Keeling's Calibration Star

What exactly did David Keeling discover that made it possible to measure CO2 levels so precisely? As Ralph Keeling explains, the secret was liquid nitrogen.

“The methods used today remain similar to what my father worked out. Carbon dioxide absorbs infrared light, and the more CO2 there is in air, the stronger the absorption. Analyzers based on infrared absorption were already available in the 1950s, and my father showed these were precise enough to measure CO2 at the low levels found in air.

“But having a precise analyzer was not enough by itself. He also needed a way to calibrate the analyzer, so that the analyzer reading in volts could be related to exact parts-per-million levels of CO2 in air. So he developed a second method of analysis that relied on liquid nitrogen, a very cold liquid, which first became commercially available in the 1950s. When air is cooled to the temperature of liquid nitrogen, the carbon dioxide freezes out. By metering out a precise amount of air and then measuring the CO2that condensed out, he was able to determine the exact CO2 part-per-million level of the air. He used this much slower liquid nitrogen method for analyzing gas mixtures that he stored in pressurized tanks which he then used to calibrate the faster infrared analyzer.

“Today, very similar methods remain in use. The analyzers are much more stable than a generation ago, (but) most precise analyzers still use infrared absorption and the liquid nitrogen method remains a preferred method for calibration. And of course we’re now measuring atmospheric CO2 in many more places on the earth.”



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