AQUAscat ^® AOBS

The sensitivity (Volts/[mg/L] ) of optical turbidity sensors varies inversely with grain size. Turbidity sensors are highly sensitive to fine particles, and insensitive to large particles.

For a constant concentration, a turbidity sensor will show a large response to fine particles (e.g. 5 µm), and – for the same concentration – a much smaller response to larger particles, e.g. 75 µm. To calibrate a turbidity sensor to suspended sediment concentration one must therefore know the grain-size, or assume that it doesn’t vary.

The sensitivity (Volts/[mg/L] ) of optical turbidity sensors varies inversely with grain size. Turbidity sensors are highly sensitive to fine particles, and insensitive to large particles.

The AQUAscat® ABS has a nearly constant sensitivity (Volts/Concentration) for particles larger than about 30 µm, out to about 500 µm. Below about 30 µm, AQUAscat® ABS response is also size dependent: relatively insensitive to fine particles, and sensitive to large particles – the opposite of optical turbidity.

The combination of acoustics and optics, when done properly, compensates these opposite tendencies of change with grain size, and produces a system that is substantially (but not totally) independent of grain size variations in this range.

Because of the near-constant calibration for the AQUAscat® AOBS Super-Turbidity sensor, the measurements of suspended sediment concentration are far more accurate, over a wide grain size range, than either turbidity (optical backscatter) or AQUAscat® ABS (acoustic backscatter) individually.

Due to size dependence, individual calibration of the turbidity and AQUAscat® ABS sensors is normally required when operating alone. Because of the fixed calibration achieved by the combination of ‘paired’ AQUAscat® ABS and turbidity sensors, the need for calibration of either sensor is greatly reduced. This is a major convenience to experimentalists!

YES!

If you have concurrent, historical AQUAscat® ABS and turbidity data you can combine them into one. A single patent pending calibration method finds a critical weight factor that then is used to combine past data. The step involves obtaining one measurement of sensitivity of the optical turbidity sensor at a given grain size (30 µm). A weight factor is found as the ratio of AQUAscat® ABS output and turbidity sensor output. This weight factor then allows the weighted sum of the two instrument outputs, thus correcting past data also.

From the calibration of the turbidity, the response (Volts/[g/L]) is measured for 30 µm particles. A weight factor is computed using this value. The two outputs – AQUAscat® ABS and turbidity – are combined with this weight factor applied to the turbidity data, resulting in the Super-Turbidity data.

No. TECPRIME will not pair the AQUAscat® ABS with any other turbidity sensor than the TurbidityPlus sensor from Turner Designs. Users can purchase the complete, paired AQUAscat® AOBS instrumentation package, consisting of a AQUAscat® ABS integrated with a Turner Designs TurbidityPlus(TM) sensor. This is the only combined package TECPRIME offers for sale.

Users who wish to use another turbidity sensor must provide their own cabling, wiring, data logger(s), and do the pairing between their AQUAscat® ABS and turbidity sensor themselves.

You don’t need to do the pairing if you buy the complete instrumentation package from TECPRIME.

No. The TurbidityPlus sensor is NOT calibrated to NTU. The output from the TurbidityPlus sensor is in V only. Please note that It is completely irrelevant for the pairing of the two sensors if the TurbidityPlus is calibrated to NTU or not. TECPRIME will not offer NTU calibration of the TurbidityPlus sensor. The customer is responsible for any NTU calibration. The Turner Designs TurbidityPlus manual provides instructions on how to calibrate the sensor to NTU if desired.

Any SDI-12 enabled data logger will work with the AQUAscat® AOBS. The AQUAscat® AOBS will output ABS data, turbidity sensor data AND the paired data in a single datastream.