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Pitot/static why ¼" tube?

Tony Spicer

Well Known Member
Planning on using Van's standard pitot and large diameter pulled rivets for static source in my current project. Any engineering reason why 1/8 poly tube won't work?

Tony
 
Just venturing a guess, but I would be concerned about a couple of things.

First, the smaller more fragile tube would I think be more susceptible to bending and kinking or being crushed, which of course just wouldn't do. The 1/4" tube supplied is pretty stiff, which seems like a good property for this application.

The second concern I would have is a delayed reaction at the instrument. I would think that although you really aren't moving a whole lot of air, that at least toward the pito end you are moving some and friction loss of the air in that small of a tube would slow things down.

Just my silly logic and free of charge. :D
 
First, the smaller more fragile tube would I think be more susceptible to bending and kinking or being crushed, which of course just wouldn't do. The 1/4" tube supplied is pretty stiff, which seems like a good property for this application.

That would be my guess, too: more resistant to being pinched or kinked, and harder to obstruct/plug.
 
Sleepy, the pitot line in the wing of my Comanche is 1/8" aluminum. I have 1/8" nylaflow lines in my Rocket. I went with 1/8" because the bend radius is smaller and easier to route thru the cockpit.
 
I just finished installing another AFS AoA in one of our planes, and they use this itty-bitty, teensy-weensy super-flexible tubing...seems to give a fast enough response for the AoA system.

That's not to say I would use it right away for all my Pitot/Static needs, but some experimentation would be interesting - I wouldn't be surprised if it works fine.
 
Considering that the pitot and static lines do not actually "flow" any air, the size should not matter. Yes, there is probably a small amount of air moving in and out of the first few inches at the open of these lines, but it is insignificant to this discussion.

All they are doing is transmitting a pressure reference.

That said, the speed of transmitting that reference may be effected by a smaller tube like Paul hinted at.

For my fat old fingers, 1/4" line is easier to work with than the tiny spaghetti tubes.
 
I wouldn't worry about response time at all, but I would worry about getting a slug of water in the line. It takes a significant amount of pressure (compared to the pressure you're measuring) to move the water around, making the measurement pretty much useless. The 1/8" line would be more susceptible to it, and it wouild be more difficult to clear.
 
Don't know the science behind it but every certified aircraft I can think of has 1/4" or larger static lines with some of the larger aircraft having 3/8". Aircraft manufactures are all about saving money and weight and if 1/8" was practical I'm pretty sure they'd be using it.
 
My only question is where will you find the fittings for connecting the tubing to the instruments?
 
If one has Van's stock pitot tube fabricated from aluminum tubing and fly thru rain they will get a slug of water in them, unless they're bent such that they will drain in taildragger configuration. Really no big deal, they still work. I don't see how water could work itself up and into the line.
 
Silicone Tubing 1/8 I. D. Works Fine for Pitot Static Lines

I have used flexible silicone tubing 1/8" I. D. pitot static lines in four RVs including my own without any problems at all. I use plastic barbed connectors with pipe threads for the instruments, barbed T's for the interconnects, and elbows for really tight places. I use red tubing for the pitot lines and black for the static. I've never had any of these lines leak or come apart, and since they are so flexible they tend not to kink. These components are so easy to work with that I typically can do a complete pitot static system behind the instrument panel in under an hour. I bought the tubing and connectors online at McMaster-Carr. I have often wondered why builders are still using 1/4" stiff poly pro lines and expensive connectors, when flexible silicone tubing and plastic connectors works so well, are inexpensive, and so easy to modify.
 
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I have used flexible silicone tubing 1/8" I. D. pitot static lines in four RVs including my own without any problems at all. I use barb fitting connectors with pipe threads for the instruments and barbed T's for the interconnects.

Pretty much the same here on my 10, except soft 1/4" rubber hose over barbed plastic fittings.
 
I have used flexible silicone tubing 1/8" I. D. pitot static lines in four RVs including my own without any problems at all. I use plastic barbed connectors with pipe threads for the instruments and barbed T's for the interconnects. I use red tubing for the pitot lines and black for the static. I've never had any of these lines leak or come apart, and since they are so flexible they tend not to kink. I can do a complete pitot static system behind the instrument panel in about a half hour. I bought the tubing and connectors online at McMaster-Carr. I have often wondered why builders are still using 1/4" stiff poly pro lines and expensive connectors, when flexible silicone tubing and plastic connectors works so well, is inexpensive, and is so easy to modify.


Hmmmmmmm.......... May have to consider this more. I'm not done with my pitot static system yet and I already found that I've had some problems getting a good airtight seal with the slip in fittings.
 
I wasn't clear in my earlier post...I'm using 1/8" OD nylaflow, which has an ID 0.096". I got the idea from the AFS AOA to use smaller tubing and just checked the manual...it says 1/16" ID tubing.
 
I don't see how water could work itself up and into the line.

Just flying to 8000 feet and back and you have already exchanged 20 % of the air in the line, not taking into account diffusion, temperature changes and other minor effects. The air in the tubes is definitely not just a "dead" mass that just sits there.

According to CS-22 (sailplanes, since they are the least strict of all certifications, but the same applies for all aviation)

(b) The design and installation of a static
pressure system must be such that:
(1) positive drainage of moisture is
provided;
(2) chafing of the tubing, and excessive
distortion or restriction at bends in the tubing,
is avoided; and
(3) the materials used are durable,
suitable for the purpose intended, and protected
against corrosion.

What mystery of science makes static tubing in experimental aviation immune moisture problems?
 
Another tubing alternative

The application is a bit different but I thought that I would throw another solution into the mix.

We provide instrumentation support for testing a variety of aviation related systems and have been using a 1/16" ID tubing that we procure from McMaster Carr. Recently it has been used successfully in a system for pitot and static measurements on some tests that were performed at velocities from a few knots to just below Mach 1. We like it because it is flexible, the antistatic polyurethane material appears to have a good life in in our rather hostile environment. The relatively thick wall cross section make it pretty difficult to collapse under dynamic loads.


Antistatic White Polyurethane Tubing
1/16" ID, 1/8" OD, 1/32" Wall Thickness
p/n: 5790K15
http://www.mcmaster.com/#5790k15/=qkd1xv

We also source a variety of reducers, tees, etc. from McMaster Carr to adapt to the pitot tubes, instruments, etc.

This is only an observation in the narrow application area that we work in and you should make your own assessment of the suitability.
 
Leak test

If you have a small leak in one of your instruments, you might still pass the pitot static leak test with large tubing but fail the test with small tubing, due to the smaller reservoir of air. The volume goes like the diameter squared, so if your leak rate is 50' per minute with 1/4" id tubing it will be 200' per minute with 1/8" tubing. The first one passes, the second one doesn't. Go to 1/16" tubing and now you're 16 times more sensitive to the leak test, compared to 1/4".
Just a thought.
 
pressure

Mike,
If pressure is all that is measured in a 1/4" line, then increasing or decreasing the dia of the line definitely changes the amount of pressure in the line. Wouldn't this pressure change reflect a gauge misreading? I don't know how this would effect a pitot tube but I process different diameters of neon tubing and different diameters require different pressures of gas to operate at the same level. Just asking
 
As long as the flow is small or zero, pressure in a tube is independent of diameter - as long as the mean free path of a molecule is small compared to that diameter. This is well satisfied for static lines; the mean free path is sub-micron for sea level pressure.
The different pressures required for different diameters in neon lights has to do with heat loss to the walls which quench the ionization process.
 
"Leak Test"

Posted by Bob Turner
"If you have a small leak in one of your instruments, you might still pass the pitot static leak test with large tubing but fail the test with small tubing, due to the smaller reservoir of air. The volume goes like the diameter squared, so if your leak rate is 50' per minute with 1/4" id tubing it will be 200' per minute with 1/8" tubing. The first one passes, the second one doesn't. Go to 1/16" tubing and now you're 16 times more sensitive to the leak test, compared to 1/4".
Just a thought."


Just hoping to clarify, the above may not be accurate if you are using "steam gauges"
The volume of the static air in the combined instruments is far greater than the static line volume. Assuming even 15 feet of 1/4" O.D. static line, the volume would be about 5 cubic inches. The combined volume of Alt, VSI, + ASI would be close to 20+20+10 cubic inches. The tube size would have very little effect on leak rate.
Of course it's a different story for a pitot only test, (also EFIS's) as their is very little volume in the ASI's Capsule/Diaphragm.
Many Pipers use AN-3 fittings for pitot/static, I.D. about 1/8".
I agree the tube size will have no effect on the pressure.
I'd say go ahead and try out the small stuff and see how you go.
Paul.
 
Mike,
If pressure is all that is measured in a 1/4" line, then increasing or decreasing the dia of the line definitely changes the amount of pressure in the line. Wouldn't this pressure change reflect a gauge misreading? I don't know how this would effect a pitot tube but I process different diameters of neon tubing and different diameters require different pressures of gas to operate at the same level. Just asking

Inside diameter only makes a difference in pressure if there's flow. Pitot and static systerms are... well, static.
 
Tuition

Well I guess I need to send in another $25 for tuition!
This class is better than any course ever taken
Thanks guys for all your years of experience.
 
That doesn't make any sense Miles, especially in the case of steam gauges. The greater the volume in the instruments, the more air you're going to have to move to change the pressure.
 
A simple test

A simple test for those that have any doubt about how well air can pass through a small I.D. tube. (if you happen to have a "steam" altimeter laying around).
Connect said altimeter to a couple of feet of small tube, connect mouth (or other gentle suction device) to other end of tube.
Evacuate/suck till altimeter reads a couple of thousand feet (this will be approx' 2" hg or about 1 PSI at sea level).
Remove mouth, (from tube:D), and observe altimeter. You will notice how rapidly it returns to the original reading.
I'm guessing you would be struggling to replicate this descent rate in your aircraft.
Give it a try. (the Altimeter test, not the aircraft descent bit:eek:).
Paul.
 
Concerns with water in the lines might well be valid, but response time of the instrument will actually be slower with larger diameter tubing. (But I doubt that there's enough difference to worry about.) When you're measuring pressure, the entire 'vessel' must change in pressure for the measuring device to see the entire change. As an extreme example, using the same air compressor (altitude change for static; speed change for pitot), how long would it take to fill a 2 gallon tank vs a 20 gallon tank?

Guys flying computer-controlled engines deal with this all the time. Manifold pressure sensors do 'sampling', and since manifold pressure is very dynamic, it can drive the computer nuts. The cure is to add a significant air volume to the MP line to allow slowing/smoothing of the pressure in the line.

Charlie
 
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Concerns with water in the lines might well be valid, but response time of the instrument will actually be slower with larger diameter tubing. (But I doubt that there's enough difference to worry about.) When you're measuring pressure, the entire 'vessel' must change in pressure for the measuring device to see the entire change. As an extreme example, using the same air compressor (altitude change for static; speed change for pitot), how long would it take to fill a 2 gallon tank vs a 20 gallon tank?

Guys flying computer-controlled engines deal with this all the time. Manifold pressure sensors do 'sampling', and since manifold pressure is very dynamic, it can drive the computer nuts. The cure is to add a significant air volume to the MP line to allow slowing/smoothing of the pressure in the line.

Charlie
This makes no sense. If this to be true, then several small diameter tubes in parallel would give slower response time than one single? Don't think so. The pressure "velocity" in a pipe is directly proportional to the speed of sound in the pipe. In fact, the pressure "velocity" IS the speed of sound in a pipe. Changes in diameter is negligible, what counts is the properties of air, and pipe material and the diameter/tube wall thickness ratio e/D.

But, as others have pointed out, the tubing is not alone. It is connected to some instrument that can have much larger air volume than the tubing itself. In those cases, positive flow of air in the tubing is needed to change pressure in the instrument. Since the volumetric flow rate of air in a tube is cross sectional area, A times the velocity v, the smaller tubing you have the greater the velocity in the tubing must be to "fill up" the instrument. A larger tubing will definitely give a faster response in those cases. If, on the other hand, the tubing is only connected to a pressure transducer with negligible volume of air, then the diameter of the tubing doesn't matter. Small large, no difference, the pressure "velocity" is decided by the parameters mentioned above.

What you are thinking about is a special case, where the the point of measurement (on the manifold) ends in a tiny hole. That hole will act as a restriction on the flow needed to pressurize the tubing. With the same size hole, the larger the tubing, the more the measurement will be smoothed out.

As far as I can see, the main issue is to be able to drain water. Both for the sake of those delicate instruments and for the sake of getting optimal readings. With a water slug in a 1mm tubing you can forget about fast, accurate and stable measurments, thats for sure. Draining requires a certain diameter tubing. Some advanced nitrogen/air purging system will also work and is used in some special cases in multiphase flow measurements.
 
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Hey! Here's an even better question!

I think the glass manufacturers should come out with pitot tubes and static ports with pressure sensors built right into them! That we can just run a wire from the port to our EFISs and do away with pitot static tubing entirely! Alternate static source? No problem! Just flip a switch and use the backup!

Course I'll be wanting my royalties guys! :D:D:D
 
Hey! Here's an even better question!

I think the glass manufacturers should come out with pitot tubes and static ports with pressure sensors built right into them! That we can just run a wire from the port to our EFISs and do away with pitot static tubing entirely! Alternate static source? No problem! Just flip a switch and use the backup!

Course I'll be wanting my royalties guys! :D:D:D

Bluetooth pitot static ports.......don't need no obsolete wires..... ;)
 
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That doesn't make any sense Miles, especially in the case of steam gauges. The greater the volume in the instruments, the more air you're going to have to move to change the pressure.

Maybe I should have said the air in the pitot and static systems is static "for all practical purposes". In going from 0 to 240 mph indicated you only increase the pitot pressure by about 1 psi. About the same in the static system for a 2000 ft altitude change. The velocity of the air in the tubing required to accomplish that change in the amount of time over which the speed or altitude changes is negligible.
 
It?s cold and snowing I?m stuck inside with a lousy virus and apparently I?m bored.
So I?ve done a little research on the web and crunched a few numbers.
At low altitudes above the sea level, the pressure decreases by about 1.2 kPa for every 100 meters
1 kilopascal = 0.145037738 pounds per square inch
1 meter = 3.28084 ft. or 100 meters = 328 feet ish.
1.2 kPa* 0.145037738 or .1740452856 psi for 328?
Or .0005306259 per ft or .5306259 psi for 1000? altitude
To get a pressure increase of .5306259 we need to apply Boyle?s law.
Boyle's law:
P1V1 = P2V2
P1v1=1. 5306259*v2
1psi * 1 cubic inch = 1. 5306259 psi *v2
V2 = 0.6533275048 cubic inches
1 - 0.6533275048 = 0.3466724952 cubic inches. This is added volume you have to shove in for every cubic inch of volume inside the altimeter to show a 1000? degrease in altitude.
Assume our altimeter static chamber contains 4 oz or 7.21875 cubic inches air.
You have to move 2.502542074 cu in of air into the altimeter case to increase the pressure by 0.5306259 psi
Now if the tubing is 1/16? id, or .0625 then the volume is Pi*Radius squared * length.
.0981747704 per in or 1.1780972448 cubic inches per ft. A 6? run of this tubing is 7.0685834688 cubic inches of volume.
So basically, if my math (which I hate) is correct, you will exchange the entire volume of air in your tube for every thousand feet in altitude you descend, or climb of course.
Having gone through this goofy exercise, I would agree, the end effect is negligible from a delayed instrument reading. It?s not like you?re going to change altitude or air speed so fast that you?ll overcome the delay caused by the insignificant friction loss in the tubing.
 
bad math

In Boyle's law the P's are the total pressure.
So if you're at sea level P1 is about 15 psi, and up 1000' P2 is about 14.5 psi.
Now using the law if V1 = 1 cubic inch, V2 = 1.03 cubic inches -- 0.03 cubic inches has to flow out, or about a 3% change.
 
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I think I learned more in this thread than in my aero engineering class. And pitot/static design is one of the first things we learned, because it's one of the simplest.

I would think avoiding capillary action is the biggest reason - but more importantly what's the benefit of using a smaller line? Because it's whats on hand?
 
Hey! Here's an even better question!

I think the glass manufacturers should come out with pitot tubes and static ports with pressure sensors built right into them! That we can just run a wire from the port to our EFISs and do away with pitot static tubing entirely! Alternate static source? No problem! Just flip a switch and use the backup!

Course I'll be wanting my royalties guys! :D:D:D

Thanks, time to go get rich! :D
 
Pitot-Static System 101

I almost hate to tell this story, but I had a substitute instructor once who argued with me about the Pitot-Static system. He asked me how it worked and when I gave him my understanding, he said I was wrong. Because the Piper he had been flying had "Vent" for the Static port, he said the air went into the Pitot probe on the wing, flowed through the air speed indicator and then vented overboard through the static "Vent." :eek:

Oh boy, and this guy had his CFII! How in the world did he get that far without understanding the Pitot-Static system? I only had him for the one lesson, and was glad to get my regular instructor back to complete my Private Pilot rating. That was over 30 years ago. I never did hear if he continued instructing. I think he was waiting to get a job with the airlines, but he probably got a job with the FAA!
 
I almost hate to tell this story, but I had a substitute instructor once who argued with me about the Pitot-Static system. He asked me how it worked and when I gave him my understanding, he said I was wrong. Because the Piper he had been flying had "Vent" for the Static port, he said the air went into the Pitot probe on the wing, flowed through the air speed indicator and then vented overboard through the static "Vent." :eek:

Oh boy, and this guy had his CFII! How in the world did he get that far without understanding the Pitot-Static system? I only had him for the one lesson, and was glad to get my regular instructor back to complete my Private Pilot rating. That was over 30 years ago. I never did hear if he continued instructing. I think he was waiting to get a job with the airlines, but he probably got a job with the FAA!

They are out there, and they walk among us. I had a BFR several years ago with a relatively young instructor that had no clue about the physics that creates lift over the wing, had never heard of Bernoulli. I ended up giving him a "from-the-ground-up" physics lesson starting with molecular Brownian motion and velocity effects on static pressure due to vector addition and on from there, he said he'd never heard of any of that stuff. Didn't charge me for the BFR though, said the lesson I gave him was worth it, so I guess it all worked out.

How guys like this get their rating is beyond me, though - and a little scary that it can happen.
 
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Well I think this falls under the of category if it aint broke don't fix it. Just because its called an experimental doesn't mean you have to experiment on every piece of it. Just my opinion.
 
How guys like this get their rating is beyond me, though - and a little scary that it can happen.

1. On the written and the oral, the FAA emphasis is on the FARs, not physics.

2. You are what you are taught. Back in the late 1980's I took a cfi ground school from one of the traveling, weekend road-shows (Sportys Academy, in this case). When it came to aerodynamics and physics, it was without a doubt one of the worst classes I have ever witnessed. The instructor got "Vx depends on thrust available, Vy depends on excess power available" backwards, but worse could not offer any insight into this. He described "Newtonian lift" and "Bernoulli lift" as if they were two different things, rather than two different ways of calculating the same thing. etc. I always wonder how the people in that class passed on anything useful to their future students.

I will say, I never heard of anyone who thought the pitot line flowed out thru the static line!
 
I understand the concerns of water evacuation but ....... has anyone had trouble with running smaller lines? water can get trapped in 1/4" line if the system is not designed properly.
 
water ?

Been reading this engineering treatise with great interest. My first reaction to post #1 was water from condensation or capillary action and pressure change ( push water into static port). Nobody has mentioned freezing. Great care must be taken to run the static line uphill to the instruments without a low spot. therefore, line size could matter a lot !

Can you fly your RV well without looking at he airspeed or altimeter?
 
How about NO static 'system'!

Hey All, I posted this in another thread, but it fits here too!

I bought a plane that doesn't have any static system installed at all! Guess what? No problems. I was very careful and bought a nice aftermarket system for the 8A I built, and expected to have to install one in this plane. I did notice that I can vary my altimeter by 25 feet by playing with the vents, but then it stabilizes. It's a 6 and I don't have the original plans (waiting for a dvd copy), was it 'optional' at some point?

Thanks, Lance
 
I have used flexible silicone tubing 1/8" I. D. pitot static lines in four RVs including my own without any problems at all. I use plastic barbed connectors with pipe threads for the instruments, barbed T's for the interconnects, and elbows for really tight places. I use red tubing for the pitot lines and black for the static. I've never had any of these lines leak or come apart, and since they are so flexible they tend not to kink. These components are so easy to work with that I typically can do a complete pitot static system behind the instrument panel in under an hour. I bought the tubing and connectors online at McMaster-Carr. I have often wondered why builders are still using 1/4" stiff poly pro lines and expensive connectors, when flexible silicone tubing and plastic connectors works so well, are inexpensive, and so easy to modify.

...This is exactly what I use, and have since 1969! I actually fabricate a manifold for pitot & static lines providing a separate devoted line for each instrument. It is incredible how much space is gained behind the panel and how easily instruments can be manipulated with this small, very flexible line. I would never use anything else. Thanks, Allan..:D
 
Hey! Here's an even better question!

I think the glass manufacturers should come out with pitot tubes and static ports with pressure sensors built right into them! That we can just run a wire from the port to our EFISs and do away with pitot static tubing entirely! Alternate static source? No problem! Just flip a switch and use the backup!

Course I'll be wanting my royalties guys! :D:D:D

To bad, already been done. You would owe royalities to Rosemount. Most of the transport airplanes are now using "smart probes" that essentailly have the air data computer built into the probe, and send digital data to the other aircraft systems. UTC Rosemount probes and others have had these in service for over 10 years.

A probe with self contained electronics would need to be powered from some reliable source so bluetooth won't get you away from wires.

BTW, the pitot and static lines on transport airplanes are typically either -4 and -5 or -5 and -6 diameters respectively. I design these systems sometimes for my work. In modern systems, water entrapment is the main concern. Without the old bellows type instruments where there are important displacements of volume, the only real issue is water entrapment.
Internal diameters less than about .18" will not reliably drain due to surface tension. Add freezing conditions and a blockage could be an issue.
 
I would want 1/4" to minimize water impact. My 6A sits outside and has the van's 1/8" at the rivet, converting to 1/4" tube. On three occasions I have had to suck water out to get air flow. I believe the surface tension of the water allows it to easily clog the 1/8" and would not so easily do so with 1/4" line. Un-scientific, of course.

EDIT: just saw the previous post. I guess my logic was right.

Larry
 
...This is exactly what I use, and have since 1969! I actually fabricate a manifold for pitot & static lines providing a separate devoted line for each instrument. It is incredible how much space is gained behind the panel and how easily instruments can be manipulated with this small, very flexible line. I would never use anything else. Thanks, Allan..:D

Of course, now many of the systems don't have the pitot and static lines behind the panel, and there are fewer places to connect them now.
 
Just installed these neat push lock connectors and 1/8 tube from MSC supply.

691iyw.jpg


Shown for comparison is the ubiqitous 1/4 tube.

The tube is a dime per foot and the fittings only a couple of bucks each.

Since a few of you have asked, the link to the fitting is here:

They are available in a wide variety of sizes and the tubing is too. I've never dealt with MSC before but they are fantastic. I'll be using them a bunch in the future.
 
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small static tubing

Posted by Bob Turner
"If you have a small leak in one of your instruments, you might still pass the pitot static leak test with large tubing but fail the test with small tubing, due to the smaller reservoir of air. The volume goes like the diameter squared, so if your leak rate is 50' per minute with 1/4" id tubing it will be 200' per minute with 1/8" tubing. The first one passes, the second one doesn't. Go to 1/16" tubing and now you're 16 times more sensitive to the leak test, compared to 1/4".
Paul.


True, but.... at the volume of air needed to fail a static test, the flow is so small that I doubt you would see a difference even in the small diameter tubing.
I have normal 1/4" OD tubing at my static ports and going to my Dynon D1000 ahrs aft of the baggage compt. I have a D6 in the panel that I needed to route pitot and static lines to. with a bunch of wires going up front, I ran out of hole space to put the 1/4" lines up front. I put 1/8" lines from under the seats up front to the D6. If you think about it, the old manifold gages had a restrictor that you could hardly blow through to keep the intake pulses from driving the MAP gage wacky. I don't think I would want 1/8" lines from the static ports up to the point that they join due to possibility of water getting in.
 
Just installed these neat push lock connectors and 1/8 tube from MSC supply.

691iyw.jpg


Shown for comparison is the ubiqitous 1/4 tube.

The tube is a dime per foot and the fittings only a couple of bucks each.

Since a few of you have asked, the link to the fitting is here:

They are available in a wide variety of sizes and the tubing is too. I've never dealt with MSC before but they are fantastic. I'll be using them a bunch in the future.

Maybe it's a non-issue with modern ADAHARS based avionics, but I'm not seeing any manifolds for 1/8" OD tubing at McMaster or MSC Direct. Using multiple T's or Y's to branch out, then?

Trying to figure out what I need to order here before the bottom wing skin goes on.
 
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