[SBE] Ultrasonic Leak Detection Revisited

[Lionel Shurtleff]

The recommendation to build a library of RBPs as suggested in this post is one that would make the SBE much more useful to persons such as myself.  I am chief for seven stations (4 FM, 3AM) stretched over 150 miles of mountainous NW terrain and the nearest active chapter is over two hundred miles away.  I don't get much out of my membership.  Something like this would be of value to me.
 
Lonnie Shurtleff CE
Capps Broadcast Group > From: billw at betterlifetv.tv> To: sbe at sbe.org> Date: Mon, 30 Jun 2008 13:01:42 -0700> Subject: Re: [SBE] Ultrasonic Leak Detection Revisited> > 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> > > > _______________________________________________> SBE mailing list> To unsubscribe, go to http://seven.pairlist.net/mailman/options/sbe> _______________________________________________> SBE at sbe.org> http://seven.pairlist.net/mailman/listinfo/sbe> > _______________________________________________> SBE mailing list> To unsubscribe, go to http://seven.pairlist.net/mailman/options/sbe> _______________________________________________> SBE at sbe.org> http://seven.pairlist.net/mailman/listinfo/sbe
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