If there are more than one instrument used for same purpose at different locations then they are numbered to differentiate between what number corresponds to which instrument.

The parameters which they control are represented by alphabets. The most important part is the indicators, transmitter and controllers. The symbolic representation of the equipments and instruments can be read by referring to legends. Isos are used during the erection and installation process and also prior to stress analysis studies. PIDs are primarily schematic used for laying out the process control installations. They are drawn along lines of equilateral triangle of form of 60 degrees. Thus it is a three dimensional diagram on a two dimensional plane. The welds are represented as certain dots The pipe may bend in different directions, so the angle of change is also represented. In Isos, the perception of depth is included although it is a three dimensional representation but pipes are indicated as solid lines for simplicity and the bends are also represented as a combination of lines. It does not give any information about the dept of the equipments or valves. The pneumatic and electrical signals are shown as dashed and dotted lines. The transfer of materials from one equipment to another equipment is shown as a solid line. The interconnection between them is shown as a line. In PID, the equipments and the process control instruments are represented as two dimensional symbols. They are also commonly referred to as isos in process and chemical industries and in academics also. A piping isometric drawing is also a detailed diagram which is used to represent pipes, pipe fittings, bends, valves, welds etc.

Iso pipe drawing