The water automatic runs along the right side of the boiler about two-thirds of the way up the side of the boiler

For the early Stanleys boiler water level was controlled manually.  It was one of the tasks the driver had to remember to attend to as the car was being driven.  A sight glass on the dash provided indication of the water level in the boiler.

For cars built in 1915 and later a water automatic was installed to control the level of water in the boiler.  The driver still had to select if one of two water pumps operated but by 1918 even that function had been eliminated.

Located at the side of the boiler, the water automatic senses the difference in the temperature of the steam and water in the boiler to operate.  When the automatic is full of water indicating sufficient water in the boiler the automatic directs the water from the pumps back to the supply tank.  When full of steam the water automatic directs water from the pumps to the boiler.



The water automatic works by the expansion of a brass tube that is connected between two steel rods.  One end of the expansion tube is connected to the bottom of the boiler while the opposite end is attached to the top of the boiler.  By being connected in this manner the water level in the expansion tube and the tubing connected to either end of the expansion tube for a closed loop with the boiler.  Whatever the water level is in the boiler will be equivalent to the water level in the expansion tube and associated tubing.

The temperature of water being turned into steam will always remain below the temperature at which enough thermal energy has been absorbed by the water to change it from a liquid state into a gaseous state or steam (called the saturation temperature).  The steam that is generated from the water however is just above the saturation temperature and as a gas can absorb additional heat as a gas.  In a closed container such as a boiler the steam thus becomes much hotter than the water that generated it since the steam can absorb additional heat.  It is this thermal difference that allows the water automatic to operate.

One end of the expansion tube is fixed relative to the two steel rods of the water automatic.  The other end of the expansion tube is attached to a block which can slide on the steel rods.  The block also has a valve stem attached to it opposite the expansion tube.  The valve stem is part of a valve assembly fixed to the ends of steel rods.  The valve is plumbed into the discharge line from the water pumps.  When the valve is open water is allowed to flow back to the water supply tank.  When the valve is closed water is not allowed to flow back to the water supply tank and if you follow the piping diagram you'll see that the only place it can go is into the boiler.  Water is forced to leave the pumps, travel through the feed water heater and then to the boiler.  The water automatic is mounted nearly horizontal at the side of the boiler so that it reacts rather quickly to changes in the boiler water level.  The valve body is fixed to the end of the steel rods with springs.  This allows the expansion tube to expand to any length and not apply undo forces to the valve stem, ball valve, or valve seat.  The valve body can slide within the adjustment limits of the mechanism on the steel rods to accommodate the expansion and contraction of the expansion tube.

If the boiler's water level is above the water automatic mounted to the side of the boiler the expansion tube will be full of water.  This doesn't put pressure on the valve stem and ball valve and thus all the water that is pumped is returned to the water supply tank.  As steam is used from the boiler the water level of the boiler drops.  The water level in the expansion tube and associated piping seeks the same level as the water in the boiler.  Once the boiler water level drops below the level of the expansion tube the expansion tube will be full of steam.  As this steam is significantly hotter than the water the expansion tube will increase in length.  The expanding motion of the expansion tube slides the block along and as it does so it pushes the valve stem into the valve body eventually forcing the ball valve against the valve seat.  This blocks off water from exiting the valve (the exit for the valve is on the backside of the drawing) and returning to the water supply tank.

When the boiler is sufficiently full of water such that the water level in the boiler is above the height of the water automatic the expansion tube is again full of water.  The expansion tube cools to the temperature of the water and returns to the length it started at.  This action relieves pressure on the valve stem and ball valve and opens the pathway for water to again return to the water supply tank.