A very unusual airship meteorologists aneroid barometer No 12 made for the Met Office in collaboration with the Royal Aircraft Establishment c1930

Aneroid barometer 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. Japanned 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 barb 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, and originally I had considered it was probably used for wind tunnel testing. However, recent research into airships, their instruments, and crew carried on many of the larger airships has persuaded me otherwise.

This barometer is certainly not a fixed wing aircraft piece but clearly related directly to aviation. We do know that RAE were directly involved in the design and construction of airships, either autonomously or in partnership with a number of smaller but private companies. There is hard evidence that altimeters carried on the R101 were similarly marked RAE. We also know for certain that meteorologists were crewed on these ships, barometric pressure being most germane not only to forecasting but an important consideration in correctly trimming out the airship.

There are aspects of the design of this instrument which would suggest operation across a wide range of temperatures, principally the very novel helical suspension of the indicating pointer mentioned below, not perhaps a feature that one might consider so important when operating within the finely temperature controlled environment of a wind tunnel or test facility.

Finally, and perhaps indicative rather than conclusive, is its diameter, much too large for a fixed wing aircraft and perhaps not quite the diameter of a reference instrument to be used in a test facility.

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 movement is far divorced in design and material terms from the original Vidie pattern, and is clearly the product of what was then cutting edge design. All considered, it is very likely that RAE worked with Short & Mason who were major government contractors for aviation instruments at that time.

Numbered “12” suggests at least eleven 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.