Erich Camerling wrote:
>
>
>Why do you advise sodium bisulfite ?
>Sodium bisulfite ( NaHSO3 . CAS : 7631-90-5 ) is very unstable.
>When you buy the expensive puriss.quality ( ~99 % )
>and store it in the best possible way (brown glass bottle with a cut
>glass stopper ) you will have after a year only ~ 20 % NaHSO3,
>the rest is Na2SO3.
> ( 2 NaHSO3 --> Na2SO3 + H2O + SO2 )
>Much better is Potassium metabisulfite ( K2S2O5 .
>CAS 16731-55-8 ).
>You can buy the puriss.quality ( ~96 % ) for ~$ 16/kg, it is stable
>when you store it dry ( after two years you will still have ~90 % )
>and dissolved in water it gives KHSO3.
> ( K2S2O5 + H2O --> 2 KHSO3 )
>Theoretically you need 11 g K2S2O5 instead of 10 g NaHSO3.
Given the small amount of sodium bisulfite used in the formula I have
been mixing my own stock solutions from a bottle of sodium bisulfite
purchased over six years ago and have not observed any change in the
working quality of the developer. Guess I need to get some potassium
metabisulfite and run some tests.
>
>
>PS 3 : What means the value "7" in the black box at PCat3 ,page 6,
>fig. 3 ?
This refers to the subject brightness range of the subject, where 7
would be considered a normal scene, or N in zone speak. SBR of 6
would represent a value of about N+1, while SBR of 8 would be about
N-2. The SBR terminology is from Phil Davis' Beyond the Zone System,
and is based on a system of metering using the incident rather than
reflected readings.
>What are BTZS type tubes ?
BTZS is a specific tube that you can buy from the View Camera Store
in Phoenix Arizona with caps on one end that allow you to develop
sheet film in the light, either rolling them over a surface or
allowing them to bob around in water. I use the term BTZS type tubes
to refer to home-made tubes that are like the original BTZS tubes.
> and at page 8 : what means "SBR"
> of 7 (equivalent to N development in Zone speak ) ?
See answer above about the 7 in the black box.
>I hope you know the answers on my questions .
Hope the answers are clear.
Sandy
Received on Wed Nov 19 09:02:06 2003
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