The notation of a circle with a horizontal bar inside, encountered in discussions of the Venturi effect, represents the distinction between measured values and actual physical quantities due to measurement interference. This notation indicates that the pressure measurement affects the values of temperature, pressure, molar mass, density, and concentration in the fluid. The confusion arises from the way these quantities are presented in literature, particularly in the context of pressure-based meters. Additional resources, such as an engineering article on pressure-temperature compensation, may provide further clarity on this topic. Understanding this notation is crucial for accurate interpretation in fluid dynamics.