testing smtk2ssrf_4.6.2-33-g14971e912875_x86_64-20170224.AppImage

[Alessandro Volpi]

My knowledge about the technical details of pressure transmitters is quite
limited. Nevertheless I think it is worth while to explain why I would like
to test some kind of ultrasonic transmitter.

My personal experience is limited to UWATEC transmitters; I have been using
them since 1998 and I have thus tested at least three generations of such
devices. I have also asked  the opinion of some friends of mine about
Suunto dive computers and pressure probes. They WERE NOT the latest
generation of Suunto transmitters  but, according to my friends,  that they
did not seem to be better than the UWATEC devices as far as link
reliability is concerned.

I have never been satisfied with UWATEC transmitter performance. The VLF
link is apparently quite marginal:  moving my left wrist so that the steel
tank is interposed between transmitter and receiver is enough to compromise
the integrity of the link:  the system is working  with a magnetic antenna
and no other solution is possible, since the wavelength is more than 10 km
. The very short range communication is more similar to magnetic coupling
than to true VLF radio communication.

The Galileo Trimix Manual (05/2010), at page 4, says that the UWATEC
transmitters are automatically switching off if the tank pressure remains
constant for 5 minutes. This is obviously a power save mode. On the other
hand this turns out to be quite annoying for a diver wanting to see at a
glance a fully populated tank pressure table. The cited Galileo manual also
says that a new version of pressure probe is being developed, which should
avoid the inconvenience. In September 2016 I asked to friend of mine, who
is a UWATEC dealer, to check whether the new devices were available; I was
told that they were not available and that no date had been established for
their availability. This was one more reason for NOT buying new
transmitters to replace my malfunctioning devices.

In my opinion the reliability problem of VLF may be related to the poor
efficiency of the small coil antenna ( inductive load ) and to the low
power or the VLF transmitter. Obviously the power has to be low in order to
avoid a quick exhausting of the small button cell.  As Linus Torvalds is
pointing out, the issue of the device's power save modes plays a
significant role.

It is perhaps worth while to point out that the past and present UWATEC
transmitters are based on static pairing with the dive computer. Each
transmitter is characterized by a unique code and, theoretically, the
interference from other devices should be impossible, although some data
loss may occur due to data collisions, like in coaxial cable ethernet
networks.

Ultrasonic wave devices are at present the only option for underwater
telephony with full face masks. The ultrasonic transmission links are very
reliable, from diver to diver and from diver to/from surface crew.

It is true that a tank pressure transmitter is more critical, since it
power supply has to be a small button cell; moreover high voltage is
required for operating piezoelectric transducers and it is probably not so
easy to design a miniaturized high voltage power supply. On the other hand
the quality of an ultrasonic link is very good even at very low power.

In any case the Liquivision dive computer was relying on ultrasonic
pressure transmitters and I have read that such transmitters are able to
work at relatively long distance, so that a diver is able to monitor also
the tank pressure of the buddy; moreover the device can be used as a
directional location beacon.

Unfortunately Liquivision seems now to be out of business. It a pity, since
a reliable pressure transmitter together with a system providing
directional location would BE A SIGNIFICANT SAFETY IMPROVEMENT, specially
when instructional dives are being carried out.

Very best regards.


Alessandro
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