o ring groove 96 and o ring 98 may be fastened to the end of each column by way of bolts 100 the end plate 94 may have ports 102 and 104 extending therethrough for receiving a transducer unit 106 and high pressure fitting 108 the transducer unit 106 may contain the pzt 5a transducers mentioned above it should be understood that essentially any cap assembly may be used to seal the top of each column and anchor the transducers at the top thereof refer again to fig2 each column 14 and 16 is partially filled with a high density working liquid such as mercury the amount of mercury is such that the level in each column is located approximately midway up the height of the columns in any event the amount of mercury should not exceed the amount which when the mercury level in one column is maximally depressed the level of the other mercury column preferrably should not immerse the collar flange 48 a second liquid 112 fills the remaining volume of each column above the mercury it is preferred that the second liquid be an oil such as odorless kerosene the kerosene is essentially non reactive with mercury and less dense the kerosene not only fills each column but is also contained in each conduit 20 and 22 and in each pressure source such as 24 and 26 shown in fig1 it is preferred that gas pockets not be present in the manometer 12 and associated conduits or pressure sources any gas in the system may be allowed to escape through valve 84 or similar valves in the hydraulic system gases because of their.