DEFECTIVE DIAGRAMS.
In seeking the causes that may produce a defective diagram, the following points should be remembered:
The indicator must be kept in perfect order, thoroughly clean and well lubricated, so that its parts will move freely. It should always be cleaned throughout after using.
The motion of the indicator drum should be an exact copy, on a reduced scale, of that of the piston at every point in the stroke.
The steam pipes from the cylinder to the indicator, if any are used, must be large enough to give a free and full pressure of steam, and care must be taken that the water of condensation does not obstruct them or enter the indicator.
The cord should be as strong as possible, or if long, fine wire should be substituted.
The pencil should be held to the card with just sufficient force to make a fine line with a sharp pencil.
The diagram should be as long as the atmospheric line, any difference in this respect showing unequal tension of the cord, probably from unequal pressure of the pencil to the paper or card.
A fall in the steam line could arise from too small a steam pipe, and this could be tested by a diagram taken from the steam chest. It could also occur from too small a steam port or an obstructed steam passage as well as from a leaky piston.
An expansion curve that is higher than it should be may arise from a leaky valve, letting in steam after the cut off had occurred, or if at the later point of expansion curve, it may be caused by the steam being wet or containing water, which evaporates as the temperature falls from the expansion.