June Perry on sun 17 aug 97
I'm getting ready to buy a digital gram scale and there are some which have
to accuracy ratings - .01 for up to about 300 grams and .1 for 3200 grams. I
do a lot of glaze testing and I'm tired of waiting for that triple beam to
balance itself!:-(
My question is, is the .1 accuracy good enough for the larger batches? My
large batches are usually the 5 -10,000 grams. My feeling is that I don't
think that the large glaze batches will suffer with that level of accuracy.
Is this correct?
Thanks for any input.
Warm regards,
June
Gavin Stairs on mon 18 aug 97
At 06:54 PM 17/08/97 EDT, June wrote:
....
>My question is, is the .1 accuracy good enough for the larger batches? My
>large batches are usually the 5 -10,000 grams. ...
Hi June,
The critical thing is the relative precision, expressed in %, for example.
So if the whole batch of glaze weighs 200 grams, then 0.01gram precision is
0.01/200, or 5/100,000, or 0.005%, or 50ppm (parts per million). A 10kg
batch weighed to 0.1g gives 0.1/(10*1000), or 1/100,000, or 0.001%, or
10ppm. The relative precision of the latter measurement is five times
greater than that of the former, even though the absolute precision is ten
times worse, because the total mass is 50 times greater.
That said, you need to read the fine print on your scale's spec sheet.
There's probably a relative precision term there too. The number of 0.1g
or 0.01g is what we call the least count in a digital scale. That is, it's
the smallest difference the scale display reports. We can measure no finer
a precision than plus or minus the least count. (There are ways around
this, but I won't get into this now.) But the precision claimed for the
scale is bound to be greater in magnitude than the least count, so that
will dominate. Repeatability over a short time scale may be quite a lot
better than claimed, though.
The accuracy of the scale is usually expressed as a relative precision, and
an additional term is often related to the maximum scale value. Accuracy
means the capability of the instrument to reproduce the standard quantity.
Thus, a scale's accuracy reflects its ability to read 1kg when a 1kg
standard mass is placed on the scale. This is different from the
repeatability of the scale, which is just the capability of the scale to
read the same value every time a constant test mass is placed on it.
If you are measuring all the parts of a glaze batch with the same scale,
then it is the repeatability which is of most importance to you. You find
this by adding the relative precision of the scale to the least count,
expressed in the same terms (e.g., relative to the mass being weighed, or
the whole batch mass, or as an absolute value with the same units, like
grams). Actually, you're supposed to add them in quadrature, but that's an
unnecessary refinement here.
If you are measuring some parts of the batch with one scale, and some with
another, then it is the accuracy of the scale which counts. Add the
relative accuracy to the least count. Actually, what counts is the
relative accuracy, which is the difference between a standard mass on the
one scale compared to the other scale, but you probably don't need to know
about that.
The important thing to realize is that the batch properties relate to the
relative proportions of the constituents, and not to their absolute masses.
So the scale property of most interest it that which relates to these
relative proportions of masses.
As with all measurements, if you want to approach the performance limits of
the instrument you have, you need to pay close attention to it's actual
behaviour, and apply some elementary theory of measurement. The easiest
technique to apply is to have a test mass which you then weigh repeatedly,
noting carefully any changes in the reported mass. If the test mass always
comes out at the same number, both before and after weighing your batches,
then you can be relatively sure that your actual precision is close to the
least count. If they vary, then the spread of the variance is a measure of
the actual precision of the measurement. There is a standard method for
calculating this variance, but I won't give it here, now. Likewise, the
relative accuracy, or, if you actually know the true mass of the test mass,
the absolute accuracy, can be determined from the mean of the test mass
measurements. Again there are standard methods for these measurements.
Pay particular attention to the zero reading after a weighing session. If
the zero drifts, then your scale is exhibiting some form of zero drift or
hysteresis. All scales do this: it is merely a matter of degree. The
amount of zero drift should be added to the error term as with the other
factors.
Most scales come with a standard mass for these purposes. Keep it clean
(by not handling it with your bare fingers, etc., and not by cleaning it
after you get it dirty), and it will serve you well for as long as you use
the scale.
Hope this is of some use to you,
Gavin
| |
|
