IM-25 Mercury Battery

[Fred Olsen]

 >>Just picked up an IM-25 bench volt meter and it is missing the 1.35
 >>volt mercury battery that is used for resistance measurements.

 >We always modified our bench meters by removing the batteries and

The mercury cell in the IM-25/IM-5225 is not the ohmmeter source.  It's
used as a bias reference for a current regulator.  A merc was chosen for
its very flat discharge curve.  A different reference voltage changes
the regulated current, changing the voltage across the tail of the
ohmmeter string, and hence ruins the accuracy of the ohmmeter.

Unfortunately, "those who know better about all things" decided that
there was no longer any excuse to produce mercury cells in this country.
The Heathkit meters were by no means alone in being rudely obsoleted
by that action.

The original cell is specified to be an Eveready E1N (ANSI/NEDA 1109M,
IEC MR50) a "1.4 V Mercuric Oxide" cell.  I have no explanation for the
difference between Heath's specified voltage of 1.35 and Eveready's of
1.4, except that the 1.35 may be 'rated' at nominal draw and 1.4 'open
circuit'.  It has a diameter of 16.0 mm, and a height of 16.5 mm.
Eveready states that "No Replacement" exists.

Just as unfortunately, there is no other primary cell process I know of
which has similar characteristics and a 1.35 V output.  The commonly
suggested sub is some sort of manganese dioxide or zinc-manganese
dioxide ("alkaline") cell, or perhaps a silver oxide cell.  The former
types are 1.5 V and have quite steep discharge curves, and the latter is
1.55 to 1.6 V and has a considerably shorter flat discharge duration
than a merc.

Lacking a better replacement for mercs, it would seem that using a
silver oxide cell is the best of poor choices.  Circuit changes may well
be needed to get the operating point of that current regulator back
where it's supposed to be.  The Heath manual has a fairly good
explanation of the circuit and its function, including operating parameters.

Don't feel like the Lone Ranger.  The "camera people" have been going
nuts over this problem for years.  Many onboard light meters used merc
cells, and are quite inaccurate with anything else.  There's even
something of a black market in imported mercury cells.

Fred
--
Outgoing checked by Norton AV.

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