Nortek instruments do not require periodic factory calibration to maintain the accuracy of current speed and direction measurements. Under normal use, there is therefore no requirement from Nortek to return the instrument to the factory for recalibration at regular intervals.
Whether you choose to return your instrument to Nortek for service and verification, and how often you choose to do so, is entirely up to you. You may establish a regular service interval based on your own quality procedures, operational requirements, or application, or return the instrument only when there is a specific need for inspection or service. A full description of our service procedure can be found here: Nortek instrument service description
Why is factory calibration not required?
Each instrument has a unique head matrix that describes the geometry of the sensor head and the orientation of the acoustic transducers. As long as the sensor head has not been physically damaged or deformed, this geometry does not change and the head matrix remains valid.
Velocity measurements are based on the Doppler shift—the change in frequency between the transmitted acoustic signal and the signal reflected by particles in the water. Small changes in the transducer center frequency over time therefore do not affect the measured Doppler shift or require periodic recalibration of the velocity measurement.
What should be checked or calibrated regularly?
While periodic factory calibration is not required, there are some checks and calibrations that should be performed regularly by the user in the field or before deployment. These do not require the instrument to be returned to Nortek.
Compass
The compass should be calibrated by the user before each deployment to compensate for magnetic influences from the deployment setup and surrounding equipment. This is a user calibration and does not require any special equipment or factory service.
For instructions, see Compass calibration.
Pressure sensor
The pressure sensor is supplied calibrated to its specified accuracy, and its performance is verified by Nortek during production. Periodic factory recalibration is not required.
However, because the pressure measurement is not compensated for changes in atmospheric pressure, an offset may be present before deployment. The user should check the pressure reading and perform an offset adjustment when necessary.
Tilt sensor
The tilt sensor is calibrated by Nortek during production and does not normally require periodic recalibration.
Temperature sensor
The temperature sensor is supplied calibrated to its specified accuracy, and its performance is verified by Nortek during production. It does not normally require periodic recalibration.
Instrument maintenance
Regular maintenance is important, but should not be confused with calibration. Keep the transducers clean and inspect the instrument for biofouling, damage, or other changes that could affect the measurements.
Biofouling can reduce acoustic signal strength and consequently reduce profiling range. Severe fouling, such as barnacle growth directly on or around the transducers, may also affect velocity measurements.
The required maintenance interval depends strongly on deployment conditions, including water temperature, depth, season, and local biological activity. Monitoring changes in signal strength over time can help determine an appropriate maintenance interval for a particular deployment location.
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