The methods are based on the measurement of the force which it is necessary to exert vertically on a stirrup or ring, in contact with the surface of the liquid, in order to separate it from the surface, or on a plate, with an edge in contact with the surface, in order to draw up the film that has formed.
There are four different methods: the plate method, the stirrup method, the ring method and the OECD harmonized ring method. They are described in detail in the ISO Standard 304-1985. The methods described are applicable to aqueous solutions of most substances regardless of their degree of purity. The concentration should be 90% of the saturation solubility, but must below 1g/l. This shall therefore be carried out under a protective cover to avoid interference at 20°C approximately. The ring is immersed below the surface of the solution. Then the table top, where the measurement vessel is placed, is lowered gradually and evenly at a rate of approximately 0.5 cm/min to detach the ring from the surface until the maximum force is reached. The force is read on the tensiometer. After completing the first measurement, measurements are repeated until a constant surface tension value is reached.