The first instrument in this control system is the level transmitter, or "LT". The purpose of this device is to sense the water level inthe steam drum and report ("transmit") that measurement to the controller in the form of a signal. In this case, the type of signal is pneumatic: a variable air pressure sent through metal or plastic tubes. The greater the water level in the drum, the more ari pressure output by the level transmitter. Since the transmitter is pneumatic, it must be supplied with a source of clean, compressd air on which to run. This is the meaning of the "A.S." tube (Air Supply) entering the top of the transmitter.