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Table of Contents

By Wesley Howard,2014-01-20 04:03
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Table of Contents

    Table of Contents

    1. Light Scattering Sensor Installation 1 1.1 Light Scattering Sensor Wiring 1

    2. Operation 3 2.1 Light Scattering Sensor Care 3

    3. Description 4 4. Specifications 6 5. Calibration 7 5.1 Optical Measurements of Suspended Particles in Water 8

    5.2 Calibration Stability 10

    Light Scattering User’s Guide (LSS) Revision C 26 November 2002 i

1. Light Scattering Sensor Installation

    The LSS is designed to measure light scattering from particles in the forward hemisphere relative

    to the front surface of the sensor. The sensor should be installed at the outermost diameter of the

    user’s system. This should be done such that all light scattering objects will be behind the plane

    defined by the front surface of the LSS (Figure 1). The LSS should also be placed as far away

    from light reflective objects as possible. If significant zero offset is observed in the LSS data for

    very clean water, the LSS is probably not mounted correctly.

    Figure 1. LSS Installation Diagram

    1.1 Light Scattering Sensor Wiring

    Do not remove the protective cap during installation and wiring of the LSS. The protective

    cap will prevent accidental damage to the sensor surface and can be used to test the device

    during the installation and wiring process. The cap is designed to allow reflected light to be

    measured permitting sensor operation to be verified with the protective cap in place.

    The sensor is supplied with a five-conductor bulkhead connector. The five-conductor

    interconnecting cable should be connected to the user’s system (Figure 2). The LSS should

    be operated with a power supply capable of delivering 50 milliamps of current at nominally

    +12 VDC. The sensor voltage output is 0 to +5 VDC with an output impedance of 1000

    ohms. The sensor has two gain ranges, high and low. For the user who chooses to select gain

    manually, high gain is selected by leaving pin 3 open. Low gain is selected by connecting pin

    3 to power ground. (Gain can be remotely controlled by the user’s equipment. Pin 3 is tied to

    +V, through a 100K-ohm pull-up resistor permitting a logic high to select high gain and a

    logic low to select low gain.) Gain can also be controlled manually if a special cable is

    fabricated (contact WET Labs for details).

    Light Scattering User’s Guide (LSS) Revision C 26 November 2002 1

Figure 2. LSS Connectors

2 Light Scattering User’s Guide (LSS) Revision C 26 November 2002

2. Operation

    1. Measure the LSS power supply voltage and make sure the voltage is between +9 and +18

    VDC.

    2. Measure the LSS current drain. It should be less than 20 mA DC (1x units) or 24 mA DC (3x

    units). Refer to individual calibration sheets for other units.

    3. Remove the LSS protective cap and block the LSS solar blind signal detector using black

    electrical tape, then measure the LSS zero output. The voltage should be near zero, less than

    10 mV DC, or at factory values. The sensor RMS electrical noise should be less than 0.01

    percent of full scale voltage. Gently clean the head with alcohol afterward to remove any

    residual adhesive.

    4. Remove the black tape from the sensor and verify that the sensor output varies with

    reflectance from objects in the sensing volume. In air the sensor detector can be easily

    saturated by ambient light and the output will either not change or be negative or both. This

    is not a problem in water greater than 1 meter in depth. The LSS output will generally go to

    full scale, approximately 5.5 VDC if you cup your hand in front of the sensor to block

    ambient light. The LSS is very sensitive in air and will have significant output due to the

    reflection of light from surrounding objects and/or ambient light.

    2.1 Light Scattering Sensor Care

    The LSS housing material is ABS plastic filled with black epoxy and the optical windows are

    clear epoxy. The LSS sensor surface may be cleaned with soap and water or alcohol. Use a

    non-abrasive paper or cloth to clean the sensor front surface to avoid scratching the clear

    epoxy windows. Clean the optical windows before and after use in water. Be sure to place the

    protective cap on the instrument when not in use.

    Light Scattering User’s Guide (LSS) Revision C 26 November 2002 3

3. Description

    The Light Scattering Sensor (LSS) measures light scattered from suspended particles in water. This measurement is often referred to as turbidity, but is also a measure of water clarity, visibility and particle concentration. The basic scattering sensor design is capable of linearly measuring nearly all turbidities found in natural waters from 0.0005 NTU to 750 NTU. WET Labs produces four different models of LSS to cover various sub-ranges of this range. The optical design of the LSS is shown in Figure 3. The optical sensor consists of two 880 nm wavelength light sources and a solar-blind light detector mounted in adjacent cavities where the mutual cavity wall forms a light stop between the light source and the light detector. The light stop is used to block the direct transmission of light from the light source to the light detector such that only scattered light is measured. Light source and light detector cavities are filled with clear epoxy to form an optically clear, watertight window. Depths of the cavities are such that the light transmitted from the LED’s and the scattered light received by the detector are maximized.

    Figure 3. Dual light source and detector geometry

    Light transmitted by the LED is refracted at the sensor surface to radiate over the entire hemisphere. Similarly, light scattered from particles is received from the entire hemisphere. The directional sensitivity within this hemisphere is approximately as the cosine of the angle from zenith. For this reason, the sensor needs to be mounted at the periphery of the package such that no reflective objects are in the forward hemisphere.

    The refraction described above allows scattering at almost all angles to be detected by the sensor. The signal received by the sensor is proportional to:

    ?

    , where ?(?) is the Volume Scattering Function, and w(?) is a weighting ????????w?sin?d??0

    function determined by optical modeling of the sensor design to be as shown in Figure 4. Maximum sensitivity is for scattering angles near 100 degrees.

4 Light Scattering User’s Guide (LSS) Revision C 26 November 2002

    Figure 4. LSS response in water Each model of LSS has two ranges selected by setting a fifth conductor to open or ground. For

    instance, our standard model has nominal ranges of 0.007525 NTU on low gain and 0.00257.5

    NTU on high gain. These two ranges cover almost all open ocean and coastal waters. The ratio

    between the two ranges is always about 3.33. Our 3x sensitive model similarly has nominal

    ranges of 0.00257.5 NTU and 0.00072.25 NTU for very clear waters. We also have 1/10x and

    1/30x models to cover more turbid inland waters

    The sample volume of the LSS varies with the turbidity of the water. The volume is limited by

    the rapid attenuation of the infrared light by water. In clear waters, roughly 50 percent of the

    signal would originate from particles less than 11 cm from the sensor face, 90 percent would be

    from within 40 cm and 99.75 percent from within one meter from the sensor face. In more turbid

    waters, all those ranges would be shortened. This applies to homogeneous distributions of

    particles. If there is a large reflective item out front, it could overwhelm the signal from the

    particles. We recommend at least two meters clear space in front of the LSS in clear waters.

    Light Scattering User’s Guide (LSS) Revision C 26 November 2002 5

4. Specifications

    Measured Parameters Turbidity and suspended solids

Mechanical

    Size length: 5 in (12.7 cm)

    diameter: 1.25 in (3.2 cm)

    Weight in air: 0.57 lbs (0.26 kg)

    in water: 0.29 lbs (0.13 kg)

    Housing material ABS plastic housing filled with epoxy

    Window material clear optical epoxy

    Rated Depth 6000 meters

Electrical

    Power 918 VDC ~20 mA Signal Output 05 VDC

    Temperature Stability ~ 0.5%, 050 deg C Power Supply 1x units: 918 VDC @ 20 mA;

    3x units: 918 VDC @ 24 mA Power Consumption ~200 mW

    Sensor Output 05 VDC thResponse Time 1/10 second

Optical

    Resolution < 0.03% full scale

    Measurement Range

     1x unit ~ 7.5 NTU on high gain; ~ 25 NTU on low gain

     3x unit ~ 2.25 NTU on high gain; ~ 7.5 NTU on low gain

    Sample Volume Varies. Large for clean water; small for turbid water

Specifications are subject to change without notice.

6 Light Scattering User’s Guide (LSS) Revision C 26 November 2002

5. Calibration

    Figure 5 shows typical correlations that can be expected in natural waters when the LSS output is compared with beam attenuation, “c,” measured with the 650 nm, 25 cm pathlength Sea Tech transmissometer. The data shows that the LSS output depends on the nature of the particles suspended in the water. The LSS output, like all sensor outputs used to measure inherent and apparent optical properties, will be dependent on particle size distribution, particle shape, index of refraction, organic vs. inorganic ratio, wavelength of the measurement, etc.

    Based on these limitations, calibration of the LSS from a scientific perspective implies that the LSS should be calibrated with the particulate material characteristics of the study region since the LSS output is a function of the nature of the suspended particles. Calibration of the LSS from an engineering perspective is less demanding. A light scattering standard that is stable and repeatable such as Formazin can be used to calibrate the LSS and is recommended since this material is readily available and has been used in the past to calibrate light scattering instruments that measure turbidity. Calibrated using Formazin, the LSS output will be a linear measure of suspended particle concentrations in water. The relationship between light attenuation and light scattering from Formazin, clay and organic material is shown in Figure 4. This relationship is approximate and can be expected to vary depending on the nature of the particulate matter suspended in the water column.

    The standard 1x LSS is calibrated at the factory using a 5 NTU concentration of Formazin in 16 liters of particle-free water. Particle-free water for calibration purposes can be made by filtering water with a 0.2-micron filter. The calibration water container used at the factory is a 5-gallon black bucket. The LSS is immersed in the water to a depth of approximately 2 inches and centered in the top of the black bucket. Refer to your instrument’s calibration sheet for the output of the LSS for a 5 NTU concentration of Formazin.

    The 3x sensitive LSS is calibrated at the factory using a 1.0 NTU solution of Formazin so that the high-gain range will produce an on-scale reading. The calibration of the high-gain range can also be computed from the low-gain calibration and the ratio of the gray-scale card readings for the two ranges.

    Light Scattering User’s Guide (LSS) Revision C 26 November 2002 7

    Figure 5. LSS calibration

5.1 Optical Measurements of Suspended Particles in Water

    A good correlation exists between any of the optical properties and suspended particulate

    concentration in water given that the nature of the particles remains constant. In most cases

    the nature of the particles does remain relatively constant in a given area and because of this

    optical instruments measuring the optical properties of water have been widely used to

    measure suspended particulate concentrations in water. The data in Figure 6, a profile of the

    inherent optical properties found in Green Peter Reservoir, shows typical correlations found

    in nature. Green Peter Reservoir is a fresh water lake near Sweet Home, Oregon. The data

    demonstrate that the inherent optical properties correlate well with the exception of beam

    absorption near the surface where a large amount of chlorophyll-a fluorescence particles are present. Exceptions occur, but in general this data demonstrates that the LSS will measure

    nearly the same relative profile of suspended particle concentrations in the water column as

    would be measured with a transmissometer. The beam attenuation coefficient was measured

    at two wavelengths: 650 nm and 880 nm with Sea Tech 25 cm pathlength transmissometers.

    The beam absorption coefficient was measured at 880 nm wavelength with a Sea Tech 25 cm

    pathlength reflective tube absorption meter. Light scattering was measured with an

    expendable light scattering sensor at a wavelength of 880 nm. Temperature, depth and

    chlorophyll-a fluorescence were also measured using Sea Tech Instruments.

    8 Light Scattering User’s Guide (LSS) Revision C 26 November 2002

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