In engineering, an indicator is an instrument for obtaining a diagram, often called an “indicatory diagram”, of the pressure-volume changes in a running positive displacement heat engine, compressor, or pump cylinder during the working cycle. [1] Said another way, an indicator is the instrument which produces a graphic record of the pressure (vertical axis) of the fluid in a cylinder for every position of the piston, a position that can be formulated as volume (horizontal axis), as it reciprocates. [2]

Overview
In 1796, Scottish engineer James Watt and his employee John Southern developed a work measurement tool called an "indicator diagram", one variation shown adjacent and below, used to exactly quantify the work produced by a steam engine, which made a chart of the pressure of the steam in a cylinder plotted out against the steam's volume, as shown adjacent.

In particular, it was Southern that developed the simple, but critical, technique to generate the diagram by fixing a board so as to move with the piston, thereby tracing the "volume" axis, while a pencil, attached to a pressure gauge, moved at right angles to the piston, tracing "pressure". The gauge enabled Watt to calculate the work done by the steam while ensuring that its pressure had dropped to zero by the end of the stroke, thereby ensuring that all useful working potential had been extracted.

The detailed operation of the indicator, however, becomes a bit more complicated. In the following Watt indicator, as found in a 1857 textbook Mechanica, the indicator consists of a small hollow cylinder A that is screwed on the cover of the steam cylinder. [4] Steam can enter through cock G. Inside the cylinder A is a piston connected to rod B. The incoming steam pushes the piston upward. Fitted to rod B, outside the cylinder, is pencil D. D touches a strip of paper tightened around a second cylinder E. When the piston travels up and down, E revolves, alternately clockwise and counterclockwise. Steam pressure in the cylinder varies, and higher and lower pressure differences cause D to move more or less, which makes D draw a curve. From this curve the change in pressure during a specific stroke of the cylinder can be calculated.


The following plot, from the same textbook, shows a curve (diagram) that was taken with the indicateur of Watt. Part ABCD was drawn on the strip of paper during the down going stroke and DEF during the up going stroke of the steam cylinder.

Thermodynamics
The functionality of the indicator diagram was later incorporated into a graphical analysis, shown below (right), by French engineer Émile Clapeyron in 1834 to determine the work done by the steam, as signified by the area found within the region abcd, using calculus: [3]


This amount of graphical area soon came to be known as "pressure-volume work" and this graph became a central fixture in the science of thermodynamics, especially through the detailed work of German physicist Rudolf Clausius. [5]

References
1. Licker, Mark D. (2003). Dictionary of Engineering. McGraw-Hill.
2. Low, David A. (1920). Heat Engines, (ch. XV: Indicators and Indicator Diagrams, pgs. 282-305). Longmans, Green and Co.
3. Clapeyron, Émile. (1834). “Memoir on the Motive Power of Heat”, Journal de l’Ecole Polytechnique. XIV, 153 (and Poggendorff's Annalender Physick, LIX, [1843] 446, 566).
4. Indicator – CruquiusMuseum.nl
5. Clausius, R. (1865). The Mechanical Theory of Heat – with its Applications to the Steam Engine and to Physical Properties of Bodies (URL). London: John van Voorst, 1 Paternoster Row. MDCCCLXVII.

External links
● Indicatory diagram – Wikipedia.