which shows a preferred embodiment of the present invention it is first pointed out that the design of this sensor was effected taking into account the following eight principal objectives 2 an improved seal of the cathode electrode which at the same time allowed versatility with respect to number of cathode elements materials size and shape 3 a pure silver anode of very great area compared with that of the cathode which totally surrounded and shielded the cathode and which would not allow leakage of oxygen from internal or external sources 4 a body which all owed a complete electrostatic shield of the cathode 5 a body with excellent structural stability and which was biological and chemically inert 6 a body which could be precisely machined to fit standard taper leak proof fittings 7 a leak proof membrane retention system resistant to changes caused by variations in external environment and with no residual membrane which can trap air bubbles and increase the difficulty of cleaning and 8 a modular design which allowed flexibility of construction testing of key components and straightforward assembly it may be appreciated that although some of the above principles have been previously identified their actual implementation has often been compromised even with these compromises many patented devices have never been produced commercially this undoubtedly stems from the extraordinarily complex machining and assembly requirements of typical previous sensors and a further objective of the present design was to reduce this complexity without compromising the other objectives the preferred embodiment of the sensor as described below comprises several component parts or sub assemblies namely an anode body sub assembly shown as its component parts in fig3 a cathode sub assembly shown in fig4 a connector sub assembly.