All the devices like transmitters, gauges, and other equipment etc., in a control loop, cannot be picturised to understand the logic. So in a P& id diagram, all the devices are symbolised.
Letters and numbers are written inside the circle to show the function of the instrument and it’s identification, circles used are of different types:
The instrument described in the blank circle is mounted in the field
The instrument is located on the front of a control room panel.
Circle with the single broken line. The instrument is located behind a control panel. The instrument is mounted in the field control panel. Circle with double dotted line. The instrument is mounted behind the field control panel. The above symbol tells about 1. where the instrument is located 2. what the instruments is, here it’s a level indicating controller. 3, what part of a control loop it is.
There are different types of abbreviations used to know the type of instruments here. Such abbreviations are listed inthe table below