Tubes, diaphragms and discs



Many late-nineteenth-century inventors were determined to provide indicators relying on something other than a coil spring, not only because they sought something that was sufficiently different to patent but also in a genuine pursuit of simplicity.

The advent of the Bourdon tube and the Schaeffer & Budenberg diaphragm revolutionised the design of the pressure gauge, which had soon become a universal fitting on boilers and similar pressure vessels. It was only a matter of time before the principles were applied to indicators. Bourdon himself is said to have made a pressure-tube indicator in the 1850s, but no details have been found. More successful was the instrument protected by British Patent no. 2249/69, granted in August 1869 to Arnold Budenberg 'of the Firm of Schäffer and Budenberg, of Manchester, in the County of Lancaster, for the Invention of "Improvements in Apparatus for Indicating and Registering the Pressure of Steam in Steam Generators, and the Pressure in Hydraulic Presses and other Vessels or Chambers, which Improvements are also applicable to Indicating and Registering Pressure and Vacuum in Condensing Apparatus; also to Indicating and Registering the Combined Pressure of the Steam or other Power employed to give Motion to an Engine, and the Speed of such Engine or other Machinery; also to Indicating and Registering Barometrical Variations"…'

The essence of the design was a conventional Bourdon-tube pressure gauge, connected to a toothed sector to rotate the pointer and a vertical rod to actuate a Richards-like amplifying mechanism. The pointer recorded pressures directly onto paper around the drum, which could be driven by clockwork to provide a permanent record, but could also be operated by 'the motion of the engine or other machinery to cause by band and wheelwork or otherwise a cylinder to revolve and complete…one revolution in a given time…' Indicators of this type were made in the late 1860s, and an improved design, introduced c. 1889, was described by Charles Budenberg during a lecture given to members of the Owens College Engineering Society in February 1890.


The short-lived Kenyon indicator was basically a Richards-type instrument with a Bourdon tube instead of a conventional piston and spring. This is no. 111, made in Manchester, England, by Isaac Storey & Sons.
Museum of Making collection.

The subject of British Patent no. 1278/78, granted on 1st April 1878 and sealed on 13th September 1878 to 'John William Kenyon, of Manchester in the County of Lancaster, England, Engineer', the Patent Pistonless Indicator was made in small numbers by Isaac Storey & Sons of Manchester. Though fitted with a Richards-type parallel motion, it also had a Bourdon Tube, bent into a semi-circle, instead of a conventional piston and cylinder. The tube expanded in relation to pressure, and, as the upper or 'free' tip of the tube was connected to the upper link bar, the pointer operated in the usual manner.

Indicators of the 'tube' class included that shown in U.S. Patent no. 185773, granted on 26th December 1876 to Thomas Minor and John Rae, which had a simple amplifier with a pointer sliding in a vertical channel cut in the mounting plate. Dreyer, Rosenkranz & Droop made not only diaphragm-type indicators embodying a Thompson-type amplifier, but also a few, patented in Germany by Hadicke, with a pantograph amplifier and an additional spiral spring above the diaphragm. The former was intended specifically to indicate Lindes Eismaschine (which used ammonia as a refrigerant) and the latter was suited to steam engines.

The Bourdon-tube and diaphragm indicators excited controversy when they first appeared. However, though they seem to have worked quite well when new and were at least the equal of a standard piston-type indicator, problems developed with age. Bourdon tubes in pressure gauges had to cope only with gradual changes in pressure; in an indicator, however, the change was sudden, violent and unusually rapid. Perpetual stressing of the brass caused the 'work-hardening' that altered the rate at which the tube responded to stimulus and degraded accuracy. In addition, though they advantageously isolated corrosive or poisonous gases from the atmosphere, Bourdon-type indicators proved to be unusually susceptible to vibration.

Left: the Rae indicator of 1886 was based on a Bourdon tube. It is assumed that a few prototypes were made, but there is currently no evidence that the design was ever exploited commercially. Conversely, the 'improved' Schaeffer & Budenberg design of c. 1889 (right) was made in small quantities. The example shown here, despite its Richards-type amplifying mechanism, dates from 1897. Courtesy of the U.S. Government Patent Office, Washington DC, and Pieter Knobbe, the Netherlands.

Public interest (lukewarm at best) had soon waned in Britain. John Rae and Walter Brown continued work in the USA, patenting several improved designs as late as 1887, but there is no evidence that large-scale distribution was ever undertaken; no surviving specimen of Rae's or Brown's later indicators has been traced. They resemble the Kenyon version superficially, though the amplifying mechanism usually proves to be a pantograph (Rae) or a Thompson (Brown). The Clarke and Clarke & Low optical indicators of the 1880s and 1890s also relied on a diaphragm to rock a mirror to deflect a beam of light.