[rollei_list] Re: Best/Worst Rollei accessory was Re: Google
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <rollei_list@xxxxxxxxxxxxx>
- Date: Mon, 6 Aug 2007 15:22:34 -0700
----- Original Message -----
From: "Marc James Small" <marcsmall@xxxxxxxxxxx>
To: <rollei_list@xxxxxxxxxxxxx>
Sent: Monday, August 06, 2007 7:22 AM
Subject: [rollei_list] Re: Best/Worst Rollei accessory was
Re: Google
At 09:29 AM 8/6/2007, Allen Zak wrote:
>While agreeing that the built in Rollei meter is capable
of sufficient
>accuracy, that was true decades ago when the meters I
encountered were
>freshly minted. These days, the big problem is with
deteriorated
>selenium cells behaving in a rudely non-linear manner.
Allen
With respect to your knowledge and experience, it
is a bit of a myth that Selenium cells
deteriorate over time.
I agree that just time does not deteriorate the cells
but many Selenium cells are damaged by heat or moisture due
to the seals leaking. These are not just down market stuff
but good GE, Weston, and other high quality meters. I don't
know the chemical mechanism of the change, however, the
symptoms are compression of the high brightness range. Most
meters of this type have two ranges with some overlap. A
measurement made in the overlap area will usually show up
the problem. The measurement made at a high scale value (top
of the low range) will be a stop or more less than one made
at the low end of the high range. This is an easy test to
make.
Other problems can be caused by poor contacts or damaged
meter mechanisms. Weston meters seem to be vulnerable to
oxidized contacts on the cell. This gives very low or
intermittant readings. The cure is often simply to press
down on the lens and rotate the cell a little. This will
often bring bad meters back to life. AFAIK, there is no cure
for a non-linear meter other than replacing the cell. There
no simple way of correcting this error because it changes
with the light level at the meter cell.
Occasionally, one encounters a sticky meter mechanism.
These can sometimes be cured by working the zero adjustment
a few times. The meters in GE and Weston instruments are
very well made and are generally trouble free. I've found
that Weston meters seem to have fewer bad cells than the
lower priced GE DW series, the GE PR-1 also seems to have
fairly reliable cells.
Any Selenium meter should be checked occasionally by
using the overlapping range method.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
---
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