The Standard Tar Viscometer (often abbreviated as STV) is a specialized laboratory instrument used in civil engineering and materials testing to measure the viscosity of cut-back bitumen, road oils, and fluxed bituminous binders. It operates on the principle of efflux time, determining how long it takes for a fixed volume (typically 50 ml) of the sample to flow through a standardized orifice under controlled temperature conditions. This helps assess the flow properties of tars and bitumen’s for applications like road construction and paving