Maintaining standards: The triple-point cell

Our society depends on standards in countless, mostly invisible ways.   If we can’t agree on how to measure weight, length, temperature, or time we can kiss manufacturing and our manufactured world with its specified, engineered, and interchangeable parts goodbye.   Actually creating and maintaining these standards is hard and the methods change over the years.  The meter was once defined in relation to earthly distances until the realization that the earth changes over time.  Now it is defined in relation to the speed of light which we are pretty sure does not.  The second used to be defined in terms of the day, now it is based on a fundamental property of the cesium atom.  The kilogram has just been redefined in terms of Planck’s constant which then turns into a combination of the second and the meter.

Making any of these measurements is difficult and requires a lot of fancy equipment, often involving lasers, vacuum chambers, electromagnets, and/or racks of electronics.  Here’s how NIST’s new F2 atomic clock works schematically and here’s a package from its inventor on the details.   As the F2 becomes a practical albeit sophisticated standard, even fancier methods are under development for the future.

The Kelvin, fundamental unit of temperature, is a nice exception to this complexity.  It is defined in relation to the triple point of water; that temperature at which the liquid, solid, and vapor phases of isotopically controlled, gas and contaminant-free water are in equilibrium.  Measure this and the Kelvin is 1/273.16 of that.   The aptly named triple point cell requires appropriate water, a skilled glassblower, and some patience.   Thermometers can be calibrated against this standard within and across laboratories.

The Fluke Corporation, despite its name, has long been a respected supplier of a wide variety of test and measurement equipment and they sell such a triple point cell.  In the right hands, it can allow the temperature of 0.01C (the Centigrade and Kelvin are equivalent) to be measured with an uncertainty better than ± 0.0001 °C.  Here’s Fluke’s Matt Newman showing how it is done and not a laser to be seen.

Addendum 20 February 2019: The Kelvin has also been redefined as of November 2018.  It is now tied to Boltzmann’s constant, k.  NIST says that not much will change for the moment since the triple point cell is a known, reliable tool.


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