next message in archive
next message in thread
previous message in archive
Index of Subjects
Hi Patrick & All, Oct 1, 2012
Having had a bit of time to reflect, I see that my comments of Sept 22
are a form of stattic; noise without information content. Hopefully this
edition will not be entirely stattic.
I have gone back to first principles which, if applied with care,
should lead to correct conclusions.
The simple equal-beam analytical balance is what it says it is; a
balance. Assuming the pans to have been balanced when empty, by adjustment
of a counterweight, then a reference mass on one pan will balance an unknown
mass on the second pan when they are equal to each other (and at the same
temperature). Being equal in mass the two pans of the balance will
experience an equal gravational force and thus be in balance.
The above analytical balance is a refined form of weighing devices that
have been around since the dawn of trade. These progress from the simple
hand-held scales (still used in some parts of the world), to steelyards and
bar scales. All of these devices balance a reference mass against an unknown
mass but the operation (in English) has for many years been called weighing
and the measurement so obtained has been called a weight.
Consequently, within the mundane context of trade or operations that
require determination of mass (for analysis, prep. of reagents, etc.), the
terms weight and mass refer to the same class of measurment and weight in
this context is not at all a measure of absolute gravitational force (F).
As the authors (Blaedel & Meloche) of my 1956 Quantitative Chem text
observe (p. 543) "The standard masses used for comparison are erroneously
called "weights," but this usage is so common that it is futile to attempt
correction." In this text " ... common usage is followed by using the term
"weight" rather than "mass...".
The use of the term weight to refer to F does introduce confusion and I
am not yet clear how this can be resolved, apart from using the term force
for this purpose rather than weight.
The 1979 Canadian Metric Practice Guide (ISBN 0317-5669) advised the
abovein section 4.7.3; "In commercial and everday use, the term "weight"
nearly always means mass. In science and technology, "weight" has primarily
meant a force due to gravity. In scientific and technical work, the term
"weight" should be replaced by the term "mass" or "force" depending upon the
application."
Spring scales (and so far as I know) torsion scales do directly sense F
but are none-the-less commonly used for "weighing" by being calibrated with
"reference weights". They differ from balances in only one respect,
reference masses are used to calibrate the spring at the time of manufacture
(some scales can be checked and adjusted later using reference masses) and
subsequently the calibrated spring is used to measure mass. If g at the
point of calibration differs from g at the point of use then the
determination of mass will be in error.
Now at this point I am on unsure footing (having been out of the loop
for 17 years) but, so far as I know, many modern top-load scales (commonly
called top-load balances) and even analytical balances use a load cell or
equivalent to directly measure F and this is subsequently converted to a
readout in mass presumably by electronics that somehow divide F by an
assumed or measured value of g. Logically such scales would include one or
more additional load cells with standard masses imposed so that g could be
sensed by the unit. [Our last electronic analytical balance tended to drift
over time and, for precise weighing, had to be adjusted at intervals.]
.
If someone can confirm or refute the contents of this last paragraph
then I would like to hear from them. Learning how things work by consulting
information on the internet yields rather superficial results.
Yours truly, Dave Webster, Kentville
next message in archive
next message in thread
previous message in archive
Index of Subjects