Turbidimeter, nephelometric

These measure turbidity, usually by sensing light scattered at 90° to the direction of the incident light beam. This type of turbidimeter is frequently termed a nephelo-meter or nephelometric turbidimeter. A typical arrangement is shown in Fig. 6.46. The nephelometer is commonly used for detecting solid particles present in water (e.g. waste water) or in air (e.g. smoke). The instrument requires periodic cleaning due to fouling of the optical system. 

Nephelometric turbidimeters measure the radiant power, Isc, of the scattered radiation at 90° from the incident light path, a scattering angle that is least sensitive to the presence of relatively few large particles. A calibration curve is obtained by... 

Nephelometric turbidimeters are more accurate for measuring dilute suspensions and less sensitive to minor changes in instrumental design. Sensitivity increases with path length however, linearity is sacrificed at higher suspension concentrations and self-quenching can result in anomalously low turbidity levels.9 Obviously, dirty, scratched, or etched glassware, air bubbles, and vibration can all interfere with the accurate determination of turbidity. 

Defining these units of measurement and establishing the formazin standard for turbidity measurement has not prevented the use of other units. For instance, nephelometric turbidity unit (NTU), identical to formazin turbidity unit (FTU) and equal to the FNU, are displayed by many types of commercial turbidimeters used in Romania. Some instruments also indicate Jackson turbidity units (JTU) or the silica unit (mg/1 Si02) [6], that are still found as references today. 

For example, Getcher Fish s turbidimeter is aimed at the sport fishing market, so it does not have to be as accurate as similar instruments used for environmental measurements. At a level of 10 NTUs (nephelometric turbidity units), the measurement variation can be within 2.5 NTUs and still be acceptable. 

The nephelometric method involves illuminating the sample in a turbidimeter and measuring the amount of light scattered at 90 to the incident beam. The higher the intensity of scattered light, the greater the sample turbidity. A formazan polymer suspension is used as the standard and results are reported in nephelometric turbidity units (NTU). The greater precision, sensitivity, and wider applicability of this method make it preferable to the visual method. 

Operationally, the optical components used in turbidimeters and nephelometers are similar to those used in fluorometers or photometers. For example, the light sources commonly used are quartz halogen lamps, xenon lamps, and lasers. He-Ne lasers, which operate at 633 nm, have typically been used for light-scattering applications, such as nephelometric immunoassays and particle size and shape determinations. The laser beam is used specifically in some nephelometers because of its high intensity in addition, the coherent nature of laser light makes it ideally suited for nephelometric applications. 

Nowadays, optical instruments, known as turbidimeters, provide objective measurements of light diffused in a given direction. If the measurements are made perpendicularly to the incident light, the apparatus is called a nephelometer. Results, expressed in NTU (nephelometric turbidity units), are correlated with the wine s appearance (Section 11.3.1). These instruments are very sensitive, which is especially useful in assessing the effectiveness of a treatment, e.g. filtration. 

Turbidity is measured by appreciating the disturbance in the diffusion of light caused by contact with particles in a liquid. A turbidimeter measures the intensity of the diffused light. A turbidimeter that makes measurements at a 90° angle is also known as a nephelometer. These apparatus are calibrated in NTU (nephelometric turbidity units). 

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