Stu Gotts wrote:

I think that wherever he takes a voltage reading, there will be a
difference. What he needs to do is to hook up the Davtron and JPI to
the exact same post, both + and -, use the exact same type of
connector and insure that the same wire size is used as lead.
Incidentally, I've seen about a dozen Davtron/JPI equipped planes, and
none agree on the voltage. One may have built this "flaw" to drive
some of the flying population crazy.

Theoretically, if you have a sensitive enough voltmeter, that's true,
but for the currents drawn by modern solid state avionics if you get
..1V drop accross a connection, it's probably a bad connection.

The biggest source of error, assuming both the Davtron and JPI are installed
correctly, is their basic accuracy.

Most of these things read to .1V resolution, but to get .1V accuracy at
13V means the meter accuracy must be better than .1/13 which is about
..8%.

To further complicate the discussion, since these things are digital, you
need to have half last digit accuracy to ensure the last digit changes
halfway between two values. That is, you need to have .05V accuracy to
ensure the last digit switches from .2V to .3V as the voltage passes
through .25V for example.

At 13V, that means .05/13 which is about .4% accuracy. Most bench test
equipment is only good for about 1% accuracy; .4% is starting to get into
the realm of lab equipment.

On top of all that, there are also the problems of temperature coefficient,
how much the reading changes as the temperature of the instrument changes,
and periodic calibration to compensate for compenent aging.

Anyone taken their Davtron or JPI out to check calibration lately?

The practical thing to do about the Davtron and JPI is to get the most
accurate meter you can find and note the difference between the 3 meters.
For the range of interest, i.e. about 11-13V the difference is going to
be linear so all you need is a single correction factor for each meter
as opposed to a correction card covering a range.

The bottom line is these in panel meters are just reference, not lab
equipment, unless someone went to heroic lengths (and a heroic price to
match) to get the accuracy and calibration that close and the periodic
recalibration to ensure it stays there.


--
Jim Pennino

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