Vacuum Instrument Installation Expert needed...

[4004]

John C:

I responded to you on the "other" page. Subsequent to that, I checked the current Spruce catalog and "yes" it does have the info and actually includes all the AN fittings typically required. So, grab a catalog and search around in the vacuum/instruments pages and you should find it amongst them - I don't think its actually addressed in the index.

Big Edd

[6673]

Took a double take on this thread. JC is always ANSWERING the Qs. Never seen you ask one!

Nick

[4006]

I am no expert; however if both instruments require aprox. 5 "Hg then they must be plumbed in parallel as per your second drawing. Otherwise the total vacuum pressure will be divided between the two instruments; and each will receive exactly the same airflow as the other.

Look at the system as 2 resistors (instruments)
voltage ( Vacuum pressure)
and current (air flow)

[6643]

So, grab a catalog and search around in the vacuum/instruments pages and you should find it amongst them

What is the date on that catalog? I couldn't find it in mine. (I'll look again...)

Took a double take on this thread. JC is always ANSWERING the Qs. Never seen you ask one!

A man's got to know his limitations...

Look at the system as 2 resistors (instruments)

I'm tempted to do exactly that, but, I'm not 100% convinced it's a valid comparison. The gauge is plumbed into the second (closest to the venturi) instrument and is showing 5.5 inches. Does that mean the horizon is only getting 3.5 inches (out of 9 total)? Or, is the whole system sharing 5.5 inches (the gauge is referenced to ambient pressure.)

Following that analogy, if I plumb them in parallel the venturi will have to move twice as much air (current)...

[4004]

John C:

I was wrong about the Spruce index - listed under 'Instrument Hook Ups" in my current, just out 2018 -2019 catalog page 614.
Just checked this out - go to their web page and you can down load PDF of new catalog - just go to the PDF download and scroll down the page before download and you'll see box allowing one to enter a page # to go to - enter page 614 and the discussed diagram should come up -print if desired.

I did check an ancient 90s catalog and same info was indexed under "Instrument Panel Layouts" so its probably in most catalogs.

Bit of trivia, in one of my Bonanzas years ago , it had the old Tactair all pneumatic auto-pilot and in troubleshooting a problem, I plumb jury rigged a big shop vac to get vac in hangar without having to run engine, not enough to fully operate the system but enough to do what I needed at the time. Not adequate for what you are doing but novel at the time.

Big Edd

[6643]

Thanks, Edd! I'm sure it's in my catalog, too. I guess I should have looked closer...

I think I should re-plumb the system before I do anything else.

[4006]

Look at the system as 2 resistors (instruments)

I'm tempted to do exactly that, but, I'm not 100% convinced it's a valid comparison. The gauge is plumbed into the second (closest to the venturi) instrument and is showing 5.5 inches. Does that mean the horizon is only getting 3.5 inches (out of 9 total)? Or, is the whole system sharing 5.5 inches (the gauge is referenced to ambient pressure.)

Following that analogy, if I plumb them in parallel the venturi will have to move twice as much air (current)...

If the gauge is plumbed into the instrument closest to the venturi then it is reading pressure (voltage) at the venturi. The two instruments are sharing the 5.5" because the pressure at the inlet side of that instrument is the same as the vacuum pressure to the other instrument. Both instruments plus the filter = 5.5" ( For example : Instrument #1: 2.1 "HG, instrument #2: 3.3"Hg, Filter : 0.1" Hg). Both instruments will have the same volume air flow(current).

[6643]

If the gauge is plumbed into the instrument closest to the venturi then it is reading pressure (voltage) at the venturi. The two instruments are sharing the 5.5" because the pressure at the inlet side of that instrument is the same as the vacuum pressure to the other instrument. Both instruments plus the filter = 5.5" ( For example : Instrument #1: 2.1 "HG, instrument #2: 3.3"Hg, Filter : 0.1" Hg). Both instruments will have the same volume air flow(current).

I'm inclined to agree. If the electrical analogy holds, then it makes perfect sense. And, 5.5 inches isn't enough divided between the two instruments. I haven't had a chance to work on it since I first posted. I think the plane is coming back in the second week of October. I'll let you know how it goes.

[6643]

Well, here we are, a year later...

I have the plane back in my hangar again. The DG still works and the Horizon doesn't. Figured I'd replumb the gauges and see if that helps..

Imagine my surprise when I stuck my head under the panel and found someone (the outfit that did his annual this year) had cut the hoses and stuck some "T"s in. Now it has a single 3/8" hose from the venturi to the suction regulator, a single 3/8" hose to a plastic "T" and from there to the two instruments' outlets. The inlets go to another "T" and then to the filter. Second port on the filter is plugged.

Now, I figure I can replace the AN fittings with RAPCO vacuum fittings, upsize the line to the regulator to 5/8" and use the second port on the regulator and the filter to eliminate all the 3/8" lines servicing both.

So, I pulled the line off the venturi. Here's the fitting attached to the venturi.

KIMG4402.jpg (312.21 KiB) Viewed 4142 times

Here's what the inlet to the fitting looks like:

KIMG4403.jpg (260.59 KiB) Viewed 4142 times

I removed the hose from the fitting and found this:

KIMG4404.jpg (293.86 KiB) Viewed 4142 times

I wasn't able to measure this directly, but a 5/32" drill bit fits and a 3/16" bit doesn't.

The port on the venturi is 1/4 pipe, so I'll be putting a 1/4 to 3/8" pipe adapter and a 3/8" pipe to 5/8" hose vacuum fitting in place of this mess. 3/8" pipe to 5/8 hose into the regulator and individual 1/4" pipe/3/8 hose from the regulator to the instruments and then on to the filter. I'm reusing whatever fittings are on the DG now, since it works, and replacing the AH fittings with RAPCO vacuum fittings.

I'll let you know if it works...