Help With Leak Test Results

Hi,

After reading as much as I could find, and assembling the necessary
tools, I have just started a leak testing program on my 1980 LS4.

I verified that my ASI is well sealed by creating sufficent vacuum on
the static input to show 100kt on ASI and observing no appreciable
airspeed decrease after several minutes.

After verifying the ASI, I used it to test my aft static lines. With
all 4 fuselage static ports blocked with tape, I T'd my 60cc syringe
into the static line, connected the assembly to the ASI and then pulled
suffient negative pressure to measure 100kt on the ASI. After clamping
off the syringe line I verified that the clamp worked by noting that I
could no longer change the ASI reading with the syringe, and then
recorded a 37kt ASI reading decrease over a 6 minute period (just over
6kt/minute)

My question is, do I need to start tearing up the seat pan and tracing
this line back into the fuselage, or is a 6kt/minute ASI reading
decrease OK?

One of the fine articles I read give a subjective threshold for
'goodness' but although the article talked about using the ASI as the
mesuring instrument, the stated threshold was "20ft/minute", (I'm
guessing they used the altimeter instead) ;-).

TIA,

Frank

No leak is good. You should see no appreciable movement if the system
is well sealed. The rate of leakdown is a function of the total volume
of the system, so it's hard to say what is OK.

Static leaks are not nearly as important as pitot leaks. The cockpit
pressure is close to static anyway, so a small leak has a very small
effect. A pitot leak is much more troubling because it will cause your
airspeed to read low. The higher the speed, the worse the error, so you
can easilly exceed Vne thinking you are well below red line. It can
also cause you to come in faster than expected, leading to an
overshoot.

To test the pitot side of the system, you need to use your syringe to
pressurize the pitot. Again, you should see no appreciable movement.

To fix the leaks, first make sure the tubing is still flexible. Then
you need to cut off the stretched ends, and reinstall the tubes. Do not
yank the tubes off because you can damage instruments like varios with
the sudden pressure spike. Cut the tubes off, but do not nick the
barbs.

But most important, you need to install tubing clamps over the barbs.
Without clamps, the tubes will leak. The best ones are the mini
stainless steel pinch clamps (you can crimp these with an end nipper
set of pliers). You can also use the nylon wraparound clamps, which are
reusable. Tie-wraps are not suitable because they leave a gap at the
right angle bend.

One other thing you can try to find the leak is bubble solution. You
can get plumbers bubbles from a plumbing supply or home center, or just
use your kid's bubble wand stuff.

Frank wrote:
Hi,

After reading as much as I could find, and assembling the necessary
tools, I have just started a leak testing program on my 1980 LS4.

I verified that my ASI is well sealed by creating sufficent vacuum on
the static input to show 100kt on ASI and observing no appreciable
airspeed decrease after several minutes.

After verifying the ASI, I used it to test my aft static lines. With
all 4 fuselage static ports blocked with tape, I T'd my 60cc syringe
into the static line, connected the assembly to the ASI and then pulled
suffient negative pressure to measure 100kt on the ASI. After clamping
off the syringe line I verified that the clamp worked by noting that I
could no longer change the ASI reading with the syringe, and then
recorded a 37kt ASI reading decrease over a 6 minute period (just over
6kt/minute)

My question is, do I need to start tearing up the seat pan and tracing
this line back into the fuselage, or is a 6kt/minute ASI reading
decrease OK?

One of the fine articles I read give a subjective threshold for
'goodness' but although the article talked about using the ASI as the
mesuring instrument, the stated threshold was "20ft/minute", (I'm
guessing they used the altimeter instead) ;-).

TIA,

Frank

"Doug Haluza" wrote in message
ups.com...
But most important, you need to install tubing clamps over the barbs.
Without clamps, the tubes will leak. The best ones are the mini
stainless steel pinch clamps (you can crimp these with an end nipper
set of pliers). You can also use the nylon wraparound clamps, which are
reusable. Tie-wraps are not suitable because they leave a gap at the
right angle bend.

Doug:
Do you have a brand name or source of supply for these?
Quick Google search does not yield the product you describe.
Thanks


Hartley Falbaum

I've found these at places like VWR, Fisher Scientific or Cole-Parmer.

Chip F

I've found these at places like VWR, Fisher Scientific or Cole-Parmer.

Chip F

Here's a leak testing article that I found helpful:
http://www.borgeltinstruments.com/Leaks.pdf
Sure, it's commercial, but it gives a nice detailed procedure for leak
testing.

Note paragraphs 3 and 4 near the front. Mike recommends using rubber
donuts to prevent leaks (I thinking these donuts are similar to the
grommets used for feeding power cables into metal boxes). He also
concurs with Doug that cable ties or twisted wire are not good for
pneumatic systems.

-John

http://www.mcmaster.com

Page 233 for pinch clamps

I think one day I will just buy all of the bits needed to build my own
glider from these guys - it's pretty much the default source for
anything that I can imagine needing.

P3

Yup, one of my goals in life is to be able to afford to call 'em and have
them ship me 6 of everything . . .

Seriously, placing an order with them is a problem, 'cause there's always
something you should have included in the order but forgot.

When you order the pinch clamps, don't forget to get a roll of UHMW
polyethylene tape. Great for anti-chafe etc. Their number is 76445A12 for
the 1" wide X .0065" X 18 yds.

bumper
"Papa3" wrote in message
oups.com...
http://www.mcmaster.com

Page 233 for pinch clamps

I think one day I will just buy all of the bits needed to build my own
glider from these guys - it's pretty much the default source for
anything that I can imagine needing.

P3

"jcarlyle" wrote in message
oups.com:

Here's a leak testing article that I found helpful:
http://www.borgeltinstruments.com/Leaks.pdf
Sure, it's commercial, but it gives a nice detailed procedure for leak
testing.

Note paragraphs 3 and 4 near the front. Mike recommends using rubber
donuts to prevent leaks (I thinking these donuts are similar to the
grommets used for feeding power cables into metal boxes). He also
concurs with Doug that cable ties or twisted wire are not good for
pneumatic systems.

-John



The donuts he talks about are like "O" rings. Roll them an inch or so
onto the tubing, place the tubing onto the barb then roll the donut back
over the connection. This gives good sealing pressure with no voids.



With good tubing and barbed connectors I have seen precious few leaks
without any sort of clamps. Once the tubing gets hard then, yes, you
could have a leak. If I were going to use any sort of clamps, I would
use donuts.



Larry

"01" USA

those "donuts" are, in real life, used for castrating sheep (cheers to
those Australians for finding a second use for everything). however,
they make great tubing clamps. they will crack after a few years
exposure, but are cheap to replace.

the electric wire grommets have nearly no elastic properties.

Ken

In article ,
"01-- Zero One" wrote:

"jcarlyle" wrote in message
oups.com:

Here's a leak testing article that I found helpful:
http://www.borgeltinstruments.com/Leaks.pdf
Sure, it's commercial, but it gives a nice detailed procedure for leak
testing.

Note paragraphs 3 and 4 near the front. Mike recommends using rubber
donuts to prevent leaks (I thinking these donuts are similar to the
grommets used for feeding power cables into metal boxes). He also
concurs with Doug that cable ties or twisted wire are not good for
pneumatic systems.

-John



The donuts he talks about are like "O" rings. Roll them an inch or so
onto the tubing, place the tubing onto the barb then roll the donut back
over the connection. This gives good sealing pressure with no voids.



With good tubing and barbed connectors I have seen precious few leaks
without any sort of clamps. Once the tubing gets hard then, yes, you
could have a leak. If I were going to use any sort of clamps, I would
use donuts.



Larry

"01" USA