Vacuum instruments

[Richard O]

How many inches of vacuum are required for vacuum instruments to work correctly? I'm having some trouble with a DG that was just installed in January (it was an overhauled exchange). I'm showing about 4.7" on the guage. For some reason I thought it was over 5" a few months ago.

 

[Tom-D]

How many inches of vacuum are required for vacuum instruments to work correctly? I'm having some trouble with a DG that was just installed in January (it was an overhauled exchange). I'm showing about 4.7" on the guage. For some reason I thought it was over 5" a few months ago.

New Style 4.8 - 5.2 Hq

Old Style 4.0- 5.0 Hq

Did you change the vac filter ? remember clogged filter = higher vac readings.

badly worn / dirty instrument innerds = high vac readings

worn or broken vac motors vanes will = low vac readings.

 

[Richard O]

Well that would explain the sudden lowering of vac on the guage. We just had an annual done a couple weeks ago. The filters were changed during annual and it seemed lower after the annual.

 

[poadeleted20]

Given the inaccuracies of vacuum gauges, and the new filters, the difference between 5.0 inches and 4.7 inches on the gauge should not affect the operation of an AI or DG. Due to flow restriction across a dirty filter, you should be seeing more suction across the gyro even though a lower gauge pressure (measured between ambient and pump inlet) is recorded now. If the problem cropped up after the annual, I would suspect either a dislodged hose or other problem around the new filter.

 

[Richard O]

Actually the new DG never did work properly. Since neither myself nor either of my two co-owners is instrument rated we just tolerated it. It has to be reset frequently and is sometime off by 20-30 degrees every 30 minutes or so. I asked the IA that did the annual to check the vacuum system since I wasn't sure where the problem was. He found a few vacuum leaks including in the fittings on the back of the DG so I was hoping that would fix the problem but it didn't. The place I bought the DG is offering to look at it under warranty but I wanted to make sure I had done everything on my end before sending it back to them.

 

[poadeleted20]

I wanted to make sure I had done everything on my end before sending it back to them.

Sounds like you have.

 

[Tom-D]

Given the inaccuracies of vacuum gauges, and the new filters, the difference between 5.0 inches and 4.7 inches on the gauge should not affect the operation of an AI or DG. Due to flow restriction across a dirty filter, you should be seeing more suction across the gyro even though a lower gauge pressure (measured between ambient and pump inlet) is recorded now. If the problem cropped up after the annual, I would suspect either a dislodged hose or other problem around the new filter.

Vac pumps cause a flow of air, the resistance to that flow causes pressure.

When the filter causes a restriction to flow the Hq readings in the systems will rise, but the flow will decrease.

When the flow across the gyro motor is decreased the instrument will slow down and not reach stability speeds, thus it will precess as this one as doing.

 

[Richard O]

Vac pumps cause a flow of air, the resistance to that flow causes pressure.

When the filter causes a restriction to flow the Hq readings in the systems will rise, but the flow will decrease.

When the flow across the gyro motor is decreased the instrument will slow down and not reach stability speeds, thus it will precess as this one as doing.

Ok that's not completely Greek to me but close. Are you suggesting that I may have a restriction in the system? I can take it back to the IA before I have the instrument pulled but what should I have him look for?

 

[mikea]

Actually the new DG never did work properly. Since neither myself nor either of my two co-owners is instrument rated we just tolerated it. It has to be reset frequently and is sometime off by 20-30 degrees every 30 minutes or so. I asked the IA that did the annual to check the vacuum system since I wasn't sure where the problem was. He found a few vacuum leaks including in the fittings on the back of the DG so I was hoping that would fix the problem but it didn't. The place I bought the DG is offering to look at it under warranty but I wanted to make sure I had done everything on my end before sending it back to them.

When I had my DG replaced by a "yellow tagged" DG, I had it swapped twice more until I got one that didn't precess. The first replacement would be off by 10 degrees when taking the runway on the next takeoff when I was doing pattern work. When I say the last one doesn't precess, I mean it. The only time I have to adjust the DG is when taking the plane out of the hangar. It's only off because the plane got turned with the engine off.

 

[Tom-D]

Ok that's not completely Greek to me but close. Are you suggesting that I may have a restriction in the system? I can take it back to the IA before I have the instrument pulled but what should I have him look for?

The only way to tell is to place the gyro on a bench and test it.

Suck through a straw, as it free flows is there much pressure? Place your finger over the end and suck, did the pressure go up and the flow go down? If the filter is clogged it acts as your finger over the end of the straw, the vac pressure will go up, and the flow stop, thus the gyro will not work with out the proper amount of air passing thru it. When you replace the filter the flow goes up, and the pressure is now depandant upon the restriction caused by the gyro its self.

If your Gyro is bad it may cause the same symptoms as a blockage in a line. the gyro operates on a certain amount of air drawn thru it at a certain vac pressure read in inches of mercury on a manometer. 4.8 - 5.2 is not a PSI reading it is read in Hq or 4.8 inches of mercury.

And we can throw in the vac pressure regulator, it allows air to enter the system when the restriction in the gyro or system is too high, your filter may be OK, the gyro could be stuck or bad causing the regulator to open and control the pressure at what you see on the gauge.

I'd pull the gyro and send it in.

 

[flyingcheesehead]

When I say the last one doesn't precess, I mean it. The only time I have to adjust the DG is when taking the plane out of the hangar.

He ain't kiddin'. I flew from Franklin, PA to Lock Haven, Williamsport, Hartford (Robertson), CT and then back from Hartford to Wellsboro, PA to Toledo, OH and back to Mike's home field and never touched it. Amazing. I've never seen such a solid DG.