Turbidity Measuring Facs & Knowledge
Measuring Principles
Standards & Guidelines
Turbidity Measuring: Direct light Principle
Direct beam turbidity measurement is probably the most used turbidity measuring technique and is suitable for measuring ranges of approximately 40 up to 40000 TEF. The turbidity is measured typically in NIR spectrum at a wavelength of approximately 750nm, then color influences (colors absorb within the visible spectrum between 400-700nm only) cannot affect the turbidity values. This measuring method is e.g. recommended in the German standard DIN 38404 part 2 and its European version EN 27027.
The standard recommends a manual measurement method to measure turbidities above 40 NTU, using a transparent cylinder and an insertable disc. Using 90° scattered light method is recommended to measure lower turbidity values. The standard contains as well a detailed procedure for the production of a 400 NTU Formazin suspension and a guideline for the calibration of turbidimeters.
Typical turbidity units based on the principle of direct light measurement:
% (percent transmission) A* (spectral Absorption value)
* The spectral Absorption value will be also designated as EXT (Extinction), CU (Concentration Unit), AU (Absorption Unit).
Turbidity Measuring: Scattered light Principle
The most common scattered light turbidity measuring methods are:
- Side scattering (90°) The detector is positioned in a right angle (90°) to the light beam
- Forward scattering (12°) The position of the detector is 12° shifted to the axis of light beam
An intense collimated beam of light is projected through a sample contained within the sensor. The intensity of this light beam is measured by the direct beam detector, located opposite to the light source. The light, scattered by particles inside the sample is measured by a scatter light detector Depending by sensor specification, this detector is located 12° and/or 90°, displaced from the beam axis.
The ratio of scattered and direct light signals, compensate for changes in product color, lamp ageing and window coatings. The signals will be processed as followed:
Forward scattered light / Side scattered light = Turbidity value
The particles inside the liquid flow decrease the intensity of direct light, and increase the intensity of the scattered light, that means the turbidity rises. Product color, lamp ageing and window coatings decrease the intensity of direct and scattered light in same ratio that means the turbidity value stays constant.
Comparison of the different measuring methods
The two different measurement methods (12° forward scattering / 90° side scattering) are not comparable.
The deviations of the measuring results are caused by the different particle size distributions in the measured liquid. Both measurement methods respond different, depending by particle size distribution in the sample.
Please take notice:
Make sure that your turbidity meters operate according to the same principle of measurement, if you compare measuring results.
You can compare 90° vs. 90° scattered light turbidity meters, or 12° vs. 12° scattered light turbidity meters.
Never compare turbidity meters with different methods of measurement!
Context between particle size, measurement methods and measurement results
The most common Calibration standard for turbidity is based on Formazin liquid.
In case of calibration with Formazin, 12° forward- and 90° side scattered turbidy meter will provide identical measurement results with Formazin.
By observation of a real sample, such as a filtrated liquid, the different methods will show different measurement results.
The measuring results of the 90° side scattered method in this case is typically by factor 3 to 10 above the measuring results of the 12° forward scatter method.
The reason is, that filtrate typically contains a higher number of small particles and a few large particles only.
The small particles will be overvalued with the 90° method, due to the fact that this method is more affected by the quantity of the particles and not by particle size.
The 12° method gets its highest scattering signal large particles inside the filtrate the small particles create a small signal only.
90° side scattered method:
Is more relatet to the number of particles, because small particles create low scattered light intensity and large particles create low scattered light intensity as well.
12° forward scatter method:
Is a more mass related, because a small particle creates low scattered light intensity and a large particle creates a high scattered light intensity.
At a particle size of approx. 0.3 µm (Formazin) both methods will create about equal scatter light intensities.
The combination of both measurement results informs about the tendency of the particle size distribution.
In case the 90° measuring results are higher compared to the 12° measuring results, means that the average particle size is smaller than 0,3 µm.
In case the 90° measuring results are lower compared to the 12° measuring results means that average particle size larger as 0,3 µm.
DIN/EN 27027 (ISO 7027)
International Organization for Standardization: ISO homepage
Deutsches Institut für Normung E. V. (German Institute for Standardization): DIN/EN homepage
The standardization work DIN supports the economical goal of a Common Market in Europe as well as a global market, free of technical obstacles. This requires an European and an international standardization. The European standardization is accomplished by three organizations CEN (Comité Européen deNormalisation), CENELEC (Comité Européen de Normalisation Electrotechnique) and ETSI (EuropeanTelecommunications Standards Institute).
National member organizations co-ordinate the European standards and implement these. Standardization organizations have – excluded ETSI – for each country only one member, who has to represent the entire interests of standardization of his country. The vote of the members during an election depends by the economic power of their nation. German interests are represented by DIN (German Institute for standardization).
The DIN EN 27027 is the standard for turbidity measurement of potable water in Europe. Turbidity measurement acccording EN 27027
Turbidities lower than 40 FNU | |
Principle of measurement: | ratio measurement: scatter light/transmission |
Measurement angle: | 90° +-2,5° |
Beam dimension: | 0° with maximum 1,5° convergence |
Measurement wavelength: | 860nm / tolerance +-30nm |
Calibration standard: | Formazin |
Turbidities higher than 40 FNU (see chapter „direct light tubidity“ ) | |
Principle of measurement: | Absorption (direct light measurement) |
Measurement angle: | 180° |
Beam dimension: | 0° with maximum 1,5° convergence |
Measurement wavelength: | 860nm / tolerance +-30nm |
Calibration standard: | Formazin |
EPA
Turbidity Standards and Guidelines
Environmental Protection Agency: EPA homepage
The US Environmental Protection Agency is an organization of the government of the United States, with the responsibility to coordinate and observe the protection of the environment and the protection of the human health. EPA was founded under president Nixon on 2 December 1970. Seat of the organization is Washington, D.C. The administrator of the EPA is Stephen L. Johnson, its representative is Marcus Peacock. The organization counts currently approximately 17,000 employees.
One of the major areas of reposibility is to accomplish the environmental laws. The Office OF Criminal Enforcement, Forensics and training, shortened OCEFT, has its own execution and determination authority since 1982. It received the full police authorization in 1988. (OCEFT homepage)
EPA standard for turbidity measurement of potable water is different from the standard of the ISO/EN/ or DIN.
Turbidity measurement according US EPA standard.
Principle of measurement: | ratio measurement: scatter light/transmission |
Measurement angle: | 90° (no tolerance specified) |
Beam dimension: | not specified |
Measurement wavelength: | full spectrum of a tungsten lamp |
Calibration standard: | Formazin |
MEBAK
Turbidity Standards and Guidelines
Mitteleuropäische Brautechnische Analysenkommission (Central European brew-technical analysis commission): MEBAK homepage
MEBAK members are mainly the national and private brew-technical institutes of Germany, Austria, Czech Republic and Switzerland as well as the national breweries and their suppliers.
Purpose of the association is the development of basic procedures to guarantee the quality of raw materials, intermediate, secondary and finished products as well as in the development of tools and equippement preferably used for malt- or beer production.
The MEBAK published a five volume edition of brew-technical analysis methods, which are internationally established and used as teaching material for the education and training of the brewers in institutes like technical university Munich Weihenstephan, Doemens institute or Charles University in Prague.
Stability of beer (90° Side scatter)Recommendation | |
Principle of measurement: | ratio measurement: scatter light/transmission |
Measuring angle: | 90° +-2,5° |
Beam dimension: | 0° with maximum +-1,5° tolerance |
Measuring wavelength: | 650nm / tolerance +-30nm |
Calibration standard: | Formazin / alternative: AEPA- turbidity standard (Styrene Divinylbenzol) |
Filtration control (forward scatter) in addition to the above specified 90° measuring angle | |
Principle of measurement: | ratio measurement: scatter light/transmission |
Measuring angle: | 11° – 25° |
Beam dimension: | not specified |
Measuring wavelength: | not specified |
Calibration standard: | Formazin |
Lauter wort (forward scatter)Recommendation | |
Principle of measurement: | ratio measurement: scatter light/transmission |
Measuring angle: | 11° – 25° |
Beam dimension: | not specified |
Measuring wavelength: | not specified |
Calibration standard: | Formazin |
EBC
Turbidity Standards and Guidelines
European Brewery Convention: EBC homepage
The European Brewery Convention was founded in 1947 and supports the brew-scientific activity. Goal of the EBC is, the development of analytic methods to guarantee beer quality and
to ensure continuously high quality of raw materials and brewing procedures. Parameter like turbidity measuring, color value of beer are measured in EBC units.
The current technology and engineering forum has been founded 1990 by the EBC, with the task to develope simple technical procedures to optimize production an management in malt-houses and breweries. Successful work has caused the standardisation processes as well as the optimization of the hygienic design of process equippement and therefore product quality.
Stability of beer (90° Side scatter) | |
Principle of measurement: | ratio measurement: scatter light/transmission |
Measuring angle: | 90° (no tolerance specified) |
Beam dimension: | not specified |
Measuring wavelength: | not specified |
Calibration standard: | Formazin / alternative: AEPA- turbiditystandard (StyrolDivinylbenzol) |
Filtration control (forward scatter) in addition to the above specified 90° measuring angle | |
Principle of measurement: | ratio measurement: scatter light/transmission |
Measuring angle: | 11° – 13° |
Beam dimension: | not specified |
Measuring wavelength: | not specified |
Calibration standard: | Formazin / alternative: AEPA- Standard (Styrol-Divinylbenzol) |
ASBC
Turbidity Standards and Guidelines
American Society of Brewing Chemists: ASBC homepage
The ASBC publishes like the EBC standard procedures to ensure quality of raw materials, brewing procedures and beer. ASBC turbidity units are not common in Europe and has been replaced partially as well in the USA by the EBC turbidity unit.
Principle of measurement: | not specified |
Measuring angle: | not specified |
Beam dimension: | not specified |
Measuring wavelength: | 580nm (no tolerance specified) |
Calibration standard: | Formazin / measuring unit ASBC1 |
1 conversion rate: 69 ASBC = 1 EBC
IOB
Turbidity Standards and Guidelines
Institut of Brewing & Distilling: IOB homepage
The Institute of Brewing & Distilling (IOB or IBD) is an organization engaged in the education of brewers and distillers. IOB recommends as well as EBC and ASBC guidelines for turbidity measurement instruments, which agree with the EBC guidelines.
Principle of measurement: | ratio measurement: scatter light/transmission |
Measuring angle: | 90° (no tolerance specified) |
Beam dimension: | not specified |
Measuring wavelength: | not specified |
Calibration standard: | Formazin / alternativ: AEPA- turbidity standard (StyrolDivinylbenzol) |
Filtration control (forward scatter)(in addition to the above specified 90° measuring angle) | |
Principle of measurement: | ratio measurement: scatter light/transmission |
Measuring angle: | 11° – 13° |
Beam dimension: | not specified |
Measuring wavelength: | not specified |
Calibration standard: | Formazin / alternative: AEPA- turbidity standard (StyrolDivinylbenzol) |
Calibration Standard Formazin
Formazin is a poisonous chemical substance!
Please take notice to the safety instructions before handling Formazin. Formazin is an aqueous suspension of an insoluble polymer and results from the reaction between a Hydraziniumsulfate and a Hexamethylentetramine. The median particle size of Formazin is 1,5 µm and the standard deviation of size is 0,6 µm.
A 1000 EBC / 4000 FTU Formazin suspension is recommended to make calibration standards for turbidimeters. The 1000 EBC / 4000 FTU suspension has a better shelf-life of up to 6 months compared to lower concentrations. In case you follow EN27027 (ISO 7027) you have to use a 400 TEF suspension to prepare your calibration standards*.
Stability:
The stability of the Formazin suspension depends on the storage conditions. Formazin should be stored in darkness at a temperature of 8°C – 10°C to guarantee maximum stability. These suspension has to be shaked carefully for at least 3 minutes before use.
After shaking you have to wait for approximately 15 minutes to allow a degassing of the suspension. Then you have to stir the suspension carefully for homogenisation. Make sure that no air bubbles occur during stirring of the suspension. Now you can make calibration standards with turbidities lower than 1000 EBC / 4000 FTU by diluting the suspension.
Please take notice:
- It is very difficult to prepare calibration standards lower than 0,5 EBC, this standards will be typically made by using specially distilled and filtered water and a high precision scale.
- Diluted calibration standards are substantially less stable than the 1000 EBC suspension. They should prepared and used immediately at the same day*.
- The turbidity of Formazin is named in different measurement units (FAU, FTU, FNU, NTU, EBC, ASBC, ….)
The undiluted suspension has a turbidity value of:
- 1000 EBC (European Brewery Convention)
- 4000 TEF (Trübungseinheiten Formazin)
- 4000 FTU (Formazin Turbidity Units)
- 4000 FNU (Formazin Nephelometric Units / unit used for 90° side scatter turbidimeter only!)
- 4000 FAU (Formazin Attenuation Units /according to EN27027 direct light/absorption for turbidity >40 FNU)
- 40000 Helm units
- 69000 ASBC (American Society of Brewing Chemists)
* suspension of 100 EBC (400 FTU) has a shelf-life of maximum 2 weeks according DIN/EN 27027 (ISO 7027) guideline.
Diatomatious earth SiO2
Please take notice to the safety instructions before handling of Diatomatious earth
Diatomatious earth (DE), is a whitish, powdered substance, which consists mainly of the silica dioxide shells of fossil algae (Diatomeen). The Diatomatious earth is used frequently for the calibration of turbidimeter. You can find different conversion factors in the literature compaired with Formazin, because Diatomatious earth is available in different qualities and particle sizes. The dry Diatomatious earth powder has the advantage that it can be stored for an almost unlimited period of time.
Please use the prepared calibration standard for 1-2 hours only, because Diatomatious earth pours in water. Take notice to a homogenious diviation during a calibration with Diatomatious earth (frequently stirring) because Diatomatious earth has the tendency to sedimentate. Diatomatious earth is available from different vendors in different qualities and particle sizes you have to use the same vendor, quality, particle size and batch number to reproduce your calibration.
Typical calibration standards based on Diatomatious earth: JTU* (Jackson Turbidity Unit) JCU* (Jackson Candle Units ppb (parts per billion) ppm (parts per million) g/l (gram per litre) mg/l (milligram per litre) %TS (percent total solids)
* The units JTU and JCU are based on the Jackson Candle Turbidimeter (Jackson candle tube) the forerunner of modern forward scattered light turbidimeter and not very common anymore.
