Turbidity Measurement

What is Turbidity?

Measuring turbidity is important for ensuring the quality of the final product, but also for monitoring and controlling individual process steps in the beverage and food industries.

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With a reliably measured turbidity value obtained during liquid analysis, you can sustainably increase the efficiency of your process.

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Depending on the application, the measurement of turbidity is a colour-compensated one- or two-angle scattered light measurement or a simple absorbance measurement.

Turbidity is a measure of the degree to which the water loses its transparency due to the presence of suspended particulates.

How to select turbidity sensors?

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Sensor selection depends on the application:

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• Nephelometric sensors for media with low turbidity such as potable water.
• To measure high turbidity, for example in wastewater, apply suspended solids sensors.
• For applications with extreme fluctuating turbidity values use absorption sensors to attain unambiguous measurement results e.g. to reliably detect product losses.

Turbidity Standards

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• The turbidity standards are an equally important portion of the measurement.
• Most modern turbidity standards are made from formazine, a synthetic polymer with a uniform particle size.
• The polymer is made from hydrazine and hexamethylenetetramine.
• The consistency of this compound has led to its adoption by nearly all standards organizations, such as ISO, EPA, and ASBC.
• A suspension of 1.25 mg/L hydrazine sulfate and 12.5 mg/L hexamethylenetetramine in water has a turbidity of one Formazin Turbidity Unit (FTU).
• Nephelometric Turbidity Units (NTU): Unit that is equal to FTU but measured using a turbidity meter design that complies with EPA standards.

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General aspects

Turbidity analysis provides information about the performance of filtration processes but is also used to ensure product quality in a variety of industries.

Turbidity, also called haze, is not a well-defined physical entity like temperature or the density of a liquid, but is always expressed with reference to a well-defined standard. Haze has nothing to do with color.

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The juice shown on the left side in the photograph is turbid, the juice on the right is clear, but both are yellow: the color is the same, the turbidity is different.

Turbidity measurement is an optical analysis method, which detects the intensity of scattered light emitted by a light source. The intensity of scattered light is detected by photodetectors positioned at specific angles around the measuring cell.

The turbidity result reported for a substance strongly depends on the method used to determine it. The methods for determination of turbidity in a liquid often differ due to the following parameters:

Turbidity measurement

Wavelength of light source

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The wavelength of the light, emitted by the incorporated light source, is a significant parameter which influences the reported turbidity. Which wavelength is used is specified by the standard the measurement needs to comply to.

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Whereas a light source emitting light at 650 nm is common for the analysis of beverages, other wavelengths are used depending on the application they serve.

Evaluation of signals

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Light scattering which is measured to derive a turbidity value strongly depends on the size of the particles in the sample to be measured.

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Scattered light at 90° is used to detect the presence of small particles.

25° (forward scattering) is used to detect large particles.

Transmission at 0° is used to compensate the sample’s color.

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Using the analysis of all three of these angles in one measuring method enables turbidity determination which considers particles of any size, independent of the color of the sample.

Measuring temperature

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Turbidity greatly depends on temperature.

Turbidity increases with decreasing temperature. Depending on the sample and its ingredients this effect can change.

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The analysis of spirits such as whisky is suitable as a practical example:

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At low temperatures, the fatty acids and their esters contained in spirits partially precipitate forming a haze as they are less soluble at lower temperatures. This unwanted occurrence is called “chill haze” and should be avoided as consumers prefer a chilled drink which is clear.

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Due to this effect, turbidity analysis of samples at low temperatures is common.

Turbidity measurement with absorption sensors

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Absorption turbidity meters measure the light absorbance of particles in a medium. They feature an LED lamp and a light detector that is positioned directly opposite the lamp.

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Between them, particles in the medium attenuate the emitted light, which is converted by the detector first into an electric signal and then into the final turbidity value.                                                

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Turbidity measurement with nephelometric sensors

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• Sensors based on nephelometry measure turbidity according to ISO 7027 / EN 27027.
• LED lamp and a light detector that is positioned in a 90 degree angle.
• The lamp emits a beam into the medium, where it generates a scattered light as soon as it hits a particle in a defined area.
• The detector converts the received light intensity into an electric signal.
• This signal is then translated into the final turbidity value and can be displayed by the transmitter in the desired unit.

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Total solids and suspended solids measurement

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• Total solids sensors or suspended solids sensors use the back scattered light method.
• LED lamp and two light detectors positioned in 90 and 135 degree angles.
• Solid particles in the medium cause the incident light emitted by the lamp to scatter.
• The turbidity or solids content of the medium is calculated from the amount of the scattered light received by the detectors.
• A transmitter displays the final total or suspended solids value in the desired unit such as g/l or %TS.

Brix measurement (or Brix scale) is a well-known application in the food and beverage industry, among others. Strictly speaking, the Brix measurement determines pure sucrose content in water

1 degree Brix (°Bx) = 1g of sucrose / 100g of solution

The Brix scale is used to measure sugar content in substances such as soft drinks, fruit juices and tomato concentrates

Refractive Index (RI) is the tendency of light to bend as it passes through a liquid. A beaker of pure water will bend light that passes through it. As solids are dissolved in a beaker of water, light will increasingly bend as the concentration increases.

Measurement of Brix

Why is Brix important?

The exact determination of Brix is very important for costs and quality control in the beverage industry. By determining an accurate Brix level, you can not only ensure the best quality required but also optimize costs

What Brix should honey have?

A good honey should be between 70 and 88 degrees Brix. However, honey’s quality depends on its moisture content. In short, this is because the higher the moisture content, the higher the probability that the honey will spoil during storage. According to the AOAC 969.38 standard, moisture should not be above 20%.

Optical refractometer

When light enters a liquid it changes direction; this is called refraction. Refractometers measure the degree to which the light changes direction, called the angle of refraction. A refractometer takes the refraction angles and correlates them to refractive index (nD) values that have been established.

The prism in the refractometer has a greater refractive index than the solution. Measurements are read at the point where the prism and solution meet. With a low concentration solution, the refractive index of the prism is much greater than that of the sample, creating a large refraction angle and a low reading ("A" on diagram). The reverse would happen with a high concentration solution ("B" on diagram).

Applications

• Quick checks of a "rough" refractive index value, especially for Brix measurements in beverages, fruit, jam, honey and other sugar-based products
• Testing blood and urine
• Heat transfer fluids as ethylene and propylene glycol (antifreeze chemicals)
• Cutting fluids and urea.

Advantages

• Simple method
• Inexpensive instrument
• Special instruments with direct readings in Brix or salinity
• Ideal for academia / education
• Some Abbe refractometers can be connected to a water bath for temperature-controlled measurements.

Disadvantages

• Small measuring range (requires several portable refractometers to cover a wide range)
• No or external thermostating
• Operator-dependent readings, therefore limited accuracy
• No built-in measurement protocols.

Digital Refractometers

A high-resolution optical sensor measures the total reflection of a light beam emitted by a special LED light source after it hits the sample. This total reflection is converted into refractive index, Brix, HFCS or user-defined concentrations. A built-in Peltier thermostat controls the temperature of benchtop instruments.

Main Applications

Quality control of raw materials and final products in a wide variety of industries and segments, including:

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• Food and beverage
• Chemicals
• Pharmaceuticals
• Cosmetics
• Flavors and fragrances
• Petroleum products.

Advantages

• Easy to use
• Small sample volume
• Automatic measurement means results are operator-independent
• Built-in product management with automatic quality control
• Possibility to connect with other instruments for multiparameter measurements
• Applicable to benchtop meters only.

Disadvantages

• Higher cost in comparison to hydrometers, pycnometers or optical handheld refractometers
• Portable refractometers have no thermostating, only temperature compensation.

pH Measurement

• pH is defined as the negative of the logarithm to the base 10 of the hydrogen ion concentration
• (pH = –log10 [H+]).
• A pH meter is an instrument used to measure acidity or alkalinity of a solution - also know as pH.
• It is measured on a scale of 0 to 14.
• If the H+ concentration is greater than OH-, the material is acidic; i.e., the pH measurement is less than 7.
• If the OH- concentration is greater than H+, the material is basic, with a pH value greater than 7.
• If equal amounts of H+ and OH- ions are present, the material is neutral, with a pH of 7.
• A pH tester system consists of three parts: a pH probe, a reference pH electrode, and a high input impedance meter.

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