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Subject: mbishop: Practical difference for measurements?
From: CCCE ConfChem <[log in to unmask]>
Reply-To:CONFCHEM <[log in to unmask]>
Date:Mon, 1 May 2017 08:48:49 -0500

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To reply: [1]

I understand the interest in defining the SI base units in terms of
“physical constants believed to be invariant of nature”, but I wonder if
it will make a difference in real-world measurements. For example, are there
ways to determine temperature that would yield different values based on the
current and proposed definitions of the kelvin? In other words, at least for
temperature, is this an abstract exercise with no practical effect?

I also find it curious that the definitions are based in part on defining
nature’s constants as exact, even though we don’t know what the exact
values are. For example, the kelvin would be described as, “The kelvin, K,
is the unit of thermodynamic temperature its magnitude is set by fixing the
numerical value of the Boltzmann constant to be equal to exactly 1.380 65…
10-23 when it is expressed in the unit s-2 m2 kg K-1, which is equal to J
K-1.” Wikipedia says that the Boltzmann constant is 1.38064852(79)×10−23
J/K. If this value (or some other more accurate value) is used to calibrate
temperature measuring instruments, what happens to this calibration when we
develop ways to determine the constant more accurately? Are we still stuck
with the problem of a drifting definition?

Mark Bishop

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