A possibly unique aneroid manometer no. 12, probably by Short & Mason, formerly installed at the Royal Aircraft Establishment, Farnborough c1930

Aneroid manometer having 5¼” silvered and die struck dial calibrated in millibars with a range from 400 – 1050mb (approx. 20,000ft – 1,000ft, divided down to 1/5th mb. The upper part with Meteorological Office “MO” logogram and instrument no. “12,” the lower marked “RAE” (Royal Aircraft Establishment) and “Millibars.” Three dial adjustment slots and corresponding clamping screws. Japaned steel knife edge pointer with helical temperature compensation. Bevelled glass under heavy screw down “O” sealed brass bezel.

The much developed precision movement driven from a single flux welded 2” capsule captured within a U-shaped tensioning spring, the top spigot  bored and threaded with adjustment screw, fine adjustments to spindle and levers. All contained within a stepped and spun aluminium case, mounting flange, the verso with single stub port, fine movement positioning screw.

Condition: The subject of a full service, conservation, and calibration under laboratory conditions, see performance chart from dynamic test. The movement with evidence of minor corrosion, though working exceptionally well with little or no adjustment required. Accuracy within 3mb across the scale.

Owing to the poor condition of this piece, and because of its great interest, the case has been correctly re-painted, the glass seal replaced. The dial is marked with some staining, the case showing signs of pitting.

Comments: This is a very interesting instrument, almost certainly fitted to monitor pressure gradients within one of the three wind tunnels at the Royal Aircraft Establishment (RAE). Basic physics tells us that as velocity increases pressure drops, which perfectly explains the lower pressures to the scale.
The dial bears the logogram “MO,”(Meteorological Office) – this department, then part of the armed forces, was responsible for all such pressure measuring equipment.

The movement is of particular interest for its development, encompassing the then latest technology in the production of pressure sensing capsules, made in two halves and flux welded around the circumference. This pattern movement with its captured capsule, ie the base of the capsule being affixed to the lower part of the U-spring, the upper part with its adjustable spigot thereby altering the tension on the capsule, affords a micro adjustment to be made, an innovation not seen elsewhere with this level of intricacy. A further and very unusual fine adjustment may be found at the head of the primary lever making alteration to angle of rotation and length of first drop link.

Another really unusual feature is the approach to temperature compensation afforded by fitting a helical bi-metallic strip fixed at the top to the pointer and at the bottom to the arbour. At time of writing, this is the only such example of this novel form of temperature compensation noted.

There is no calibration screw fitted, the only possible adjustments made at the head of the capsule spigot or on the spindle. Some small setting up latitude is afforded by releasing the three dial screws, the dial may then be rotated approx. +/- 2mb relative to the pointer.

The instrument is sealed principally with a single deformable large “O” seal sitting between the glass and the case. This allows the instrument to remotely monitor pressure by means of a connecting pipe.

The origin of this instrument is most likely Short & Mason, the movement is so far divorced in design terms from the original Vidie pattern that some might consider it to be of continental manufacture, however this is unlikely since Short & Mason were major government contractors and certainly capable of producing a movement of this quality.

Numbered “12” suggests at least 11 others were produced for RAE. We have no means of assessing numbers produced, only that this is, as far as I know, one of only two known examples.