TSI recommends a full annual calibration. Ask for TI-112 for details. The Model 8702 and 8705 read a nominal velocity that is neither standard nor actual, but a combination of both. This is the velocity that is read by any Pitot tube and pressure device that doesn't do a density correction. The Model 8705 allows entry of barometric pressure and temperature.
Standard velocity is velocity that is referenced to standard conditions.
Actual velocity is the speed at which a microscopic particle of dust would be traveling as the air flow carries it along. The two measurements give the same readings if actual conditions equal standard conditions. However, if the temperature increases, the air would expand and become thinner. Actual velocity would then be higher than standard velocity. The Model 8705 DP-CALC can display either actual or standard velocity when barometric pressure and temperature are entered.