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A333-26 · Aneroid
1969
Object Overview
The Spring-Suspended Marine Aneroid, a marine aneroid barometer on a spring anti-vibration mount, was made by the Tokyo firm Nihon Kisho Sokki Co., Ltd. to the order of the Yokohama firm Utsuki Keiki Seisakusho, which specialised in ship's instruments. This is stated in the inscription at the bottom of the dial: SPECIALLY MADE FOR UTSUKI KEIKI SEISAKUSHO, YOKOHAMA, JAPAN. The instrument's serial number is 22782. The DATE field is stamped "44—10". In the Japanese calendar this is October of the 44th year of the Shōwa era, that is, October 1969.
The barometer was intended for service at sea. It was installed in the wheelhouse or chartroom of merchant ships and fixed to a bulkhead or shelf. Utsuki supplied such instruments to shipyards and shipowners. The whole design, from the mount to the movement, is built for shipboard conditions: continuous engine vibration, wave impacts, rolling and pitching, and changes in temperature.
self-elastic stacked capsule movement
aluminium, brass, copper-beryllium, glass, silvered brass, steel
The case is cylindrical, cast in aluminium alloy and finished in pale green enamel, typical of ship's equipment of the period. At the front the cylinder widens into a broad flange. A heavy bezel screws onto it; its side is finely straight-knurled so that it can be unscrewed by hand. The bezel holds a flat mineral glass. A chrome-plated knurled knob is fitted at the centre of the glass. Through the glass it turns the set hand, a thin brass pointer with a red tip. Before a watch it is aligned with the main pointer, and the divergence of the two pointers then shows whether the pressure is rising or falling.
The instrument consists of two parts that are not rigidly connected:
They are joined only by springs, so the cup with the movement effectively hangs on the dish-shaped base. Between them is a soft stop or damper of rubber or felt. The springs are arranged in V-shaped pairs. The upper ends of each pair meet at a single point on the body, at a lug under a screw. The lower ends diverge to two holes in the base ring. With such pairs set around the circumference, they pull the body in different directions and their pulls balance one another:
All the springs are pre-tensioned and work only in tension. The body is therefore held in every direction, and no spring ever goes slack. The design is cheap and almost indestructible.
A ship vibrates constantly. The main engine, shafting, propeller and auxiliary diesels produce vibration ranging roughly from a few to several tens of hertz. Shocks come on top of this from time to time: a wave against the hull, berthing. All of this is harmful to an aneroid:
Here the mount acts as vibration isolation. The spring-suspended body is a mass-spring system with its own natural frequency.
The dial is of silvered brass and is assembled from three parts. Two silvered brass pieces, an outer ring and an inner disc, are laid over a metal disc polished to a mirror finish. Between them a narrow annular band of the mirror remains exposed. It serves as an anti-parallax device: the reading is taken with the pointer aligned with its reflection, so that the viewing angle introduces no error.
The outer ring carries the barometric scale in millibars, from 920 to 1050 mbar. The inner disc carries the scale in millimetres of mercury, from 690 to 790 mm. Both scales are large, with closely spaced intermediate marks. They are numbered every 10 units, with twenty divisions between adjacent values. The scale interval is therefore 0.5 mbar on the outer scale and 0.5 mmHg on the inner. Since 0.5 mmHg corresponds to about 0.67 mbar, the millimetre scale is slightly coarser in terms of pressure than the millibar scale. On a large scale with a mirror band the eye readily halves a division, so readings can be taken to a quarter of a millibar. This is the resolution of the scale, not the error of the instrument: accuracy is determined by the movement and its calibration. Such fine graduation nevertheless shows that the instrument was conceived as an observer's working tool rather than a weather indicator.
Around the dial, under the glass, lies a white-painted aluminium ring that fills the space between the glass and the dial.
The barometer has an unusual system for adjusting its readings. The readings are corrected not by an adjustment inside the movement but by rotating the whole dial relative to the pointer. The dial rests on three thin, springy rods arranged horizontally. Each rod is fixed at one end in the case, its other end is free, and the rod itself lies in a special groove. The springiness of the rods presses on the dial and holds it in place while allowing it to slide around its circumference.
A brass stem projects from the dial. It is brought out through an arc-shaped slot in the case behind the bezel, and a chrome-plated knurled nut is screwed onto it. With the nut loosened, the dial can be moved in either direction over the full length of the slot, a range of about 6 cm of arc, and then locked in its new position with the nut. In this way the instrument could quickly be checked against a standard barometer or against data from a shore weather service without opening the case or disturbing the movement.
The main pointer is long and needle-like, with a crescent-shaped counterweight. This is the so-called Japanned aluminium counterbalanced pointer: an aluminium pointer balanced by a counterweight and finished in black lacquer. The light metal and the counterweight reduce the load on the pointer arbor and make the pointer insensitive to tilting and shocks.
The barometer is driven by a high-grade precision movement built as a symmetrical, preloaded kinematic system in which two opposing branches of the transmission act on a common pointer arbor. At its heart are two capsule stacks mounted in the main frame in opposing directions; each stack consists of two aneroid capsules of bronze alloy. The cast aluminium frame carries the two stacks facing each other: each is fixed by its outer central boss to the brass bar of a side pillar, while the free bosses face the centre of the instrument. As pressure rises, both stacks contract and their free ends move apart; as it falls, they move together.
The design of the movement follows a single principle: no arbor in the measuring train may carry an unbalanced load, and almost every force in the movement is opposed by an equal and opposite one. Hence the characteristic duplication of parts, which may at first sight seem superfluous. It is not redundancy but symmetry.
From the free ends of the stacks, links run to a brass rocker mounted on a central arbor of blued steel. The arbor turns at its lower end in a brass bush set in a boss on the central pillar of the frame, and at its upper end in a bridge on the end face of that pillar. The links are attached to the rocker on opposite sides of its arbor, so the two opposing forces form a pure couple: their moments add, while the side loads on the bearings cancel out. This matters most at the first stage, which carries the full forces of the capsules, the largest in the entire movement.
The opposed mounting of the stacks brings a second benefit: insensitivity to common-mode disturbances. A shock, vibration or tilt along the axis of the stacks moves the free ends of both stacks in the same direction, and their moments on the rocker subtract. Pressure, by contrast, brings the ends together or moves them apart, and the moments add. The instrument thus distinguishes a change in pressure from shaking.
Two stacks acting on one rocker double the force but not the travel: with equal lever arms, the rocker turns through the same angle that a single stack would give. In return, the error due to any friction or load in the train, expressed in units of pressure, is halved. A stack of four capsules would give double the travel, but it would be long, laterally compliant and sensitive to accelerations. Rigidly coupled through the rocker, the two stacks also average out each other's individual errors.
When the atmospheric pressure changes, both stacks deform in a physically identical way. Their moving ends, however, travel in opposite directions in space. This gives rise to an important point. Although the moving ends of the capsules travel towards or away from each other, their forces must not cancel each other out. The geometry of the linkage converts these opposite translational movements into a single, coordinated rotation of the rocker.
From the rocker the motion divides into two symmetrical branches. Two links lead to the cranks of two intermediate arbors standing on either side of the pointer arbor. The cranks are threaded studs passed through the arbor and locked with nuts, so their effective length, and with it the transmission ratio of each branch, can be adjusted. The intermediate arbors turn on centre bearings: a conical point running in an adjusting screw with a locknut. Each arbor has its own hairspring. The copper colour of the hairsprings most likely indicates bronze (phosphor or beryllium).
In an ordinary aneroid the hairspring always sits directly on the pointer arbor. Its purpose is to return the pointer and keep the single transmission chain constantly taut. Here the design is fundamentally different. There is no hairspring on the pointer arbor. The two spiral springs are mounted on separate intermediate arbors. Their action can therefore be used to set up two predetermined torques acting on the pointer arbor from opposite sides.
This makes it possible to create a mechanical system in which the transmission chains do not go slack even when the direction of motion reverses. The springs can serve both as return springs and as preload elements at the same time.
Each intermediate arbor carries a thin profiled metal lever with a widened balancing blade at one end and the attachment for the fusee chain at the other. The lever has a characteristic longitudinal V-shaped fold that forms a stiffening rib. This three-dimensional form greatly increases its resistance to bending without a significant increase in mass, ensuring accurate transmission of small movements and reducing the effect of parasitic deformations. The angle between the chain arm and the crank defines the portion of the swing over which the lever operates, and the linearity of the scale depends on it.
The two branches converge again on the pointer arbor, which turns coaxially with the rocker arbor. The motion thus first travels out from the centre to the periphery and then returns to the centre. The pointer arbor carries a drum with a helical groove, onto which the two chains are wound in opposite directions. The groove keeps the turns from overlapping; otherwise the working radius of the drum would change, and with it the scale interval.
The point of this transmission becomes clear when it is compared with an ordinary aneroid, which has only one thin chain winding onto the pointer arbor. A chain can work only in tension: it transmits a pull but cannot push. The chain must therefore stay taut when the lever moves back. This is usually the job of the spiral spring on the pointer arbor.
That arrangement has three weak points. First, the full torque of the hairspring is passed back through the train to the capsule. Second, the single chain pulls the pointer arbor sideways, so its bearings work under load. Friction on this arbor is the most harmful of all: it is the last in the train, and its friction is referred back to the capsule through the full transmission ratio, which largely explains the habit of tapping the glass of a barometer. Third, the hairspring winds up over the whole scale, so the tension, and with it the friction, varies from one end of the scale to the other.
In the movement described here, all three shortcomings are eliminated by a single device. The two oppositely wound chains act like a belt passed over a pulley and fixed to it, with the ends of this "belt" held by the levers of the intermediate arbors. The rocker turns both intermediate arbors in unison: one lever takes up exactly as much chain as the other lets out, the "belt" shifts, and the drum turns. Motion in both directions is transmitted by a taut chain, so the pointer arbor needs no hairspring of its own.
The tension is supplied by the hairsprings of the intermediate arbors. Each pulls its own chain, and through the drum they work against each other like antagonistic muscles. The torques of the two chains on the pointer arbor are equal and opposite, and the chains pull the arbor in opposite directions almost along a single line. The arbor receives neither torque nor side load from the tension, and the last link in the train, the one most sensitive to friction, turns practically freely.
The capsules, too, barely feel these springs. In the symmetrical arrangement, at any movement of the mechanism one hairspring unwinds by exactly as much as the other winds up, and the work of one cancels the work of the other. The rocker receives only the difference of their torques, which is zero for identical, identically wound hairsprings, plus a small linear contribution from their stiffness, which is absorbed by the calibration. At the same time, play is taken up in every link: in the chains, in the link joints, in the bearings. Preload without loading the sensing element is the central idea of the design. And since the intermediate arbors turn through only a small angle, the hairsprings work at an almost constant torque, and the chain tension is the same over the whole scale. The loop "rocker — secondary twin-arbor transmission — pointer arbor" is closed, and the branches must be precisely matched geometrically: over the entire travel, one chain must pay out exactly as much as the other takes up.
The following point is particularly important: even when the torques of the two chains balance each other, their tensions may remain quite high. For example, if both chains are equally tensioned, the pointer arbor experiences no resultant torque. Yet each chain is still under load. This is the principle of the preloaded transmission.
Two tensioned transmission elements also prevent free oscillation of the pointer arbor. A single chain with insufficient preload may momentarily lose tension under a sharp acceleration. With a twin transmission such a condition is less likely. It would be wrong, however, to claim that the two chains damp vibration by themselves. That is the job of the full external spring damper on the case.
The movement is temperature-compensated by means of a bimetallic strip. As the temperature rises, the elasticity of the capsule metal decreases, and under the same pressure the capsules deflect further; without correction the instrument would show a pressure that depended on the temperature of the room. The bimetallic strip, made of two metals with different coefficients of thermal expansion bonded together, bends when heated and displaces the link of the train connected to it in the direction opposite to the temperature drift of the capsules. With a correctly chosen strip the two effects cancel each other, and the readings remain unchanged over the working temperature range.
As the pressure rises, both capsule stacks contract. Through the system of articulated links they change the position of the central rocker. A downward force from the first capsule stack is applied to its left end, and an equal upward force from the second capsule stack to its right end. The forces are opposite and equal in magnitude. Their vector sum is zero. Yet the rocker begins to turn clockwise, because both forces produce torques of the same sign. It turns the two intermediate arbors, changing the positions of the bent levers and the tensions of the chains. Both chains are under preload. The first produces a clockwise torque on the pointer arbor, the second an anticlockwise one, and as long as the torques are equal the arbor is in equilibrium. When the atmospheric pressure changes, the central lever system turns the intermediate arbors. The geometry of the two branches then changes in unison: one chain takes up additional length from the drum or is pulled tighter, while the other simultaneously yields the corresponding movement and remains taut. The pointer arbor turns to a new position of equilibrium. One chain thus provides the pull in one direction, and the other a controlled pull in the opposite direction.
When the pressure falls, the whole sequence runs in reverse. In a sound, correctly adjusted movement, neither chain should ever lose tension.
Outwardly the Spring-Suspended Marine Aneroid looks modest: a pale green ship's cylinder, a plain dial, no ornament whatever. Behind this restraint, however, lies a carefully thought-out engineering system in which each level of the design combats its own source of error. The spring mount cuts off the ship's vibration before it can reach the movement. The opposed capsule stacks distinguish pressure from shaking. The preloaded twin-chain transmission relieves the pointer arbor of side load, friction and play. The bimetallic strip cancels temperature drift. The mirror band on the dial eliminates parallax when reading, and the rotating dial allows the instrument to be checked against a standard without opening the case.
The instrument reflects well the time and place of its making. Nihon Kisho Sokki, founded in 1943 in Ginza with the participation of the clock house of Hattori and the trading house of Tamaya, produced it as a small-batch working instrument. The customer was Utsuki Keiki Seisakusho, a Yokohama firm that had grown out of the repair of marine chronometers. By the end of the 1960s Japanese shipbuilding was among the largest in the world, and barometers like this one went to sea aboard merchant ships, where safe navigation depended on accurate pressure readings. This example, numbered 22782 and dated "44—10", has preserved not only a working movement but also the full markings of maker and customer, which makes it a valuable witness to Japanese instrument-making of the Shōwa era.