Discussion»Statements»Rosie's Corner» How do you verify monitor calibrate TESTING FACILITIES? What standard of measure is used? How do we know the yardstick is correct?
On my first ship, I worked in an electronics calibration lab. We tested all electronic measurement devices...voltage, current, impedance, frequency , resistance and time. The devices we used were at least ten times more accurate than what we were testing, and our devices were tested with devices ten times more accurate that ours.
Seriously? Ten times and then another ten times? How is that double ten times tested? I don't mean to be a smarta** about this but there is some start or basic standard which is created from nothing that went before, right? The first ever created can't be measured against anything or tested can it? It becomes a standard for other measuring devices but what went before originally initially I still find hard to grasp. Sorry. Thank you for your reply E! :)
"the times then another ten times" is not double, but 10 X 10, i.e. 100-fold.
So if Element needed to measure voltages to no finer than 0.1V, his lab's test-meters had to be capable of reading to 0.01V, and they were calibrated against instruments sensitive to 0.001V (a milli-Volt, or one-thousandth of a volt).
There are or were standard pieces of metal for the primary units; but the ISO definitions are not quite as clear-cut as that. Although you can say a particular, carefully-preserved platinum bar is THE standard one-metre long, that is not enough for many scientific measurements so the metre is described in terms of numbers of wavelengths of light of certain wavelength would occupy the length of that bar.
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Effectively, the system forms a chain traceable to what are called National Standards, but they in turn have to conform to definitions agreed and controlled internationally by the International Standards Organisation (ISO).
(The USA and UK, and many other countries are full ISO members. A lot of the others are in a sort of associate membership arrangement. There are very, very few not in the ISO at all.)
The level of accuracy or precision (I forget the difference) needed at point of use depends on the work done there; but as Element says the measuring equipment has to be appropriately more accurate than what is being measured.
Many organisations send their measuring-equipment to independent test-laboratories for calibration. Others test it locally but against master instruments that are sent away to the test-houses, usually annually. If you work to recognised national standards or the international ISO9001 system, the calibrations are meaningless unless "traceable", i.e. accompanied by an auditable paper-trail showing they were performed to the required quality within that chain.
It's all quite a rigmarole, and like Element I was involved in it at work. We did calibrate a few of our own laboratory standards, but against measuring-instruments set away annually for calibrating.
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You ask about the history. Basically, all the measuring units we use are arbitrary and their modern definitions seem to create odd circular arguments!
They do not exist in Nature, even though many natural processes work to mathematical rules. Our measures are artificial divisions of real things; originally based on practical work like farming and building; nowadays rather more abstract.
The Metre is the international standard unit of length. It was invented by France (hence the spelling) in a post-Revolution programme to replace their utter chaos of old trade and regional units; but it based on a fraction of the Earths' circumference as best as could be measured at the time. The French made a Standard Metre Bar, kept in Paris, and that's it - The Metre; along with equivalents for mass and liquid measures. Electrical units came later as that science developed.
The standard unit of time is the second; but all our time units are artificial divisions of the time it takes for the Earth to orbit the Sun. Even then the second is now defined in nuclear-physics terms.
France's neighbouring Continental countries realised it would help trade to follow suit, fairly rapidly. The UK retained Imperial units until the 1970s, and they have not gone away entirely. I think the USA is now the only major country not using metric units for everyday domestic trade, but does so in science and engineering.
The ISO has reduced the Metric System to the Systeme Internationale (French spellings still) for mathematical regularity, helping science but at cost of practicality for everyday use. SI has, I think, only 6 primary units of measurement plus several permitted, derived units.
Element 99 