Just to elaborate on units I use, here's a hypothetical (not stuff that actually happened today, but very commonplace nature):
I drove 20 minutes up the road to by a bushel (US, not British) of U.S. No. > 1. apples, to make apple cider. On my return trip, I stopped at the > hardware store to buy a 2 lb box of 1-3/4" ring shank 12 penny nails. I > used my 7/8 hole kitchen planer blade to grate the apples, then squeezed > them for an hour and a 15 minutes at 30 psi to extract the juice. For good > measure I added 2 tablespoons of vanilla and a pinch of salt. Then I drove > the nails into grade C 2x4 joists (whose sizes are 1.5 x 3.5 inches, with a > 1/16th inch permissible tolerance in sizing). Please express that description in SI units! ;-) On Mon, Apr 4, 2022 at 11:55 AM Ken Kundert <pyt...@shalmirane.com> wrote: > I think there is one more point worth making here. There is a suggestion > that > dimensional analysis can underpin a units system. Actually, the idea that > all > units can be broken down into a small set of fundamental units is very > limiting > and results in many vexing issues. > > For example, consider currencies. There are currently hundreds of > national > currencies and thousands of cryptocurrencies. They all have the same > basic > fundamental unit of “value”, but value is only loosely defined. > Furthermore, > there is no fixed ratio between the currency and its value. It varies > over > time, over location, and from person to person. > > Consider units of a particular commodity. The example of a ream of paper > was > recently mentioned. A ream is 500 sheets of paper. However two reams may > not be > comparable. They may refer to a different size of paper or a different > quality > of paper. So all prices for reams of paper would have the same > fundamental > units of “value per each”, but both “value” and “each” are not necessarily > comparable. In effect, the fundamental unit system is not complete. You > also > need to include information about what you are measuring. For example, > “each” > could represent a single item of anything. The unit is not complete until > you > include a description of what that anything is, and in effect, there is an > unlimited number of things it could be. > > Now consider the issue of “unitless units”. In electrical circuit we > often talk > about gain, which it the ratio between the signal level at the output of > a circuit relative to the signal level at the input. But you need to be > specific about how you measure signal level. Typically, it would be a > voltage, > current, or power level. Consider a circuit where both the input and > output are > measured in volts. Then the gain would have units of "V/V", which is > unitless > in dimensional analysis. But units of "V/V" (voltage gain) is much > different > from units of "A/A" (current gain) or "W/W" (power gain), even though they > have > the same dimensions. Mixing them up results in errors and confusion. An > additional complication is that sometimes logarithmic units are used. For > example, decibels in voltage, or dBV, is 20*log(Vout/Vin). Again, > a dimensionless quantity, but nonetheless "dBV" much different from "V/V". > > The same issue occurs with the arguments to trigonometric functions like > sin(), > cos() and tan(). Generally, we assume the arguments are given in radians, > which > is a dimensionless number. But it could also be given in degrees, another > dimensionless number. Radians and degrees are indistinguishable from the > perspective on dimensional analysis, but mixing them up results in errors. > > This is not to knock the idea of dimensional analysis. It is just not > something > that would be done in most programs that process physical quantities. > Rather it > is something that is largely done as a one-time check on your analysis. > It is > a “second opinion” on whether your hand calculation are correct, or at > least > plausible. So dimensional analysis packages such as pint have their > place, but > dimensional analysis is not something that belongs in the base language or > even > the standard library. > > However I do believe that a case can be made to allow numbers to be easily > tagged with units in the base language, and then allowing those units to > be > accessed as an attribute of the number. Packages such as pint and > QuantiPhy > could then use that attribute to provide processing of units that is > appropriate > for the particular application. > > -Ken > _______________________________________________ > Python-ideas mailing list -- python-ideas@python.org > To unsubscribe send an email to python-ideas-le...@python.org > https://mail.python.org/mailman3/lists/python-ideas.python.org/ > Message archived at > https://mail.python.org/archives/list/python-ideas@python.org/message/A6BVGOUYE2BPO54M63KXFNDOE6FUSJ3F/ > Code of Conduct: http://python.org/psf/codeofconduct/ > -- Keeping medicines from the bloodstreams of the sick; food from the bellies of the hungry; books from the hands of the uneducated; technology from the underdeveloped; and putting advocates of freedom in prisons. Intellectual property is to the 21st century what the slave trade was to the 16th.
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