This boiler level measuring method uses a
differential pressure measuring system with "wet leg", that is, the
impulse pipes to the positive measuring point below the lowest water level and
the impulse pipe to the negative measuring point above the highest water level,
are both filled with water. The differential pressure signal is normally
elevated, electrically or pneumatically, so the output signal corresponds to
the level in the boiler. 0% of water in the level glass corresponds to 4 mA (3
psi) and 100% corresponds to 20 mA (15 psi).
The boiler level measuring result is insensitive to
foam in the boiler drum since the measuring method is based
on the weight of the water in the boiler drum.
If you connect the boiler level transmitter this way, with the high pressure
leg to the boiler's lower measuring point, then the output signal will be
direct, i.e., raising water level gives an increasing output signal, and hence
a signal cable failure will cause a burner-trip due to low water level. That
is, if you use the water level signal to activate the emergency
level trip of the burners.
Boiler level differential pressure transmitter is a common method for
measuring the water level in a boiler.