‘Super Turbidity’ Explained

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This article was originally published by Sequoia Scientific. It details the advantages of their new ‘Super Turbidity’ sensor; the LISST-AOBS

 

The short explanation

Briefly, the difference between turbidity and Super-Turbidity (Patent Pending) can be summarized as follows:

Turbidity (NTU) = Optical Backscatter; Needs Calibration for SSC; less accurate
Super-Turbidity (mg/l) = Optical & Acoustic Backscatter; No Calibration for SSC; more accurate

  • NTU: Nephelometric Turbidity Unit
  • SSC: Suspended Sediment Concentration

 

The slightly longer explanation

Traditional turbidity sensors deliver a reading in NTU, which must be calibrated to SSC with samples. Turbidity sensors are highly sensitive to fine particles, and insensitive to large particles. In contrast, the LISST-ABS delivers a DIRECT reading of SSC in mg/l and has a nearly constant sensitivity for particles larger than about 30 µm, out to about 500 µm. But, below this range, the response is also size dependent.

LISST-AOBS Super-Turbidity – the combination of a turbidity sensor and a LISST-ABS – delivers a direct reading of SSC in mg/l. This SSC reading is far more accurate for both fine and coarse particles than either sensor on its own.

LISST-AOBS Super-Turbidity compared to turbidity and LISST-ABS

The figure below shows a real-time plot of Super-Turbidity compared to the optical and acoustic backscatter sensors individually.

 

AOBS Super-Turbidity Real-Time

AOBS Super-Turbidity Real-Time

 

On the top plot we see time history of turbidity sensor output in V (orange) and LISST-ABS concentration in mg/l (blue). Each sample is at 1 s interval. The two sensors were installed together in a beaker with equal amounts of 5-10 and 75-90 µm particles. The particles were kept in suspension and well-mixed with a magnetic stir-bar. Around sample number 30 the stir-bar was turned off. A few seconds later the 75-90 µm particles have fallen out of suspension. We see the LISST-ABS concentration dropping almost 1,000 mg/l, but the fine 5-10 µm particles stay in suspension. Crucially, there is almost no response from the turbidity sensor; it does not see the coarse particles disappearing, only the 5-10 µm particles in suspension.

The bottom plot shows the COMBINED LISST-AOBS Super-Turbidity response from the two sensors, paired according to Sequoia’s patent pending method. The output is directly in mg/l. It is evident that the COMBINED output shows the concentration decrease from the large particles settling AND the concentration of the remaining finer particles.

 

The longer explanation

The figure below shows the response of the LISST-ABS and an optical turbidity sensor for a range of standard particles with varying grain-sizes. Each curve shows the response of the LISST-ABS (circles) or turbidity sensor (plusses) for a given grain-size as a function of concentration. Note that the concentration varies three orders of magnitude from ~5 mg/l to a few 1000 mg/l.

Let us look at the four LISST-ABS curves at the top of the plot. These are for particles from 40-80, 63-75, 75-90 and 125-150 µm in size. We can see that they are all very close together. We can also see that these four curves are on or very close to the 1:1 line (line not shown). This shows that the LISST-ABS measures the correct concentration, regardless of grain-size, as long as the particles are coarse. Note that the LISST-ABS output is directly in mg/l from the factory, without the need for any further calibration.

Let us now look at the two red, two green and one blue line from the OBS that plots together. These are for particles from 4-8, 5-10, and 10-20 µm in size. They all plot close together. This shows that the OBS output is constant for a given concentration, as long as the particles are fine. If desired, the OBS output for these fine particles can be calibrated to concentration with a high degree of accuracy.

 

Now, let us look at the second figure below.

This figure shows ALL data points from the previous plot. The data has been converted to LISST-AOBS Super-Turbidity using Sequoia’s patent pending methodology. It involves combining the data from the turbidity sensor and the LISST-ABS using a weight factor for the turbidity sensor output.

The units for the turbidity sensor output are completely irrelevant for the sensor pairing. It doesn’t matter if the turbidity sensor output is V, mV, FNU or NTU or any other unit. When pairing two sensors using Sequoia’s patent pending method, their COMBINED output will be in units of mg/l – as shown on the plot – regardless of the turbidity sensor unit.

Also shown on the plot is the 1:1 line. We can see that all data plot within a factor of two from the 1:1 line, regardless of changes in grain-size and concentration. This means that the combined output from the two sensors is far superior than each sensor on its own.

AOBS vs concentration

 

LISST-AOBS vs actual sediment concentration for a range of sediment grain-size

 

‘Super-Turbidity’ is a Sequoia-developed patent pending methodology.

 

CHECK OUT THE NEW SUPER TURBIDITY SENSOR

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