Aqua-Data RV4X Calibration
July 2020 Procedure
Each flow sensor is individually calibrated at a hydrodynamic towing tank at QinetiQ, Gosport, UK.
We use a specific RC3 measuring unit that is dedicated to the measurement of the sensor, and this has previously been calibrated against a fixed precision unit that generates electrical signals in direct proportion to the current excitation, so as to simulate an ideal sensor with a fixed flow of 3.64m/s. This precision calibration unit uses ultra-stable 0.01% precision parts for the critical components, and we check this annually using a calibrated current source and a verified voltmeter (the voltmeter is verified internally against a calibrated voltage source).
Having calibrated the RC3 display unit so it displays the ‘nominally’ correct speed (as per design criteria) then a number of measurements are made at various speeds, logged every 50 milliseconds from a calibrated calibration rig onto a laptop, and assigned against the unique serial number previously assigned to the flow sensor. The flow sensor is rigidly bolted to a heavy trolley that is moved by a regulated electric motor and gearing at a constant speed, parallel to the side of a 270 metre tank of depth 5.4 metres. The sensor is held at a depth of between 1 and 2 metres. The depth is not a calibrated value; it is nominal, so as to reduce side effects to a negligible level.
The trolley is moved at an exact speed, as the trolley has its own timer system which displays the velocity, calculated from an encoder wheel mounted to the trolley. We verify the encoder wheel and thus velocity measurement by comparing it to a calibrated Dimetix laser distance sensor and a precision steel rule. The ambient temperature is fairly constant at a nominal 20 C, and even allowing for worst case thermal effects on the steel rule, a length error of more than 0.03% cannot occur over a 20C temperature variation. Hence we have a high level of confidence that the velocity is extremely accurate.
The time spent at each speed is a minimum of 10 seconds, and a nominal time of 20 seconds. Readings are taken at various exact speeds, as follows:
These readings are converted into a table of 13 data points. These points in turn are used to create a piecewise linear curve that corrects the sensor readings to be correct at the given sample points, and linearly interpolates in between. The graph being interpolated is very nearly linear in any case, so the interpolation is very precise. For speeds from 3.8 m/s to 4.0 m/s the above data is extrapolated upwards.
Since the sensor contains no moving parts, and the sensitivity is a function only of the magnetic field generated (in turn only a function of the excitation current) and the hydrodynamic effects of the geometry, then provided the sensor is clean and no physical damage has occurred there is no mechanism whereby drift can occur, except in the drift of internal gain setting resistors potted inside the sensor.
These are all precise and high stability parts, and are guaranteed free from significant drift by the manufacturer. The combination of all the possible errors mentioned above is such that the precision can be guaranteed within its working range to +/- 1.0% of range, and +/- 0.03m/s over a temperature range of -20 C to + 60 C.
Special calibration is possible for units that are intended to be used in more extreme temperature environments.