Re: formula

On Wed, 26 Dec 2001, Art Kerbs wrote:

> Wayed, 10 oz of water = 282 grams, yes if you then take your graduate at
> zero it out and pour water to 282 grams it comes out to 10 oz.
>
> convert ml to oz and oz to grams on the conversion site.

I've tried using the conversion site you've suggested. The problem as
I've already tried to point out is that you are running into the issue of
conversion between volume and mass. These are not equivalent units and
you can't simply convert a volume to a mass without knowing the density of
the material. Saying just "convert ml to oz and oz to grams" is
problematic since the fluid ounce (1/16 of a pint) and the dry measure
ounce (1/16 of a pound) are two different quantities (see
http://ask.yahoo.com/ask/20000717.html). Converting milliliters to ounces
gives you fluid ounces. You'd still need to use a density factor to
convert from fluid ounces to ounces of mass. and this is not something
that the conversion calculator has available. In my post yesterday I
referenced everything back to the national standards.

As for your experiment, I can only suggest that you may be seeing the
uncertainties in your measurement process. There is no such thing as a
perfectly exact measurement. The zeroing of your scale isn't going to be
perfect, the act of weighing your sample won't be perfect, your ability to
read the scale on your graduated cylinder also won't be perfect, and it
isn't likely that the temperature you are measuring at is exactly 20
Celsius (the temperature I used for my conversion) or 4 Celsius ( the
temperature at which water has a density almost exactly equal to 1).

For your nominally 10 ounce sample of water it looks like you are getting
a discrepancy of roughly 13 grams between what I've computed and what
you've measured that is an uncertainty of roughly 4% which is fairly
typical for most consumer level measurement processes. (Calibration labs
struggle to get uncertainties of 1%.)

Anyway, you had asked someone to look at your computations
<http://rmp.opusis.com/pipermail/carbon/2001-December/002781.html> so I've
attempted to do so. I think you may be misusing the conversion tables.
That makes your values deviate from the national standards. However,
for you own personal use your numbers will at least be consistent. In
many cases consistency rather than absolute accuracy is the key.

> In my case oz to grams..........yes your only working with 100 points of
> percents (could be 1000 if you want to use 01 of a percent, but that is more
> than fine enough with grams. I suppose you could go for grains, but not
> necessary.

I really don't understand what you are getting at here? Percentages
aren't discrete 100 point quantities. It is an infinite numerical scale.
You can have fractional percentages. Fundamentally this nothing more than
a fraction normalized for a denominator of 100. It is useful to compare
percentages since the fractional relationship is constant regardless of
the relative quantities: 8 parts in 100 is 8% just as 80 parts in 1000 is
also 8%.

> I believe that when we all talk about "how much should one use" etc. we
> should be using percents of the total volume when dealing with photographic
> formulas. Do you agree or have a problem with that?

I'll have to think about it a bit more. Using the total volume of
solution as the reference certainly has its merits. Of particular
importance is that it is basically the status quo. On thing that bothers
me about this is that this adds yet one more measurement to the process.
You have to measure all of the ingredients and then adjust the final
volume at the end. That introduces another uncertainty term in the
process, and depending on the ingredients used "may" mean that different
people are using varying amounts of water since that is usually what one
would add to adjust the final volume. I'd probably standardize on adding
a measured quantity of water and use that as your percentage reference as
opposed to the practice of adding a certain amount of water, the
ingredients, and then topping off with an unknown amount of water to reach
a target volume.

In any case, the key is consistency and when sharing recipe's, indicating
how you obtained the values noted.

- Wayde
  (wallen@lug.boulder.co.us)

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Received on Thu Dec 27 10:11:44 2001