Barometers Realm

This Servo-Operated Precision Aneroid Barometer by Mechanism Ltd., manufactured circa 1965, represents an automated version of the company’s earlier manual (human-operated) Electro-Mechanical Precision Aneroid Barometer. In this design, the micrometer screw is no longer driven by the operator but by a servo motor. In addition, the electron-beam tube (CRT) used in earlier models is absent: pressure is determined by the equilibrium position of the servo system and is read electrically and/or in digital form.

The barometer’s case is elongated horizontally and made of heavy cast metal, giving the instrument a distinctly industrial, monolithic character. The surface is finished in a grey-olive hammered enamel, typical of post-war laboratory and aviation-meteorological equipment. This coating not only protects the metal from corrosion but also effectively conceals minor casting imperfections and traces of use.

Electrical connectors for external devices are located on the side panel of the housing. The multi-pin circular connectors are intended for power supply (24 V DC), connection to an external servo unit, and transmission of the measurement signal to recording or indicating systems. Their design clearly indicates that the instrument was intended to operate as part of more complex measuring installations—meteorological stations, calibration facilities, or aviation laboratories.

On the front face of the housing there is a small rectangular window containing an electromechanical counter-type indicator that displays the current atmospheric pressure in millibars.

Particularly noteworthy is the protruding steel coupling on the end face of the case. This coupling is rigidly fixed to the housing and serves as a guide for an internal shaft. Inside it runs a thinner steel shaft mounted on bearings and capable of free rotation. This shaft constitutes the mechanical input interface of the barometer: when an external servo drive is connected, rotational motion is transmitted into the instrument, where, via a gear train, it acts upon the micrometer screw that compensates the aneroid unit.

Changes in atmospheric pressure cause microscopic deflections of the aneroid capsules, which are converted into an electrical error signal. This signal is fed to an external servo unit, where it is amplified and used to control the servo motor. The motor, in turn, rotates the shaft and micrometer screw until the mechanical state of the system reaches equilibrium with the ambient pressure. Measurement is thus carried out according to the null method—one of the most accurate and stable principles in precision metrology.

This instrument is, in essence, an aneroid pressure comparator with a closed-loop servo system. Inside is a three-capsule aneroid stack composed of self-elastic beryllium-bronze capsules. The output of the aneroid does not drive a pointer; instead, it generates an error signal. The servo system eliminates this error by rotating the micrometer. The aneroid capsules, connected in series, respond to pressure, and their micro-deformation is transmitted to a lever fitted with thin metal vanes on both ends, ultimately conveying motion to the micrometer screw. Here, the aneroid does not measure directly; it merely indicates whether the system is above or below the equilibrium point. In place of a CRT, an electrical null detector is employed (typically inductive, LVDT-like, contact-bridge, or capacitive), sensing the deviation of the aneroid from its ideal equilibrium position. The servo motor then rotates the micrometer screw, which mechanically compensates the aneroid deformation. The process continues until the error is reduced to zero. This is classic servo-null measurement: it is not the deflection that is measured, but the compensating action itself. The position of the micrometer screw, strictly proportional to pressure, is read via the electromechanical counter visible through the window in the housing.

The principal advantage of the servo-driven scheme over manual micrometer operation lies in the elimination of the human factor. The system provides continuous automatic balancing, high repeatability, the possibility of remote readout, and straightforward integration with recording equipment. This makes the instrument especially valuable for long-term observations, laboratory calibration, and scientific experiments where stability and high accuracy are paramount.

This Servo-Operated Precision Aneroid Barometer is an outstanding example of the transition from manual precision instruments to automated servo-compensated systems in the mid-twentieth century, embodying the shift from nineteenth-century mechanics to the cybernetics and servo systems of the twentieth. It combines classical aneroid mechanics with electromechanical control and vividly illustrates a key stage in the evolution of meteorological measuring technology—from optical and manually operated methods to fully automated solutions.