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Object Overview
This is the SundO Baro Discograph, a disc barograph made by the Hamburg firm of Stäcker & Olms around 1980. The model continues the firm's pre-war line and is a direct descendant of the instrument that the company's catalogue of the 1930s called Die schreibende Wetteruhr (The Writing Weather Clock). Even then, the catalogue stressed its two chief merits: "the instrument is hung on the wall like an ordinary aneroid barometer and therefore takes up little space" and "it is particularly suitable for ships, being little affected by vibration". Half a century later the design retained both features, changing only the clock movement, the pen and the units of measurement.
The case is square, with rounded corners and edges, and is made of mahogany. Its massive walls and the deep, warm tone of the polished wood give the instrument a restrained, almost furniture-like solidity. A keyhole slot on the back allows it to be hung. The instrument can be mounted on a wall, as the pre-war catalogue prescribed, or laid on a table.
self-elastic capsule movement
brass, cast iron, copper-beryllium, glass, paper, steel, wood
The front carries a large door: a brass bezel fitted with a domed mineral glass. The dome stands clear of the chart, so the entire barogram is open to view, from the edge of the disc to the central rosette, with no glare from frames and nothing obscuring the record.
The back is closed by a blackened metal panel. It bears the firm's label — SundO, Stäcker & Olms, Margaretenstraße 43, 2000 Hamburg 6 — and the adjusting screw of the aneroid movement, marked "−" and "+", with which the pen is set to the actual atmospheric pressure. The panel also has a round removable cover concealing the clock movement and the battery compartment.
The chart is driven by a West German quartz clock movement powered by a single 1.5 V AA cell. The movement has a STOP/START switch that allows the drive to be halted while the chart is changed. Through a gear, the movement turns a solid metal disc on which the paper chart is laid. The chart is held to the disc by a knurled brass nut at the centre.
The chart is printed in ochre on white. The central rosette carries the inscriptions BARO DISCOGRAPH, the model numbers 113/115 and the address Stäcker + Olms, Hamburg 6. Around the rosette run the Roman numerals I–VII, marking seven daily sectors: one revolution of the disc takes a week. Along the outer edge an hour scale repeats, labelled every two hours from 2 to 24.
The pressure scale covers the range from 960 to 1060 and increases from the edge of the disc towards the centre: low values lie at the periphery, high values next to the rosette. The units are given with the dual label "mbar / hPa". The millibar and the hectopascal are numerically equal (1 hPa = 1 mbar), and the dual designation is a sign of a transitional period: on 1 January 1984 the hectopascal officially replaced the millibar in meteorological practice. The lines of constant pressure on the chart are concentric circles. The lines of constant time are curved arcs that follow the path of the pen.
The sensing element is a single self-supporting, corrugated, evacuated capsule of beryllium bronze. The capsule needs no separate counter-spring: its own walls balance the pressure of the atmosphere. The rigid centre of the upper diaphragm is hinged to the main lever of the movement. The arbor of the main lever turns on centre pivots between the two pillars of a brass bridge spanning the capsule.
The long arm of the main lever is connected by a link to the bell crank of the main arbor, which also turns on centre pivots. The vertical arm of the bell crank passes the motion on to an unusual part — a bent steel connecting link whose ends are spatially offset, lying in different planes. The link is hinged to a short lever fixed directly on the pen arbor.
This lever-and-crank system replaces the fusee-chain transmission traditional in aneroid barometers. It converts the small linear movements of the capsule into the angular motion of the recording mechanism. A flat spiral spring on the pen arbor provides the restoring torque and keeps the transmission constantly loaded, taking up play in the joints.
The entire pen arbor assembly is mounted on a separate bridge with two pillars. The arbor carries the long pen arm, at whose end a felt-tip recording pen is fitted. The felt-tip pen is another mark of the new age: in the pre-war catalogue, spare „Schreibfeder“ (writing pens) and bottles of „Registrier-Tinte“ (recording ink) were sold as consumables.
The disc barograph is a highly interesting alternative to the classic drum barograph. Its chief distinction lies not merely in the round chart but in an entirely different organisation of the record. Instead of a cylindrical drum, a flat rotating disc is used, and the movement of the pen builds up a graph of atmospheric pressure in polar coordinates.
Disc barographs appeared above all as a more compact variety of recording instrument, convenient for direct observation. The flat instrument hangs on the wall like an ordinary barometer and shows the whole week's record at once, without the need to walk round a drum. They did not, however, displace drum barographs.
Two independent motions must be distinguished in the instrument. The first is the rotation of the paper disc by the clock movement, representing the passage of time. The second is the movement of the pen, which depends on atmospheric pressure. Combined, they form a continuous curve — the barogram.
Since one full revolution of the chart takes seven days:
These values are determined solely by the clock movement and do not depend on changes in pressure.
Pressure remains constant. The pen holds one position while the chart turns beneath it, and the trace follows the circle corresponding to a single pressure value. Ideally the pen would move strictly along a radius. On the Baro Discograph, however, the pen is mounted on a pivoting arm, and its working path relative to the disc is an arc rather than a radial straight line. The chart ruling takes this geometry into account: the lines of constant time are printed as curves. One therefore cannot automatically assume that any printed line is a circle or a radius.
Pressure rises gradually. As the disc turns, the pen moves towards the parts of the scale corresponding to higher pressure, that is, towards the centre. The trace gradually departs from the line of constant pressure and becomes a kind of spiral curve. When pressure falls, the deviation runs the opposite way, towards the edge of the disc.
Pressure changes rapidly. The pen moves noticeably over a comparatively small angle of disc rotation, and a section appears on the chart that cuts steeply across the pressure lines. The more pressure divisions crossed within the same interval of time, the higher the rate of change.
Here lies one of the most interesting mathematical shortcomings of disc recorders. On a drum barograph, the unrolled chart has a practically straight time axis: with the drum turning uniformly, equal intervals of time correspond to equal horizontal distances. On a disc, equal intervals of time correspond to equal angles, but not to equal arc lengths at different radii.
The arc length is given by s = r × Δθ, where s is the arc length, r the distance from the centre of the disc, and Δθ the angle of rotation in radians.
A three-hour interval corresponds to an angle of about 0.112 radians. At 70 mm from the centre it gives an arc of about 7.9 mm; at 35 mm, about 3.9 mm — half as long. On this instrument the pressure scale increases towards the centre. In the high-pressure region, therefore, time is compressed on the paper, and the same change in pressure looks steeper there than it does at low pressure near the edge of the disc.
Two pressure changes of identical rate may thus look quite different on different parts of the disc. It is therefore wrong to compare the geometric slopes of the lines as one would on an ordinary rectangular graph. What must be assessed is the rate of pressure change, ΔP / Δt, where ΔP is the change in pressure in hectopascals and Δt the interval of time.
The most convenient comparison is over three hours: this interval is traditionally used in meteorology to assess the pressure tendency. If pressure has changed from 1008 to 1002 hPa in three hours, the pressure tendency is −6 hPa per 3 hours, regardless of how steep or gentle the line looks on paper. In the marine terminology of the British Met Office, such a fall is already on the threshold of "falling very rapidly" — a telling boundary for an instrument that the firm was recommending for ships as early as the 1930s.
The Baro Discograph is an instrument at the junction of several eras. Its concept goes back to the pre-war "Writing Weather Clock", and the name of that model proves almost literal here: time on the chart runs round in a circle, as on a clock dial, while the weather writes its line into that circle. From the 1930s the instrument inherited its wall-mounted format and its indifference to vibration. Its eight-day winding gave way to a quartz movement running on a single battery, the writing pen and bottle of ink to a felt-tip, the millibar to the hectopascal.
What remained unchanged was the mechanics of reading the weather itself: the evacuated capsule, the chain of levers on centre pivots and the fine spiral holding it all in balance. A polar record asks a little more attention of the observer than the unrolled band of a drum, but in return it gives what a drum never can: the whole week of the atmosphere at once, at a single glance, on a single circle of paper.