A sling psychrometer with two bulbs, one dry and the other wet, is shown in Fig. 2.5a. Both dry and wet bulbs can be rotated around a spindle to produce an airflow over the surfaces of the dry and wet bulbs at an air velocity of 400 to 600 fpm (2 to 3 m / s). Also a shield plate separates the dry and wet bulbs and partly protects the wet bulb against surrounding radiation.