On a piping and instrumentation diagram (P&ID), a balloon is used to indicate an instrument or control device that is used to monitor or control a process variable, such as temperature, pressure, or flow rate. The balloon contains information about the instrument or control device, such as its tag number, type, and location. The purpose of the balloon is to provide a clear and concise way to identify the various instruments and control devices used in a process, which can help operators and maintenance personnel to quickly and easily locate and troubleshoot problems.