G3X Touch Static Pressure Calibration question.

[bjdecker]

Please read the below in its entirety before "killing the messenger" -- just asking a question in an attempt to better understand some things...
---snip---

The instructions in the G3X Touch Installation manual Revision AU for Static Pressure Calibration (Section 30.4.7.4, Page 30-21) describe how to connect the various ports to the calibration test set -- specifically:

"To avoid damaging the ADAHRS pressure sensors, the pitot, AOA (for GSU 25), and static ports must be connected to the test set."
-and-
"It is acceptable to connect multiple pneumatic ports together. The AOA port should typically be connected to the static port so it is always exposed to the same pressure as the static port during the test."
(ed. The "...connect multiple pneumatic ports together..." in the above is poorly worded -- it should be eliminated from the text IMHO)

Unlike the Airspeed Indicator, VSI, Altimeters of old, there's no diaphragm, bellows, eye-of-newt-attached-to-feather mechanism in the GSU 25. The Pitot, Static and AOA ports each have their own transducer and they are mechanically/pneumatically isolated from each other --- No? The pressures, in absolute terms, are on the small side - 14.695PSI at sea level, standard day, decreasing to 10.2 PSI @ 10Kft, 6.4 PSI @ 20Kft and so on.

The AOA is supposed to work by reading impact air pressure -- which at 10Kft and at 200 KTS is about 11.05 PSI. If you were to leave the AOA port "open" (ambient) it would be exposed to the air pressure at the field elevation, which at our field of 800ft is about 14.29 PSI.

Given this - why the warnings?

If it's a software/algorithm thing that the G3X/GSU 25 needs, then so be it, just say that in the docs.

 

[Brantel]

It’s the differential pressure they are concerned about. When the guy test your altimeter, he will suck down the pressure on the static port and if you don’t connect your AOA port to the same suck, the differential will grow rapidly to the point of damaging the differential pressure sensor that the AOA and Static ports are connected to.

Before Garmin came out with AOA, I built an AOA unit out of two differential pressure sensors with each of them tied common to the static. It was fairly easy to exceed the limits of the sensors.

I imagine the inside of the Garmin unit is something similar.

 

[bjdecker]

It’s the differential pressure they are concerned about. When the guy test your altimeter, he will suck down the pressure on the static port and if you don’t connect your AOA port to the same suck, the differential will grow rapidly to the point of damaging the differential pressure sensor that the AOA and Static ports are connected to.

Why? The pressure differential is measured by software, not a mechanical/pneumatic interconnection.

 

[Brantel]

Why? The pressure differential is measured by software, not a mechanical/pneumatic interconnection.

Study up on gauge pressure vs absolute pressure and you will find the answer.

 

[jaypay]

Unlike the Airspeed Indicator, VSI, Altimeters of old, there's no diaphragm, bellows, eye-of-newt-attached-to-feather mechanism in the GSU 25.

You are making an assumption about the internal design of the GSU 25.

Differential pressure sensors/transducers are frequently used when measuring small pressure deltas, since they are less susceptible to measurement noise and various errors. There is a point where an excessive pressure differential will permanently damage the device.

 

[Avanza]

You are making an assumption about the internal design of the GSU 25.

Differential pressure sensors/transducers are frequently used when measuring small pressure deltas, since they are less susceptible to measurement noise and various errors. There is a point where an excessive pressure differential will permanently damage the device.

The pressure differential is measured by a mechanical/pneumatic interconnection inside the GSU 25.
Just follow the instructions on page 30-22 sec 6 to avoid damaging the unit.

 

[bjdecker]

You are making an assumption about the internal design of the GSU 25.

Differential pressure sensors/transducers are frequently used when measuring small pressure deltas, since they are less susceptible to measurement noise and various errors. There is a point where an excessive pressure differential will permanently damage the device.

Correct -- I've since been in contact with some "folks in the know" and now have a good understanding of the transducer design and internals of the GSU 25.

The AOA transducer is connected to AOA port *and* Static port; if you "suck down" the static and leave the AOA port open to ambient you may damage the AOA transducer.

Thanks all for the clarification.

 

[M McGraw]

on a similar subject…
By use of the NTPS and a Kitplanes article I have determined my CAS/TAS reads fast by a few knots at cruise speed. The solution appears to be a small disturbance aft of the two static ports.

Two questions:
(1). Does anyone have a suggestion of the size and mechanical attachment they have done?
(2). Is there a software adjustment/correction within my Garmin G3X system? I need to decrease my CAS/TAS.

 

[bjdecker]

on a similar subject…
By use of the NTPS and a Kitplanes article I have determined my CAS/TAS reads fast by a few knots at cruise speed. The solution appears to be a small disturbance aft of the two static ports.

Two questions:
(1). Does anyone have a suggestion of the size and mechanical attachment they have done?
(2). Is there a software adjustment/correction within my Garmin G3X system? I need to decrease my CAS/TAS.

Might be able to TLAR our way to a solution -- can you post a couple of pictures of the static ports? include some kind of scale (micrometer, ruler, etc.) for reference.

 

[M McGraw]

Might be able to TLAR our way to a solution -- can you post a couple of pictures of the static ports? include some kind of scale (micrometer, ruler, etc.) for reference.

They are standard Vans pop rivets. I have already verified there are no leaks in the static system. The depth is 0.0305”. The diameter is 0.398”. I need to reduce the CAS/TAS by 3.5KTS, Paint build up could be the issue? There is a slight ridge at the center hole on one side of the aircraft.

UPDATE: Photo #2 is after removing the paint. There was a bit of buildup. Sadly, it will be more than a week until I can flight test. Work is such a time suck!

 

[Walt]

your rivet does not look like the correct one?

 

[bjdecker]

They are standard Vans pop rivets. I have already verified there are no leaks in the static system. The depth is 0.0305”. The diameter is 0.398”. I need to reduce the CAS/TAS by 3.5KTS, Paint build up could be the issue? There is a slight ridge at the center hole on one side of the aircraft.

Yes -- remove the paint; get down to the bare steel of the MSP-42 rivet. I would remove the paint chemically -- Acetone/MEK or Methylene Chloride. Don't upset the shape of the rivet "dome" or the transition from the dome shape into the orifice by sanding.