As a watch stander, you monitor systems and tanks for liquid levels. Sometimes, you are only required to know if a level exceeds or falls below a certain preset parameter. At other times, you need to know the exact level. If only a predetermined limit is needed, you can use a float switch. When the set point is reached the float switch will make contact and sound an alarm. If you need to know a specific level, you must use a variable sensing device. The sensor used to indicate a tank level is commonly called a tank level indicator (TLI). This sensor tells you the exact amount of liquid in a tank. In the following paragraphs, we will describe the operation

Figure 11-21.-A phase-sequence indicator.

Figure 11-22.-Magnetic float switch.

of each of these sensors and their applications. Refer to the manufacturers' technical manuals for more information on the procedures you should use to adjust each type of device.