Steam passing into the valve chest or chamber, through the cylinder and exiting from the exhaust nozzle is represented by areas shaded with dots of four different colors. The appearance of the density of the dot pattern is intended to represent the relative steam pressure.

The colors of the dots are intended to represent the following:

RED - Steam at after throttle pressure or at a further diminished pressure within the cylinder caused by various restrictions to its flow before entering the cylinder.

MAGENTA - Steam expanding in the cylinder and diminishing in pressure after admission of steam into cylinder has been cut-off by the valve and before the point of release where the valve allows steam to exhaust from the cylinder to the exhaust nozzle, smoke stack and thus to atmosphere.

BLUE - Steam exhausting from the cylinder to the exhaust nozzle, smoke stack and thus to atmosphere.

GREEN - Exhaust steam remaining in cylinder compressing after the valve has closed the cylinder to exhaust and before the valve has opened to admit steam from the throttle into the cylinder.

Links to Additional Steam Locomotive Valve Gear Information