GLI Method 2

The Great Lakes Instrument Method 2 doubles the number of photodiodes and photodetectors used in the average turbidity instrument 16. It also doubles the number of measurements taken. As such, this design is also known as a modulated four-beam turbidimeter. By using two measurements, two light sources, and two detectors, this method can compare results between the detectors and cancel out errors 18.

A modulated 4-beam turbidimeter alternates light pulses from two light sources into alternating primary and secondary detectors

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This method requires 860 nm LEDs, which allows for color compensation, much as the single beam ISO 7027 method does 18. The LEDs alternate light pulses every half second. The photodetectors take simultaneous readings, providing an “active signal” and “reference signal” 16. The detector directly across from the active LED is considered the active signal, while the detector at a 90-degree angle is considered the reference signal. Every half second, the active and reference signals switch as the second LED pulses 16. Thus the GLI2 method provides two active and two reference measurements to determine each reading. The ratiometric calculations used to determine turbidity mean that the light input and the output are directly proportional. Any errors that may appear are thus mathematically canceled out 16.

A modulated-four beam ratiometric algorithm can use the following formula 37:

NTU = Calslope * Sqrt((Active1 * Active2) / (Reference1 * Reference2)) – Cal0

turbidity_equation_cal

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Calslope = Calibration coefficient
Active1 = 90 Degree Detector Current (Light Source 1 ON, Light Source 2 OFF)
Active2 = 90 Degree Detector Current (Light Source 1 OFF, Light Source 2 ON)
Reference1 = Transmitted Detector Current (Light Source 1 ON, Light Source 2 OFF)
Reference2 = Transmitted Detector Current (Light Source 1 OFF, Light Source 2 ON)
Cal0 = Calibration coefficient

As fouling, sediment or color interference affect both detectors equally, any potential errors are nullified 37.

Based on the ratiometric (directly proportional) algorithms used to calculate turbidity, the GLI2 method allows for increased sensitivity and error cancellation in the 0-100 NTU range 16. However, this method loses some accuracy as turbidity levels rise above 40 NTU 18. This is due to the the increased light intensity. As turbidity increases, the intensity of the scattered light will also increase 17. GLI2 instruments are ideal for lower turbidity ranges, and when measuring in the 0-1 NTU range in particular, they are extremely accurate 16.

Due to its multi-beam design, the USGS recommends using Formazin Nephelometric Multibeam Units (FNMU) instead of NTU as the turbidity units for this method 18. Instruments with this design are still classified under nephelometric technology as they use photodetectors at 90-degree angles.

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