Photometers & Turbidimeters

The design of photometer instruments has a significant impact on their applicability. Recent design improvements provide better overall measurement performance and long-term measurement stability. So, when evaluating photometers, it helps to understand how certain features affect the validation and verification methods.

In our new tech brief, Validation of Inline Photometers for Increased Measurement Confidence, we explore the designs more in-depth as well as discuss their pros and cons. For a quick introduction to a few aspects of photometer instrument design, keep reading this post.

Simply put, an industrial photometer is used to measure light after it has passed through an industrial process stream. Photometers can be configured for a variety of purposes, with the configuration and technique used depending upon the specific measurements needed. This makes them applicable to a wide range of applications in process analytical chemistry.

Understanding the impact of Beer’s law

The science at the core of photometer instrument design is Beer’s law. Also known as the Beer-Lambert law, it relates the attenuation of light to the properties of the material through which light is traveling. Since different chemicals absorb light at different wavelengths, the application of Beer’s law helps determine the accurate concentration of a chemical solution.

For the majority of applications where absorbance characteristics obey Beer’s law, a simple zero-plus-one sample point will be sufficient to calibrate an online photometer. The key to obtaining a signal that is proportional to concentration is that the light reaching the photodetector must be monochromatic (single wavelength).

But there are exceptions that require non-linear calibration: Beer’s law does not describe the behavior of concentrated solutions (greater than 10-2 M) due to interactions between the absorbing molecules. It also does not apply when measurement is made at a wavelength where the slope of the sample absorption curve is very steep. In such cases, a non-linear calibration should be used. Industrial photometers typically include non-linear and piecewise linear calibration models.

Considering a few photometer instrument designs

Let’s take a look at a few design variations:

Spectrophotometers

Spectrophotometers produce monochromatic light by using either a single photodetector with mechanically moving grating or prism, or a diode array detector if the instrument is more modern. They use almost exclusively broadband light sources such as halogen for visible (VIS, 400-700nm) and near-infrared (NIR, 700-2000nm) and deuterium for ultra violet (UV, 200-400nm).

Traditional process filter photometers

Traditional process filter photometers use narrow bandpass optical filters in conjunction with broad spectrum, high energy incandescent, halogen (VIS, NIR) and mercury vapor (UV) light sources to generate a facsimile of monochromatic light. In traditional designs, the light source is hot and a filter in close proximity to it is subject to potential damage over a relatively short period of time.

Modern filter photometers

Modern filter photometer designs utilize high performance specific peak wavelength light emitting diodes (LEDs) instead of broadband light sources and narrow band filters to produce tight, narrow band light emission. Unlike traditional process filter photometers, the LED source is cold and and does not damage or degrade other sensitive optical components in close proximity. A further advantage is that the process itself is no longer exposed to high energy light that can be potentially damaging to the process itself. This is particularly true for applications such as protein detection in bioprocessing that utilize UV light for the measurement

Addressing the issue of stray light

A problem with process filter photometers is the issue of stray light. It is the effects of stray light that often lead to dissatisfaction with photometer performance when deployed into a process environment. Typical bandpass filters only block to approximately 3 AU (0.1% of the total transmitted light) outside of the pass band and all remaining light below this level passes freely through the optical filter, creating non-linearity in absorbance measurement and limiting instrument range.

Narrow bandpass filters

Narrow bandpass filters are constructed to allow light to pass through a very small region or slot, typically 10nm wide, around a specific peak wavelength (the wavelength of interest), while blocking light from passing at all other wavelengths. When narrow bandpass filters are used in combination with process filter photometers that utilize LED light sources, there is virtually no stray light outside of the pass band region around the wavelength of interest.

As a result, LED light source photometers adhere to Beer’s law and provide native units of absorption as their baseline measurement without the need for additional absorption calibration. This means that any engineering unit can be correlated to absorbance while leaving a possibility for confident validation of the instrument’s performance.

Watch for decay

But a word of caution is warranted. Bandpass filters, while generally stable, can exhibit signs of decay over time with changes in peak transmission and stray light blocking capability caused by environmental factors e.g., moisture or erosion from the light source itself. This is particularly true in traditional design filter photometers. Since bandpass filters essentially define the operating wavelength and performance of a process photometer, it is important to ensure they meet or exceed the original specification from the manufacturer.

Impact of broad spectrum light sources

As mentioned above, when the light reaching the detector is not monochromatic, Beer’s law does not apply. Photometers that use broad spectrum light sources necessarily produce a significant amount of stray light passing beneath the out-of-band blocking capability of the filter and onto the detector. Such photometers must, consequently, be calibrated at multiple points for the instrument to work in units of absorption, noting that absorption is proportional to concentration. This requirement for signal correction presents some specific challenges when validating instrument performance.

Summing up

In sum, an understanding of the stray light issues surrounding filter-based photometers can greatly improve the approach to field validation and can enhance measurement accuracy. Using multiple validation points can help describe non-linearity exhibited by a photometer and allow action to be taken to correct it. Provided stray light impact is understood, highly reliable validation measurements can be obtained with a resulting high confidence in the measurement results of the instrument itself.

Do you have questions for us about photometers, or other measurement instrumentation and analyzer solutions? Please contact us. Our experts stand ready to assist.

Few factors are more important to operators than ensuring an instrument is working to specification and providing the correct measurement to an overall control scheme. 

Finding the right method for process photometer validation is therefore a priority. Operators can realize significant gains by achieving that aim, including ease of equipment maintenance, replication of maintenance efforts, and increased confidence in the instrument’s measurement performance. 

The most typical validation method that operators now use is verification of instantaneous reported instrument reading against a process sample measured offline. But the method comes with certain drawbacks:

  • It is limited to only one sample concentration at a time 
  • It may not provide a true indication of how the instrument will perform under changing process conditions. 

To more closely examine the pros and cons of various photometer validation and verification methods, we created a tech brief for download, “Validation of Inline Photometers for Increased Confidence”. It provides an in-depth look at key variables, including:

  • Photometer Method & Design 
  • Validation
  • Validation of Wavelength and Blocking
  • Validation of Photometric (Absorbance) Performance
  • Optical Pathlength and Error Assessment

In this post, we will feature a sample of the various methods, but before we begin, let’s look at the definitions:

What is a photometer?

A photometer is an optical instrument designed to measure the absorption of light at a given wavelength after it passes through a fixed distance of sample. Different chemicals absorb light at different wavelengths, and through the application of Beer’s law, the concentration of a chemical in solution can be accurately determined. Photometers are highly applicable to a wide range of applications in process analytical chemistry.

What are different types of photometers?

Filter photometers

Filter photometers are instruments designed to operate at one or more fixed wavelengths e.g. 280, 300, 330, 400nm. 

  • They are used as an online, real-time process instrument. 
  • They are a simpler and therefore more cost-efficient instrument configured to continuously monitor a fixed process stream as part of an overall plant control system.
  • The expectation is that they will operate unattended over long periods of time without operator intervention or the need for maintenance and servicing. 
  • Process photometers use specialized industrial in-line measurement cells or immersion probes which are designed to withstand harsh process conditions e.g., extreme pressures and temperatures, hazardous/explosive and radioactive environments.

Spectrophotometers

Spectrophotometers allow monitoring of a range of wavelengths within instrument specifications, e.g. 400-800nm. As they provide flexibility when working with a wide range of different samples, they are commonly used in chemical laboratories.

  • They are precision instruments, designed to be manually configured and operated by a trained technician in the relative comfort of a laboratory. 
  • Spectrophotometers measure individual samples that are prepared and typically placed in a standard size 12.5 mm square glass cuvette before being inserted into the measurement path of the instrument.
  • An offline sample-based measurement methodology operates, in nearly all cases, laboratory spectrophotometers. Feedback from measurements taken cannot be considered real-time.

Validation Basics

Two methods: 

  1. Vary process concentration

The best way to validate a process photometer is to vary the process concentration itself between certain maxima and minima, taking samples at cardinal points and comparing the process photometer reading to laboratory data. 

Potential con: This may not be practical for a continuously running production system. 

  1. Remove in-line process photometer measurement

An alternative method would be to remove the in-line process photometer measurement cell from the process line and introduce a series of verified samples or standard solutions in a controlled manner to confirm agreement. 

Potential con: This method is not always desirable as the process must be interrupted and it may not be possible where line fluids are potentially hazardous to health or present disposal and other challenges. Both of these methods require process disruption and a considerable labor overhead.

Wavelength Validation

Validation of the wavelength accuracy of a spectrophotometer is undertaken using a liquid sample containing a number of distinct peaks such as a solution of holmium perchlorate or a holmium oxide and/or didymium doped glass filter.  Emission peaks enable some spectrophotometers to automatically calibrate the instrument, offer methods to validate the wavelength, and automatically generate a validation report.

Solid glass type validation filters provide a convenient and safe method to quickly validate wavelength accuracy. However, it is not possible to validate wavelength accuracy of a filter photometer using wavelength calibration standards. Most filter photometers operate at one or two fixed wavelengths and are therefore not able to resolve the distinct peaks that the wavelength validation standard provides.

For filter photometer designs that utilize fiber optic connections to the inline measurement cell, a portable fiber optic type spectrophotometer can validate wavelength accuracy. 

Photometric Performance Validation

Let’s look briefly at three methods for validating photometric performance: 

  1. Process samples
  2. Certified optical filters
  3. Certified standards

1. Process Samples

This is probably the most typical validation method employed in the field. A sample of the process stream is taken along with a simultaneous reading of the photometer output on the instrument display. The process sample is analyzed offline and the photometer reading is compared to the result of the analysis. 

Potential con: This method is limited to only one sample concentration and may not provide a true indication of how the instrument will perform under changing process conditions

2. Certified Optical Filters

Certified filters are used to validate that a filter photometer is working to its absorbance specification.  NIST have specified a range of robust glass type validation filters that are suitable for use with both laboratory spectrophotometers and field process photometers that use a standard 12.5mm type square cuvette filter holder.

This type of non-intrusive traceable reference standard verifies photometric accuracy and linearity, both of which are critical to measurement result quality. 

Pros: The glass type validation filters specified by NIST exhibit long term stability and are simple and convenient to use. Metal on fused silica glass filters exhibit a relatively flat transmission profile over a wide wavelength range of 250nm-2200nm, compared to the neutral density type filters that are limited to 360-1100nm (do check the filter is suitable for the instrument under test). 

Note: Process photometers that can be verified without the need to interfere with the process line allow regular trouble-free validation, assuring confidence while saving valuable time and resources.

3. Certified liquid standards

Liquid standards are commercially available to calibrate and validate a wide variety of optical instrument measurement units. They can be introduced into a photometer light path in one of two ways: Directly into the sample cell or using standard cuvettes or proprietary insertable liquid holding devices. 

Pro: Although messy, using liquid solutions is specific

Con: Liquid Standards can exhibit quite wide variability in value, so care must be taken when using them..

Summing up: The pros of photometer validation and verification methods

  • Reduced production costs: Filter type photometers provide valuable real-time measurements into an overall control scheme and assist in reducing production costs by improving product consistency and quality. 
  • Enhanced measurement accuracy: An understanding of the stray light issues surrounding filter-based photometers can greatly improve the approach to field validation and can enhance measurement accuracy. 
  • Improved confidence: The ability to use photometric standards certified by third parties to nationally maintained standards for measurement and performance validation allows a transparent correlation between offline and online measurements and therefore provides a greater confidence in the results provided by inline photometers. 
  • Enabling of corrective action: Using multiple validation points can help describe non-linearity exhibited by a photometer and allow action to be taken to correct it. The use of filters and absorbance standards to validate photometers removes the need to take photometers offline and handle potentially hazardous materials as test samples. 

Do you have questions for us about photometers, or other measurement instrumentation and analyzer solutions? Please contact us. Our experts stand ready to assist.

One of the primary concerns when using an inline process analyzer for real-time liquid or gas analysis is validating the instrument is working to specification. Process photometers are often calibrated using liquid or gas calibration standards or directly to the process. These calibration methods can require considerable resources, so the preferred maintenance approach is often to validate performance as opposed to full recalibration of the instrument.

The most typical validation method is verification of instantaneous reported instrument reading against process sample measured offline. This method is limited to only one sample concentration at a time and may not provide a true indication of how the instrument will perform under changing process conditions. This paper will discuss the pros and cons of various photometer validation and verification methods and the impact photometer instrument design has on these methods.

Download Paper

The Kemtrak NBP007 measures the amount of suspended solids in a sample using backscattered light. While the use of backscattered light is typically applied to measuring suspended solids, the technique is equally applicable to emulsions, suspensions, dispersions and foams. The suspended phase may be either solids (e.g. calcium carbonate) or two immiscible liquid phases, such as oil in water or gas in a liquid (i.e. foam).

The measurement of backscattered light is also a measurement of turbidity. However, one must be aware of significant differences between backscattered light (180°) and other turbidity measurement technologies such as attenuated light measurement (light passing directly through the sample at 0°), forward-scattered light (typically 10-15°), and side-scattered light (at 90°).

Comparison of backscatter, side-scatter-forward-scatter, and attenuated light

Attenuated, forward, and side-scatter turbidity measurement techniques require light to enter into a sample, be scattered by particulates suspended in that sample and then emitted from the cell where the intensity of light can be measured by one or more photodetectors. As light must pass through the sample before being emitted, it is essential the optical density (i.e. concentration of particulates in the sample) is not too high or no light will be emitted. Such techniques are therefore typically used for low and very low sample concentrations or turbidities (<1% total suspended solids down to 0.0005% or 0.01-4000 FNU/NTU).

Backscatter differs in that the sample is illuminated at the window’s surface, and the light scattered back is measured. Light does not have to pass through the sample before measurement. As the concentration of the sample varies, the amount of light scattered back to the detector will vary, making this measurement technique suitable to measure just about any level of solids, even all the way up to 100%. The lower limit or measurement resolution of backscatter is typically around 0.001% suspended solids or 10 FNU/NTU. Backscattered light measurement can provide a wide dynamic range of measurement for applications where suspended solids levels can vary dramatically.

It should be noted that scattered light measurement is a function of concentration, particle size, and—to a lesser extent—refractive index and color. If you intend to measure concentration using any scattered light measurement technology, then it is essential that particle size is consistent. Samples where the refractive index of the suspended particles and continuous phase are similar will not scatter light—light will pass straight through the sample—and should be avoided (e.g. sweetened condensed milk). In the NBP007, measurement is made in the near infrared (NIR, 850nm and higher) and sample color therefore generally has no influence. However, if the sample is known to be highly colored, the influence of the color should always be checked in advance.

Applying Backscatter Measurements to Real-Life Applications

There are a plethora of applications where an accurate measurement of solids concentration is a real advantage. But measurement in many of these applications has historically been performed with instruments that do not have the dynamic range necessary for the job. A prime example is the measurement of spent yeast slurry in breweries. In large scale breweries, spent yeast is sold for animal feed, providing a revenue stream for the brewery while disposing of a large amount of biomass—a win-win for both the brewery and the processor taking the spent yeast.

However, there is a cost associated with transporting spent yeast in trucks, so it is important that the concentration of yeast in the slurry is high enough for the economics to work out. On the other side, too high a concentration can cause pumping and pipe transport difficulties. A reasonable target for solids concentration is around 50%.

A well-known brewery had implemented spent yeast solids concentration measurement using attenuated light probes. These probes, reading in an arbitrary CU unit, worked to some degree at low levels of solids concentration, but exhibited a nonlinear response to higher levels and quickly became blinded over 20% solids. Furthermore, when they were reading, they exhibited continuous drift. This resulted in several batches of spent yeast leaving the brewery with low solids concentration, and charge backs from the processor. Technicians in the plant tried to remedy the issue and operators attempted to work within the limitations of the system. But eventually, the measurements were abandoned, and the operators reverted to manual measurements when transferring spent yeast.

A Kemtrak NBP007 12mm OD backscatter probe was installed in the same Tuchenhagen VariVent “ball” as the transmission probe using an Exner EXstatic port adapter to improve the measurement and resolve the back charge issues the plant was experiencing. Using the native BU (backscatter unit) readout, the NBP007 provided a linear response to solids concentration changes across the entire range with no baseline drift. The unit was then correlated to actual solids concentration and set up in the DCS system as a live solids concentration measurement. Several years later, it continues to provide accurate and reliable readings of solids concentration, and is used to prompt the addition or removal of water from the spent yeast slurry circulating in the spent yeast loop. Savings to the brewery are estimated to be in the hundreds of thousands of dollars over the time it has been in operation, compared to the costs associated with the previous methods, and additional NBP007s have been installed in the plant where high solids levels are experienced.

A unique benefit of the Kemtrak backscatter (or reflectance) probes is that they will not go blind at any concentration of suspended solids. The output of the Kemtrak NBP007 will continue to increase with sample concentration, ensuring a reliable measurement at any concentration. The NBP007-L (low range) analyzer is recommended for process concentrations up to 10% total solids, while the NBP007-H (high range) analyzer should be used for accurate monitoring of suspended solids exceeding 10%.

The dairy industry makes widespread use of clean-in-place (CIP) systems in plants that process milk and produce yogurt, ice cream, and other milk-based products. CIP systems have revolutionized food production, making it easier and less costly to clean and sterilize manufacturing systems. These systems avoid the need for dismantling, and allow for the capture and reuse of water, detergents, and disinfectants.

But there is still a great deal of room for improvement. Dairy manufacturing facilities are still losing thousands or even tens of thousands of dollars per month from unnecessary product shrinkage, and the excessive consumption of water and cleaning agents.

Clean-in-place systems typically use a combination of timed cycles and turbidity sensors for system cleaning—a process which is often unnecessarily inefficient.

First generation CIP systems relied on timed cycles to ensure the cleanliness of product processing systems. Using predetermined intervals, the system was first rinsed out, then flooded with hot water mixed with detergents and disinfectants circulated through the system, followed by a clean water rinse cycle. However, the efficiency of CIP cycles relies heavily on accurate interface detection within the pipe. In systems where detection is either manual or relies solely upon time, loss of product and water can be costly, and product quality can be compromised.

Installing one or more turbidity sensors in the process pipe system greatly improves the efficiency of each cleaning cycle.

When a CIP cycle starts, there is still product in the line and the initial water rinse is used to push it through the system. A turbidity monitor allowed operators and/or automatic control systems to detect the arrival of the rinse water, signaling the end of the “batch” and helping avoid dilution of product in the pipeline by indicating a divert of the oncoming rinse water to a drain. The rinsing process continued until turbidity dropped to a specified point, indicating that most of the product was removed from the line. The drain was then closed and water containing cleaning agents introduced into the system and recirculated for a predetermined time to complete the wash out of the system pipelines. Finally, the cleaning solution was removed from the system by re-introducing clean water to do a final rinse, again relying on a change in turbidity reading to identify the arrival of the clean water to trigger the closing of valves. Clean water, retained in the lines until production resumed, was pushed out by raw product fed into the system. This water, being totally clean, was diverted to a holding tank for future use until the turbidity sensor ‘saw’ the product arriving, signaling the start of the new batch.

The goal of all of this was to get away from old, strictly time-based cycles, and instead act based upon the actual conditions existing in the pipelines. This had a direct impact on product quality, as product contamination/dilution was reduced, and overall consumer satisfaction in the market heightened.

This approach also helped reduced operating costs as well. Water and product were saved, less energy used for heating water and speed of the operation generally increased. Lastly, wastewater discharge volume was lessened, lowering the load on wastewater treatment and reducing sewer system costs.

The implementation of sensor-based CIP systems has saved dairy product plants vast amounts of money, sometimes hundreds of thousands of dollars annually. But there’s still room for improvement. Existing turbidity monitor designs on the market can be unreliable, resulting in ruined product, excess water and cleaning solution usage, higher energy costs, and increased waste discharge. Small investments in improving turbidity measurement and reliability in existing plants can result in fast returns for the bottom line.

Existing turbidimeters are often fooled by bubbles and tricks of light in process fluids, fouling measurements.

Traditional turbidimeters are installed into process pipelines so that their sensors emit light out through a flat window into the process stream. The light is then reflected by particulates, back through the process stream and flat window, to the eye of the sensor.

Competitor Turbidimeter Measuring Process

This approach has long been considered to work well enough, as it represents a vast improvement over strictly time-regulated CIP processes. But there’s an inherent flaw with turbidimeters using this design: they often have an optical focus point that is somewhere in the middle of the pipe.

This light is essentially diffracted out into the pipe. As the light travels through the process fluid, it can encounter bubbles, non-homogeneous mixtures, pipe wall reflections, and other disturbances. In fact, bubbles often collect and linger directly on the window surface, as a low-pressure zone forms over the window when fluid is flowing past it. Bubbles collecting on the window in this way disrupt the light beam and impact the resulting measurement. Turbidity indications affected in this way can be noisy and erratic, producing false positives or negatives that the control system and/or operators act upon.

Competitor Turbidimeter Flawed Measurement Due to Bubbles

The consequences of this should be obvious: noisy and inaccurate measurements result in the interfaces between product and water/cleaning agent not being detected correctly. Product is contaminated. Water is wasted. More wastewater is discharged. Money is lost. Furthermore, operators lose faith in the instrument and ignore it, reverting instead to manual control of CIP cycles and defeating the object of installing the monitor in the first place.

Exner has introduced a new sensor package in their EXspect 271 turbidity sensor, eliminating this serious measurement flaw, making it perfect for measuring turbidity in milk products.

The Exner EXspect 271 features a unique, patented ball lens over the sensor, which directly interfaces with the process fluid. The surface of the spherical lens is designed to repel bubbles and other disturbances.

Exner EXspect 271 Turbidity Sensor

The second significant different is the point of focus. Instead of the optical focus point being somewhere in the center of the pipeline, the EXspect 271’s focal point is exactly at the interface point between the process fluid and the surface of the ball lens.

Thus, only the turbidity of the fluid itself is measured—the measurement is not marred by bubbles collecting on the surface of the lens, reflections from the interior of the pipe, or other disturbances.

Exner EXspect 271 Turbidity Sensor Measurement Process

Because the light beam is focused on the surface of the lens, another key issue is solved—light absorbance. As mentioned previously, competing products focus light on the center of the pipeline. This means that the light encounters a very ‘thick’ cross-section of fluid, and the light beam loses energy as photons are scattered by the fluid. Less of the light beam is reflected, producing a weaker signal from the turbidimeter, indicating that the process fluid is cleaner than is really the case.

The Exner turbidity sensor does not have an appreciable light path through the liquid, and thus doesn’t suffer from this issue. The curved shape of the lens also resolves the issue of generating a low-pressure zone, so bubbles and other debris do not ‘stick’ to the lens.

Exner EXspect 271 Turbidity Sensor - Advantages of Ball Lens Design

This new design represents a significant step forward for CIP operations producing milk-based products. To learn more about how the Exner EXspect 271 can improve the cost efficiency of your operations, contact South Fork Instruments today.

Assessing a beer—whether as a finished product or at various points in the brewing process—is largely a matter of measuring visual and flavor-based cues. Does it taste right? What color is it? Is it clear? If it’s hazy, is the level of haze such that it signifies a production problem?

Haze is of key concern in beer brewing, as many mass-produced beers are known for their clarity and brightness.

The presence of haze is typical in many traditional beers, such as Hefeweizens and other wheat beers. In addition, hazy IPAs have become increasingly trendy in recent years.

But many brewers and macro-brewers produce bright, filtered beers that are sold in very large volumes throughout vast distribution areas. For these products, ensuring consistency and product recognizability is crucial to maximizing the visual impact of the product, as well as consumer satisfaction.

Measuring Beer Haze with a Turbidimeter
Beer brewing at the macro scale poses serious challenges in ensuring product consistency and quality, including the appropriate level (or lack) of beer haze.

One of the most common forms of haze is “chill haze.” This results from proteins and polyphenols (micronutrients found in vegetables, fruits, and cereals) bonding together to form large particles which reflect light and are thus visible. Chill haze gets its name from the fact that these particles dissolve when the temperature of the beer rises above 68 degrees Fahrenheit (20 C).

As long as the particulates are soluble at warmer temperatures, chill haze does not pose a problem. However, if chill haze is left unremedied for long enough, or the particulates oxidize, they will form larger particles which are no longer soluble. The result is a beer which is permanently hazy.

More seriously, haze can be an early sign of an infected beer, compromising the taste of the product.

Visual aesthetics are not the only concern when it comes to haze. Haze can be a sign of a beer batch infected with Brettanomyces (wild yeast), Lactobacillus, Acetobacter aceti, or other unwanted organisms. While most forms of infection don’t pose a health concern, infection can still result in a product with significant off-taste.

For brewers that produce beer in extremely large quantities—or smaller brewers that wish to scale up their production—relying on visual inspection to evaluate beer haze is not feasible. This is especially true when brewing occurs in multiple locations, where the subjectivity of multiple brewmasters inevitably results in inconsistent product.

Turbidimeters offer an objective, automated means of measuring beer haze and ensuring consistency across multiple batches, or in beer produced in multiple locations.

While “haze” is the term most often used to describe the foggy quality of a beer, more generally the opacity of a fluid is defined in terms of “turbidity.”

Turbidity is the measure of the quantity of particles suspended in a fluid. As the number of particles in a fluid increase, they begin to scatter light passing through. This is interpreted by the eye as cloudiness or haziness.

In many day-to-day situations, we unconsciously evaluate the turbidity of fluids we interact with. The water that comes out of your tap typically has very low turbidity—it’s clear—and you don’t think twice about drinking it. But if you were to one day run the tap and found that you had a glassful of cloudy water, you likely be instinctively repulsed by it.

This is because we associate clarity (low turbidity) with purity, and a lack of clarity (high turbidity) with contamination. While turbid water may simply have high mineral content, turbidity can also be a sign of something more sinister, such as bacterial growth.

Haze and turbidity are most often controlled or removed from fluids—including beer—via filtration.

Old school homebrewers may crack an egg in their beer—the egg white is placed in the brew, and as it sinks the albumin acts as a mobile filter—or use some other another fining additive to remove haze from their beer. At a large scale, macro brewers pump beer through large filtration setups to get that desirable polished clarity before going on to storage and/or packaging for sale.

Large scale filtration often uses plate and frame equipment with a filter media. Diatomaceous earth is a classic type of filter media—often encountered in swimming pool equipment—although there are other materials, such as perlite. In more recent times, filtration cartridges have become more popular.

Plate and frame filters consist of vertical plates covered with a filter cloth. In between these plates are frames that alternately are hollow or contain filter media. Beer is pumped into the frames filled with filter media, where the solids that cause haze are trapped. Clear beer flows through the media and cloths into the hollow plates, and out through collection tubes.

But what happens if a cloth fails and allows unfiltered beer to flow through into the clear stream? This can be a costly problem, as the beer may be spoiled or require reprocessing to bring it to specification. Sight glasses on the clear outflowing liquid have traditionally been used to spot when the beer is no longer clear, but these require the constant attention of an operator to ensure all is going well. Furthermore, using a sight glass is a subjective measure. It’s easy to see when a gross failure in the filter has happened. But smaller problems can be easily missed until late in the process.

Turbidimeters offer an objective, quantifiable means of measuring turbidity in beer and other fluids.

Enter turbidimeters. A turbidimeter is essentially an electronic eyeball that stares continuously at the flowing stream and reports the amount of particulate it sees. The output from the turbidimeter is fed into the overall control system in operation and can trigger a warning or alarm when the particulate level surpasses a predetermined setpoint, alerting the operator to attend to the problem. Sophisticated systems will also stop the flow of beer or divert the flow back to its holding tank while the issue is taken care of.

Some care has to be taken when applying turbidimeters to a beer haze application. A turbidimeter must have enough sensitivity to see low levels or particulate in the beer while being stable in operation, resilient to optical fouling, and insensitive to color change. The ideal instrument is a 90° scatter device, for sensitivity, with an NIR light source for color independence, and a scatter to attenuation ratio measurement technique for window fouling tolerance. This approach will deliver readings that correlate well with offline methods. Forward scatter and backscatter instruments can also be used to measure beer haze, but will deliver results that differ from those of standard offline instruments.