Barometers Realm

Before us is the famous Thommen Classic Altimeter, a pocket altimeter produced by the Swiss company Revue Thommen AG, dating from approximately 1970 — a period when compact aneroid altimeters had reached the peak of their engineering maturity while still retaining a fully mechanical construction, without any electronics. Among mountaineers, mountain guides, surveyors, and pilots, instruments of this type were regarded as a benchmark of reliability and accuracy. In its official instruction manual, the company itself described the THOMMEN CLASSIC, with no false modesty, as “one of the finest instruments of its kind in the world.” Moreover, the firm’s marketing materials stated that during the famous 1978 Everest expedition, when Reinhold Messner and Peter Habeler first proved that the summit could be reached without oxygen apparatus, weather changes were monitored using a Thommen pocket altimeter.

The instrument has a compact, almost square case made of dark, impact-resistant plastic. The entire altimeter is housed in a fixed protective leather case with a press-stud fastening and a cord for wearing it around the neck. The design of the case is highly practical: the front has a circular opening corresponding to the diameter of the dial, allowing the readings to be taken without opening the case. However, in order to set the initial reading, the case must be opened, since access to the setting mechanism — the large knurled adjusting ring — is only possible when the case is open. This ring effectively occupies most of the outer perimeter of the instrument and serves as the main interface for operating the mechanism. A transparent acrylic crystal is fitted into the front of the rotating ring, protecting the dial.

The dial of the instrument is a complex multilayered system, executed with the precision characteristic of the Swiss instrument-making tradition. It consists of two extremely thin aluminium discs placed one over the other. The inner disc, carrying the coloured barometric scales, is rigidly screwed to the mechanism and remains fixed. The outer disc, carrying the main burgundy-coloured altitude scale, rotates together with the knurled setting ring. In the upper part of the inner disc there is a small oval window through which the counter wheel can be seen, displaying altitude marks in kilometres against a white background. This wheel also has additional colour coding corresponding to the colours of the barometric scales.

The main concentric altitude scale is calibrated in metres. It is divided into 100-metre intervals, each subdivided into ten smaller divisions, so that each division corresponds to 10 metres. The range of the main circular scale is 0–1000 metres; after the pointer completes one full revolution, the reading continues again on the same scale, while the kilometres are read from the counter-wheel window. In essence, this counter wheel is an indicator of the pointer’s revolutions. This system allows for a relatively large and easily legible scale without making the dial excessively dense or overloaded. This particular example is designed to measure altitude up to 5000 metres — the limit embossed on the upper part of the leather case.

The barometric scale of the instrument is calibrated in millimetres of mercury and consists of three concentric coloured sectors. The inner blue sector corresponds approximately to the range of 560–620 mmHg, the middle yellow sector to 630–700 mmHg, and the outer red sector to 710–790 mmHg. This colour division is necessary because the same pointer is used for different pressure ranges depending on the position of the counter wheel.

The principle of operation is based on the standard relationship, used in aneroid altimeters, between atmospheric pressure and altitude above sea level. Before starting a route, the user sets the known altitude of the starting point — taken from a map, marker, station, or other reference point — by rotating the knurled ring. After that, the instrument continuously indicates changes in altitude. According to the company’s instructions, the THOMMEN CLASSIC maintains uniform accuracy regardless of altitude thanks to the special construction of its mechanism and the system of multiple pointer revolutions.

The instrument is also highly functional as a weather barometer. The user can rotate the crystal so that the red mark coincides with the pointer. If the pressure then falls, the pointer moves toward the tip of the red arrow, indicating deteriorating weather and the approach of a storm. Movement in the opposite direction indicates rising pressure and improving weather conditions.

When measuring atmospheric pressure, the instrument shows the absolute barometric pressure at the point where the user is located — a value known in aviation as QFE. To determine which of the three coloured scales should be used for reading the pressure, one must look at the coloured line in the counter-wheel window beneath the kilometre indicator and then read the scale of the corresponding colour. In addition, the instrument can also determine pressure reduced to sea level — QNH. To do this, it is sufficient to set the geographical altitude of the user’s location on the altitude scale. Once this has been done, the zero mark of the altitude scale indicates the sea-level pressure on the barometric scale, while the pointer itself continues to indicate the absolute pressure at the measuring point.

Particular attention should be paid to the mechanism, which is an example of high-grade Swiss micromechanics. Its construction is based on techniques long perfected in watchmaking. All pivot bearings — nineteen in total — are fitted with standard watch jewels: synthetic rubies, that is, corundum (Al₂O₃) doped with chromium and grown by the Verneuil process. Their characteristic pinkish-violet to raspberry colour is clearly visible in the photographs of the brass movement plate.

At the heart of the mechanism is a sensitive aneroid capsule made of beryllium bronze — an alloy with excellent spring properties and high fatigue resistance. It is opposed by a strong flat return spring, which prevents the capsule walls from collapsing. Across the centre of the capsule, a nickel-steel bimetallic compensation bridge is clamped by a screwed plate, allowing the instrument to maintain stable readings under significant changes in ambient temperature.

Through a central boss, the movable wall of the capsule acts on the first lever, which operates on an almost frictionless knife-edge-and-pivot bearing with a ruby and a brass regulating endstone screw that sets the minimal axial clearance. From this first lever, the motion is transmitted by a thin flat steel ribbon link to a toothed sector — a curved lever with teeth cut along its edge, also mounted on a ruby bearing. This is already the first stage of mechanical amplification: the long arm of the sector transforms the microscopic deflection of the aneroid capsule into a much more perceptible angular movement.

The toothed sector meshes with a small pinion, which turns the next gear, then another intermediate gear, which in turn drives the kilometre counter wheel and the pointer arbor. All pivots — those of the first lever, the sector, the intermediate gear arbors, and the pointer arbor — are set in synthetic corundum, including both pierced jewels and endstones. For an altimeter, this is even more critical than for a watch: the mechanism amplifies the microscopic deflection of the aneroid capsule hundreds of times, and any friction or hysteresis immediately becomes an error in the reading. This is precisely why almost every bearing point in the amplifying gear train is jewelled.

This construction allows the same circular scale to be used over several successive revolutions of the pointer, each representing 1000 metres / 3000 feet. It is this engineering feature that ensures the instrument’s consistent accuracy regardless of the current altitude.

Overall, the Thommen Classic Altimeter is a characteristic example of the late flowering of fully mechanical portable altimeters — instruments in which Swiss horological culture, aviation practice, and the needs of mountaineering merged into a single engineering object. Despite the arrival of electronic devices, such mechanical altimeters remain highly respected among collectors, historians of technology, and mountain enthusiasts thanks to their autonomy, repairability, and almost watch-like level of execution.