Try cocks are
small valves installed on steam
boilers at
safe high-water level and at the safe low-water level. When the boiler water column is
inoperable, try cocks function as a back up
system to determine the water
level. The water level is determined by opening slightly first one try cock and then the other. The water level is indicated by the
color of the plume escaping from the try cock
orifice.
A steam plume is characteristically colorless, and will indicate that the water level is too low (i.e. below the level of the try cock orifice). A white plume, on the other hand, characterizes water. A false water level reading will be obtained if the try cock is opened too wide when the level of the water is only slightly below the level of the try cock orifice. The violent agitation of the water caused by a wide open try cock will result in some of the water escaping, giving the false impression that the level of the water in the boiler is at or above the try cock fitting.
On large steam boilers, a third try cock is often installed between the other two. Sometimes a try cock on these larger boilers is at a level too high to reach. When this is the case, chain operated lever-type try cocks are installed.