Nortek instruments generally do not require calibrations for current speed and direction, but rather regular maintenance. Maintenance should be performed often enough to ensure biofouling does not influence the return signal strength, and thus profiling range, of the instruments. There have been cases of barnacles causing erroneous velocity data, but other biofouling has no effect other than limiting the range. The time this takes depends completely upon the local conditions (warm vs cold water, shallow vs deep, season, etc.) so we recommend looking for changes in signal strength over time to estimate how often maintenance should be performed.
Calibration is not needed because all the instruments have what we call head matrixes that are unique to each instrument. These head matrices describe the geometry of the sensor head, or the transducer orientation to be more specific. As long as the sensor heads are not physically deformed, the head matrixes will, consequently, remain the same. Further, what is measured by the sensor is the change in frequency from the acoustic pulses sent out through the water to those being reflected back. A slight change in center frequency will, in other words, not affect the measured frequency shift, or the Doppler shift, which is entirely dependent on the velocity of particles in the water.
The other sensors such as tilt, temperature and pressure are not drifting in time, and need not be calibrated unless they are damaged. Be aware that the pressure sensor is not compensated for atmospheric changes and might need an offset adjustment before use. The compass, however, may need calibration before each deployment.