Precision aneroid barometer Type M1991

The barometer is fixed into a wooden box by means of two locking screws, but is designed to be used without removing it. The box contains a set of printed instructions and accessories. A set of basic operating instructions is also stuck inside the box lid. To take a reading, the operator holds down the switch and turns the counter knob until contact is just broken. The barometric reading is then given on the digital dial in millibars. The makers, Mechanism Ltd, specialized in the manufacture of precision barometers, including barometers used in research by organizations such as the British Antarctic Survey.

These instruments were produced in the 1960s. There were four variants of the basic design. They were developed for use on board ships, or wherever portability was desired. The aim was to produce an aneroid barometer having an accuracy, over several months, equivalent to a Kew Pattern mercury barometer, but without their transportability problems of mercury.

The instrument used a three stage beryllium-copper capsule (two stage on one of the variants), rigidly fixed at one end, the other free to move. A pivoted bar, mounted in jewelled bearings, has one end held in contact with the free end of the capsule by a light spring, similar to a watch hairspring. A micrometer screw operates on the other end of the bar and a 'magic eye' type of indicator shows when an electrical circuit through a gold contact on the bar has just broken.
The specification is that the instrument is has a range of 930 to 1055mb and is capable of indicating changes in pressure of 0.02 millibars. At a fixed pressure of 1000 millibars, the change in correction is less than 0.02 millibars per degree C over the range 0 to 40 C.

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