Aneroid ambient breathing air pressure gauge having 4½” die struck and silvered dial calibrated in millibars with a range from 400 – 1050, equivalent to 12” to 31” of mercury or, in terms of altitude, -1000ft – 23,000ft, over dual concentric scales, the upper portion marked “Compensated,” with “MO” – Meteorological Office – logogram, and having serial “No 7343/41,” the lower annotated “Mark I” and signed by the maker “T. Wheeler Scientific Instruments Ltd, London,” with the designation “Millibars.” Blackened alloy pointer contained below a bevelled glass set in a bronze rotating bezel having diametrically opposed turned projections for actuation.

Special long range movement constructed in alloy, cast steel and lacquered brass and driven by a hardened 3” diameter capsule based on a conventional “C” spring system  with Wheeler pattern recuperation spring, twin heavy bronze shrouded recoil hair springs, bi-metallic temperature compensating strip to primary lever, the arbour set with deep capstan affording take up of fusee over two rotations. Drum form alloy and brass backed case with fixing flange to upper part, compensation port to verso, bezel clamping screw to 12 o’clock position.

All contained within its original timber case, flip catch to front, hinged lid with applied paper label to inside top right annotated “Do not unlock or alter bezel position unless duly authorised seamen (Met),” and hand-annotated “Do Not Tap,” broad arrow government property mark struck to top right, the lower case rim struck with instrument serial “No 7343.”

A very unusual instrument, issued to the Royal Navy in 1941. The open and wide ranging scale afforded by two almost full rotations of the indicating needle predominates toward lower than normal atmospheric pressures – conventional barometers generally having a millibar range from 960 – 1060, this instrument will record pressures as low as 400 millibars, the equivalent altitude of 23,000ft.

Applications for such an instrument within the Royal Navy would indicate submarine use, for monitoring of boat pressures. This was of great importance when running dived on diesels with snorkel induction, as the possibilities of air being drawn from the boat, through a blocked or partially blocked or faulty induction mast or snorkel, needed to be constantly monitored. High levels of vibration coupled with varying fluctuations of compartment or boat pressure meant these instruments needed to be regularly checked for calibration. This was easily achieved with this design since unlocking and rotating the bezel caused the whole dial to move relative to the movement, an appreciably easier operation than fiddling with a small screw on the back of the instrument.

We have noted three similar instruments, all of which have surfaced within a short period of time. Of the three, this is the only such that is complete within its original case.

This aneroid instrument falls with the remit of the Met Office since it measures ambient air pressure. It was not however intended as a meteorological instrument by virtue of its extreme range and lack of any index point. Clearly the instrument was intended to read air pressure in the moment of reading only, and not designed to track rise or fall in pressure. The portability of this instrument was important for two reasons: the first, that it clearly could be read in compartments that lacked the requisite instrumentation; and second, it might be used as a reference for fixed instruments, particularly within the control room and engineering spaces.

Disassembled, conserved and calibrated, checked for full deflection. Some losses to grey painted finish, particularly to verso, the dial with slight mark to centre, the wooden case in sound condition.

A very unusual and interesting instrument of which we know around 400 were manufactured between 1941 and 1944. Very much a collector’s piece.

Dimensions: 6¼” wide x 6¼” deep x 3¼” high

Stock No: BA0622

Price: SOLD