referring to the drawings in detail , wherein like numerals designate like parts , fig1 depicts schematically a tank liquid level measuring system substantially according to the disclosure in the above - referenced prior application and including plural underground storage tanks 10 , 11 and 12 for gasoline or the like , each having a vertical purge or sensing tube 13 , 13 &# 39 ; and 13a coupled therein so that the bottom of the tube terminates near and above the bottom of the particular storage tank . these vertical sensing or purge tubes are connected through associated conduits 14 , 14 &# 39 ; and 14a with a three - way rotary selector valve , not shown , which selector valve is connected through a t - fitting 15 to the bellows compression chamber of a pressure gage 16 whose measuring dial 17 is calibrated in inches or other suitable linear distance units to indicate depth of liquid in the storage tanks 10 , 11 and 12 selectively . through a simple manual pumping means 18 , atmospheric air is pumped simultaneously to the compression chamber of the gage 16 and through the three - way selector valve , not shown , to any selected one of the tubes 13 , 13 &# 39 ; or 13a to purge the latter of liquid and obtain an indication on the instrument dial 17 of the depth of liquid stored in any one of the tanks . this entire structure and mode of operation thus far described is identical to the disclosure in the referenced prior application and need not be further explained herein for a full understanding of this invention . as in the prior application , the gage 16 , pump means 18 and selector valve are held within a security housing 19 conveniently mounted on a wall in a service station office or the like . in order to impart to the tank level measuring system the added capability of detecting and indicating remotely the presence of water 20 , or other contaminating heavier liquid in the bottom of any tank beneath the main liquid body 21 of gasoline or other lighter liquid , the following simple means is provided . on the lower end of each sensing or purge tube 13 is sealingly mounted a preferably plastic valve casing 22 having an internal upwardly facing valve seat 23 surrounding a purging orifice 24 at the bottom wall 25 of the casing 22 . the main chamber 26 of valve casing 22 communicates with the tube 13 through an offset or loop passage 27 . a stop surface 28 or shoulder is provided adjacent to the loop passage 27 in spaced opposing relationship to the valve seat 23 . a preferably stainless steel float ball 29 secured to an arm 30 , pivoted between its ends at 31 to a depending extension 32 on valve casing 22 carries an upturned arm extension 33 . the arm extension 33 projects through the orifice 24 and has a rubber - like valve disc or seal 34 secured to its upper end within the chamber 26 . when the float ball 29 is unsupported , as when submerged in a liquid whose specific gravity is such that the float ball will sink , the valve disc 34 will be elevated , fig2 and unseated and the purging orifice 24 will be open as shown . when the float ball 29 encounters liquid 20 of higher specific gravity , such as water , whose buoyant force will elevate the float ball , the arm 30 will pivot at 31 and the valve element 34 will move down and engage the seat 23 sealingly and the purging orifice 24 is closed so that no further liquid can be purged from the particular tube 13 by the action of the pump 18 . in fact , further air pressurization of the tube 13 by the pump 18 will only tend to close the valve more tightly against the seat 23 . the tank liquid level measuring system operates in exactly the same manner described in the above - referenced application and involves the coordinated activities of the gage 16 , pumping means 18 , selector valve not shown , and purge tubes 13 , 13 &# 39 ; and 13a . by virtue of the present invention installed in the system , whenever water 20 or other liquid of sufficient specific gravity reaches a certain level in one of the tanks , the float 29 for that tank will begin to be lifted and eventually the valve element 34 will close and seal the purge port 24 . as a consequence , continued air pressurization of the associated tube 13 , 13 &# 39 ; or 13a by the pump 18 will only seal the valve 34 more tightly and this will prevent evacuation of any further liquid from the bottom of the associated purge tube . the resulting air pressure trapped in the tube will produce on the gage 16 by its indicator needle 35 a maximum reading at the dial face zone 36 indicating that water is present in the particular tank . the advantage of this detection and indication to the service station manager and to the company owning the service station in terms of decreased liability or damage caused by polluted fuel should be completely obvious to those skilled in the art . the essence of the invention , therefore , resides in the provision of the purging or sensing tube of the tank level measuring arrangement of a valve which is actuated by a float , the float becoming buoyant in a contaminating liquid of one weight and remaining non - buoyant or sinking in another liquid of lesser weight , such as gaoline , contained in the storage tank above the contaminating liquid . the arrangement is highly simplified , very reliable in operation and inexpensive to manufacture . it forms a simple attachment to an existing system or may be incorporated into other systems of a similar nature but for specifically different applications . it may also be noted in connection with fig2 that when the valve disc 34 is in the raised or open position shown in full lines , fluid flowing downwardly in the tube 13 and through the loop passage 37 cleanly bypasses the disc 34 which is sheltered beneath the shoulder 28 and thus exerts no closing force on the float valve . however , when the valve disc 34 is in the closed position shown in broken lines caused by the presence of water in one of the tanks , the fluid flow from the tube 13 will impinge on top of the disc element 34 and assist in closing or sealing the valve even more securely . when the water has been removed from the tanks or tank , the weight of the float 29 is sufficient to unseat the valve disc 34 by gravity action . in some instances , it may be desirable to install some pressure relief means in the gage 16 but in most cases this will not be necessary to the proper operation of the system . it is to be understood that the form of the invention herewith shown and described is to be taken as a preferred example of the same , and that various changes in the shape , size and arrangement of parts may be resorted to , without departing from the spirit of the invention or scope of the subjoined claims .