The Dwyer AP3-24 Thermo-Hygrometer Probe measures air velocity, air flow, and temperature when combined with the Model UHH6 Universal Handheld. In order to prevent damage to the sensing element, the AP324 has a protective sleeve that slides over the sensor when it is not in use. Markings on the AP324 allow users to know the insertion depth to obtain better accuracy when traversing the duct.