switch 33 which generates a signal to acutate the power cylinder 25 upon the detection of complete insertion of joint 20 of second nozzle 16 into the joining part 18a of first nozzle 15 and switch 35 which gives a signal to actuate the high pressure air system not shown at the detection of the engagement of clamping plate 24 with neck 22 of second nozzle 16 the structure of the present invention thus being constructed the cover is placed on the upper brim of the principal water tank in the water contained in the auxiliary water tank after a container to be tested is immersed in the water contained in the principal water tank consequently the air adhering to the inner surface of the cover is free to escape so that the deaeraring operation is eliminated furthermore the time required for the operation of setting the zero point of the buret is reduced as a fixed quantity of the water in the principal water tank and the auxiliary water tank is restored during the replacement of the containers after a series of measurement operations reducing the time expended for resupplying water in the water tanks still further the junction of the first nozzle and the second nozzle is disposed outside of the principal water tank the high pressure air leaked from the junction is released into the atmosphere without producing any effect on the measurement and the leakage of the high pressure air into the principal water.