Comparison Type NephelometerWilliam Finley Wright Citation: Review of Scientific Instruments 28, 129 (1957); doi: 10.1063/1.1715832 View online: http://dx.doi.org/10.1063/1.1715832 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/28/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Aerosol scattering optical properties by nephelometer measurements at the El Arenosillo site (SWcoastal area of Spain) AIP Conf. Proc. 1531, 572 (2013); 10.1063/1.4804834 Novel bistatic polarization nephelometer for probing scattering through a planar interface Rev. Sci. Instrum. 67, 2089 (1996); 10.1063/1.1147020 Vehiclemounted nephelometer for use in desert environments Rev. Sci. Instrum. 67, 1725 (1996); 10.1063/1.1146984 A comparison of three types of scale invariance Am. J. Phys. 63, 474 (1995); 10.1119/1.17883 A Photoelectric Nephelometer Rev. Sci. Instrum. 18, 665 (1947); 10.1063/1.1741024

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded

to IP: 129.24.51.181 On: Fri, 28 Nov 2014 10:34:22

HIGH-FREQUENCY-INDUCED ELECTROLUMINESCENCE 129

light fluctuations when used on BaTiOa with best results being obtained from thin plates of ceramic rather than single crystals.

VOLTAGE SOURCES

The voltage sources employed were conventional sine-wave or square-wave signal generators, with fre­quency ranges up to about 100 kc. Above this, power signal generators were used to 400 Mc. These generators deliver about 10 watts over their entire range. The power, as such, is not important in making the various measurements. However, the relatively high voltage obtainable is necessary. With proper coupling and impedance matching it is possible to obtain 100 volts rms over much of the frequency range. Zinc sulfide crystals, in particular, require this voltage at high fre­quencies and the titanates require it at low frequencies. Any generator capable of supplying the above voltage will work satisfactorily for many purposes. An amateur­type, variable frequency oscillator (about! watt) with proper impedance matching could be used. However, light emission measurements on ferroelectric materials may involve frequency-dependent piezoelectric reson­ances as well as temperature-dependent Curie-point studies. Each produces large changes in the crystal dielectric properties. For these measurements a low

THE REVIEW OF SCIENTIFIC INSTRUMENTS

impedance (power) generator that will maintain a constant voltage and frequency is essential.

In general, voltage is measured with a diode probe at a "T" which is placed on the connector (2) (Fig. 1).

Dc and low-frequency ac voltages have been super­imposed on therf by simple means that are external to the crystal mount and will not be discussed here. Various klystrons and small magnetrons have been used as power sources for microwave measurements.s

Measurements of the rise and decay time of light emission are made by pulse modulating an rf generator and observing the wave shape of the photomultiplier output on an oscilloscope. The equipment should be capable of measuring decay times in the order of 1 fJ.sec when studying SiC and the titanates.

ACKNOWLEDGMENTS

The author wishes to thank members of the National Bureau of Standards Electron Tubes Section for many valuable suggestions and R. L. Raybold who worked on problems associated with the optical system. The di­chroic mirrors and high-efficiency beam splitters were kindly supplied by Liberty Mirror Division of the Libby-Owens-Ford Glass Company.

8 Typical units are the Varian X-13 klystron and the Microwave Associates 6444-series magnetrons.

VOLUME 28, NUMBER 2 FEBRUARY, 1957

Comparison Type N ephelometer* WILLIAM FINLEY WRIGHTt

The Johns Hopkins University, Baltimore 18, Maryland

(Received March 18, 1952; and in final form November 27, 1956)

A comparison type nephelometer has been designed and constructed with which turbidities are measured by both absorption and scattering of light. The especially designed cells permit accurate measurement of small changes in intensity of either transmitted or scattered light over a wide range of intensities. A single light source with a sector scanner alternately illuminates two separate optical paths for simultaneous comparison of two cells, one of which may be a standard. The intensities of the two optical paths can be varied independently for comparison measurements. A light integrating sphere with phototube assembly furnishes a single photoreceiver for the two optical paths. One such sphere compares the transmitted beams, another, the scattered beams. The reproducibility of resetting in the normal operating range is two parts in ten thousand. The apparatus will operate over a range of more than five density units. When making series of measurements, it was found possible to make and record readings for different cells every 15 sec. The apparatus was used successfully for two years in bacteriological, silica gel, and colorimetry studies. A slight modification permits measurement of the depolarization of the scattered light as well as the usual Tyndall scattering. This depolarization is measured for both polarized and unpolarized incident light, thus characterizing the size and optical anisotropy of the particles or scattering centers.

T HE effects of certain chemical compounds on the growth rates of various bacteria have been fre­

quently studied by turbimetric techniques. Com­mercially available nephelometers and those previously described in the literature proved inadequate to detect

* In partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Johns Hopkins University.

t Present address, 601 Winston Drive, Vestal, New York.

small changes in light intensities, since they lacked adequate sensitivity, reproducibility, and density range. The instrument described herein corrects these deficiencies.

OPTICAL APPARATUS

Figure 1 is a scaled plan view of the optical apparatus. A 100-w mercury lamp A (type AH-4) is surrounded

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded

to IP: 129.24.51.181 On: Fri, 28 Nov 2014 10:34:22

130 WILLIAM FINLEY WRIGHT

ALIGNMENT PIN

TRANSMITTED LIGHT INTEGRATING SPHERE

CELL HOLDER

"I'"'' '"'''" Efrx PENTAGONAL

~~ .L- TRANSM TED "V ,-- ~I BEAM ....L PRISM !:t.l --,--- ,-

I ~.,"'" 1& >C§ ATTENUATOR I

I ROTATING POLAROID

;/ i:~ -~;: ,

() o

ROTATING POLAROID

FIXED POLAROID

)L-""';-_IItl-_LENS HOLDER ASSEMBLY tl

SCATTERED LIGHT INTEGRATING SPHERE

SYNC. PHOTOTUBE

2 3 4 5 I ! , ,

LENSES

AH4 LAMP !l

FIG. 1. Nephelometer -plan view.

SCALE IN INCHES

by cylindrical light sector scanner B. A single light source obviates the necessity for close line regulation. The drawing shows scanner B rotated to pass maximum light to Path I and none to Path II. The synchronizer phototube D is not necessary if a synchronous motor is used to rotate the light sector scanner B. The synchro­nizer phototube D was included in the experimental design where various motors were to be tested. Appro­priate lenses E render the light beams essentially parallel. The removable filters render the light mono­chromatic, if required.

The attenuators consist of pairs of Polaroids per­mitting a compact design, with the advantage that the light always passes through the same sections of glass, so any dirt or irregularities on the Polaroid always remain in the light beam. The Polaroid closest to the light source is rotated to vary the intensity of the light passing through its assembly. The other Polaroid is fixed to pass only vertically polarized light, thus resulting in the greatest intensity of the light scattered laterally from the turbid suspension in the cell. The attenuation of the light, with rotation of the Polaroids,

is not linear over their entire range, but follows to a close approximation the equation

1= kIo cos28,

where I is the transmitted intensity, k an empirically determined constant, 10 the intensity of the incident light, and 8 the angle measured between the optical axes of the two Polaroids.

The cells I-I and I-II have optically flat and parallel surfaces for the transmitted light, and an optically flat window at right angles for the scattered light. Light from cells I-I and I-II pass into the transmitted light integrating sphere K (coated with titanium dioxide) through rectangular apertures. A third aperture, located in the top of the sphere, permits the internal repeatedly reflected light to pass into the phototube assembly M. Likewise, scattered light from cells I-I and I-II passes into sphere L through circular apertures, and thence to the phototube assembly N. The scattered light phototube assembly "sees" the light scattered between approximately 50° and 130°.

Figure 2 is a photograph of the completed optical

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded

to IP: 129.24.51.181 On: Fri, 28 Nov 2014 10:34:22

NEPHELOMETER 131

CELL

FIG. 2. Optical apparatus of the nephelometer. Letters refer to the designations in Fig. 1. For clarity's sake, the following were removed: the motor and sector scanner; the cover to the Sync. phototube; the scattered photo tube assembly; the cells J-I and J-II. A quarter view of a cell, in its holder, is shown on the left side of the apparatus.

apparatus. The various components are mounted on a i X 13 X 18-in. base with the nephelometer covering an area 12X12 in.

The scattered light integrating sphere, Fig. 3(a), consists of a metal cylinder 1, which is the support for a hemisphere 2 (top), and a hemisphere 2 in. (bottom, not visible). Aperture 3 is one of the two one-inch diam. openings located at 90° to each other for the scattered light beams. The phototube 10 is held in tube 4, which has an opening into the top of the hemisphere 2. Light

(a)

shield 8 fits inside tube 4 through opening 6. A pin in shield 8, fitting in slot 5 permits the rotation of the shield to either open or block off the light from the integrating sphere to the phototube. The multiplier pho­totube 10 is held in a common mount with its cathode follower tube 12. Photo tube 10 and base 11 are in­serted into tube 4 through opening 7. A slot in 4, near 7, permits the rotation of the phototube in its housing to a position of maximum electrical response. Where two brass tubes are fitted into each other, telescopic tubing was used.

A completed Pyrex glass turbidity cell is shown, without its holder, in Fig. 3(b). The cell consists of a central body 1, to the parallel ends of which are fused two optically flat windows 2 and 3, through which the transmitted light beam is passed. The scattered light passes out through window 4, positioned parallel to the transmitted light beam axis. The material to be tested is introduced into the cell through the 24/40 ground glass joint 5 and the side arm 6. When bacterial contamination is not involved, a simple rectangular cell may be used. Such cells have been employed in studies of silica gels.

Figure 3(c) depicts the light sector scanner which consists of a synchronous motor 1 (Bodine KYC-23, 1800 rpm, 8.5 w), with a 2-to-l gear reduction, mounted on assembly 2; around it is wound a copper water­cooling coil 3 to cool the lamp and motor. The motor drives the light shield 4 (made of 0.Q10-in. thick

(b)

(e) (d)

FIG. 3. Nephelometer components. A scattered light integrating sphere; B optical turbidity cell; C light sector scanner; D attenuator II. Refer to the text for a description of the numerical designations.

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions.

Downloaded to IP: 129.24.51.181 On: Fri, 28 Nov 2014 10:34:22

132 WILLIAM FINLEY WRIGHT

phosphor bronze), with an aperture 5 which is used to "scan" Paths I and II, at 900 rpm; Paths I and II "see" either all positive or all negative pulses from the 60 cy AH-4 lamp, thus eliminating the error due to unequal light intensity on the positive and negative pulses. The base assembly 2 is rotatable to synchronize the scanner with the lamp's peak intensity.

Figure 3 (d) is a quarter view of the attenuator II from the side facing the light source. This assembly consists of a base plate 1, into which one Polaroid (not visible) is permanently fixed to pass only vertically polarized light; the other Polaroid 2 is centrally mounted inside a cylinder to which are also attached two 3 in.-32 pitch anti-backlash gears 3 with springs 4. This cylinder is supported by a precision ball-bearing assembly mounted in plate 1. Gears 3 are driven by the worm 5 whose shaft is attached to calibrated dial indicator 6, with 100 divisions and a tenth's vernier. A Veeder counter 7 driven by 8 indicates the integral rotations of the worm shaft. Twenty-four worm revolutions rotate the Polaroid 90° which effects a maximum change in the light's intensity. Each vernier division of dial 6 indicates 13.5 sec of polaroid rotation.

Attenuator I (not shown) is a brass block with two polaroids; one permanently clamped to pass vertically polarized light, and the other in a ring which may be rotated to a convenient operating position with respect to attenuator II. No accurate angular indication of this second polaroid was used.

ELECTRICAL CIRCUITS

In this nephelometer, the electrical output of the photomultiplier is placed on an oscilloscope screen for comparison of the amplitude of the two incident beams i the oscilloscope also indicates any condition of over­loading or malfunctioning. The electrical circuits are shown in Fig. 4.

Due to the high impedence of the phototube load resistance, a cathode follower was mounted on the same phototube assembly, thus lowering pick-up noise, etc., and permitting a low impedence input to the diode amplitude choppers.

eSI A

82t( + Soovoe REGULATED SUPPLY

........ -NM~ 20.

OSCILLOSCOPE )

220 K HELIPOT

A VTVM

20.

FIG. 4. Electrical circuit of the nephelometer. The identical circuit is used for the scattered and transmitted light photo­receivers. (VTVM=Vacuum tube voltmeter.)

Silica gel is used as a dehydrating agent. Care must be taken to insure that moist and therefore conductive silica gel does not come into direct contact with elec­trical components.

In "nulling" the two incident signals by operation of the light attenuators, only the peak amplitudes of the two signals need be observed on the oscilloscope. Therefore, the output of the cathode follower is fed into an amplitude chopper, a 6AL5 type diode, which eliminates any desired base portion of the signals, leaving only the peaks for visual comparison on the screen. The operating voltage of a 6AL5 plate is determined by R-5, a ten-turn, 20000-ohm Helipot, which permits a rapid and accurate "chopping," thus permitting "nulling" of signals of various amplitudes with great accuracy, e.g., only 0.1 v of the top of a 50-v input signal need be presented on the screen. By proper manipUlation of the light attenuator II, the intensity of the two light beams entering the phototube assem­blies can be made identical to a small fraction of the 0.1 v. A double-pole, double-throw switch permits the operator to select either the scattered or transmitted light beams for comparison.

OPERATION, CALIBRATION, AND RESULTS

The light intensity of the AH-4lamp, when operated from an ac supply, increases during the first five or ten minutes of operation. However, for less than maximum accuracy, the nephelometer may be used satisfactorily during the warm-up period. The AH-4 lamp has been operated from a suitable dc supply; however, its life is then relatively short. With a tungsten lamp, no warm-up period is required. However, one disadvantage of the tungsten lamp is that it cannot furnish as intense mono­chromatic light as the discharge type of lamp. Cooling air is forced through perforations in a copper coil located at the lamp's base. This air blast, plus the copper water-cooled coil (3, Fig. 3), furnishes adequate cooling.

When making transmitted light measurements, it was found advisable to close off the scattered light phototube assembly. When scattered light measurements are taken, it is necessary not only to close off the trans­mitted light phototube assembly, but to place black shields (both black velvet and specially prepared black opaque disks have been used) in the paths of the trans­mitted beams, between the cells and the transmitted light integrating sphere. An incident light beam passing successively through one of the cells, the transmitted light integrating sphere, and back out through a second cell, gives two scattered light beams., one from each of the two cells. The intensity of the scattered light from the second cell may be several percent of that from the first cell and perhaps, therefore, several times the change of light intensity that is to be measured. By suitably blocking off the transmitted light integrating sphere, this false scattered light signal from the second cell is eliminated. These shields should not reflect an

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions.

Downloaded to IP: 129.24.51.181 On: Fri, 28 Nov 2014 10:34:22

NEPHELOMETER 133

appreciable amount of light back into their respective cells, otherwise another error may result in the scattered light measurements.

The light paths within the apparatus must be ade­quately shielded to avoid any "hash" and/or modula­tion from external light sources disturbing the sensi­tivity or accuracy of measurement. There is no change in the accuracy of the instrument due to normal fatiguing of the photosensitive surfaces.

There are several different methods for making the turbidity measurements. The two cells, I-I and I-II, may be used as illustrated in Fig. 1; one cell, I-I may be considered as the standard or reference cell, and I-II, as the "unknown cell"; or in place of I-I, a "dummy brass cell" may be employed, consisting of a rectangular box with two large apertures for the trans­mitted beam, and an aperture in its side positioned close to the scattered light integrating sphere. Flexible light shields attached to this cell exclude stray light from the scattered light sphere. The cell has no glass windows to become dirty, or any liquid medium which might change its composition or become contaminated. The inside of the cell is painted an opaque black. Only the small amount of light scattered by the air and the dust particles in this cell enter the scattered sphere. The apertures for the transmitted beams are considerably larger than those in the adjoining .shields; therefore, no effect is noted on the intensity of the transmitted beam when this dummy cell is inserted. However, because the dummy cell cuts down stray light, the intensity of the scattered light entering its sphere is greatly reduced. For rapid and accurate "standard" check on the relative transmitted or scattered intensities of the two light paths, a dummy cell is placed in each path.

The limitations of accuracy of the apparatus are due, to a large extent, to the external conditions under which it is operated, e.g., smoke from a cigarette held several feet away differentially affected the intensity of the two light beams, resulting in a transient "error" of several percent. An electric typewriter in an adjoining office sent large signals back through the ac lines, which made very accurate nulling difficult. For work involving very small changes of light intensities, a dust-free en­closure for the whole instrument would be convenient, though more adequate dust shielding of the light paths alone would suffice.

As previously noted by many observers, it is im­possible to maintain completely dust-free sterile water in a Pyrex cell; therefore, corrections should be applied to any turbidity measurements of extreme sensitivity to compensate for this source of error, if other means of maintaining dust-free solutions are not practicable. The treatment of Pyrex cells with a silicone, e.g., G.E. # 87 Dri Film, has been found to decrease this error.

Figure 5 is a plot of a calibration of attenuator II made independently of the nephelometer, using the Hg green line (S461A). The log of the intensity is plotted versus log (J (the angle between the optical axes of the

104r--------------__ ---..

,03

-;;;',02 ~

Z :J

>­a: ..: a: t: 10 OJ a: -.!­I

>-t: ~ 0.1 w ~ Z

o EXPERIMENTAL

• THEORETICAL

0.001.'7"'-L-L.J...J-':'::"--'-~--'-...L...I...L..Li.L---'---'--L-.1.-J...J...LJ 0.3 1.0 3 10 30 90

ANGLE BETWEEN POLARIZING AXES OF' POLAROID PAIR

FIG. S. Calibration of variable light intensity assembly II with the mercury green line. "0" observed relative intensity values. "0" plot of y=K cos2(J, where k is arbitrarily determined from the observed value for 0= 15°,

two polaroids). On the same graph is plotted the equation 1= kIo cos28, where the value of k was deter­mined from the empirical value of I for 1J=60°. The divergence of the two curves at low values of 8 is quite apparent. For this particular pair of Polaroids, the in­tensity of the transmitted light at IJ=O° is approxi­mately 1/20000 that for 1J=90°. This calibration data may be subject to an experimental error of approxi­mately 0.2%. The deviation of the intensities from the empirical equation 1= kIo cos21J is less than the experi­mental error of measurement.

This apparatus, with a slight modification, has been used to determine the state of polarization of the trans­versely scattered light, in addition to the measurement of the intensity of the Tyndall effect. Light attenuator I is removed, and a Polaroid is inserted in the scattered light beam between the scattering cell and the scattered light integrating sphere. This Polaroid is oriented to pass either the vertically or horizontally polarized com­ponent of the scattered light. In addition to these measurements with unpolarized incident light, another series of similar measurements is made with polarized incident light by inserting a second Polaroid in the incident beam between the cell and the light source. These three series of measurements give the quantities P'" pv, and pH. The importance of measuring all of these

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions.

Downloaded to IP: 129.24.51.181 On: Fri, 28 Nov 2014 10:34:22

134 WILLIAM FINLEY WRIGHT

.J 1.0 ::>

"'-II)

w d 0.3

f­a: ~ "' 0.1

"' « .J

" ~0.03 « a:

" o 5 0.01

i

0.003

0.0010:---'-1 ~2-~3-4~--'5'--~6--::-7-8~-!9:--""IO--J1I

I/INTENSITY SCATTERED LIGHT -(ARBITRARY UNITS)

FIG. 6. Observed values of the scattered light beam for various concentrations of Pyrex glass particles; size, approximately two microns.

quantities is discussed in detail by R. S. Krishnanl •2

and G. Oster.3 A simple optical change in this apparatus gives a measurement of forward light scattering.

Refrigeration of the photo tube improves the signal to noise ratio and permits an even more accurate null to be made. This refrigeration can be accomplished by

1 R. S. Krishnan, Proc. Indian Acad. Sci. (A)l, 211 (1934). 2 R. S. Krishnan, Proc. Indian Acad. Sci. (A)l, 717 (1935). 3 G. Oster, Chern. Rev. 43, 319 (1948).

THE REVIEW OF SCIENTIFIC INSTRUMENTS

surrounding the phototube housing with dry ice. Care must be taken to avoid condensation of moisture in the optical path. Figure 6 is a curve showing the effect on scattered

light of the addition of small amounts of Pyrex glass particles, approximately 2 microns in diameter, to a cell containing distilled water. The changes in the scattered light intensity were noted by the rotation of the polaroid assembly (attenuator II). The sensitivity of the apparatus permits one to measure accurately the addition of less than 1.6X 10-9 glml of these glass particles. The deviation of the empirically determined points from a smooth curve was, in major part, caused, as shown by subsequent measurements over a narrower range of concentrations, by inaccurate addition of the serial aliquots.

ACKNOWLEDGMENTS

The author gratefully acknowledges the assistance and encouragement of his father, the late Dr. Fred E. Wright, and also that rendered by many of his friends and associates at The Johns Hopkins University; in particular, Dr. H. M. Crosswhite and Dr. D. W. Stein­haus for calibration of the Polaroid assemblies; Dr. J. D. Strong and his associates for data relating to the use of light integrating spheres; Dr. Walter A. Patrick for assistance in the studies of silica hydrogel; Dr. A. H. Corwin for assistance in bacteriological studies; Mr. W. R. Asher for making the integrating spheres j Mr. J. Lehman for making the glass turbidity cells; Dr. J. P. Hervey, Dr. E. F. MacNichol, Jr., and Dr. C. F. Miller for advice regarding the electrical circuits; and C. H. Weber for the photographs. Dr. R. P. Teele of the National Bureau of Standards assisted the author with the proper internal coating of the integrating spheres.

VOLUME 28, NUMBER 2 FEBRUARY, 1957

Plane Parallel Plate Transmission Line Stark Microwave Spectrograph S. A. MARSHALL, U. S. NavaJ Ordnance Laboratory, White Oak, Silver Spring, Maryland

AND

J. WEBER, U. S. NavaJ Ordnance Laboratory, White Oak, Silver Spring, Maryland, and The University of Maryland, College Park, Maryland

(Received October 29, 1956)

A parallel plane transmission system is described for use in high Stark field microwave spectroscopy. The planes of the system are optically flat silvered glass plates and are separated by plane parallel fused quartz spacers. The instrument gives intense sharp lines out to Stark field strengths of the order of 25 statvolts/cm.

INTRODUCTION

I N certain microwave measurements, such as occur in the study of dipole moment determinations and

second-order Stark field effects, a highly uniform Stark field is required.1.2 This is especially the case when

1 Gordy, Smith, and Trambarulo, Microwave SPectroscopy (John Wiley and Sons, Inc., New York, 1953), Sec. 7.2.

molecular information is to be determined with great precision.

For such measurements, a rectangular wave guide with Stark septum insulated from the guide wall (see Fig. 1) has been used rather extensively. However,

2 Townes and Schawlow, Microwave Spectroscopy (McGraw-Hill Book Company, Inc., New York, 1955), Sec. 10.5.

This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions.

Downloaded to IP: 129.24.51.181 On: Fri, 28 Nov 2014 10:34:22