[rollei_list] Re: Krimar speed


----- Original Message ----- From: "Eric Goldstein" <egoldste@xxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Monday, October 16, 2006 8:52 PM
Subject: [rollei_list] Re: Krimar speed


I will not quote from a web site without express permission. The
referenced sites show/state that alkaline cells show an irregular drop
in voltage over the life of the cell from about 1.55 volts to about .9
volts. The voltage drop begins early in the life of the cell. Unless
the meter circuit is regulated, this will result in a multiple f/stop
drift for a given quantity of light. Mercury and Silver Oxide cells
are virtually consistant and flat in their discharge voltage for the
life of the cell. If Krikor's recalibration were done quickly with an
absolutely fresh alkaline cell, then it should be ok for Silver Oxide
cells which will give consistent readings.


Eric Goldstein

I have been suffering from a pinched nerve in my back again and have been away from the computer.
Eric is right about Mercury cells and Alkaline cells. Mercury cells have been used for applications requiring very stable, constant, voltage for perhaps 70 or more years. They were common as a supply of grid bias in portable electronic instruments until the advent of transistors. In these applications they were often soldered into circuits as permanent elements.
Alkaline batteries of all types have a constant diminuition of voltage even when not loaded much and also have relatively short lives. The meters using Mercury cells did so to avoing having to use some method of voltage regulation.
Generally, Mercury cells and batteries (technically a battery is more than a single cell) have the property of maintaining their voltage until very nearly the end of life at which point the voltage drops very rapidly to virtually nothing. This was a great problem with the early radio microphones used in television and motion pictures because one never knew where the battery was in terms of life so it was common for mics to fail suddenly on air. The answer was simply to put new cells in right before air time and to have lots of spares (both mics and batteries).
Since these microphones drew considerable power from the battery the life was not as long as in low drain applications like bias cells and light meters.
AFAIK, no other battery type has the combination of long shelf life and constant voltage of the Mercury cell but Silver cells come close. The use of other batteries with a simple regulating diode and resistor will result in even more stable voltage than the Mercury cell and is probably the best solution. The voltage drop across the diode is extremely constant. To some degree, this system has the same trouble as the Mercury cell, namely, when the battery voltage drops too low the regulator no longer works and out put can become very low or unstable.
The main difficulty of using a regulator diode is finding a place for it. Some adaptors have been made to fit existing Silver Oxide batteries, which are smaller than the Mercury cells, but the diode and resistor can be installed inside a lightmeter or camera if the insides are at all accessible.

---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx 

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