Turbidity Meter

The term turbidity is used in different meanings. For example, turbidity can refer to the reduced gloss of surfaces, but it can also refer to deteriorated prospects for success or to psychological moods. Frequently, the term turbid is associated with weather phenomena and with the optical perception of liquids and gases. In this case, turbidity stands for less translucent.

The more turbid liquids or gases are, the more undissolved particles they contain. These particles can be normal components of the substance, for example in cloudy air, drinking water or milk. On the other hand, turbidity can also be caused by foreign substances. Since turbidity is directly related to the number of particles contained, conclusions about the particle content in liquids can be drawn by comparing the light transmission.

In the meantime, special photometers with measuring cuvettes are mainly used to determine the intensity of turbidity; these are also referred to as turbidimeters for this application. Possible applications for such a turbidimeter include, for example, checks on drinking water and wastewater, but also liquid analyses for beverage production and in the pharmaceutical and chemical industries.

Turbidity measurement methods can be roughly divided into quantitative and semi-quantitative methods. The semi-quantitative methods include those measuring methods in which visual impressions of the tester are also important for the result. In the case of optoelectronic methods, on the other hand, which belong to the quantitative methods, the user's vision has no direct influence on the measured values.

Semiquantitative turbidity measurement
Semiquantitative methods are mainly used to measure the transparency of water bodies. With these methods, the turbidimeter does not measure a turbidity value, but usually the depth of visibility. The depth of visibility is the distance to which light can penetrate the water. Outdoors, the measurements are usually made in daylight, and in the laboratory – under normal room lighting. The turbidimeters used in this process therefore do not require their own light source.

In deep, stagnant waters, a Secchi disk is lowered into the water until its black and white colour markings are indistinguishable. This depth can be marked on the line and then measured. For shallow waters and for flowing waters, the water is filled into a measuring tube instead. At the bottom of this measuring tube there is the same colour marking, a circle with the circle quarters alternating in black and white. In contrast, transparency test tubes are used for drinking water tests in the laboratory. Their use is described in the ISO 7027-2 standard.

Optoelectronic turbidimeters according to ISO 7027-1
The international standard ISO 7027-1, which has been converted into national standards in many countries, describes two quantitative methods for turbidity measurements to determine water quality. Both methods use the turbidimeter that transmits light through the sample and then measures and evaluates the light intensity. The first method is when a light sensor is used to measure the light scattered at 90 °. This measurement arrangement should preferably be used for only slightly turbid water, in order to avoid the measurement errors caused by multiple scattering.

The second method of ISO 7027-1 measures the light that has passed through the sample without changing direction, i.e. that has not been scattered or absorbed. For this purpose, the light sensor is located on the other side of the sample directly opposite the light source. In the specifications of these turbidimeters, the measuring angle is specified as 0° and the measuring method – as transmitted light measurement. This method is also suitable for more turbid water.

Industry-specific turbidimeters
For concentration and purity measurements in the food and beverage industry, optoelectronic turbidimeters have been developed that are specially adapted to the respective examination of the substances. If you need an instrument for such measurements, please be sure to obtain detailed information on which choice is best suited for your measurement in terms of wavelength, measuring angle, measuring volume, measuring vessel and other specifications.

Suitability for mobile or stationary use
It is almost always clear in advance whether the turbidimeter is to be permanently installed for the measurements, permanently set up at a specific location, or transported more frequently for on-site measurements. Most instruments are designed and equipped for only one of these applications.

Light source
The light source plays a role in the measurement result because of the wavelength of the light. While visible light can be used for turbidity measurements for certain areas of use, infrared light is recommended for the majority of applications. There are also models of turbidimeter, with multiple or adjustable light sources.

Measuring angle
The measuring angle used has a great influence on the measured light intensity. Which measuring angle is best suited for an application depends, among other things, on the amount of particles present, but also on their size, shape and colour. Some turbidimeters offer both 0 ° transmitted light measurement and 90 ° scattered light measurement according to ISO 7027-1 for turbidity measurement in water. Other instrument types measure specifically at 11 ° or 25 ° scattered light or the reflection at 180 ° because this is the most accurate way to determine the concentration of certain foreign particles in the substance under examination.

Measuring range and units of measurement
Important specifications always include the measuring range and the associated units of measurement. Choose a turbidimeter that can output the values in the unit you need and make sure that the measuring range fits your application.

Calibration options
Many turbidimeters can be calibrated with instrument-specific calibration standards and can also be adjusted via the instrument software in case of deviations. If there are high requirements for the measurement accuracy, select an instrument with several calibration points and matching calibration standards.

Memory, interfaces
If the turbidity measurement values are to be stored in the instrument, pay attention to how large the instrument memory is and whether the data can be read out from the memory via an interface.

Determination of water quality is important and is carried out not only by means of the visual analysis, but also with the help of certain tests and application of the auxiliary measuring devices. Turbidity is one of the parameters that indicate what kind of water is under test and whether is may be appropriate for particular applications. This parameter is directly dependent on the presence of the substances of the organic and non-organic origin. There may be various substances that do not get dissolved in the water and the level of turbidity depends on their amount and density of the accumulation. When measuring turbidity, it is essential to detect not the fact of turbidity, but the amount of the substances which became the reason for turbidity in the particular sample.

Dirt, living organisms, contaminants, minerals, metal particles, organic elements – all these and many other particles may be present in the water, whether it is surface water, or groundwater, or sewage water that is likely to penetrate into the ground and bring additional elements into the already present ones.

Determination of turbidity is absolutely essential at the water treatment plants, in the foodstuffs production or wastewater facilities. For each particular branch different parameters and norms of turbidity have been set. For example, for consumption, water should be not only optically appropriate, but also safe, and that is why the turbidity should be at its minimum level. The measuring principle and determination of turbidity has been taken into consideration by the Health Organizations and there is ISO standard covering the issue of water quality, in particular turbidity determination.

Since a few measuring methods have bene applied and there are a few measuring units used for description of turbidity, it may seem to be a bit challenging task to choose the correct one for the particular application.

Among the most famous methods is a photometric method, with the formazine suspension taken as standard. In this case, the unit is FTU and is in mg /l or turbidity unit per dm3. Among the commonly similar applied units only for formazine are also FNU and NTU (Formazine Nephelometric Unit and Nephelometric Turbidity Unit correspondingly). Besides, there is another group of the turbidity units, which refer to the concentration of the exact substances. Here, no standard may be applied, since each particular technology may have its peculiarities and it is necessary to take into consideration each individual measuring task.

Even though, all the devices seem to carry out the same measurement, there are differences which make the measurements carried out by different meters not comparable. It is impossible to always provide the same source of light for the measurements in different samples and obviously the devices with different probes cannot be positioned in the same way. Also, the number of the particles and their nature are very different and the scattered light coming from these particles also has different angles. Principally, the difference between measuring devices for turbidity is the wavelength of the light and the angles at which the devices are applied. For example, the above mentions FNU and NTU are basically the same (measurement at 90 °), but the light sources will be different in a way that it is infrared for the FNU (in compliance with European ISO norm 7027) and white light for NTU (in compliance with EPA method).

Depending on the light source and the angle, formazine units, relatively speaking, may be classified into separates subclasses. The angles may be 90°, 90°+X (besides nephelometric detector at 90° the other detectors are applied at other angles 135°, 45°, providing a wide measuring range and requiring special ratio), 180°, backscatter light, and depending on that the unit change, for example, FBU for formazine backscatter light or FNRU for formazine nephelometric ratio unit.

A special attention to the light wavelength is required when collecting the data, since the white light and IR light do not give comparable results. Application of the white light source may require the application of the light colour filter, to compensate the influence of the coloration of the sample under test, because the latter may cause a serious error in the measurement results. At the same time, the coloration of the sample does not influence the measurements when the light source is IR.

Turbidity definitely spoils the optical look of the water. Besides that, the higher the turbidity, the quicker the growth of the bacteria in the water may be, since turbidity creates the most favourable conditions for them. Detection of the turbidity is easy; the measurement of it is a task requiring the correct choice of the conditions and equipment. The type of sample, conditions (light source and the wavelength) and the measuring device (its positioning and the number of the detectors) influence the result of the measurement.

The turbidity of water can be caused by organic or inorganic foreign substances. Not in every case, the reduction of light transmission in drinking water is a sign of poorer quality. The degree of turbidity determined by the turbidity meter alone does not usually allow any conclusions to be drawn about the nature of the foreign particles. Therefore, other values are recorded in addition to the turbidity. However, the measured turbidity value is in any case an important indicator value for the content of dissolved additives or undissolved particles.

Typical measured values for the turbidity of water are listed in the table below. Even with a high-precision turbidity meter, realistic values below 0.016 NTU cannot be measured for water because water molecules also cause turbidity.

Limit value of turbidity according to Drinking Water Ordinance
The Drinking Water Ordinance (TrinkwV Annex 3, Part I, No. 18) stipulates that the water at the outlet of the waterworks may have a maximum turbidity of 1.0 NTU. All the turbidity values determined with the turbidity meter on the treated drinking water must be below this limit. Sudden or continuous increases must be reported immediately by the operator of the water supply system to the appropriate authority to avoid endangering the consumers. Turbidity is one of the values that must be routinely tested. The frequency of checks depends on the volume of water discharged. The higher the volume of water, the more frequently the indicator parameters and foreign substances must be determined so that no one is harmed by the contaminated water.

In-house measurements of the turbidity value during the drinking water treatment
Even though the raw water used for the drinking water production is usually of good quality, it is treated. During the treatment, undesirable constituents or those present in excessive concentrations are removed by means of suitable processes. The treatment can be adjusted to the initial values and the desired target values by means of the measurements. The values for the parameters to be monitored are checked and documented several times. For the turbidity value determined with the turbidimeter, an in-house target value of 0.2 NTU applies in most waterworks.

A statistical evaluation of 17,684 samples published in the 2008 Drinking Water Report for NRW showed that 50 percent of the measured values for the turbidity were below 0.07 NTU and 90 percent of the samples were below 0.23 NTU. Only 0.9 percent of the values were above the limit of 1.0 NTU. However, the maximum value was 43.6 NTU. High turbidity values are caused, for example, by fine suspended solids such as silt and organic particles or by discolouring constituents such as iron and manganese. To remove these components, the raw water is filtered. If microbial contamination is present, disinfection processes are also used.