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Index of Subjects Hi Dave, I see you got no takers so far, re your penultimate paragraph. I don't really know for sure either but I thought that modern electronic balances relied on a strain gauge based device either for all of it, or for the last fine adjustment after balancing out most of the inequality to some reference mass along some kind of beam. They would therefore be based wholly or in part on estimating the force required to overcome all or a bit of the gravitational pull on the mass being 'weighed'. Any non-linearity or known variation in g could be compensated by an approximation algorithm in the microprocessor these things all seem to have, while a second matched unit in a bridge circuit would help with temperature compensation. So at least part of it would depend on taking account of the exact value of the gravitational constant at the particular location with some calibration, as opposed to being strictly independent of g with the old type of beam balance. But I don't recall a technician with a screwdriver ever coming round to adjust any newly purchased recent balance, so presumably each somehow autocalibrates itself straight out of the box, analogous to when you hit the 'tare' button to recalibrate for an unwanted extra mass like a weighing dish. I was away/off line until Sept 25 so didn't see your original post or replies to understand your actual interest in this. Provided that you stay within the stated tolerances of your measuring device for 'weighing'/estimating the mass of something, does it actually matter whether the balance/scale compares masses directly on the old g-independent balance beam principle, or instead uses a gravitional force-based estimator like a spring (I'd recommend the cheap but useful digital readout one from Lee Valley that we carried with us, for checking that our flight luggage wasn't overweight)? Steve (Halifax) ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Quoting David & Alison Webster <dwebster@glinx.com>: > 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 static; noise without information content. > Hopefully this edition will not be entirely static. > > 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 gravitational 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 above in 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 -- Stephen R. Shaw Ph.D Dept of Psychology & Neuroscience Dalhousie University 1459 Oxford Street Halifax, Nova Scotia, Canada B3H 4R2 e-mail: srshaw@dal.ca fax: 1-902-494-6585 phone: 1-902-494-2886
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