Acoustic flow meter for measuring very slow fluid flow

Claim 1

IN A DEVICE FOR MEASURING THE VELOCITY OF FLOW OF FLUID, THE COMBIANTION COMPRISING: FIRST SONIC MEANS FOR TRANSMITTING AND RECEIVING ACOUSTIC ENERGY THROUGH A FLUID IN A DIRECTION OPPOSITE THE DIRECTION OF THE FLOW OF THE FLUID; SECOND SONIC MEANS FOR TRANSMITTING AND RECEIVING ACOUSTIC ENERGY THROUGH THE FLUID IN ACCORDANCE WITH THE DIRECTION OF THE FLOW OF THE FLUID; SAID FIRST AND SAID SECOND SONIC MEANS HAVING SEPARATE MEANS FOR TRANSMITTING AND FOR RECEIVING ACOUSTIC ENERGY; SAID TRANSMITTING AND SAID RECEIVING MEANS IN SAID SONIC MEANS HAVING INTERCHANGEABLE FUNCTIONS; SAID TRANSMITTING MEANS BEING ADAPTED TO CONVERT ELECTRICAL ENERGY INTO ACOUSTIC ENERGY; SAID RECEIVING MEANS BEING ADAPTED TO CONVERT ACOUSTIC ENERGY INTO ELECTRIC ENERGY; PHASE METER MEANS OPERABLY CONNECTED TO EACH OF THE RECEIVING MEANS ON SAID SONIC MEANS FOR INDICATING THE PHASE DIFFERENCE BETWEEN THE ELECTRICAL SIGNALS RECEIVED FROM SAID FIRST AND SAID SECOND SONIC MEANS; CALIBRATING MEANS CONNECTED TO SAID RECEIVING MEANS TO COMPENSATE SAID PHASE METER MEANS FOR EXTRANEOUS ERRORS IN THE FLOW MEASURING SYSTEM INCLUDING SWITCH MEANS FOR INTERCHANGING THE FUNCTIONS OF THE TRANSMITTING AND RECEIVING MEANS IN SAID SECOND SONIC MEANS WHEREBY SAID SONIC MEANS GENERATES IN THE SAME DIRECTION AS THE ACOUSTIC SIGNAL FROM FIRST SONIC MEANS; AND PHASE SHIFT MEANS OPERABLY CONNECTED TO SAID SONIC MEANS FOR SHIFTING THE PHASE OF THE ELECTRICAL SIGNAL RECEIVED FROM SAID SECOND SONIC MEANS, WHEREBY THE PHASE DIFFERENCE BETWEEN THE SIGNALS RERECEIVED FROM SAID FIRST AND SAID SONIC MEANS IS PROPORTIONAL TO THE VELOCITY OF FLOW OF SAID FLUID BEING MEASURED.