On Sun, 12 Apr 2009 03:00:06 +0000, DoN. Nichols wrote:
On 2009-04-09, James Waldby ... wrote:
On Thu, 09 Apr 2009 22:31:31 +0000, DoN. Nichols wrote: ...
Just for the fun of it -- let's see what I can remember picking
up at hamfests which might be of interest he

1) B&S 0-6" digital calipers (now not usable because they need
PX-13 mercury cells in a holder which would not be happy with any
modern cell, even if the slightly higher voltage does not harm
it.)
I'll try that someday.
...

A WeinCell MRB625 battery is supposed to be a proper $4 substitute for
a PX-13 mercury cell,

But since the B&S caliper needs *four* of them, that is a $16.00
replacement. (It is a "DIGIT-CAL II", FWIW.) (At least it is better
than some adaptors which were supposed to convert something like a SR357
to fit the PX-13 profile, which were selling for something like $25.00
each. Five Chinese calipers are cheaper enough of those to run the
caliper. :-)

I can't quite parse that last sentence, but get the drift that
the batteries get pricy (even if you buy 12/$40 from earlier link,
http://www.micro-tools.com/store/item_detail.aspx?ItemCode=MRB625-12),
possible costing more than a new cheap caliper.

[snip re shape]

They claim "WeinCell is a revolutionary new zinc/air battery designed
to replace environmentally-unfriendly (banned) mercury batteries. ...
Only WeinCells deliver exact voltage and stable output consistent with
mercury batteries." ... "the WeinCell is not activated until the
pull-tab is removed (removing batteries from equipment and replacing
tabs prolongs battery life)." ...

Snapping off the battery holder, removing two small Phillips
screws, removing the four cells, and relacing the tabs -- certainly
sounds convenient. :-) Well ... three of the four cells are held + side
up, visible through apertures in the holder, but (assuming that is the
surface with the holes to be covered) that still leaves one which
requires the same disassembly sequence.
....

True, putting the tabs back on doesn't sound like an easy fix for
longer battery-life in that caliper. I included that bit in my
earlier post not because it's useful, but because it's amusing
that the seller suggests it. However, it might be practical to
store the caliper in a box purged with nitrogen, argon, or CO2.

Since this does not require the stable voltage that an exposure
meter (or one built into a camera), I'm more likely to machine a
replacement holder using two CR3032 3V cells, with a silicon diode and
two germanium diodes in series with them (for a total of about 1 V drop,
since four mercury cells add up to 5 V, and the CR3032s are 3V each.)
I'll first check how forgiving it is using a bench power supply to apply
a full 6 V (working my way up from 5 V) and see whether it has problems
with that.
[snip rest]

Depending on what the current draw is and how stable it is, you
might be able to use something like the dual-diode BAV199 in a
0.05"x0.12" SOT-23 package. Under a milliamp, the drop per diode
would be about right. Or perhaps use a couple of Schottky diodes
at about .4V each, a little low. Of course if you've got those
germanium diodes already, wouldn't 3 in series be about right?
(Per, eg, (http://baec.tripod.com/articles/crystal.htm) If
the current doesn't vary, just use a resistor.

--
jiw