Technical Bulletin: APS360-1
25 September 2003 Revision 03
The advantages of the Quad Beam (4-Beam) Alternating Light Principle for measuring the components of liquids versus the traditional cell (cuvette) or reflectance type instruments.
The 4-Beam Alternating Light Principal was developed to improve quantitative measurement of the components in liquids. Liquids with known constituents such as fat (oil), protein, starches (sugars) and water can be accurately measured using the 4-Beam Alternating Light Principal. By measuring the absorption and scatter of specific wavelengths of light that are known to be absorbed by the constituents in the liquids, accurate quantitative values can be determined for those constituents. In any analyzing system, naturally occurring variations can introduce error. The optical method of liquid component measurement depends on several variables, including light source intensity and detector sensitivity.
Causes of light source variation:
1. Temperature of the environment around the light source
2. Aging of the light source
3. Voltage variations creating surges of light
Causes of detector variation:
1. Temperature of the environment around the detector
2. Aging of the detector
3. Voltage variations creating changes in the current output
Variables that affect the liquid product itself:
1. The contamination of the cell by improper cleaning or product contents such as chocolate particles that become lodged in the cell.
2. Scratching and deterioration of the cell by erosion changing the path length of the cell or in other words changing the density of the product in the cell.
3. Introduction of air particles.
4. Temperature of the liquid while it is being analyzed.
5. Variations in the particle size of the component being analyzed within the liquid.
6. Enzymes and other chemical contaminants that may have been accidentally introduced into the liquid being analyzed.
Analyzing systems such as the Advantec that uses the 4-Beam Alternating Light Principal compensates for most of these variations and provide more accurate test results than other infrared analyzing systems.
When all of the variables discussed above are constant, the traditional infrared transmission analyzing instrument is very accurate. However the most critical question is "how can one consistently maintain: a. minimum amount of variation in the product itself; b. the temperature environment of the instrument; c. stability of an aging source and detector; d. using an electrical system that often surges when large motors are activated; e. maintain a baseline or zero value; f. keep the sensor clean? ".
In the following discussion one can see the advantages of the 4-Beam Alternating Light Principal.
A. One Light - One Detector
The traditional method for analyzing suspended components in liquids using light is shown in Figure 1. Light ( L ) strikes a photocell detector ( E ), which generates an output current ( I ). The detector output current ( I ) is a function of the intensity of the light source ( L ), the detector characteristics ( E ), the distance ( X ) better known as pathlength between the source and the detector, and absorption/light scatter by the medium or product itself (a).
One light and one detector
Although the distance between the light source and detector is fixed, uncontrollable variations in both light intensity and detector sensitivity can still be sources of error. There is no compensation for reduced system performance due to component aging or contamination. For example, if the light intensity is reduced by accumulated product buildup within the path of the light, the smaller signal received by the detector will be interpreted by the system as an increase in the concentration of the components in the product itself. This is what happens when the cell is contaminated by chocolate particles, buildup of product components on the cell wall, trapped air bubbles, and expansion or contraction of the product by changes in the temperature of the product itself.
B. Attempts to overcome these variations: One light - two detectors
The analyzing accuracy of a one light - one detector system can be improved by adding a second detector at a greater distance ( X2 ) from the light source (Figure 2). Since physical law dictates that light intensity decreases as a function of distance, the output of the more distant detector ( E2 ) will always be lower than the output of the closer detector ( E1 ) because it will receive less light. The output of both detectors expressed as a ratio, (Ix1divided by Ix2), is a measurement value that depends on absorption by the product but does not depend on light source intensity.
One light and two detectors
If for example:
Ix1 = 80uA (eighty micro amps)
Ix2 = 40uA (forty micro amps)
Ix1 / Ix2 = 80uA/40uA = 2 (Ix1 divided by Ix2 equals 2)
The ratio increases with an increase in absorption. Distance is not a variable because both distance values are fixed. This method compensates for light variation. The ratio remains constant if light intensity is reduced by 25%, because the light reaching each detector will also be reduced by 25%.
Ix1 / Ix2 = 60uA/30uA = 2
However, the only flaw in this system is that a contaminated cell or aging detector could produce inaccurate measurements because they would affect the output of only one detector. This constant variable could be interpreted by the system as a change in the component concentration.
C. Two Lights - One Detector
The analyzing accuracy of the one light, one detector system can also be improved by using one detector with two light sources switched on and off alternately (Figure 3). The detector signals expressed as a ratio, Ix1 divided by Ix2, provide a measurement value that depends on absorption or scatter of the light by the product but does not depend on detector sensitivity.
Two lights and one detector
Ix1 = 80uA
Ix2 = 40uA
Ix1 / Ix2 = 80uA/40uA = 2
Again, the ratio increases with an increase in absorption, and distance is not a variable because both distances are fixed.
This method compensates for changes in detector sensitivity. Any reduction in detector sensitivity lowers the input from each light source by the same factor and results in a constant ratio. However, light intensity variation caused by contamination of the cell, air bubbles etc. aging of the light source or fluctuation in voltage, could produce inaccurate analysis because it might reduce the intensity of only one light source.
D. Two Lights - Two Detectors
The 4 Beam Alternating Light Principal compensates for variations in both light intensity and detector sensitivity. It features two detectors with two light sources switched on and off alternately (Figure 4).
When L1 is on, light is transmitted through the product or process media, and each photo detector (E1 and E2) receives the light, via paths X1 and X2, respectively. E1 and E2 generate signals based on the amplitude of light each receives. A ratio (RA) is created by comparing these two signals.
Two lights and two detectors
Advantec 4-Beam Alternating Light Principle
RA= Ix1 / Ix2 = 80uA/40uA = 2
Because any change in the characteristics of L1 (due to age or contamination of the cell) will affect E1 and E2 equally, this ratio will remain constant. For example, if the light output of L1 is reduced by 25%, the signals generated by E1 and E2 will both be reduced accordingly, but the ratio will remain the same. This eliminates the effects of contamination on L1.
RA = Ix1 / Ix2 = 60uA/30uA = 2
The same procedure is performed using L2, across paths ( X3 ) and ( X4 ), and a similar ratio (RB) is created. This eliminates the effects of contamination in the cell affecting L2.
RB= Ix3 / Ix4 = 30uA/60uA = 0.5
To eliminate the effects of contamination or component aging on the detectors, a ratio comparing RA and RB is make.
RA/RB = 2 / 0.5 = 4
Because a change in the sensitivity of E1 or E2 (due to age or contamination of the cell) will reduce the light received from L1 and L2 equally, this ratio remains constant even if the characteristics of E1 and E2 change. For example, if contamination covers 25% of E1, the light received from both L1 and L2 will be reduced by 25%, but the ratio will remain the same.
RA = Ix1 / Ix2 = 60uA/40uA =1.50
RB = Ix3 / Ix4 = 22.5uA/60uA = 0.375
RA/RB = 1.50 / 0.375 = 4.00
By using these ratios, rather than the direct output from E1 and /or E2, the effects of cell contamination and component aging are eliminated.
Ambient light is detected by E1 and E2 while L1 and L2 are off. The resulting "offset" signal is used to correct the measurement. All signals are linearized and combined to produce a reliable output signal which is equal to the component concentration being analyzed.
4-Beam Sensors of different sensitivity and measuring ranges, created by changing the distances between the sensor's light sources and detectors, are available for liquids with very low concentrations as well as liquids with very high concentrations. By placing the light sources and detectors further apart, the sensor becomes more sensitive to product concentrations because the longer optical path lengths increase the sensors ability to detect small changes in component concentrations.
Because an increase in distance reduces the amount of detected light intensity, optical path length also determines the measuring range. For example: in two sensors with identical light sources, the sensor with longer path lengths will detect less light; therefore, the sensor reaches the upper limit of its measuring range at a lower concentration and has a smaller measuring range. The sensor with shorter path lengths can measure higher concentrations and has a larger measuring range.
In terms of value of ownership over other instruments on the market:
The traditional instruments should they be in line, at line or laboratory instrumentation all have one thing in common. All of them use either a cuvette, only one source of light or essentially only one detector. All suffer from the same problems which can be one or all of the following:
a. changes within the path of light from source to detector
b. contamination and fouling of the source window or detector window
c. changes in path length d. temperature of the electronics and
d. power fluctuations.
All of these variables affect the final results determined by the instrument.
The NIR Systems (Perstorp,Foss), DairiTech, L T Industries and Infra Red Engineering instruments all receive raw data from within the process line. All have the problem of changing path lengths, deteriorating seals and gaskets, and fouling of the source or detector windows with caramelized proteins.
The On-Line and MMS instruments must first extract a sample from the process line. The sample then passes through a tube that has the potential to become blocked with product buildup. The sample must then be heat tempered or in the case of the On-Line the protein in the sample must be dissolved, before an analysis of the sample can be made. Other problems associated with the above named instruments include air bubbles, power fluctuations, plugging of the flow system, improperly prepared diluent, temperature variations, and contamination of the flow system (cell). Sensitivity to CIP chemicals that can be accidentally introduced into the cell can cause major repair and maintenance problems. Product wasted during the sampling process contributes to plant shrinkage and waste water treatment costs.
The 4-Beam Alternating Light measuring system can also be affected by certain variables that can cause incorrect readings. Placement within an area in the process where the temperature is above 60 degrees Centigrade can cause fouling due to the buildup of protein on the probe tips. Air bubbles can cause variations in the readings however they are normally factored out due to the large number of readings that are taken each second Variations in the homogenization of the product can cause variations within the test results but will be readily seen by the operator. This not a negative however because it also alerts the plant that the process system is not working properly. Failure of any one of the components within the 4-Beam system will instantly give a depletion reading on the screen thereby advising the operator that the system has failed in some way. Unlike the other systems that were mentioned earlier, that in many instances may continue giving false answers, the 4-Beam Alternating Light System will fail totally. Also and very important, unlike the other systems on the market the 4-Beam Alternating Light instrument with its modular components may be repaired quickly by in plant technicians and back on line in a matter of minutes.