A capillary tube contains a liquid that evaporates according to the Arrhenius equation for maturity. The tube is placed on a scale in a container. Following casting of the concrete the capillary tube is broken at the zero mark, threaded into the container and the container is pressed into the fresh concrete.
The temperature in the container will quickly stabilize with the temperature of the concrete. Evaporation of the liquid in the container will take place at a rate determined by the temperature. The level of the liquid - readable on the scale - indicates the maturity, the number of days (M20), the concrete is old at 20ºC (68ºF).