# B Calibrating measurements from scratch

#### atracious

Summary
I'm wondering how a person would determine accurate measurement of time, weight/mass, and volume if there were no calibrated equipment.
Assuming you are in the field and don't have any device to weigh, measure volume, or measure time. How would one establish accurate measurements? Where does one start?

BvU
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#### BvU

Homework Helper
Well, go back in history: you take the size of the king's foot as a length standard and a nice big stone for weight. Twenty centuries later you still measure feet and stones.

#### Ibix

The first question is what accuracy and precision you require, surely. Why do you ask, OP?

#### berkeman

Mentor
Summary: I'm wondering how a person would determine accurate measurement of time, weight/mass, and volume if there were no calibrated equipment.

Where does one start?
That's actually a pretty good question...

#### atracious

The first question is what accuracy and precision you require, surely. Why do you ask, OP?
I am curious as to the scientific method applied to define units of measure. And how easy/hard it would be to progress in accuracy up to, say, high school level. Mm, cm, L, mL, Kg, lbs, feet, inches, etc. Or if it would be more prudent to just define new units of measure and rebuild from there.

#### anorlunda

Mentor
Gold Member
Are you asking about actual history, or are you writing a fictional story?

I have to imagine that the balance scale was a really ancient invention. Wikipedia says 2400 BC. The accuracy and precision of a balance scale can be very good.

Something as simple as a fingernail clipping could be use to measure small distances. On fingernail, two fingernails -- 1 mm, 2 mm. Wikipedia says the oldest measuring rod dates to 2650 BC.

#### Nugatory

Mentor
Or if it would be more prudent to just define new units of measure and rebuild from there.
We do redefine our units of measure, actually somewhat routinely (if any change requiring decades of thought and thousands of man-years can ever be called “routine”) as measuring technology improves. In 1983 the meter was redefined to be the distance that light travels in 1/299792458 seconds. Before then it had been defined to be 1650763.73 wavelengths of the radiation associated with a particular atomic transition of the gas krypton, and before that it was the length of a reference bar kept in a vault in a laboratory in Paris.

In each case, the new standard was something that could, because of technological advances, be measured more precisely than the old standard. Calibrations made with the new standard are better than with the old standard; the worst-case difference between measurements of the same thing made with two different but ostensibly properly calibrated instruments is smaller.

However, the new standard is deliberately kept compatible with old one, in the sense that the length of the new meter is always within the range of error of measurements of the old one. Thus, anything calibrated against the old standard still works as well as it ever did - we aren’t obsoleting hundreds of billions of dollars of worldwide investment in machine tools, measuring devices, and manufacturing equipment.

And this last is why we keep improving the meter instead of defining something new and rebuilding from there. Sure, we could create a new unit of length... call it the “greeple” and define it to be the distance that light travels in $10^{-8}$ seconds. But why bother? The downside is the enormous economic cost of retooling the entire world economy, and the upside is.... what? Whether we use greeples or meters, our ability to precisely specify distances (share specifications among manufacturers, repeatable manufacturing processes, spare parts that fit, ....) is still determined by our technology for measuring fractions of a second.

You should not be surprised to hear that the 1983 definition of the meter was adopted precisely because we can measure time more precisely than distance. Maybe in a few decades that will change, and then we will take advantage of the new technology to further improve the meter standard.

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"Calibrating measurements from scratch"

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