[PSUBS-MAILIST] Mechanical Depth Gauges

[Douglas Suhr via Personal_Submersibles]

All, as you may've been aware, in late 2015 I purchased Snoopy (a
K-250) from Alec Smyth. Regrettably I have been quite neglectful of
performing much upkeep on the sub, save for keeping the batteries
topped off. My schedule is finally allowing me to spend some real time
working on the sub and getting to know the specifics of each system.

As you can imagine, I've been bugging Alec with a question here and
there, but thought with this topic I should start a discussion here on
the list for the benefit of anyone else who might have questions /
curiosities about mechanical depth gauges.

Snoopy's depth gauge is nice and large (about 5" in diameter) so it's
easy to read. Alec created a nice label for the gauge face which reads
from 0 to 300 (in feet) with the last fifty feet being marked in red
to indicate 'too deep.'

Problem is, when I looked at the depth gauge the other day, I noticed
that the transparent cover plate on the gauge looked to be quite
opaque (to the point where I couldn't quite read the numbers or count
the hash marks). I assumed that there was some moisture trapped in the
gauge, causing the obscuration. I decided to remove the gauge from the
sub and open it up to let it dry out.

After removing the gauge from the sub, I unscrewed three small screws
around the bezel and popped off the transparent cover plate. It was
only then that I realized the opaqueness was not caused by moisture
but a distortion in the clear plastic. The distortion (like super fine
scratches) appears to be on the outside of the cover, as though it was
cleaned by something that started to melt the plastic or was left out
in the sun too long. I don't think either of these explanations make
sense, but I'm puzzled as to what might have caused it. I might try to
buff / polish the plastic, or perhaps just replace it with glass.

I think it's a good idea to stick a dive computer in viewing range of
a viewport for a depth reading, but I like the idea of having a
mechanical gauge in the sub as well. A couple of questions that come
to mind: what is the pressure variation between salt water and fresh
water? Also, although I didn't find moisture in the gauge, The label
on the face is wrinkled a bit as though it has seen moisture. Although
water should never actually reach the gauge, I can see how moisture
from the inlet tube could find its way up into the gauge body. Should
I be concerned about drying it out somehow after a dive (especially in
salt water)? If so, how should I go about it? ~ Doug S.