Turbidity using New Dual-Angle Light Scattering Instrumentation

Model DSB 

For 90° and forward scatter at levels, sizes, and particle types typically found in the brewery filter room beer line.

By John Byrnes & Andrea Valentine


There is a recent interest in multiple angle simultaneous in-pipe turbidity measurement. This is in addition to the 90° ASBC/ EBC or forward scatter measurements now made.

This paper presents actual simultaneous measurements by a single instrument showing the various sensitivities of the two angles to yeast, diatomaceous earth, and protein size particles. The paper includes a description of the apparatus, experimental method and laboratory technique with photographs.

The conclusion is the presentation of the actual data in graphic or tabular form.


Product clarity is the single characteristic most apparent to the buyer at the time of purchase, and therefore very important to the brewer.  Beer filtration is traditionally monitored at the final filter for solids content. The instrument is usually a nephelometer, in-line and in the laboratory (see Methods of Analysis of the ASBC, Beer-27).  These nephelometers measure scattered light at a 90-degree angle to the axis of the incident light. Haze and turbidity are measured by the nephelometer as turbidity in FTU (Formazin Turbidity Units) and these FTU, in turn, are proportional to particle concentration.

The most common particles that make up this haze measurement are yeast and proteins/tannins. Filter aid, including diatomaceous earth, may also be present and contribute to beer haze.

Some brewers measure haze using an instrument that employs a "forward scatter" technique. Instead of placing the scattered light detector at a 90-degree angle to the axis of the incident light, it places the detector at a smaller angle (10 - 25 degrees) from the axis. The for­ward scatter measurement is intended to be more sensitive to larger particles.


This paper presents data showing the measured light that is “side scatter” (90º), and “forward scatter" when measuring typical beer particles.  The equipment used is a new in-pipe hazemeter developed by McNab Incorporated.  This design was developed to measure both side scatter and forward scatter simultaneously; using a common light source and optics.  This eliminates the ambiguity of measurements taken at different points and with different optical systems.  The variation due to the transportation of samples to a laboratory hazemeter is eliminated as well.  The operator can observe each reading 90º (FTU) or forward scatter as desired.


The instrument used for the test includes a pipe section flow cell with integral light source and sensor, and an electronic analyzer.  The analyzer (Figure 1B) provides simultaneous readout of haze, in FTU per ASBC, with a digital readout for each channel and an analog 4-20mA output.  A three-inch diameter flow cell was used with the optics mounted directly in the pipe section, per standard practice.  The flow cell (Figure 1C) is 12 inches long.  The in-line turbidimeter is usually located after the beer filter.  An additional location would be after PVPP and trap filters.

The instrument, the Model DSB, uses a single polychromatic and near-infrared collimated light source to illuminate the liquid stream in the pipe.  Readings shown as 90º scatter in the graphs are from a detector located at an angle of 90º from the axis of the collimated light.  Readings marked forward scatter on the graphs were made be a sensor located directly opposite the light source, on the axis of the collimated light.  A metal blocking disk stops the directly transmitted light.  A series of non-imaging optics collects light which has scattered at a small angle (10º - 25º, typically) from the axis of the light source.  This light is then measured by the forward scatter sensor (Figure 1A)

A third sensor is located in front of the blocking disk, directly on the axis of the light beam. This sensor collects light transmitted directly from the light source. The resulting signal from this reference detector is compared by the instrument to the signals coming from the side scatter and forward scatter sensors.  Ratio division of the measured signal by the measured signal is performed to eliminate common mode problems, such as the loss of light due to change in color of the carrier liquid.


Particles tested have been chosen to be representative of particles found in a beer filtration line:

  • Yeast particles
  • Diatomaceous earth particles
  • Protein sized particles

In each of the graphs shown (Figures 2, 3, and 4), the particle population is limited to one of these three types and the concentration of the type chosen for each graph is varied.  The sample is illuminated by the hazemeter light source, and both side scatter and forward scatter are measured simultaneously.  The result is plotted on the graph.  Units of measure are shown in FTU (ASBC).  The 90º data can also be shown in equivalent EBC turbidity units. The forward scatter signal is usually shown in ppm silica, but is imposed on the EBC scale here for comparison.

In this test, yeast samples were taken from a brewery after fermentation, before storage or filtration.  For the beer protein graph, polymer beads were used with typical sizes under 200nm. This approximates the small size of beer proteins present after filtration. Continuous agitation was provided where needed to keep the solids in suspension.


Figure 2 shoes the results of test using yeast particles.  These particles are typically 20,000nm in diameter.  Both forward and 90º sensors measure the changes in particle concentration, and respond to increased concentration.  The slope of the line shows the gain or response of the system.

Figure 3 shoes results from the same test, using particles of diatomaceous earth.  Figure 4 shows results on protein-size particles.


Per the data, the 90º measurement shows response to each of the particle types, and can be expected to alarm at high concentrations of any of the particle sizes.  The forward scatter shows higher gain to large particles.  Forward scatter alone shows inadequate response on protein-size particles.

For the first time, measure at two angles allows simultaneous center-of-pipe monitoring with both the traditional 90º measurement method and the forward scatter method.  This allows direct comparison of readings on both scales, in the pipe during filtration or product transfer.

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