Replacement Battery

[Deane D McIntyre]

In message  <19980303.161336.3286.0.DFCSENIOR at juno.com> DonaldF F Christensen
writes:
>
> I have an old Heath Model IM-25 VOM which uses a 1.35 or 1.5 volt(forgot
> which)  mercury cell for calibration. I am told mercury cells are no
> longer available. Can I
> use a lithium battery in place of the mercury? Can anyone provide a
> source for the
> replacement lithium battery? Thanks. Don W8WOJ Midland, Mi.
>

Mercury cells are still sold here in Canada, but I understand that they
have been banned in most if not all states in the US. I remember using
one years ago to recalibrate my Heath IM-13 VTVM. (1.35 volts)

Heath used to recommend a 1.5 V Carbon-Zinc (not alkaline) cell for
calibration at 1.56 vots, in in fact both my IM-11 and IM-13 VTVM's
have a red bot at the end of the 1.5V scale for this purpose.

Today a better way is to use a known accurate Fluke or other quality
digital meter; put both the digital meter and the VOM/VTVM across
any DC voltage source (a battery is fine) and adjust the cal pot until
the meter reads correctly. Another way us to use a garden variety
5V regulator as a voltage reference; I have one connected to a 9V battery
with a resistor (few k ohm) to provide a light load. I checked this
with a quality digital meter from work to confirm I got 5.00 V; some 5V
regulators are a bit off, so I selected one that gave 5.00 V. This is
how I check/calibrate my meters.

73, Deane D McIntyre VE6BPO
dmcintyr at acs.ucalgary.ca

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