Quite some time ago I described a simple way to use size 675 zinc-air hearing aid cells to replace mercury cells in cameras and exposure meters. I have been keeping track of the cells I put into the lightmeter of my Nikon-F. This meter draws no current when not in use. I have just replaced the cells after a full year. Curiously enough one of the cells still measured 1.35 volts, the original value, but the other was down to about half a volt. The low cell was the one closest to the cover, the other is flat against the first so perhaps it does not get as much air as the top cell. I have not tried plugging up most of the holes in the cells to see what happens. Until the seals are removed the cells are inactive so the shelf life is pretty long. The package of cells I have has my date of May, 08 on it and the cells came right up to value when the seals were peeled off. Mercury cells last for several years, perhaps decades if not loaded. No other cell will match this. However, they are unavailable so something elese must be used. Zinc-air cells have a very flat voltage curve and are very close to the value of mercury cells, so they will work in items like exposure meters who's calibration was based on having a very stable and constant supply voltage. At the time I began this experiment I had a supply of good mercury cells so I tried using both in a couple of different devices that used them. These included a Luna-Pro exposure meter and the finder-meter in the Nikon. In both cases both the battery check indicator and the actual exposures were identical. The 675 is smaller in diameter than the original mercury cell. I overcame this by using a common "O" ring with the cell pushed into its center. The resulting diameter is just about right. The Nikon finder picks up the positive voltage from the top battery with a side contact. Of course the O ring insulates the cell from this contact. I made a contact surface by wrapping a small bit of aluminum foil around the O ring on one side. This contacts the cell wall and also the side contact. It takes a bit of dexterity to get the foil in place but its simple, works well, and costs nothing. There may be a better way to do this but, since the cells do not have to be disturbed for long periods of time it seems to be satisfactory. So, the life of the zinc-air cells seems to be at least a year. Not bad for something available pretty cheaply at any Radio Shack store and probably most drugstores too.
-- Richard Knoppow Los Angeles, CA, USA dickburk@xxxxxxxxxxxxx --- Rollei List - Post to rollei_list@xxxxxxxxxxxxx- Subscribe at rollei_list-request@xxxxxxxxxxxxx with 'subscribe' in the subject field OR by logging into www.freelists.org
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