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A fascinating Air Ministry portable calibrator MK1 for aircraft altimeters No 295 AM Ref 6C/63 by R.W. Munro Ltd c1940

Portable calibrator constructed in japanned brass, the pressure vessel with 3 x 2” viewing ports, external electrical connections, twin internal balloon bulb holders, removable end hatch with wheel operated expanding O seal, correct internal instrument frame bearing “AM” property mark, twin apertures for altimeters having a 3½” diameter, diagonally positioned spring loaded mounting clips. The frame positioned on an internal rail.

External end mounted valve gear comprising: dump valve, twin selector valves isolating or bringing online base mounted hand operated vacuum pump, bicycle pump connected at tyre valve or external autonomous pump. All raised on attached cast cradles mounted to a mahogany base with hand operated vacuum pump, instrument ID plate marked “Ref No. 6C/63, Portable Calibrator MK1 for Altimeter,  R.W. Munro Ltd, Serial No. 295/40.” Twin brass threaded ferrules for screw down pins attached to removable cover, the top with similar ID plate. Carry strap fixings to ends.

Condition: Generally exceptional, the subject of extensive restoration including the air and electrical systems. The exterior repainted in precisely correct cellulose based paint, the mahogany base with some repairs and refurbishment.

Fully working, and capable of achieving and holding low pressure.

Comments: These instruments were delivered to the Air Ministry as a stop gap, at a time when new small operational airfields were coming on line across the country. This enabled the onsite testing of critical equipment away from the established test centres.

It is unknown how many were made, though it is likely that only a very few hundred were delivered. Examples have been found in an Australian museum, at a military fair in the US, and two here in the UK. Numbers noted include 263/40 and 220/40.

Bearing in mind the extreme pressure that the AM was under in the late 1930s and leading into the war, and especially as the battle of Britain raged in June 1940, this is truly an item of great historical interest.

In working condition, this equipment may be used in the testing of any barometer or altimeter provided it will fit within the chamber. To that end it could be of great interest to the serious collector of barometers or militaria alike.

NB: This equipment has been designed to operate principally at reduced pressure, thus simulating an increase in altitude. The original fitment of a bicycle valve suggests that it may also operate at increased pressure. Under no circumstances should this instrument be pressurised above standard pressure.

NB: Two altimeters shown are for display purposes only

Dimensions: 19½” long x 9½” high x 9½” deep

Stock No: SI1567

Price: £2750