Barometers Realm

Before us is the Thommen Precision Altimeter Type 3B4.01.2.3000.T, Model MR0 2000 — a specialized Swiss barometric altimeter (barometric leveling instrument) manufactured approximately between the 1960s and 1980s by Revue Thommen AG of Waldenburg, one of the best-known European makers of aviation, navigational, and precision measuring instruments. Unlike the well-known portable mountaineering altimeters produced by Thommen, this instrument belongs to the class of high-precision reference barometric altimeters intended primarily for barometric leveling — a method of determining differences in elevation through variations in atmospheric pressure. Such techniques were widely employed in geodesy, topographic surveying, geology, forestry, mountain services, meteorology, and military applications wherever classical geometric leveling proved too slow, labor-intensive, or practically impossible under field conditions.

It is known that instruments of this type were routinely used by geodetic units and by the West German Bundeswehr under the designation Bodenhöhenmesser for barometric determination of elevations, artillery reconnaissance work, and mountain troop operations. The measuring range extends from −400 to +3000 meters, the negative portion of the scale being intended for locations below sea level and for conditions of elevated atmospheric pressure. The accuracy class “0.1” marked on the dial already approaches laboratory-grade precision for portable aneroid field instruments of this category.

The entire altimeter mechanism is mounted on a massive base and enclosed within a cylindrical instrument-style plastic housing. The rear section of the housing contains an air inlet channel leading to a hose connection located on the main front panel of the instrument. The front side is protected by a flat mineral glass retained by a thin screw-on bezel. In essence, this is a typical cockpit-style aviation instrument installed within a rectangular transport case made of painted steel finished in characteristic dark green hammer-tone enamel. The case is fitted with a hinged lid and a chromed latch.

Mounted inside the lid is a mercury thermometer required for accounting for air temperature during barometric altitude calculations. The thermometer is calibrated in degrees Celsius over a range from −30 to +50 °C. The lid also contains the instrument identification plate bearing the THOMMEN SWITZERLAND logo, the manufacturer’s name Revue Thommen AG, the Waldenburg address, and the instrument designation 3B4.01.2.3000.T. Arranged on the main panel are the altimeter dial itself, a circular spirit level for precise horizontal positioning, an inlet connection protected by a brass cap, and a large black knurled adjustment knob used for setting the zero or reference value.

The hose connection plays an important role in the operation of the instrument: through it, the internal cavity communicates either directly with the atmosphere or with an external static-pressure source when the instrument is used as a reference altimeter within a measuring system. For this reason, the device could serve not only as a portable field leveling barometer but also as a stationary reference instrument for control and calibration work.

The dial is manufactured from a thin metal plate finished in matte black paint. Along its outer edge runs an extremely detailed circular altitude scale graduated in meters with linear divisions and a resolution of only 2 meters per division. Immediately inside this scale is a mirrored anti-parallax band intended to eliminate reading errors caused by viewing angle. Further inward is the principal altitude scale with larger divisions, calibrated from 0 to 500 meters per revolution of the pointer: major graduations correspond to 50 meters, while minor divisions indicate tens of meters. One complete revolution of the long main pointer corresponds exactly to 500 meters of altitude.

The dial bears the marking “Kl. 0,1,” indicating the instrument’s accuracy class. At the center is a small auxiliary subdial — a revolution counter (Zählscheibe) marked with the numerals 1–2–3–4. This mechanism indicates the number of complete 500-meter revolutions performed by the main pointer. Thanks to this arrangement, a comparatively large and easily readable scale is capable of covering the entire range from −400 to +3000 meters without loss of resolution or excessive crowding of the graduations. In the lower portion of the dial is a short arcuate scale for absolute atmospheric pressure in millibars (mbar/hPa).

The operating principle of the instrument is based on barometric altimetry — the direct relationship between atmospheric pressure and elevation above sea level. As altitude increases, atmospheric pressure decreases, and the instrument scale is therefore calibrated directly in meters of altitude, although in essence the device remains an exceptionally precise aneroid barometer converting pressure into altitude according to the standard atmosphere model.

All practical manipulation during measurement is reduced to setting the current elevation and barometric pressure by means of the adjustment knob. In practice, the procedure is as follows: the instrument is first positioned horizontally using the built-in spirit level, the mechanism is allowed to stabilize, and the air temperature is recorded from the built-in thermometer. At a point of known elevation, the zero or reference value is then set using the knurled adjustment knob. After moving the instrument to the measuring point, the reading is taken from the long pointer and the revolution counter, and, if necessary, the current pressure is read from the lower arcuate scale. Corrections for air temperature and changing weather conditions are subsequently applied, since atmospheric pressure depends not only upon altitude but also upon meteorological conditions.

At the heart of the mechanism lies a stack of aneroid capsules made of beryllium bronze — a material distinguished by extremely low aging effects, hysteresis, and elastic after-effect. Temperature compensation is provided by a bimetallic element. The sensitive assembly consists of a stack of three evacuated capsules connected in series. The use of multiple capsules is necessary specifically for precision: the deflections of the individual capsules are additive, allowing the mechanism to obtain a substantially greater working displacement for the same pressure variation than would be possible with a single capsule.

The free end of the aneroid capsule stack acts through a system of levers upon a toothed sector, which then converts the small axial movement of the capsules into rotation of the pinion mounted on the main pointer shaft through a multiplying gear train. The main indicating pointer is mounted directly at the output of the multiplying transmission and operates with a rapid ratio: one full revolution corresponds to 500 meters on the scale. The small revolution-counter pointer is derived from the same axis through a reduction gear and moves much more slowly, advancing by one division for each complete revolution of the main pointer — essentially in the same manner that an hour hand relates to a minute hand in a clock movement. Together, these two indicators provide an unambiguous reading of absolute altitude over the full operating range of the instrument.

All moving components of the multiplying gear train are mounted in ruby bearings. For instruments of this class, this is critically important: the mechanism amplifies extremely small movements of the aneroid capsules hundreds of times, and any friction immediately translates into measurement error. The use of watchmaking technologies — including synthetic ruby bearings, fine pivots, and high-precision gear trains — places this altimeter in close relation to aviation instrumentation and sophisticated Swiss micromechanics.

Overall, the Revue Thommen MR0 2000 represents an exceptionally characteristic example of the late flourishing of high-precision mechanical barometric altimeters — instruments in which aviation engineering, geodetic practice, and Swiss instrument-making culture merged into a single technical object. Even after the emergence of electronic sensors, such mechanical leveling barometers continued to be valued for their autonomy, stability, lack of external power requirements, excellent serviceability, and extraordinary precision of mechanical execution.