This work contains the review of the literature about influence of frequency rate of foam on effectiveness of fire extinguishing. It is known that the frequency rate of foam significantly influences key parameters of fire extinguishing. Foam generators do not allow to maintain up-to-the-mark frequency rate of foam with changing pressure. As a result the frequency rate changes during time of its giving, and for its monitoring when carrying out tests it requires the device which is capable to measure frequency rate in real time. Some ways of determination of frequency rate of foam in laboratory, and in the conditions of carrying out actual tests are analysed. In the main way is weighted one. Its weak spot consists in inaccuracy of determination of values of frequency rate of foam. At suppression with ready foam in laboratory a key indicator is its stability. During actual tests with application of high-head foam generators the essential error arises because of air barbotage. Earlier methods of determination of frequency rate on a direct-current conductivity of foam were used. However, despite accuracy of these methods, they did not find application in the standardized techniques. Common weak spot of the existing ways of determination of frequency rate of foam is that they do not consider a counter pressure in system which arises at sublayer fire extinguishing. The purpose of this work - to offer the device, which is able to measured of frequency rate of foam at sublayer fire extinguishing uninterrupted, throughout all time of giving. There were two steps of researches. At the first stage the calibration cell was used. The dependence of coefficient of a cell on resistance between electrodes is established, using an aqueous solution of chloride of a potassium with the known direct-current conductivity was fixed. Then the specific resistance of working solution of the fluorinated film-forming foamer was defined. At the second stage a set-up which allowed to reveal dependence of frequency rate of foam on the size of the relation of a direct-current conductivity of foam to a direct-current conductivity of working solution was piked up. As a result of the conducted researches the mathematical dependence allowing to determine values of frequency rate of foam by the relation of a direct-current conductivity of foam to a direct-current conductivity of working solution was offered. The constant of proportionality for determination of low frequency rate of foam is established. The technique allowing to define frequency rate of the given foam at sublayer fire extinguishing is offered, throughout all time of giving taking into account a counterpressure at the exit of a high-head foam generator.