Flow Diagram Instruments

Instruments function by sensing changes in the variables they monitor. The four basic instrument groups are:

Flow (F)
Level (L)
Pressure (P)
Temperature (T)

The types of instruments used to sense, control and monitor these variables are:

Controller (C)
Indicator (I)
Gauge (G)
Alarm (A)
Recorder (R)

The first letter in the symbol indicates the instrument group, and the second and/or third letters indicate the instrument type. To indicate a change or to control the flow, level, pressure, or temperature, an instrument must first sense a change in the variable. Once a change has been detected, the instrument then transmits this information via mechanical, electronic, or pneumatic means to a control panel where it can be observed and recorded. At the same time, the instrument may activate other devices to affect and change process conditions in the facility. Some instruments are read in the plant at the instrument’s actual location. Others are displayed on a control panel located in an operator’s control room.