A mass detection apparatus for detecting leaks in a liquid storage tank comprising: a programmable logic controller, PLC, operably connected to: a first differential pressure transmitter having a transducer low side and a transducer high side; said transducer high side being configured to measure tank bottom pressure within a liquid in which a bubbler is immersed; and said transducer low side being configured to receive data indicative of atmospheric and vapor pressure substantially at the surface of said liquid; a barometer configured to measure atmospheric pressure at the surface of said liquid; a second differential pressure transmitter for contents density measurement having a transducer low side and a transducer high side; said transducer high side being configured to measure tank bottom pressure within the liquid in which the bubbler is immersed; said transducer low side being configured to measure fluid pressure just below the liquid surface; and ambient temperature measurement means for measuring ambient temperature near said storage tank; wherein the PLC is configured to: receive first and second data reflecting, respectively, said tank bottom pressure and of said barometric pressure at said surface of said liquid, and adjust said first data to substantially eliminate variations upon said measurements of said tank bottom pressure caused solely from atmospheric pressure variations to yield an atmospheric pressure adjusted tank bottom pressure; wherein the PLC is configured to: receive third data indicative of said atmospheric pressure adjusted tank bottom pressure, receive fourth data indicative of ambient temperature measurements by said ambient temperature measurement means, receive fifth data indicative of expansion characteristics of said storage tank, and to adjust said third data with reference to said fourth and fifth data to substantially eliminate variations upon measurements and calculations of said barometric pressure adjusted tank bottom pressure, caused solely by dimensional changes in said storage tank resulting from atmospheric temperature variations, to yield a tank dynamic adjusted tank bottom pressure; wherein the PLC is configured to: calculate average fluid density throughout a hydrostatic column based on measurements of the second differential pressure transmitter, wherein the second differential pressure transmitter measures the differential pressure between the transducer high side and the transducer low side just below the liquid surface; and wherein the PLC is configured to: calculate mass contents of said storage tank based on said tank dynamic adjusted tank bottom pressure, average fluid density, operator input data reflecting tank shape and configuration and operator input data reflecting physical characteristics of said contents of said storage tank. a programmable logic controller, PLC, operably connected to: a first differential pressure transmitter having a transducer low side and a transducer high side; said transducer high side being configured to measure tank bottom pressure within a liquid in which a bubbler is immersed; and said transducer low side being configured to receive data indicative of atmospheric and vapor pressure substantially at the surface of said liquid; a barometer configured to measure atmospheric pressure at the surface of said liquid; a second differential pressure transmitter for contents density measurement having a a first differential pressure transmitter having a transducer low side and a transducer high side; said transducer high side being configured to measure tank bottom pressure within a liquid in which a bubbler is immersed; and said transducer low side being configured to receive data indicative of atmospheric and vapor pressure substantially at the surface of said liquid; a barometer configured to measure atmospheric pressure at the surface of said liquid; a second differential pressure transmitter for contents density measurement having a transducer low side and a transducer high side; said transducer high side being configured to measure tank bottom pressure within the liquid in which the bubbler is immersed; said transducer low side being configured to measure fluid pressure just below the liquid surface; and