[SBE] Ultrasonic Leak Detection Revisited

[Bill Whitt]

That's awesome man -- I'm glad that it turned out for the best. I'm also
wondering if you will ever use soap and water again ... LOL. Ohhh, and
double thanks for revisiting the forum and giving a update. Sometimes these
things drop and never get revisited.

Bill Whitt

-----Original Message-----
From: sbe-bounces at sbe.org [mailto:sbe-bounces at sbe.org] On Behalf Of Chris
Spacone
Sent: Saturday, June 28, 2008 10:04 AM
To: 'sbe member discussion mail list'
Subject: [SBE] Ultrasonic Leak Detection Revisited

Several weeks ago I launched a discussion regarding the detection of leaks
in waveguide systems. Several viable methods were put forward and I
eventually came to the conclusion that ultrasonic leak detection was
appropriate for my situation. Several concerns were noted and I am back to
report on my experience.

 

After much searching I settled on the Whisper from Inficon (
http://www.inficonultrasonicleakdetectors.com/en/whisperultrasonicleakdetect
or.html ). This unit features single piece construction and comes with a
carrying case that holds a battery, extension tube, headphone and a thin
instruction manual. I was initially concerned that the instruction manual
was so thin. I would have expected something with an exotic nomenclature to
be extremely complicated and difficult to use. I'm happy to say that my
concern was unfounded.

 

Taking the unit from the box and setting it up was as simple as installing
the battery and turning the unit on. It has a knob to adjust the sensitivity
of the unit and not much else. On the front is a black, truncated cone with
an opening covered by a screen. This is the ultrasonic detector which I am
guessing is really nothing more than a microphone with a shield in front.
The previously mentioned hose attaches over the cover and extends (acoustic
waveguide? Sorry, I couldn't help the pun!) the instruments reach into
cramped spaces.

 

Ultrasonic leak detection is predicated on the fact that an ultrasonic
disturbance is created by a gas molecule leaking from an orifice when it
impacts another gas molecules. This ultrasonic disturbance can be sensed by
using a detector that is tuned to this ultrasonic sound. In this case the
instrument is tuned to 40.5Khz. Without knowing the internal construction or
electronics of the instrument I can only guess that there is some
combination of mechanical and electronic filtering that isolates the 40.5Khz
ultrasonic signature of a gas leak which is then processed by a circuit that
indicates the presence of a leak. Earlier I mentioned the sensitivity
control. This function is described in the manual as a squelch control that
allows the stronger or weaker signals to be detected by the unit. It sounds
complicated but in practice is as easy to use as a squelch knob on a
transceiver. 

 

Time to get down to business. I took the unit to our uplink room which
houses 5 HPA's and a variety of other gear that makes additional noise. An
earlier contributor to the email chain I generated stated some concern that
the instrument would be overwhelmed by the environmental noise which would
make it unusable. That turned out to be of absolutely no concern. The
environmental noise is broadband white (pink, brown, fuchsia?)  noise. The
signal created by the leak is centered at 44.5Khz which probably makes it
stand out like a sore thumb. I started searching the manifold by looking in
places that I had exhibited other problems. If I had the 'squelch' wide open
the unit beeped, as it I turned the 'squelch' down the unit became quiet. By
adjusting the 'squelch' and probing the threads on the manifold I eventually
found a loose compression nut and interestingly enough, an atmospheric port
on a regulator that had a leak. More about that later. Using the instrument
was pretty straightforward and required that I hold the black microphone tip
almost on top of the region to be inspected. This was not a bad thing as
turning down the sensitivity and moving the tip of the instrument around
allowed me to precisely locate the source of the leak. I didn't need to use
the extension tube but I'm certain that the instrument  would have performed
in precisely the same manner. At no time did the background noise inhibit
the use of the instrument.

 

Regarding that leaking regulator port mentioned earlier. It turns out that
this regulator is actually designed to dump overboard any excess
pressurization gas as a method of maintaining regulation. The size of this
'leak' was directly related to the incoming gas pressure. At 60 PSI it could
be felt by hand. At 5-10 PSI it could only be detected using the instrument.
Now it became clear why I was going through bottles of N2. The 1st stage
regulator was deliberately designed to leak. Any gas pressure, in excess of
the outlet pressure target, presented to this unit would eventually find its
way to atmosphere thus draining the tank. But this only accounted for a
portion of the leak rate that I had calculated. It turns out that the
'pressurization system' designed for this installation wasn't really a
pressurization system at all, it was designed as a continuous waveguide
'purge system'. At the feedhorns where the elliptical waveguide meets the
LNA port was a small petcock much like you would see if you were trying to
install an icemaker line for a refrigerator. This tiny petcock went
completely unnoticed because I had expected the waveguide system to be
completely sealed thus maintaining the pressure of the applied gas.
Apparently this is not an unusual arrangement given the moderate demands
(environmental and RF) of our system. Eventually I spoke with the person
that installed the purge system and he explained that in small
installations, this type of approach is entirely adequate given the low
power levels and fairly benign installation environments (about 400 watts in
an air conditioned TX room going into a 5.5M dish about 50 'waveguide' feet
away). He went on to tell me that higher transmit power levels, longer
waveguide runs and more demanding environments would be more appropriate for
a fully pressurized and sealed system.

 

We went on to find several other leaks which we fixed and managed to bring
the gas flow rate down to a more manageable level which allowed us to use a
small air compressor to feed the system until we get a more permanent
solution in place. I'll report back when that solution is up and running. In
the meantime I want to express my thanks to all of the good guys (and even
the chuckleheads) that responded with encouragement and constructive advice.
It is the main reason I keep my email subscription going.

 

An editorial comment to the leadership of the SBE may be in order. I cannot
be the only engineer that has run into this kind of situation. Many of us
deal with installations that require pressurization systems (or other
systems) that facilitate our business and it would be unreasonable for us to
be experts in everything. I would like to see the SBE publish a series of
'recommended best practices' on a variety of different topics. These 'RBP's
would not serve as a replacement for qualified engineering advice but would
be a convenient template for rank and file engineering types. These RBP's
could be used by the engineering staff to explain to management the why's
and how's of accomplishing a task. If I'd had a 4 or 5 page RBP with the
weight of the SBE behind it I could have saved us a bunch of wasted time and
effort. I can see the beginning of a library of RBP's becoming the
collective wisdom of the broadcast engineering community as a whole. It
could go a long way toward the standardization and rationalization of the
installation and operation of engineering plants across the broadcasting
industry.

 

 Just my $0.02

 

Best regards,

Chris Spacone

 

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