The production and entrainment of bubbles in water as it flows around rocks in a stream or a propeller. Bubbles cause a turbidimeter to read higher than it would without bubbles.
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The regulation of light power such that the irradiance at the entrance window to a turbidity sample remains constant with time and temperature. In our sensors, APC is achieved with the output from a monitoring photodetector and an electronic feedback circuit.
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Developing a mathematical relationship between signals from a sensor exposed to known values for turbidity, temperature, pressure, or salinity so the signals from unknown values can be converted to engineering units.

The process of reducing sediment to a collection of dry particles with the smallest practical size.

An unpredictable change in sensor outputs caused by temperature and humidity fluctuations, deterioration of materials, and aging of electronic components.

The percentage of time a device is turned on. Duty cycle is an important factor in determining battery life.

A device (LED, tungsten lamp, laser diode, or IRED) that radiates energy when excited by an electrical current.

A procedure for testing OBS sensors whereby one waves a finger in front of the sensor and notes the output signal. Signal fluctuations indicate an OBS sensor is working.

A glob of particles that settles faster and scatters or absorbs more or less light than the individual would; see photomicrographs of flocs.

The physical properties, including absorption, scattering, attenuation, and the volume scattering function that affect the transfer of light through a water sample but are unaffected by ambient illumination.
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The degree to which the output of a sensor is a linear function of the detected parameter. A detector is perfectly linear if a scatter plot of its output versus the inputs has a coefficient of variation, r², equal to one.

The conversion of light to heat or some other energy form as it passes through a water sample. Unlike scattering, absorption does not alter the direction of light transfer. In a turbid sample, light is absorbed by water, dissolved material, and by suspended particles. The absorption coefficient, a, defined as the fraction of energy absorbed from a light beam per unit of distance traveled in the sample, is the measure of this IOP. It is expressed in cm^-1. The intensity of light in a narrow beam, after passing through a water sample is given by: I=e^la, where: I = the path length; is the initial light intensity; I is the intensity at a measurement location. In pure water, for example, the absorption coefficient of 650-nm (red) light equals 0.003 cm^-1 and the beam intensity will declined about 63% after the beam travels 333 cm.
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The combined effects of absorption and scattering that reduce (attenuate or extinguish) the intensity of light as it passes through a water sample. The attenuation coefficient, c, defined as the fraction of energy scattered and absorbed from a light beam per unit of distance traveled in a sample, expressed in cm^-1, is the measure of this IOP. The formula, a + b = c expresses the concept (links to a and b). For example, a 40-NTU formazin standard has an absorption coefficient of 0.01 cm^-1 and a scattering coefficient of 0.15 cm^-1 at 650 nm (red) light. The attenuation coefficient is therefore 0.16 cm^-1 and light intensity will decline by 63% for every 6.3 cm traveled in the formazin standard.
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The interaction of light with suspended particles and small-scale variations in the refractive index and density of the matrix that alters the direction of light transport in a sample without changing its wavelength. The scattering coefficient (b) is an IOP, defined as the fraction of energy scattered from a light beam per unit of distance traveled in a sample expressed in cm^-1. For example, the scattering coefficient for 650-mn (red) light in filtered drinking water is 0.00012 cm^-1. So, about 63% of the energy will be scattered out of a beam per 83,000 cm traveled in drinking water.
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Light having wavelengths between 760 nm and 2,000 nm.

The weight of particles in each of the various size ranges determined by sieve sizes or settling speeds given as a percentage of the total solids of all sizes in the sample.

Nephelometric turbidity units. A numerical scale for the cloudiness of water determined with a turbidimeter calibrated with a turbidity standard such as formazin or AMCO Clear.
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The spectrum resulting from the wavelength-by-wavelength products of source intensity, filter transmittance, and detector responsivity. The operating spectrum represents the relative contributions of individuals wavelengths to a measurement.
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The difference between the highest and lowest values of a parameter that can be detected by an instrument.

The irradiance in W cm² required to cause the photodetector amplifier in an OBS sensor to clip and not respond to changes in backscatter from particles. This is caused by mounting an OBS sensor in direct sunlight or near underwater objects that reflect direct sunlight onto a sensor, typically about 180 µW cm² or 0.2% of the sunlight in the visible-NIR band.

The angle between a light beam incident upon a scattering volume and light beam scattered from that volume is called scattering angle. Forward-scattered light fills the hemisphere surrounding the source beam and oriented away from the source (0 < θ < 90°) and backscattered light fills the opposite hemisphere (90o = θ< 180°).
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The rate of change of a sensor output per unit of input change. For example, if an OBS sensor output changes by one Volt for a change in SSC of 200 mg l^-1, its sensitivity is 5 mV per mg l^-1.

An electronic device that responds in a predictable way to light, heat, pressure, or conductivity.

This factor equals kD/2, where: the wave number, k, is 2 π λ ^-1; D is the particle diameter; and λ is the wavelength of the scattered light in the same units.

The mass of solid matter filtered from a water sample divide by the sample volume.

The percent change in a sensor output per unit change in temperature. For example, an OBS sensors with a temperature coefficients of + 0.05% per °C that reads 100 NTU at 25 °C, will indicate 101 NTU at 5 °C with no actual change in turbidity.

The cloudy appearance of water produced by light scattered from suspended particles and light absorption by dissolved matter and particles.

Light with wave lengths between 380 and 760 nm.

The volume scattering function, VSF, is the angular distribution and relative intensity of light scattered from a sample. In water, VSFs depend mainly on the size factor. Suspended particles with D > λ scatter nearly all light in a small, forward cone. About half of the scattered light is contained in a 10-degree cone oriented in the forward direction whereas less than 2.5 percent of it is backscattered.
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