Metric Instruments

"Bill Daniels" wrote in message
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"Tim Ward" wrote in message
ink.net...

"Bill Daniels" wrote in message
...
Does it strike some of the digerati here that expensive mechanical
altimeters with easily mis-read clock-like hands locked into either
the
metric or US measurement systems are archaic?

GPS provides highly accurate, although not ATC compliant, altitude.
Various
vendors provide electronic pressure altimeters with digital displays
that
can be switched between meters and feet with the push of a button.
Digital
pressure altitude sensors drive the "glass cockpits" of new GA
aircraft.

I seems to me that clock-like altimeters designed 70 years ago and
maintained by watchmakers must be nearing their well-deserved
retirement.

Bill Daniels

Yeah, now if they can just make them so they don't need batteries.

Tim Ward


What's the big deal with batteries? IMHO, batteries are at worst a minor
inconvenience easily worth enduring for the benefits of the technology
they
make possible.

Every portable gadget uses them. Most folks have a cell phone, PDA,
portable GPS, digital camera, maybe a camcorder and who knows what else.
Even your car, tug or winch won't start without a battery. They're cheap
and they work fine with a little TLC and regular replacement.

My glider uses a standard 7.5 AH 12V SLA that now sits on a shelf
connected
to a charger that quietly maintains the charge. I know for sure that it
will work at least 10 hours and still show more than 12.5 volts while
transmitting. It has a three year "replace by" date written on it
whereupon
I will plunk down $20 for another at "Batteries-R-Us" even if it still see
ms
OK. I don't trust old batteries.

Bill Daniels


Upon reflection, Bill, I'm sure that an instrument could be built that could
satisfy both of us.
Digital, easily scalable, there's no reason it can't have both an analog
display (or quasi-analog, with LCD) for trends, and a 5 digit display for
accuracy. It could have a lithium cell recharged by the expansion and
contraction of an aneroid . Several "perpetual clocks" have used that
scheme to drive mechanical gear trains with far smaller pressure changes
than you'll get going up and down in a glider or airplane.
An update rate of twice a second should be plenty fast enough. With LCDs,
and CMOS circuitry running a few microamps at two volts or so, it's probably
not impossible to build. You could probably build one with a primary lithium
battery that would only need to be changed once every ten years or so.
That would be the cheapest way to go.
What will it cost to get it approved? How many people are going to buy it?
In the small market that is aviation, what will it cost to build? If you
could sell it cheaper than a mechanical altimeter, you might have a shot.
In enough volume, you might be able to do that. I don't know if the
altimeter market is large enough for that to be possible. The combined
output of all the altimeter manufacturers is probably not as big as a run
of, say, a cheap DVD player.

I dunno. I'm afraid good enough is the enemy of best.

If you don't care whether or not it's approved, or whether it takes
batteries, then you should look at the Flytec hang glider varios. They have
a lot of options as to what they display and how they display it, and I
believe they'll display altitude both digitally and analog. They'd take up
a bit more space on a panel, though.

Tim Ward