Archived posting to the Leica Users Group, 1999/06/18

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Subject: Re: [Leica] Mercury vs. Alkaline Battery
Date: Fri, 18 Jun 1999 12:28:11 EDT

In a message dated 6/18/99 12:05:36 PM Eastern Daylight Time, 

<< To try to help with this question, I just did a little test comparing a new
 Varta V625 PX, 1.35V Mercury battery with a Varta V625 U, 1.5V Alkaline
 First I made various exposure measurements with a recently calibrated
 Leicaflex SL and the Mercury battery.  The measurements agreed with the
 other metering systems I have available.  I then substituted the Alkaline
 battery and the measurement read high to the point that it would have
 resulted in about a 1/3rd stop underexposure.  So far as I can tell, the
 error is linear but I wouldn't trust adjusting the meter to account for it.
 I tried that in the past and it didn't work for me.
 Next, I performed the same tests using a very reliable MR-4 meter and the
 results were consistent with those obtained with the SL (and an SL/2).
 Bud Cook

If I remember back to college physics, Voltage = Current/Resistance.  Since 
an analog camera lightmeter functions basically as a variable resistor, the 
error brought about by an increase in voltage should be linear, and a simple 
compensation with the "ASA" dial on an older meter should be sufficient.  If 
there is a deviance from the linearity, as long as it isn't more than about 
1/3 stop you'd be safe even with slide film.  Most camera repair techs I've 
talked to who claim to "recalibrate" meters for the alkaline replacements 
just tweak the needle or adjust the ASA dial to reflect the compensation 
needed, and of course this is a linear compensation.  
The real bugaboo with Mercury vs.Alkaline is that their discharge curves are 
different.  Hg cells maintain constant voltage throughout their life and then 
die suddenly.  Alkalines gradually lose voltage until they are exhausted.  
Meter circuits designed for alkalines compensate for this, but meters made 
for Hg batteries do not.  A *new* alkaline vs a mercury might show a 1/3 stop 
difference but as the alkaline is used it might end up 1/2 or 1 stop, and 
there's no way of predicting where it is along the curve at any given time.  
The "recalibration" by service techs does nothing for this issue.  It is my 
understanding that the adaptor made by CRIS (which takes a MS76 silver 
battery inside a shell the size of a 625/PX13) has the requisite circuitry to 
flatten the discharge curve to match a mercury battery.  At least that's what 
they told me.  I have a Nikon FTn which was only "tweaked" to accept the 
625A, so I have always made it a point to check it against a known meter each 
time before using it and reset the ASA if necessary.