Re: HP419 leaking battery?

From: Jim Adney (jadney_at_vwtype3.org)
Date: 12/01/04 

Date: Tue, 30 Nov 2004 22:17:32 -0600

On Tue, 30 Nov 2004 22:31:43 +0100 Leif Holmgren <nobody@nowhere.com>
wrote:

>Jim Adney wrote:

>> Shack single AA holder. I put a rechargable alkaline battery in mine,
>> because the don't self discharge, but you could use anything that puts
>> out 1.2-1.5V.
>
>Rechargeable alkaline??? Never heard about these before. Will start
>looking around.

Ray-O-Vac sold them for a few years, but they are now discontinued.
You just need something that has a good shelf life, so you're not
opening the meter to replace it all the time. It's not recharged by
the meter like the other cells, either, so you have to take it out and
replace/recharge it.

>Are you sure you used one? There is a "big" figure 3 on it, but I guess
>that could be some size identification since the voltage for a single
>mercury cell should be 1.35V which makes 3 volts impossible.

Okay, I checked my manual, and the original cell is shown as 1.35V.

>How about the voltage drop over time? Have been looking around for data
>on batteries and they all specify a rather large drop over time compared
>to what could be expected from a mercury cell. Is it connected so that
>the exact voltage is irrelevant as long as it does not drop too much
>when reading the nulling voltage instead of 0.

Droop with time is irrelevant, because you trim the bucking voltage to
get just enough to buck the input signal to zero. Then you measure
your adjusted "bucking voltage." The only droop that might matter
would be that amount that occurs in the 10 seconds between bucking and
measuring. That should be insignificant.

>Regarding isolation, is that a requirement due to the rather high
>impedances involved, or is one or both of the poles separated from the
>ground of the rest of the meter?

All of the above, I believe. The bucking cell floats in the middle of
the circuit, and the meter is so sensitive that you will find that
tiny amounts of leakage will show up in the oddest places. I had to
carefully take apart the input binding posts and clean off the plastic
pieces to get the leakage down. I cleaned the range switch wafers with
alcohol and still couldn't get it to calibrate in the lower ranges.

I actually gave up on this meter, thinking that it had some fatal flaw
that I hadn't found. When I went back to it a year later, it was FINE!
I suspect that I had just touched something and left a fingerprint
that finally evaporated enough that it was no longer important.

>Sometimes analog is really superior the digital stuff.

I couldn't agree more. ;-)

-
-----------------------------------------------
    Jim Adney jadney@vwtype3.org
           Madison, WI 53711 USA
-----------------------------------------------

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