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A337-26 · Aneroid
1940s–1950s
Object Overview
Snort Barometer — an aneroid barometer for submarines, made in the 1940s–1950s by the renowned British firm of Negretti & Zambra (London) for the Royal Navy to Admiralty order. The instrument was fitted in diesel-electric submarines and indicated the air pressure inside the pressure hull. The coloured sectors on its scale mark the permissible vacuum inside the boat while the diesels are running submerged on the snort. Serial number 17170.
The case is a cylindrical drum 24 cm in diameter, spun from sheet copper, with a rolled front rim. Copper is non-magnetic, so the case caused no deviation of the control-room compass and added nothing to the boat's magnetic signature — a significant consideration for a British navy operating in waters thick with magnetic mines. Copper also does not rust: in the damp, salt-laden atmosphere of a submarine, with constant condensation and fuel vapour, a steel case would soon have been destroyed by corrosion, whereas a copper one merely acquires a patina.
aneroid capsule tensioned on a C-spring
aluminium, brass, cast iron, copper, glass, nickel silver, steel
The inside of the copper case is lined with a brass plate to which the movement is mounted; the plate is stamped with the number 845, repeated alongside by hand. On the outside of the back, the heads of the three brass screws securing this plate are visible, together with the calibration port. The instrument was fixed to the bulkhead by a brass hanging lug — the only survivor of the original three; the two lost lugs are recalled by screw holes near the edge of the back.
At the front the case is closed by a brass bezel secured with screws. The bezel retains a thick flat mineral glass with a bevelled edge. A brass set hand with a knurled knob is mounted at the centre of the glass. The inner reflector ring of the bezel is painted white.
The dial is made of thin aluminium-alloy sheet, with the scales and inscriptions applied by screen printing. Its surface is finely grained and frosted, with a delicate sparkle. At the edge of the dial, in the part hidden beneath the bezel and invisible without dismantling the instrument, a "crow's foot" (broad arrow) is scratched — the British mark of government property. The mark has been in use since the 14th century and was later applied to all property of the government and the armed services.
The centre of the dial is printed COMPENSATED, NEGRETTI & ZAMBRA / LONDON and SERIAL Nr. 17170. The main pointer is of blued steel with a crescent tail.
The dial carries two concentric extended barometric scales: an outer scale in inches of mercury, ranging from 21 to 35 inches, and an inner scale in millibars, ranging from 700 to 1200 mb.
Coloured sectors — red, orange and green — are applied to the outer scale. They indicate potentially dangerous pressure values, allowing the crew to take the necessary action in good time: altering the height of the induction mast, adjusting the submarine's course or, if need be, stopping the engines. Taking normal pressure as 30 inches, the colour boundaries coincide with the regulation vacuum limits:
The uncoloured sector above 30 inches, up to 1200 mb, is the overpressure zone. Because tanks were vented inboard, the pressure inside a submarine tended to rise. Air from torpedo firing added to it: this was vented into the boat so that no bubble would betray her position at the surface. Before the hatch was opened on surfacing, this overpressure had to be seen and released.
The instrument measures the pressure inside a steel box, not the weather. That is why its scale extends far beyond any atmospheric pressure.
The snort (snorkel) is a raisable air mast that allows a submarine at periscope depth to run her diesels — to proceed and to charge her batteries without surfacing. The device was devised by the Dutch engineer Jan Jacob Wichers; the first submarines in the world to be fitted with it were the Dutch O 19 and O 20 (1938–1939). After the occupation of the Netherlands in 1940 the idea passed to the Germans and was widely used on U-boats in the last year of the war. The Royal Navy, having initially removed the Dutch "snuivers" from the boats that escaped to England, returned to the device at the end of the war and especially after it; in British naval slang the snorkel was called the snort, hence the name of the instrument.
Snort barometers were fitted in pairs in the diesel-electric submarines of the Royal Navy (or the Commonwealth navies): one in the control room and one in the engine room. Their purpose was to monitor the internal air pressure of a submarine running submerged with her engines going. The engines draw air through the induction mast, which is fitted with a ball valve to keep out seawater when waves wash over it. If the valve closes, the engines, still running, lower the pressure inside the submarine, which endangers both crew and machinery.
When a wave closed the snort head valve, the diesels began to draw air out of the compartments, and the internal pressure fell rapidly below atmospheric. For this reason the large barometers in the control room and the engine room were synchronised at the start of snorting, and the vacuum was then watched against that reference point. The build-up of vacuum itself was tolerated relatively well, but when the valve reopened the pressure returned sharply to atmospheric, and every man had to clear his ears vigorously to avoid barotrauma. With frequent wash-overs in heavy weather these swings recurred again and again, wearing the crew down. When the limiting vacuum was reached, the head valve was shut and the engines stopped.
The problem was serious enough to be studied in its own right. In 1948 the submarine HMS Ambush made a prolonged Arctic cruise to investigate the effects of snorting in rough weather and extreme cold. Between 1952 and 1957 a separate Admiralty file was devoted to pressure levels in submarines while snorting and their effect on the crew (National Archives, ADM 1/25118). The principle of colour coding survived into the next generation of instruments: the Kelvin Hughes snort barometers of the early 1960s for the Oberon-class boats (Admiralty Pattern No 750) likewise carried sectors on the dial corresponding to zones of decreasing pressure.
The instrument is driven by a conventional movement based on a metal aneroid capsule 75 mm in diameter. It is formed of two corrugated diaphragms joined at their periphery, each with an initial convexity and a slightly conical form. Unlike the more common capsules with relatively flat diaphragms, in which the corrugated zones lie roughly in the same plane as the central part, here both diaphragms project noticeably outwards from the common plane of the joint, giving the capsule a characteristic vase-like silhouette that is clearly visible from the side.
This geometry serves a definite mechanical purpose. An ordinary domestic barometer works within a narrow range of about 950–1050 mb, whereas this instrument must follow a band of 500 mb reliably and linearly, from deep vacuum to a marked excess of pressure. A flat corrugated diaphragm grows stiffer as its deflection increases: tensile stresses build up within it, and each further increment of pressure produces a smaller displacement — the scale becomes compressed towards its ends. A diaphragm with an initial convexity behaves differently: under pressure it first flattens, and this flattening "softens" it, partly offsetting the stiffening caused by the corrugations. By choosing the height of the initial convexity, the designer can control the elastic characteristic of the diaphragm — making it almost linear over the whole working stroke, or giving it the required progression to match the characteristic of the main spring. Moreover, convex diaphragms stand farther apart than flat ones, giving the capsule a greater reserve of compressive travel: at pressures up to 1200 mb the centres of the diaphragms do not meet or bottom against each other, and the metal suffers no permanent set.
The capsule is fixed by the rigid centre of its lower diaphragm to a cast-iron base painted pale green, while the upper diaphragm is held in tension by a steel C-spring anchored by its lower end to a brass cross-bar, likewise painted pale green. From the upper end of the spring runs the brass main lever with the so-called bi-metallic link system, which provides temperature compensation: over the length of the link, half the thickness of the lever is machined away and the removed brass replaced by an equal thickness of steel. The difference in thermal expansion of the two metals bends the lever by exactly the amount needed to offset the temperature drift of the elastic elements — hence the word COMPENSATED on the dial.
Through a connecting rod, the main lever is linked to the bell crank of the main arbor, which turns on pointed pivots. At the front the arbor is fitted with a mechanical stop in the form of a bent brass rod, which limits further travel of the bell crank at low pressure values so as not to damage the mechanism. From the vertical arm of the bell crank, a long fusee chain with an extension piece, about 6 cm in total length, runs to the pointer arbor, allowing the pointer to cover the whole of the extended scale. The chain winds onto a brass drum on the pointer arbor, which also carries a spiral spring (hairspring) with considerably more turns than a standard one. This reinforced spring keeps the link reliably pre-tensioned and returns the pointer to its starting position. Because of the stop limiting the bell crank, the standard bridge carrying the whole pointer-arbor assembly is shifted to the left and set diagonally towards the centre.
The instrument can be calibrated in the usual way through the calibration port in the back of the case, by turning the screw that alters the position of the cross-bar. On the underside of the cross-bar, in a hard-to-reach place, two letters are struck: TW.
The barometer was acquired in the Netherlands. Despite its evident British naval origin, it could have ended up there for perfectly understandable reasons: during and after the war British submarines were transferred to the Royal Netherlands Navy and later either returned to Britain or went to the breakers.
Three British boats are known to have served under the Dutch flag and then been broken up:
Equipment from such boats often found its way onto the local market, and could also remain in Dutch naval stores after refits and replacements. To these may be added the boats that later returned to Britain. Hr.Ms. Zeehond (1), the former S-class HMS Sturgeon, was received on lend-lease in October 1943 and returned to the Royal Navy in November 1945. From 1948 to 1953 the Royal Netherlands Navy leased two more British T-class boats — Hr.Ms. Dolfijn (2), the former HMS Taurus, and Hr.Ms. Zeehond (2), the former HMS Tapir; both were fitted with a snort. Dolfijn (2) became the first submarine to cross the Atlantic on her snort, and Zeehond (2) made a long passage snorting on her way back to Britain in 1953. These were the boats that carried British snort equipment in Dutch service.
The Negretti & Zambra Snort Barometer No. 17170 is an instrument in which the aneroid, invented to observe the weather, became an instrument of crew safety. The non-magnetic copper case, the extended scale running from deep vacuum to overpressure, the coloured sectors marking regulation limits, the capsule with convex diaphragms, the mechanical stop and the lengthened fusee chain — everything in it serves a single purpose: to warn submariners in good time that the air inside their steel box is becoming dangerous. The broad arrow links the instrument to the property of the British Crown, while the route that brought it to the Netherlands reflects the close post-war cooperation between the British and Dutch submarine services. The barometer documents a brief but important period in the history of submarine warfare — a time when a Dutch invention, having passed through German wartime experience, became standard practice in the Royal Navy, and the pressure inside a boat demanded as close a watch as the pressure outside her hull.