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accurate scales



 
 
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  #1  
Old January 22nd 09, 02:13 PM posted to rec.photo.darkroom
Jean-David Beyer
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Posts: 247
Default accurate scales

Larry wrote:
hello all,

I am looking for a scale which measures up to .001 grams accuracy (.
0001) but is inexpensive (not cheap). I only need to measure up to 50
grams at that accuracy. Does anyone know where I could find such a
scale? (Preferably in Canada, BC to be more exact)

For darkroom work, or even drug dealing, I cannot imagine needing such
accuracy. The chemicals you buy, even photo grade, are not that pure, nor do
they need to be. I use an Ohaus triple beam balance that is good to 0.1
grams up to about 600 grams, and that is good enough.

Your exposures are not that accurate. Your apertures are not that accurate.
Your developing times are not that accurate. Your solution temperatures are
not that accurate. Film speeds are accurate to only about 1/6 of a stop.

If it is simply a matter of measuring a small amount of something, make a 1%
solution of whatever it is and use only a few millilitres of it. Were I
mixing a litre of some solution that needed only 0.001 gram of something, I
bet the solution would work the same without it.

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  #2  
Old January 22nd 09, 03:35 PM posted to rec.photo.darkroom
John J
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Posts: 110
Default accurate scales

Jean-David Beyer wrote:

If it is simply a matter of measuring a small amount of something, make
a 1% solution of whatever it is and use only a few millilitres of it.


Very good point. And I will add that dissolving something to make a
solution is important. If a chemist says 'stir', he means it. Don't
shake: stir. (I wonder if everyone got the feeling I did when first
mixing Rodinal 1:200. "Is it really in there?" Now I just drop it into
the water and it incorporates within a minute.)

Were I mixing a litre of some solution that needed only 0.001 gram of
something, I bet the solution would work the same without it.


Ah, but you aren't making nerve poison!
  #3  
Old January 23rd 09, 01:22 AM posted to rec.photo.darkroom
John J
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Posts: 110
Default accurate scales

Larry wrote:

I am making a one gram batch of Kodalk which calls for .692 grams
borax and .145 grams NaOH. I realize that the ingredients are not
that pure so there is a margin of error. This recipe does not allow
for such deviation otherwise I'd be measuring out .69 grams and .15
grams or less....


Understood now. So make ten grams.
  #4  
Old February 10th 09, 11:16 AM posted to rec.photo.darkroom
Richard Knoppow
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Posts: 751
Default accurate scales


"Larry" wrote in message
...
On Jan 22, 6:35 am, John J wrote:
Jean-David Beyer wrote:
If it is simply a matter of measuring a small amount of
something, make
a 1% solution of whatever it is and use only a few
millilitres of it.


Very good point. And I will add that dissolving something
to make a
solution is important. If a chemist says 'stir', he means
it. Don't
shake: stir. (I wonder if everyone got the feeling I did
when first
mixing Rodinal 1:200. "Is it really in there?" Now I just
drop it into
the water and it incorporates within a minute.)

Were I mixing a litre of some solution that needed

only 0.001 gram of
something, I bet the solution would work the same

without it.

Ah, but you aren't making nerve poison!


I am making a one gram batch of Kodalk which calls for
.692 grams
borax and .145 grams NaOH. I realize that the ingredients
are not
that pure so there is a margin of error. This recipe does
not allow
for such deviation otherwise I'd be measuring out .69
grams and .15
grams or less....


The formula above is for the equivalent of sodium
metaborate in solution. You can always mix a concentrated
solution and measure off the equivalent of one gram. That
would allow the use of less precise scales and the accuracy
and precision of the liquid measure would not have to be so
fine.
I wonder what exactly you are trying to do.


--
--
Richard Knoppow
Los Angeles, CA, USA




 




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