A glass capillary contains a liquid for which the rate of evaporation varies with temperature according to the Arrhenius equation, which is the same function that is used to determine maturity of concrete from its temperature history (see pg. 75). The closed capillary is placed on a card with a calibrated scale indicating maturity in equivalent age at 20 ºC. The card is attached to a cap that fits into a transparent container. After the concrete is cast, the capillary tube is snapped at the zero mark on the scale, the cap is pushed into the container, and the container is inserted into the fresh concrete.