The principal of a hot wire anemometer is based on a heated element from which heat is extracted by the colder impact airflow. The temperature of the hot wire is kept constant via a regulating switch, and the current (amp draw measured internally) is directly proportional to the air velocity. When using a hot wire in turbulent air streams the measured results can be impacted by turbulent airflow striking the measurement sensor from multiple directions. This could indicate a higher measured value than a vane probe. This characteristic is typically prevalent in ducts where turbulent airflow can occur even at very low velocities.
All measurements should be made in a straight section of duct if possible. In an ideal location the duct will have a minimum of 10 diameters before the measuring spot and at least 4 diameters after before making a transition of turn. The airflow should not be inferred by dampers.
Hot wires are calibrated to a specific air density and either require the density to be input to the meter or a correction to be made. Many are calibrated to standard air which is 68F 0%Rh and 29.92mmhg. Consult the manufacturer. Hot wires are best suited for low velocity measurements at or near standard air conditions. Care must be used when measuring conditioned and turbulent air. Hot wires are not recommended for air velocities exceeding 2000 FPM unless they are specifically designed for that purpose. Heavy duty models are available that can measure in excess of 6000 FPM.