[PSUBS-MAILIST] Home-Built Pinger

["Jon Wallace" <jon@psubs.org>]

Hello,

I'd like to hear your input regarding a piece of safety equipment known as
an acoustical pinger.  Pingers are typically used to mark an underwater
location so you can return to it at a later time.  There are some very
highly developed (and expensive) pinger systems available that act more like
transponders and can map out the exact location of various underwater
vessels based upon encodings in the acoustical signal.  However, those
systems cost tens of thousands of dollars and are certainly out of reach for
the recreational submersible market.

For psubber applications, I'm interested in the use of an acoustical pinger
in an emergency situation to help rescuers locate a disabled psub.  In the
last few months we've had many discussions surrounding underwater
communication whether it be via morse code, sound-wave encoding, or direct
verbal communication.  I think those are valuable discussions but it occurs
to me that before any communication can take place at depth, a disabled sub
has to first be located.  While there are low-tech versions of location
identification available (buoy release, buoy towing), these may not always
be practical.  Surely, towing a buoy behind a sub in a finite environment
such as the pond that we used for the 2007 convention is enough.  But it
probably is not suitable for all possible sub applications, especially those
that involve deeper water or potential entanglement.

Enter the acoustical pinger.  I believe a small acoustical pinger could be a
useful device (not a perfect one) to help locate a disabled sub at depth.
It could be built into a thru-hull, or simply mounted on the hull
externally.  It could be activated prior to diving, or turned on by the
pilot in the event of an emergency.  It's physical size would be both small
and unobtrusive relative to the submersible.  It would emit an alternating
and/or timed acoustical sound within a known frequency range.

Commercial pingers are rather expensive.  The cheapest one I've located
would cost a psubber $695.00 and rated to 20,000 psi well beyond our
performance requirements.  However, for our limited application I'm
wondering if a home-built pinger would be practical enough for our purposes.
It turns out that a home-built pinger is both easy to make, and very
inexpensive.  I contacted David Bartsch who has been working on an
underwater communication device and asked him to spend a little time
back-tracking and looking at a simple pinger device.  In the meantime,
research on the web led me to this document
http://sonar-fs.lboro.ac.uk/uag/downloads/bender2.pdf that describes how to
build an inexpensive hydrophone.  David essentially came up with a modified
version of the hydrophone plans which uses a piezo element from an
inexpensive radio shack buzzer (273-070) that only costs $12.00(US).  The
buzzer comes with some small electronics that creates an ear splitting
(100db), alternating, two-tone acoustic output in air and operates from 6-16
volts.  I expect the performance under water to be somewhat less since the
buzzer uses a resonant chamber to reach the 100db output.

Of course the pinger is only one-half of the equation and we would also have
to find or develop an inexpensive directional hydrophone that could be used
to help locate the acoustical beacon of the pinger.  As the document link
above suggests, the hydrophone itself is arguably easier to construct than
the pinger.  However, an unknown is the sensitivity obtained as well as
whether a means of making it directional are possible.

But first things first.  For the moment, let's assume just for the sake of
discussion, we can develop a reasonably efficient and practical pinger for
less than $50 and distribute how-to plans for its construction.  What
benefits do you see with this idea?  What problems do you see with this
idea?  Would it be worth an RFC proposal to adopt such a device as a safety
guideline for personal submersibles?  Assuming the cost of the device is
trivial, is an acoustical pinger practical for the purposes of our
community.

Thanks for your input.
Jon





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