From the area of the diagram the engineer may also estimate the quantity of steam that is used, and supposing it to be dry steam, he may calculate the amount of water used to make the steam, and assuming one pound of coal to evaporate so much water, he may calculate the amount of coal used to produce the steam.
The indicators commonly used upon steam cylinders contain two principal mechanical movements; first, a drum revolving the piece of paper upon which the diagram is to be marked, and second, a piston and parallel motion for moving the pencil to mark the diagram upon the revolving paper.
The drum is given a motion that, to insure a correct diagram, is exactly timed with the piston motion.
The pencil is given a vertical movement; this movement must bear a constant and uniform relation to the pressure of the steam in the engine cylinder.
Fig. 3358.
An indicator may be attached to each end of the cylinder or in the middle, with a pipe passing to each end of the cylinder, as in [Fig. 3358], but an indicator of the usual construction and such as here referred to, can take a diagram, or card as it is sometimes called, from but one end of the cylinder at a time. The stop valves a and b are used, so that the communication between the indicator and one end of the cylinder may be shut off while a diagram is being taken from the other end, while both ends may be shut off when the indicator is not being used.
In the figure a piece of paper (or card, as it is commonly called) is shown in place upon the drum with a diagram upon it.