Water monitoring

meet the standard guidelines are comparable to the results of any other turbidimeter that meets the same guidelines. Both standards use a scattering angle of 90°,

which is called nephelometric arrangement, but each of them use a different wavelength and different prescriptions on the geometry of the illumination and detection. Using detection at 90° reduces the effect of stray light and absorption. The EPA standard was designed to harmonise

the design of turbidimeters that used incandescent light sources, with illuminati-on wavelengths peaking in the green region of the spectrum. These devices work well at low turbidity values, owing to the shorter wavelength. However, they suffer some limitations when

significant amounts of dissolved organic matter are present, as shorter wavelengths are absorbed quite effectively by the dissolved organics. EPA measurements are expressed in Nephelometric Turbidity Units, or NTU. The ISO standard, on the other hand,

stipulates the use of light in the near infrared region. Using a longer wavelength reduces the effect of absorption by organic contaminants on the measurement significantly. Another advantage of the ISO design is that the spectral distribution of the illumination source is more tightly defined, as well as the optical layout, reducing the possibility of differences in reading between instruments designed to this standard. ISO measurements are expressed in Formazin Nephelometric Units, or FNU. As devices either of the methods will be

measuring in different ways, it follows that any readings obtained will be different as well. It is not uncommon to see situations in final water measurement where an ISO method is measuring 0.6NTU whilst the equivalent EPA method might measure higher at 0.8 NTU. Neither instrument is wrong. Instead, the

variation can be attributed to the different wavelengths used by the two techniques.

Top Tips for opTimum measuremenT accuracy

Whichever method is used, there are certain key steps that can be taken to ensure that sensors used for measuring turbidity and total suspended solids offer maximum performance and accuracy.

1. should a flow cell be used?

For applications of lower suspended solid concentrations (perhaps below 1,000mg/l), a flow cell may be preferred. The use of a flow cell allows a sufficient and consistent flow rate to be established with ease past the sensor optics. A sufficient flow rate is important for ensuring sampling of the entire particle size distribution, whilst a consistent flow rate is important in terms of minimising the impact of air bubbles on the optical measurement. Furthermore, the use of a flow cell designed specifically for turbidity monitoring limits the influence of both refraction and stray light on the measurement, improving accuracy and repeatability. In the absence of pressure head or gravity, a pump would be required to move sampled water

Instrumentation Monthly November 2018

in to the flow cell for measurement. In this setup, costs rise as the pump will need to be maintained throughout the operational life of the instrument. A further by-product of using a flow cell is water loss; following measurement, sampled water is often sent to drain rather than being recovered. Where use of a flow cell is seen as sub-optimal,

for example in high suspended solids applications or for reasons such as those mentioned earlier, other methods for mounting the sensor can be considered. Alternatives include dip mount systems, chain mount immersion systems, and hot tap retractable assemblies, all of which remove the need for a pump. In all cases, however, sensor orientation and positioning within the flow path remain a critical consideration for obtaining good quality and consistent data.

2. is The sensor mounTed and orienTed correcTly?

Proper orientation of sensors is vital to maintaining quality data when monitoring turbidity and suspended solids. For both NTU and FNU measurements, the sensor face is at a 90-degree angle relative to the incident light beam. The

sensor face is placed in this position as it is most sensitive to the scattered light coming from the suspended particles. Particle size and distribution are not affected by this. With the sensor face being in a 90 degree fixed

position, it is imperative that the actual sensor is positioned correctly as well. Dip mounting is one of the most common

methods used to measure turbidity and suspended solid levels, if the required space is available. In an ideal scenario the measuring end of the sensor should be placed mid-channel of the liquid being monitored. Having the sensor pointing away from the chamber surface also helps to limit light refraction, which can impair measurement data. To ensure optimal data measurement, the

measuring end of the sensor should also be a minimum of 300mm or 12 inches below the surface of the fluid being measured. If the channel is too shallow, an angled elbow will be required to make sure the sensor is fully immersed in the sample. Using an angled elbow can also help to reduce the build-up of air bubbles on the sensor face if the sample is heavily aerated.

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