Pressure sensing instrument

Claim 1

A PRESSURE SENSING DEVICE COMPRISING IN COMBINATION A TUBE, OF SUBSTANTIALLY UNIFORM DIAMETER, SAID TUBE DEFINING A CLOSED LOOP AND BEING CAPABLE OF RETAINING LIQUID WHICH IS RESPONSIVE TO PRESSURE CHANGES IN ITS LOWERMOST PORTION, A VALVE WITHIN SAID LOOP SITUATED ABOVE THE LIQUID LEVEL IN SAID TUBE, SAID VALVE ISOLATING ONE PART OF THE LOOP ABOVE THE LIQUID FROM THE OTHER PART OF THE LOOP ABOVE THE LIQUID AND FROM A SYSTEM TO WHICH THE TUBE IS CONNECTED, SAID TUBE COMPRISING AT LEAST A SEGMENT THEREOF WHICH ADMITS THE PASSAGE OF LIGHT THERETHROUGH, A SINGLE OPENING IN SAID TUBE ADAPTING THE TUBE FOR CONNECTION TO A PRESSURE SYSTEM IN WHICH IT IS DESIRED TO SENSE A CHAANGE FROM A PREDETERMINED PRESSURE SET IN THE SYSTEM, A LIQUID IN SAID TUBE CAPABLE OF ACTUATING A LIGHT BEAM WHEN THE LIQUID LEVEL CHANGES IN RESPONSE TO A CHANGE IN PRESSURE IN A SYSTEM TO WHICH THE TUBE IS CONNECTED, A PHOTOCELL ON ONE SIDE OF SAID SEGMENT, AND A SOURCE OF LIGHT POSITIONED SO THAT A BEAM OF LIGHT IS DIRECTED THROUGH SAID SEGMENT ONTO SAID PHOTOCELL, ELECTRICAL SWITCHING MEANS CONNECTED BETWEEN SAID PHOTOCELL AND A MEANS COMMUNICATING WITH SAID TUBE FOR RESTORING THE PRESSURE TO THAT PREDETERMINED FOR THE SYSTEM SAID PHOTOCELL BEING ARRANGED SO THAT WHEN THE LIQUID LEVEL IN SAID TUBE IS VARIED AS A CONSEQUENCE OF A CHANGE IN THE PREDETERMINED PR%ESSURE, SAID LIGHT SOURCE IS INTERRUPTED THEREBY ACTUATING SAID SWITCHING MEANS.