Product category:
Liquid Analysis: Turbidity, Particles
News Release from: Siemens Automation and Drives | Subject: AquaScat
Edited by the Processingtalk Editorial
Team on 03 May 2007
Turbidity - in search of true values
John Kupczak of Siemens outlines the calibration and service problems on turbidity meters, and offers the AquaScat, which uses a contactless free-falling water stream to eliminate window fouling
Turbidity is a vitally important measure in determining water quality, particularly in light of increasingly stringent regulations, but how can the water industry ensure accurate measurement with resources ever more stretched? Growing environmental pollution by industry and traffic, high population density and intensive agriculture are all having an effect on water quality Reports of impure drinking water and resulting disease are unfortunately more and more common and many countries have responded, or been forced to respond, by tightening their guidelines for drinking water quality
This article was originally published on Processingtalk on 20 Mar 2006 at 8.00am (UK)
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Turbidity is created in liquids by solid particles.
When a light beam is passed through the sample, particles scatter the light and a measure of the scattered light intensity provides a turbidity measurement which can be used to determine water quality.
Turbidity is a key indicator because studies have shown a clear correlation between particles and the number of bacteria in a water sample.
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In particular, turbidity measurements can indicate the presence of cryptosporidium, a water-born parasite often found in water treatment works close to sheep and cattle: In recent years there have been numerous outbreaks of this parasite, causing severe diarrhoea and sickness.
As a result, the European Union's 1998 Drinking Water Directive was implemented in the UK as the Water Supply (Water Quality) Regulations 2000 in England and set a non-mandatory indicator parameter value for turbidity of 1NTU in the water leaving treatment works.
Whenever this value is exceeded, the Regulations require the water company to investigate the cause of the excess and report the results to Drinking Water Inspectorate.
Enhanced reduction of turbidity would help to ensure that the basis for bacterial growth is reduced.
Ideally, turbidity should remain at the lowest possible levels well below 0.5 NTU, and often down towards 0.1 NTU.
Increased regulation has brought increased pressure for the water industry to ensure that standards of water quality are kept high but traditional turbidity meters have a series of problems.
All Turbidity monitors suffer from zero point drift, and therefore need regular calibration to counter this.
The calibration regime set by most manufacturers is usually 1 to 3 months, by which time is can be extremely difficult to measure the increasingly lower values < 0.1NTU.
Such values can easily be masked by this zero point drift.
A second problem with traditional instruments, and in particular closed flowcell type turbidimeters, is the gradual fouling over time of the optical windows, even in apparently clean water samples.
Immersed probes can foul even faster simply because of flow patterns around the sensor often leading to an enhanced build up of deposits, from dissolved substances.
When turbidity is measured by determining the side scattered light density, a light beam is passed through the flow cell and the scattered light produced by the solid particles is detected at a specific angle.
However, if the medium contains absorbent substances, such as coloured constituents, these can easily attenuate the incident light beam by factors ranging from 2 to 5 and reduce the scattered light intensity thus falsify the results.
Another major problem with traditional turbidity instruments is the need for regular calibration, particularly as legislation requires that an individual meter should be used on outlet of each filter.
Since all turbidity meters suffer from calibration drift which is detrimental to zero point stability as mentioned above, costs of such calibrations can be considerable and require significant manpower resource; (monthly or three monthly calibrations are normal).
Instruments will give false high values due to zero point shift, if such calibrations are not performed, thus as an operator you will be reporting higher turbidity values than you need to.
Furthermore, there is a need for high resolution instruments when measuring ultra low turbidity values, particularly as turbidity values as low as 15mFNU are now common in UK WTW filter outlets.
Errors of 100% are not uncommon in these measurements, the principle cause being zero point drift.
One recent solution to the problems outlined was non-contact falling stream measurement which, while offering automatic calibration and ultra low costs of ownership did not suit all the applications for turbidity monitoring.
It is clear that a new solution is necessary for turbidity measurement in the water industry.
Modern sensors with semi-conductor light sources and microprocessor controls are capable of maintaining reliability and ensure maintenance free measurement.
To cover as many turbidity applications as possible any solution must be able to monitor samples under pressure.
They must have an option for reduced flow rate; use the latest DWI approved materials; offer manual calibration if required.
Any new solution should have an option to measure higher Turbidity, have a smaller footprint, and offer reduced costs.
An answer may lie in a new generation of turbidity meters, such as the AquaScat from Siemens, which utilises a contactless free-falling water stream that eliminates all problems of window fouling and, as a result, minimises service.
Furthermore, as stray light is reduced significantly it becomes possible to measure extremely low turbidity levels precisely, even below 0.01 FNU.
The AquaScat meter is capable of measuring 90 degree scattered light to ISO 7027 the international standard for water quality.
The key performance criteria of a contactless, free falling turbidimeter is factory calibration and a dual-beam measurement method which ensures the calibration remains stable, reducing the need for time consuming servicing.
Water quality is becoming increasingly important issue, particularly as the threats to drinking water quality are increasing all the time.
Reports of impure water are more frequent and traditional turbidity instruments are unable to keep pace with the threat.
Fortunately, the new generation of instruments are able to respond to the threat.
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