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Turbidity Measurement Solutions

South Fork Instruments' metrologists understand the practical realities of industrial operations. A hands-on understanding allows us to develop reliable, cutting-edge turbidity measurement solutions which minimize and mitigate common sources of measurement error, allowing for the accurate measurement and reporting of liquid turbidity in-line, and in real time.

DISCUSS SOLUTIONS WITH US

SOUTH FORK INSTRUMENTS CAN ADDRESS A VARIETY OF COMMON TURBIDITY MEASUREMENT CHALLENGES.

Do you have any of these concerns:

DRIFT

“We are constantly having to adjust our turbidity monitors because they drift all the time.”

Read More

When simple 90° scatter technique turbidimeters are used in applications with fluids that less than almost clear (i.e. drinking water), coating/fouling quickly forms on the optical window surfaces.  Fouling cuts down the amount of excitation light entering the fluid and reduces the amount of scattered light being received by the instruments’ photodetector.  This causes both drift and a reduction in sensitivity.  Using a ratio based 90° scatter instrument provides compensation for window fouling and keeps the measurement stable and online.

HIGH PRESSURE

“We want to measure very low turbidity in our process after final filtration, but our plant runs at high pressure.”

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Making turbidity measurements at high pressure can be challenging.  Optical windows in any turbidity measurement circuit must be rated to the design pressure of the plant. This is a problem for instruments that utilize windows with a large surface area.  Kemtrak inline turbidity measurement cells, because they are fiber optic based, need only small windows into the process line and are therefore able to meet extremely high pressures.

ACCURACY

“We have a turbidity probe but it gives very poor and non-repeatable results, particularly at lower turbidity levels.”

Read More

Turbidity probes, particularly small ones, use a backscatter or transmission technique to measure turbidity.  Backscatter measurement techniques do not work well at low turbidity levels.  Transmission measurement can work well at low turbidity levels, but requires a long optical path length to do so – something not generally found on optical probes.

HIGH TEMPERATURE

“We measure turbidity in a steam condensate but we see a lot of failures of components in the sensor housings. It’s very expensive to keep the measurement online.”

Read More

Many turbidity sensors have “high temperature” options, but these are really just standard sensor parts mounted with some sort of plastic insulator.  It’s not an effective way to measure turbidity at high temperature, and sensor failures are common.  The only way to resolve this problem is to use a fiber optic-based system that has no electrical or electronic parts in the sensor housings.  Kemtrak systems use fiber optics to connect the measurement cell to sensitive optical components in the electronic unit.

CALIBRATION

“We calibrate our turbidity systems using formazin standards regularly, but our results seem to vary depending upon which technician carries out the calibration.”

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Calibrating turbidity systems is not a simple task.  The main problem is that calibration solutions constantly settle, so readings are always on the move.  Agitating to re-suspend particulate and pouring into and out of measurement cells can introduce air bubbles that create errors.  Kemtrak has a simple calibration system using sealed formazin (or polymer bead) standards that are inserted into window optical paths using a unique positioning fixture, ensuring repeatable and consistent results.

“I read on my Formazin standards that they should be refrigerated. Is this really necessary?”

Read More

The chemical stability of formazin suspensions is dependent on storage conditions. Exposure to heat or direct sunlight will denature the polymer structure, while prolonged contact with ambient air will allow the fluid supernatant to evaporate while air-borne contaminants enter. Formazin standards should be stored in a cool, dark place and refrigeration will provide extended life. When using your standards, do not leave containers open longer than necessary and always seal them tightly for storage. Never pour standards back into their storage containers as this will introduce contamination and could lead to erroneous future calibrations.  As a rule, a 4000 NTU formazin suspension, when stored correctly, will be stable for about one year.

DISCUSS SOLUTIONS WITH OUR EXPERTS

SOUTH FORK PROVIDES SUPERIOR-CLASS TURBIDIMETER PLATFORMS WHICH SUCCEED WHERE LESSER MEASUREMENT SOLUTIONS STRUGGLE OR FAIL.

Many traditional turbidity measurement solutions use probes that emit light through a flat window into the process stream. Then, light is reflected/scattered back through the same window and onto a photometric detector in the sensor. While this "one-probe-does-all" approach is simple, it is insensitive to very low turbidity levels and, when installed in pipes (particularly smaller ones), wall reflections can greatly interfere with the measurement, giving false readings and erratic results. Kemtrak flow-through cells are highly sensitive to very low levels of turbidity whilst still having wide measurement range capability.

For higher range measurements, probes are used. Kemtrak probes have a 45° angled head to allow stray light to travel down pipes rather than reflect back through the optical window and create disturbance and instability.

Our systems instill a high level of confidence in the measurement by consistently producing values in agreement with those determined through offline sampling. Turbidimeters are configured to utilize the optical measurement technique—attenuated, backscatter, ratio—that best accommodates your process conditions and needs, ultimately ensuring that you know, in real time, the true state of your process stream.

What is a turbidimeter?

Turbidimeters are optical instruments used to measure the clarity of a fluid. For low level turbidity (i.e. drinking water), a light scattering technique is most often used. In this, a collimated beam of light is passed through a liquid sample. A photocell at 90° to this incident light detects light scattered by any particulate matter in the fluid. The greater the amount of scattered light, the more particulate is present in the liquid. Once the turbidity of the liquid reaches a certain point, the side scatter technique is no longer viable because the detector becomes blinded by the amount of particulate present. Once that limit is reached, attenuation and back scatter techniques can be used instead.

Each technique has a relative sensitivity, and while there is a great deal of overlap between the techniques, it is important to select the right one for the application to get the best and most satisfactory results.

What is turbidity? Expand

Turbidity is the measure of clarity that applies to all types of liquid. The lower a liquid's turbidity is, the clearer it appears. Conversely, the more turbid it is, the cloudier it appears. Turbidity in water is typically due to the presence of suspended solid particulate, although it can also be from undissolved oils or other substances, such as colloids.

How does measuring water turbidity contribute to public health? Expand

In the ancient world, people had to use their senses to test water quality. They would look, smell, and taste water to make sure it was safe to drink. The Romans, for example, brought fresh water into their cities through aqueducts. They understood the importance of protecting against water-borne diseases and used water clarity as one assessment of the suitability of water for consumption. Through the years, people adopted various methods like filtering or boiling to help improve water quality. Nowadays, water distribution in the western world is a sophisticated industry surrounded by standards - and turbidity measurement is now a primary measurement of water quality at water treatment plants

In the 1970’s, Congress passed legislation which set high standards for drinking water quality throughout the U.S. In the U.S. today, the Environmental Protection Agency (EPA) is responsible for approving water quality standards.

Dirty water, apart from being unpalatable, can contain contaminants which might poison or sicken anyone drinking it. Therefore, before it can be distributed, water must be filtered to a very high standard. Along with chlorination and other processes, turbidity measurement helps control the treatment processes that ensure distributed water is clean and safe to drink.

What causes turbidity to change? Expand

Turbidity in water is caused by a variety of factors, both natural and man-made. Particulate found in water can be any combination of silt, dirt, bacteria, or chemicals. For instance, agriculture is a common cause of increased turbidity in streams, rivers, and lakes. When farmers irrigate their fields, runoff causes cloudy water to enter water courses. Food processing plants, major construction projects, and mining can cause similar issues.

During rainy seasons, runoff into streams and rivers increases turbidity levels as it carries a lot of dust and dirt with it. When it's raining, water treatment plants have to deal with higher levels of undissolved solids in the raw water coming into their plants. Left unchecked, filters can be overwhelmed and pipes can become clogged - to avoid this, turbidity measurement throughout the treatment process is critical to keeping the plant running and water standards high.

Turbidity levels are also important to the environment. When water sources contain high levels of solids, fish and water plants suffer. Cloudier water limits light penetration into the water which makes it difficult for native plant species to grow like they normally would. This can then affect the entire food chain.

In general, the higher the turbidity of a water source, the higher chance there is that it will make people sick. Measuring turbidity is a critical function in producing clean water.

What is the unit of measurement for turbidity? Expand

The most common unit of turbidity measurement is the Nephelometric Turbidity Unit (NTU) and is used to describe the intensity of light scattered off of particulate suspended in water. NTUs are most often associated with the clarity rating of drinking water. The World Health Organization (WHO) states that drinking water should never be above 5.0 NTU.

However, light scatter magnitude is dependent upon particulate size and wavelength of light. Nephlometric Turbidity Units describe the turbidity of a liquid with a measurement method that uses a white broad band light source and 90° scatter light detection method. To standardize readings, instruments using this method are calibrated using Formazin solutions of known concentration (aka Formazin turbidity standards). However. because there are several variations on this measurement technique, there are several other units quoted in literature that are synonymous with NTU. When the broad band light source is substituted with an NIR source, the units are more often referred to as FTU (Formazin Turbidity Unit). Unfortunately, these two units are often (incorrectly) substituted for each other.

With the creation of additional turbidity standard solutions, notably polymer bead standards, additional units have been used. There are now FNU (Formazin Nephlometric Unit), PNU (Polymer Nephlometric Unit), and PTU (Polymer Turbidity Unit) to describe turbidity measured using these different standards and wavelengths.

How is a turbidimeter used to measure the turbidity of a fluid? Expand

To give a more formal definition, turbidity is the measurement of light intensity after the interaction of incident (collimated) light with suspended solids and colloidal materials in a liquid sample. This interaction results in light rays being scattered and absorbed rather than transmitted straight through the sample. As light passes through a sample containing particulate, the particles absorbs that light, then re-radiates it in all directions. Particle shape, size, color, and refractive index determine the spatial distribution of the scattered light by the particle. Particles smaller than the wavelength of light (e.g. bacteria) scatter light in equal intensities in all directions, while particles larger than the wavelength of light result in greater forward scattering.

Small particles measuring less than 1/10th of the light’s wavelength scatter light symmetrically like this:

 

Turbidity Scattering

Medium-sized particles measuring roughly 1/4th of the light’s wavelength scatter light in a more forward direction:

Turbidity Scattering 2

Larger particles, with diameters greater than the light’s wavelength, produce scattering that is extremely concentrated in a forward direction:

Turbidity Scattering 3

When turbidity is measured at right angles to the incident light beam (as in the 90° side scatter technique), particle shape and size differences are minimized, providing sensitive measurement of light scatter by all particles in the sample. International Standard ISO7027:1999(E) defines the measurement of turbidity by NIR light measurement at 90°.

The inherent flexibility of the Kemtrak TC007 turbidimeter allows its measurement components to be configured in various modes dependent upon the type and turbidity range of the sample being monitored.

 

What is attenuated detection? Expand

Turbidity - Attenuated DetectionThis detection method measures the attenuation of the incident light by a detector positioned directly in-line with the incident light beam. The greater the turbidity level in the measurement cell, the greater the attenuation of the light passing through it and the lower the signal the turbidimeter measurement circuit receives.

  • Results correlate well to the concentration of suspended solids.
  • Can be zeroed between runs to compensate for optical window fouling.
  • Large concentration range determined by the optical path length.

 

What is backscattered detection? Expand

Turbidity - Backscattered detection

Backscattered light is measured using a backscatter probe and allows for the accurate measurement of suspended solids from dilute to extremely high concentration solutions. Incident light is passed into the sample through a single window. Then, particles within the sample scatter back some of the incident light. The more particles there are, the greater the magnitude of the scattered light which returns back through the window.

  • Features good sensitivity from 10 FTU to extremely high concentrations.
  • Backscatter probe simplifies inline installation.

 

What is ratio detection? Expand

Turbidity - Ratio Detection

Ratio detection is ideal for low level turbidity measurements, particularly where optical window fouling or variable absorption of NIR light may occur. Both transmitted light and scattered light is measured and mathematically combined using a ratio algorithm to calculate the turbidity of the sample. When combined in this way, absorbance of the incident light within the cell and/or any loss due to window fouling (assumed uniform on all windows) will appear in both the numerator and denominator of the ratio algorithm and cancel out, leaving only the turbidity signal.

  • Good sensitivity at very low turbidity down to 0.01NTU/FNU.
  • Complies with ISO7027:1999(E) using scattered light at 90°.
  • Compensation for sample color.
  • Compensation for optical window fouling.

Ratio mode has the added advantage that the direct light beam can be monitored and used as a signal quality parameter to indicate the degree of optical window fouling present. Once the signal quality goes below a set value (i.e. 10%), the requirement for cleaning can be signaled.

Have more questions? Take a quick moment to get in touch with our expert sales engineers and technologists and we'll guide you to a solution.

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