The following is a detailed description of the service routine we offer and its procedures.

This will mainly cover the different tests and calibrations we perform, as these are important to our customers and assure that the instrument is performing within its specifications and tolerances.

It is important to note that the term calibration has different meaning used in the context of this document than the ISO/IEC 17025 definition of traceability to international SI standards.
The correct term for Nortek’s “calibration” according to ISO/IEC is adjustment. We will however still use the term calibration since this is the term commonly recognizes.

The result of the calibrations is saved in the instruments head file and in a report which is included when the instrument is shipped.

All sensors included in a Nortek instrument is calibrated against a reference sensor with much greater accuracy. All references are calibrated at factory recommended intervals.

The Nortek service routine is based on the topics and procedures that are mentioned under the Service Manual section in the Comprehensive Manual(s) as well as electronics revisions if necessary.
We recommend all users in general to read the manual thoroughly and perform important basic service routines like function testing and O-ring replacements regularly.

Routines may vary between different instruments and not all instruments have the below described sensors.

The service routine will first include a thorough inspection and testing of the instrument to establish the condition of the instrument, referred to as the initial check.
This is required any time an instrument is returned to the factory for an RMA.
Any discrepancies will be noted, and we will contact the customer to let them know what we find before continuing any work.

O-rings will always be replaced, and mating surfaces inspected, along with the following pressure testing to ensure that the instrument is watertight.

Service does not imply that the instrument is as good as new, but all tested values are within our specifications.

For instrument specifications please see the individual instrument’s data sheet available on the product webpage.

The following tests and procedures are performed during a service (but not limited to):

Current drain test

A current drain test is performed to make sure that the instrument is within its tolerances for how much power it is allowed to draw in its different modes.

Temperature chamber test

The temperature chamber test is a warm- and freeze test which will stress test the instrument in +50 degrees Celsius for 2 hours, and in -20 degrees Celsius for 2 hours.
During this test the instrument is measuring and must work flawlessly.

Tilt sensor verification
Our tilt sensor has got two axes and an up/down detection. The up/down detection is verified during the calibration and in the final test of the instrument. The two axis are calibrated separately and in both up and down position. This is giving 4 different calibration polynomials of 3^rd order.

The calibration is done in 2 steps. First we do a full calibration and calculate all the polynomials. Then we do an offset adjustment during the magnetometer calibration to remove issues of mounting of the instrument in the automated jig.

The calibration is done by putting the instrument in 5 different positions between -30⁰ to +30⁰. Values for both the Nortek instrument and the reference are inserted into a calculation giving out a 3^rd order calibration polynomial. Subsequent to the calibration, this is uploaded to the instrument and verification is done at 5 different positions between -30⁰ to +30⁰.

The procedure is repeated for both axes both up and down.

If the verification is approved the calibration coefficient is saved and a report is generated.

Pressure sensor verification

The instrument is put in a pressure chamber at 10 different pressure levels evenly spaced throughout the range of the specific instrument’s pressure sensor. Values for both the Nortek instrument and the reference are inserted into a calculation giving out a 1^st order calibration polynomial. This is uploaded to the instrument and verification is done at 5 different pressure levels evenly spaced throughout the range of the specific instrument’s pressure sensor.

The pressure sensor offset is not calibrated to, or corrected by the atmospheric pressure. This means that the pressure sensor reading will follow the atmospheric pressure.

If the verification is approved the calibration coefficient is saved and a report is generated.

Magnetometer/compass verification

The calibration of the magnetometer is done in two steps. The first step is a soft iron calibration referred to as magnetometer calibration. Soft-iron distortion is dependent upon the orientation of the material relative to the sensor and the magnetic field. The cause of this effect is distortion from the instrument’s electronics and mounting inaccuracies.

The compass calibration is a hard iron calibration. Hard iron distortion is constant, additive field to the earth's magnetic field, thereby generating a constant additive value to the output of each of the magnetometer axes. The purpose of this calibration is to remove the effect of hard iron (=magnetic properties are independent of orientation) objects in the vicinity of the instrument. This means that the customers should preferably calibrate the instrument when it is mounted in the frame it will be deployed in.

The purpose of the calibrations is not to measure the "true" magnetic field, it is to improve the quality of the compass measurements.

For the magnetometer calibration the instrument is mounted in a two-axis compass test jig. The instrument is rotated slowly around every three compass axes. The gain for all axes is calculated and adjusted.

In the compass calibration we rotate the compass around the vertical axes and read out the two horizontal magnetometer components (which should form a circle). We calculate new offset coefficient such that the circle is centered in origo of the circle.

Together with the tilt sensor the magnetometer data give the heading. The heading of the instrument is then verified at 45 degree steps – both up and down.

If the verification is approved the calibration coefficient is saved and a report is generated.

Temperature sensor verification

Our temperature sensor is a thermistor (a resistor whose resistance varies significantly with temperature). The thermistor element itself does not need to be calibrated, but it is not linear. In order to achieve linearity we need a 3^rd order calibration polynomial. The coefficients are static except for the offset.

The offset is calibrated by loading the circuitry with known resistances. If the read temperature value is off it is calibrated to the correct value. The readings are verified at 3 different temperatures.

If the verification is approved the calibration coefficient is saved.

Signal strength check

A signal strength check is a check of the transducers amplitude and SNR to uncover any obvious issues that might need to be troubleshooted. This is performed in a water tank and observed closely.

Probe calibration for velocimeters

Velocimeters have unique transformation matrices that will be calibrated for each service. This does only apply to velocimeters.
A velocity check will check the velocities and direction of the calculated currents. This is performed with water moving relative to the instrument in a controlled environment.

Test of optional input/output

Instruments may come with different options available (auxiliary outputs/inputs, sync lines, power output) depending on the order. These outputs/inputs will be tested to make sure they are working as intended.

Dock test

A dock test is a monitored deployment in the factory water tank. The instrument continuously records data, and the system log and data output are thoroughly analysed and approved by a technician. All test data is stored in the instrument production database.

Final test

The final test is a final quality assurance before the instrument is shipped.
It is a quick check after all the previous testing to see that all sensors are ok, all the beams are functioning, matrices are correct, harness accessories etc. are tested.

Running through a 'final test' is highly recommend for everyone to perform prior to a deployment. It can be found under "Function Test" in the respective Comprehensive Manual.

If any details are unclear or you have questions regarding the service procedures, please contact us at support@nortekgroup.com and we will be happy to help.