THE PURITY of the water in rivers, lakes and ponds can be tested by determining the coliform-bacteria count or the presence of dissolved gases and of various compounds. Tests of this kind were described in this department in March, 1970, and February, 1971. During the past year a remarkably accurate instrument for measuring turbidity (the concentration of solid matter that is carried by the water in the form of solid particles) has been developed by a group of undergraduates at the University of Rochester. The group, which included Cathie Lubell, Thomas Barry and Gregory Hearn, worked in the Institute of Optics and Materials Science Program under the supervision of Edward M. Brody, assistant professor of optics. They describe the work as follows:

"It is possible to determine the turbidity of solutions by many techniques. With a microscope one can count the number of particles in a unit volume of the specimen. With a calibrated grid in the viewing field of the microscope one can measure the relative sizes of the particles in a polydisperse suspension (a suspension containing particles of widely varying size). Turbidity can be evaluated relatively by filtering the specimen and weighing the dried filtrate. All these procedures are time-consuming. Boredom and fatigue on the part of the observer can lead to error.

"Extinction turbidimeters are doubtless the simplest instruments that have been devised for measuring the concentration of solids in suspension. They are based on the principle that turbidity is inversely proportional to the minimum length that a column of fluid must have in order to extinguish at one end of the column a source of light at the other end. A major source of error in all such turbidimeters arises from the fact that changes in the range of particle sizes in the suspension affect the pattern in which the transmitted light is reflected from particle to particle. The effect is known as multiple light-scattering. It causes apparent turbidity to decrease as the length of the light path through the specimen is increased. The influence of the multiple-scattering effect becomes increasingly significant when the attenuation of light through the specimen exceeds 15 percent. Extinction turbidimeters must therefore be calibrated with suspensions of known particle size. Indeed, reliable measurements of turbidity cannot be made with extinction turbidimeters unless the distribution of particle sizes is known.

"The substitution in turbidimeters of lamps for natural illumination and of photocells for the eye has led to improved measurements, but many sources of potential error remain. Variations in voltage applied to the lamps, fatigue of the photocathode of photocells and the effects of space charge in photomultipliers can combine to introduce an error of at least 10 percent. Moreover, it has been shown that the inherent temperature sensitivity of photoelectric detectors causes the output to vary as much as 50 percent dhrough the temperature range from zero to 30 degrees Celsius.

"To cope with such errors we built an instrument of the ratio type in which an error in one part of the system is in effect compensated for, keeping the ratio constant. We call the device a dual-beam turbidimeter. Apparatus employing dual-beam transmission is available commercially, but it is costly and needs modification for use as a turbidimeter.

"In our device light from a single source is split into two beams. One beam traverses the specimen and the other, a reference beam, is transmitted dhrough air. The two beams combine in a photo-detector, where they are added algebraically. Experience has demonstrated that the instrument measures turbidity effectively whether the liquid contains suspended particles of widely varying size or suspended particles of uniform size. The results have been highly reproducible when checked against comparable measurements made by other procedures.

"A helium-neon laser serves as the light source [see illustration at right]. We elected to use the laser because it costs little more than a high intensity lamp and provides parallel rays in the form of a monochromatic beam one millimeter in diameter. The output power of about two milliwatts exceeds the requirements of the photomultiplier detector by at least 100 times. We reduce the intensity by inserting neutral-density filters in the beams. An incandescent lamp could be substituted for the laser by inserting a color filter in the optical path (to select a range of wavelengths) together with simple collimating lenses to make the rays parallel.

"The output of the laser falls on a beam splitter, which is a rectangle of clear plate glass set at an angle of 45 degrees with respect to the beam. The transmitted portion of the beam passes through a neutral-density filter and enters the specimen cell. This vessel (a hollow prism made of sheet glass) has the form of a right triangle. The refracted rays emerge from the cell, pass through an aperture and fall on the photocathode of the photomultiplier tube. The intensity of these rays, which constitute the specimen beam, varies with the turbidity of the specimen.

"Rays that are reflected by the beam splitter fall on a front-surface mirror, from which they are reflected through a neutral-density filter and the apertures in a motor-driven chopper. The chopper interrupts the beam 160 times per second. Rays of the flickering beam recombine with those of the specimen beam at the photocathode.

"The output of the photomultiplier tube is a unidirectional electric current that varies in amplitude 160 times per second. A circuit separates the output into two parts: a direct-current signal and an alternating-current signal [see illustration at left]. The direct-current signal represents the sum of the current generated by light transmitted through-the specimen plus the direct-current component of the chopped reference beam. It is measured by meter No. 1. The amplitude of the alternating-current signal is proportional to the intensity of the reference beam. It is measured by meter No. 2.

"The electronic circuit consists of a network that includes three operational amplifiers, which boost the output of the photomultiplier to a usable level. The alternating-current and direct-current components are amplified independently for display on separate meters. The filter that separates the alternating current from the direct current consists of an inductance of 1.4 henrys and a capacitor of .7 microfarad connected in series with the input of the reference-signal amplifier, A. The filter transmits maximum current at a frequency of 160 hertz. Both the capacitor and the inductor are available commercially.

"The frequency at which the chopper operates is not critical. Choppers can be improvised to operate at other frequencies. For example, a synchronous motor that operates at 3,600 revolutions per minute could be fitted with a disk that contains two apertures to chop the beam at a frequency of 120 hertz. The input circuit of the reference-beam amplifier could be tuned to this frequency by retaining the 1.4-henry inductance and increasing the capacitor to approximately 1.25 microfarads. The values of capacitors and inductances that resonate at other chopping frequencies can be calculated by the formula , in which L is the inductance in henrys and C is the capacitance in farads.

"The experimenter is primarily interested in the intensity of the transmitted beam as it is displayed on meter No. 1. This meter, however, responds to both the direct current of the specimen beam and the direct current in the reference beam unless the instrument is appropriately adjusted. To make the adjustment connect switch No. 2 to the position shown in the circuit diagram, block off the specimen beam and vary the resistance of potentiometer R3 until the pointer of meter No. 1 falls to zero. Unblock the specimen beam. The adjustment need be made only once a day.

"The electronic components of the circuit are quite stable. The switch is not strictly necessary. It merely serves as a check for determining that the electronic components are working properly. When the switch is in position 1 and the circuit is properly adjusted, the alternating-current portion of the reference signal is subtracted at the input junction of the A amplifier, because operational amplifiers reverse the polarity of signals When the switch is turned to the other position, at which the output of the reference-signal amplifier is connected to ground, meter No. 1 displays the sum of the reference and specimen voltages instead of the difference.

"After the instrument has been adjusted meter No. 1 indicates voltage proportional to the intensity of the beam transmitted through the specimen cell. Meter No. 2 similarly indicates voltage proportional to the intensity of the light source. An easy way to use the instrument is to fill the cleaned specimen cell with clear water and record the ratio of the two meter readings: (water) The specimen is then transferred to the cell and a second set of readings is made: (specimen). The transmission of the specimen, as calibrated against clear water, is then expressed by the ratio, ()(specimen)/() (water). The apparatus automatically compensates for changes in the intensity of the light source, changes in the sensitivity or gain of the photomultiplier and most other variables that degrade the results of extinction turbidimeters.

"Ideally a specimen of water that is well mixed should be uniformly turbid, and a transmission measurement for one path length is sufficient to determine the that a series of turbidity measurements should be made through columns of fluid of varying length to improve the accuracy and as a check that only single scattering processes are involved. A plot of the logarithm of the transmission versus the length of fluid traversed should be a straight line. Systematic deviation from a straight line indicates multiple scattering and a difficulty in calculating the turbidity.

"Our specimen cell was designed expressly with this requirement in mind. As mentioned, it consists of a right triangular prism. The vessel is mounted to a traversing table with the faces of the prism perpendicular to the plane in which the transmitted and reference beams are located. The prism is moved in a direction parallel to the hypotenuse of the triangle. The light continues to enter the specimen cell at a right angle to the vertical face as the specimen cell is displaced in the plane of the hypotenuse, but the length of the path traversed by the light beam varies directly with the position of the cell. The cell is moved along the traversing table by a screw mechanism calibrated in increments of one millimeter.

"The four pieces of the cell were cut from ordinary window glass. Three rectangular sides were glued to a base piece to form a right-triangular prism. Before making the strips with a glass cutter of the wheel type we inspected the sheet and chose areas that were free of bubbles and striations. Surfaces of optical quality are not required because the measurements involve intensity, not image formation. Any convenient angle can be made between the hypotenuse and the side through which the beam enters. Our vessel was assembled with Sauereisen Adhesive Cement, an acid-proof cement that is distributed by the Fisher Scientific Company in Rochester, N.Y. Doubtless any epoxy cement would work as well if the cell is cleaned only with detergent.

"We clean the cell after most measurements with a solution of chromic acid made by dissolving from 30 to 60 grams of sodium dichromate or potassium dichromate in a liter of concentrated sulfuric acid. A comparable cleaning preparation known as Chromerge is available commercially. It must be similarly diluted with concentrated sulfuric acid.

"To clean the cell wet the inside surfaces of the glass with the acid mixture, let the cell stand for five minutes and then rinse it thoroughly with filtered water. We prepare the rinsing water with a Millipore filtering apparatus. In our opinion cells need not be cleaned with chromic acid unless extreme accuracy is desired. For the routine assessment of turbidity in natural streams the vessel can be cleaned with any good household detergent.

"We found that the results of our transmission measurements were reproducible to within 1 percent. We also found that we had to dilute specimens from rivers and lakes about six to one, even when they were taken from calm water, to avoid significant multiple scattering of the light and a corresponding loss of accuracy. Usually severe multiple scattering occurs when turbidity attenuates the light more than 15 percent.

"Our results were checked by theoretical calculations of the turbidity of known samples and also by experiment. A typical experimental check was made with a specimen prepared by suspending polystyrene spheres two microns in diameter in filtered distilled water. We counted the number of spheres in a known volume of solution by means of a microscope and estimated the concentration to be between 100,000 and 200,000 particles per cubic centimeter. Subsequent measurements with our turbidity meter indicated an actual concentration of approximately 130,000 spheres per cubic centimeter. The test is based on the fact that concentrations of spheres of uniform diameter attenuate light by a predictable amount.

"Spheres of this kind are available commercially from a number of suppliers, including the Dow Chemical Company (Midland, Mich. 48640) and Particle Information Service (600 South Springer Road, Los Altos, Calif. 94022). The spheres come in the form of a concentrated solution. Dilute the solution by adding it to distilled water. This procedure prevents the spheres from clumping. We used one drop of the sphere solution in 250 milliliters of water for a 'stock' solution that was diluted further to conduct scattering experiments. We positioned the specimen cell near the bottom limit of its traverse for the maximum optical path length and diluted the sphere solution until the attenuation was less than 15 percent.

"To verify the concentration with a microscope we counted the particles against a translucent background in the form of a grid. If the focal length of the objective lens is known, the depth of the field can be computed. If the concentration is low, the microscope views particles in a single plane. We counted the number of particles per unit length of the field in one direction and cubed the result to find the concentration per unit volume.

"A polydisperse specimen was obtained at a depth of eight meters in Lake Ontario near the inlet of the Genesee River. Turbidity was measured after the specimen had been diluted six to one with distilled water. The results were expressed as a graph by plotting the logarithm of the ratio of the transmitted light (I) divided by the unattenuated intensity of the source (I) against the length of the path through the specimen that was traversed by the light beam [see illustration at right]. Turbidity is equal to the slope of the graph. In this example it amounts to .374 unit of turbidity per centimeter of path length.

"The apparatus can be used to measure the distribution of sizes and concentrations of particles, provided that the particles are assumed to be spherical in form so that they settle at a predictable rate. The rate at which the intensity would change with time could then be related to particle diameter.

"The physical details of the construction are left to the resources of the experimenter. The optical parts of our instrument were mounted to a metal base by rugged fixtures that were made with machine tools. Most of the parts could be made of wood or plastic. We enclosed the entire apparatus in a light proof housing so that measurements could be made in a normally lighted room.

"Our apparatus is not readily portable. Moreover, the laser and the light chopper require a source of 115-volt, 60hertz power. For this reason the apparatus cannot be used in the field. It should be possible to modify the apparatus for use with battery power by exchanging a certain amount of accuracy and precision for portability. For example, a photodiode or phototransistor could be substituted for the photomultiplier. High-intensity lamps that operate on 12 volts, together with color filters and collimating lenses, could replace the laser. It should also be possible to substitute for the motor-driven chopper an oscillating chopper improvised from a 60-hertz resonant switch of the kind formerly used in the inverter circuits of automobile radios."

Bibliography

THE EFFECT OF LIGHT ON THE SETTLING OF SUSPENSIONS. C. G. T. Morison in Proceedings of the Royal Society of London, Series A, VOL 108, NO. A746, pages 280-284; June 2, 1925.

TURBIDIMETRY AND NEPHELOMETRY in Encyclopedia of Chemical Technology: Vol. XX, edited by R. E. Kirk and D. E. Othmer. Wiley-Interscience, 1969.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.