PDV is well suited for high-speed flow measurements where concerns about particle seeding make PIV impractical. Although PDV requires particles to scatter light, individual particles do not need to be imaged thus allowing the use of much smaller seed particles and making the measurements less sensitive to particle seed density. For example, in some unheated supersonic flow facilities it is possible to use condensation of a vapor, such as water, acetone or ethanol, to produce seed particles in the flow. Particles formed using this method, known as product formation, have been estimated to be ~50 micrometres in diameter.