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  1. #21
    Ralph Javins's Avatar
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    Quote Originally Posted by GRHazelton View Post
    Just be careful if you are considering an older used meter that you determine what sort of cell/battery it uses. For example, mercury cells were favored in the past for exposure meters, both hand held and in cameras because they maintained their voltage pretty constant throughout their life, unlike alkaline or silver oxide cells. However ... mercury cells can no longer be sold, at least legally. Some otherwise "legal" cells/batteries may be very difficult to find.

    OK, guys. This one got my attention; it contains one GLARING ERROR !

    Yes, the alkaline cells do change their voltage as the cell is used. However, claiming that the Silver-Oxide ( Ag-O ) cells also change their voltage over their useful life, as do the alkaline cells, is a gross error and a great dis-service to the Ag-O cells.

    In actual fact, the Silver-Oxide cells are remarkably similar to the old Mercury ( Hg ) cells in their voltage stability characteristic of their discharge curve. They are very much like the old Hg cells in that regard (with one notable difference), including even the sudden drop to Zero volts at the end of their life. The one main single difference is that while the Hg cells had a nominal voltage of 1.35 VDC, the Ag-O cells have a slightly higher nominal voltage of 1.50 VDC, or 0.15 VDC higher. Depending on the type of circuitry inside the light meter, this can be significant.

    With those few light meters using a "bridge metering circuit," there is no problem. It is with the rest of the more common and simpler light meter circuits that relied on having a constant 1.35 VDC power source where we run into a problem with the light meter no longer being accurate when we try to simply make a direct substitution of the earlier Hg cells with the newer Ag-O cells. Then we notice that the light meter usually is about one full f-stop off calibration. One simple quick way to deal with this to adjust the ASA/ISO film speed rating to compensate and have the meter read out the correct exposure value for us. Try using about 50 % of the film ASA/ISO speed rating. Then there is another way that is a little more complex, but it is also more elegant and it allows us to continue working with our light meter in exactly the same normal way as always.

    This involves taking advantage of the low Forward Voltage Drop characteristic of a Schottky Barrier Diode, which at the current levels we are discussing, will be right at about 0.15 VDC.

    There are two (2) ways to do this. The simplest way is to use the metal shell of an old PX-13 or PX-625 battery cell and mount into it a Schottky Barrier Diode and the contacts to work with a type 357 or similar Ag-O battery cell, and then this combination is put into the battery compartment of the light meter and it works as normally. These are available from some sellers on e-Bay and also from C. R. I. S. Camera in Arizona; www.criscam.com is their URL. Look for their MR-9 Battery Adapter. When I last checked, this adapter was about $35 USD or $40 USD. But it does work. I have three of them right now.

    The second way to do this is to take your camera or light meter to a camera repair technician and ask him if he can install into the wiring for the battery compartment, "an axial lead Schottky Barrier Diode" that is installed in a "forward biased" orientation in that wiring. If he understands the words in quotations, he should be able to install it correctly for you in your camera or light meter, and then you can use an Ag-O battery cell in the battery compartment (perhaps with an O-ring to keep the Ag-O battery cell centered in the space) just like you did previously with the old Hg battery cell.

    Again, my main point is to say that the Silver-Oxide battery cells DO NOT change their voltage over time and use like the common alkaline battery cells. They are very stable and stay at their voltage until they reach the knee at the end of their discharge curve and their useful life, and then they quickly drop to zero, just like the old Mercury battery cells, but at a slightly higher voltage.
    Last edited by Ralph Javins; 09-20-2013 at 01:03 PM. Click to view previous post history.
    Enjoy;

    Ralph Javins, Latte Land, Washington

    When they ask you; "How many Mega Pixels you got in your camera?"
    just tell them; "I use activated silver bromide crystals tor my image storage media."

  2. #22
    Sirius Glass's Avatar
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    I have tried a number of hand held light meters and I choose the Gossen Luna Lux SBC that I got with the case for about $100 in EX+ condition at KEH. It is accurate, easy to use, provides ELVs, filter adjustments and Zone adjustments.
    Warning!! Handling a Hasselblad can be harmful to your financial well being!

    Nothing beats a great piece of glass!

    I leave the digital work for the urologists and proctologists.

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