ways: often they're in series with the attitude indicator (or less commonly the heading gyro), and sometimes they have their own dedicated vacuum line. Feb 9, 2015 at 20:28