System for measuring changes in fluid pressure

Claim 1

AN APPARATUS FOR MEASURING SMALL DIFFERENTIAL PRESSURE CHANGES IN A WELL HAVING A WELLHEAD WHICH COMPRISES: (A) A DIFFERENTIAL PRESSURE TRANSDUCER AT THE SURFACE OF THE EARTH AND CONNECTED ON ONE SIDE TO THE WELLHEAD, INCLUDING MEANS FOR GENERATING A FIRST SIGNAL INDICATIVE OF RISING PRESSURE AND A SECOND SIGNAL INDICATIVE OF FALLING PRESSURE; (B) A REFERENCE PRESSURE CELL HAVING A PRESSURE SUBSTANTIALLY THE SAME AS THE WELLHEAD PRESSURE; (C) A CONDUIT PROVIDING PRESSURE COMMUNICATION BETWEEN THE OTHER SIDE OF THE DIFFERENTIAL PRESSURE TRANSDUCER AND THE REFERENCE CELL; (D) AN AUXILIARY PRESSURE SOURCE HAVING A SUBSTANATIALLY HIGHER ABSOLUTE PRESSURE THAN THE WELLHEAD PRESSURE; (E) MEANS INTERCONNECTING THE AUXILIARY PRESSURE SOURCE AND THE REFERENCE CELL FOR TRANSFERRING AN ACCURATELY METERED VOLUME OF LIQUID FROM THE AUXILIARY SOURCE TO THE REFERENCE CELL IN RESPONSE TO SAID FIRST SIGNAL; AND (F) MEANS CONNECTED TO THE REFERENCE CELL FOR VENTING AN ACCURATELY METERED VOLUME OF LIQUID FROM THE REFERENCE CELL IN RESPONSE TO SAID SECOND SIGNAL.