An improved nephelometer is disclosed comprising a cruciform housing having therein helical flow diverters for providing a helical water flow pattern so as to cleanse the inside of a sample tube. A regulated light beam is incident on the sample tube and sample. The transmitted portion of the light is measured and used in a feedback loop to control the incident bulb intensity and the scattered portion of the light is measured to provide a measure of the turbidity of the fluid under test. The housing of the nephelometer is a hermaphroditic molding of plastic so as to simplify construction and assembly. All parts mounted within the nephelometer are located by recesses molded into the two halves of the housing. Amplifier circuitry for the output of the scattered light detecting photosensor is disclosed, as is lamp regulator circuitry for feeding back the value of the transmitted light, so as to adjust the incident light, for ramping the bulb on and off and for keeping it in a low current condition.

FIELD OF THE INVENTION 
This invention relates to a nephelometer, which is an instrument for 
measuring the turbidity of a fluid. More specifically, the invention 
relates to a nephelometer for measuring the amount of solid material in a 
sample of water. 
BACKGROUND OF THE INVENTION 
It will be apparent that in numerous industrial and other processes it is 
essential to provide an instrument which is capable of producing an 
electrical signal indicative of the amount of solid material present in a 
sample of fluid. It has been known in the prior art for many years, for 
example, to provide such a signal indicative of the condition of 
lubricating oil in aircraft engines. See U.S. Pat. Nos. 3,736,431 to 
Childs; 3,627,423 to Knapp; or 3,892,485 to Merritt et al. Similarly, such 
apparatus is useful in water purification apparatus for determining 
whether all solid materials are being filtered from the water. See 
commonly assigned U.S. Pat. No. 4,145,279 to Selby which describes a water 
purification system in which the nephelometer of the present invention may 
play an important part. 
Usually nephelometric measurements have been made by passing an incident 
beam of light of known intensity upon the sample to be tested and 
measuring either the amount of light transmitted through the sample or the 
amount scattered by the solid particles within the sample. See, e.g., U.S. 
Pat. No. 3,869,209 to Sigrist, or 3,936,192 to Skala. A distinction is 
sometimes drawn between the use of the terms "nephelometry", to indicate 
measurements made of the scattered light intensity, and "turbidimetry" to 
indicate the measurements of the intensity of the transmitted light. In 
this application, the term "nephelometry" shall be construed to include 
turbidimetry, unless the context clearly indicates otherwise, as certain 
of the improvements made by the present invention are applicable to both 
sorts of systems. 
Nephelometric measurements have, in general, required several significant 
problems to be solved. Clearly, in order to measure the light either 
transmitted or scattered from a fluid sample, the sample tube must be 
transparent; if the tube's transparency varies over time due to, for 
example, the collection of dirt on either the inside or the outside of the 
tube, the measurement will vary over time, so that the instrument will 
require periodic cleaning and/or adjustment of its output to match a 
sample of known turbidity. This problem has been discussed in U.S. Pat. 
No. 3,861,198 but no adequate solution is suggested therein. Another 
problem which occurs is leakage into the light sampling tube of stray 
light from the surrounding environment. A further problem of numerous 
prior art systems is that they are very expensive to make due to the 
elaborate circuitry and mechanical construction required. A further 
problem with certain prior art nephelometers and turbidimeters is that the 
electric bulbs used to supply the incident light vary over time and 
moreover, do not have sufficiently long lifetimes to allow adequately 
trouble-free operation although measures have been taken to limit this 
problem; see U.S. Pat. No. 3,561,875 to Reid. Another problem which has 
occurred in numerous prior art nephelometric systems is that the 
instrument is incapable of distinguishing bubbles which are usually 
harmless in the fluid to be sampled from solid matter in the sample, thus 
giving erroneous indications of excess turbidity. A further problem 
inherent in certain prior art designs is that the photocells used to 
sample the turbidity of incident light only measure the intensity of the 
light falling on a small fraction of the sample and thus do not always 
provide accurate results. 
OBJECTS OF THE INVENTION 
It is therefore an object of the invention to provide an improved 
nephelometer. 
A further object of the invention is to provide a nephelometer which can be 
made less expensively than those in the prior art. 
A further object of the invention is to provide a nephelometer which is 
self-regulating with respect to light intensity. 
Still a further object of the invention is to provide a nephelometer in 
which the transparent tube containing the fluid to be sampled is adapted 
to be kept clean by means of water flow deflectors. 
A further object of the invention is to provide circuitry for lamp control 
whereby lamp life is increased and lamp intensity is self-regulated. 
Still a further object of the invention is to provide bubble rejection 
circuitry and whereby the effects of bubbles on the nephelometric 
measurements are minimized. 
SUMMARY OF THE INVENTION 
The above needs of the art and objects of the invention are satisfied by 
the nephelometer of the invention which comprises a cruciform housing 
containing various elements of the system. An electric lamp bulb provides 
a light beam incident on a glass sample tube the intensity of which is 
regulated in accordance with the output of a first photo-detector. The 
photo-detector output is fed back through amplifying circuitry to power 
the bulb so that the light transmitted through the sample tube is 
maintained at a constant value regardless of the color of the sample, or 
of the transparency of the tube due to dirt accumulating over time, and 
the like. A second photo-detector preferably positioned perpendicular to 
the first, measures the intensity of light scattered from the sample, thus 
eliminating effects of the color, and only measuring the amount of light 
actually scattered from solid particles in the fluid sample to be 
measured. Helical flow restrictors are placed in the input and output 
sides of the sample tube so as to force the fluid to be measured to travel 
in a spiral path. The spiral flow path persists during the sample area so 
as to provide a self-cleaning action to the interior walls of the tube. 
Additionally, the helical flow restrictors provide a light trap so that 
any light entering by the sample tube cannot reach the area of sampling. 
Furthermore, the nephelometer of the invention is contained in a 
hermaphroditic housing assembled of two identical molded parts whereby 
construction of the nephelometer is considerably simplified and made much 
less expensive than those in the prior art. Finally, bubble rejection 
circuitry is employed to enable differentiation between bubbles in the 
sample tube and actual turbidity or solid matter in the sample tube, thus 
enabling increased accuracy of measurement. Lamp ramp circuitry is 
provided so that the current to the lamp is turned on and off gradually so 
as to extend bulb life, and a keep-alive current is maintained across the 
bulb even when nominally off, also extending its life.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 1, an overall perspective view of the nephelometer 
according to the invention is shown. It comprises two identical molded 
halves 1 and 2, respectively, which are identical and are made in such a 
way as to mate without modification. Their overall shape is cruciform and 
they are adapted to have added thereonto extensions 3 and 4, respectively, 
which permit connections of conventional water tubes 5 and 6. At one end 
of the cruciform structure, a cap 12 is located, inside which is circuitry 
to be described in further detail below and from which protrude control 
and connection wires 9. Inside cap 12 is also located the detector of the 
scattered light. The light is generated at a second cap 10 covering a bulb 
which is the source of the illumination. The illumination thus travels 
towards the opposing end of the nephelometer capped by cap 11 containing 
circuitry, and to which is attached to heat sink 7. The light passes 
through a sample tube which is of glass and which contains water arriving 
at tube 5 and departing by way of tube 6. Preferably the sample tube and 
the tubes 5 and 6 are oriented vertically. Some of the light is scattered 
off and is thus picked up on a photocell concealed beneath cap 12. A 
fourth cap 8 corresponds to cap 12 and is filled with a light absorbing 
material so that there are no double reflections of the light to upset the 
measurements. Caps 8, 10, 11, and 12 are so sized as to hold the tube 
mating halves of the nephelometer 1 and 2 firmly together along parting 
line 15. 
Referring now to FIGS. 2 and 3, which are cross-sections taken along lines 
2--2 and 3--3 of FIG. 1, respectively, the internal construction of the 
nephelometer will be made clear. The bulb 20 appearing at the left side of 
FIG. 2 directs light through two lenses 22 and 24 thus focusing the beam 
of light through an aperture plate 26 at the center of the glass sample 
tube 28 which serves to minimize stray light. The lenses are arranged so 
that the incident light is normal to the tube 28, thus avoiding 
refraction. Thence, the light is largely transmitted through the sample 
onto a diffusion screen 30 angled to reduce the reflection back into the 
sample tube. The diffused light is then passed to a first photocell 32 
which controls, by means of feedback circuitry to be described in further 
detail below, the intensity of light emitted by bulb 20 so that the amount 
of light incident on the photocell 32 is essentially independent of the 
condition of the sample and of the sample tube. In this way, they are 
automatically compensated for. Some fraction of the light is scattered 
from any solid material present in the sample tube 28, and is passed 
through a second pair of lenses 34 and 36, onto a second photocell 38. The 
output of photocell 38 thus determines the amount of solid material 
present in the sample tube, since the amount of light falling on the 
photocell 38 is a function of the amount of light scattered in the fluid 
contained with the sample tube 28 which is, in turn, a function of the 
amount of solid material present therein. The output of photocell 38 is 
not affected by any other variations in light transmissivity through the 
fluid since these are automatically compensated for by the feedback of the 
signal generated by photocell 32 to bulb 20 thus increasing its intensity 
in the event that, e.g., the sample tube gets dirty with age or the like. 
It will be noted that the nephelometer body part 2 is molded with various 
recesses and cavities to accommodate the various parts of the nephelometer 
assembly itself. Specifically, the lenses 22 and 24 and 34 and 36, the 
aperture screen 26, the diffusion screen 30, the panels holding the two 
photocells 32 and 38 and that holding the bulb 20 are all held directly by 
the body. This is done so as to enable simplified assembly, thus reducing 
the cost of the nephelometer. Similarly, as noted above, the nephelometer 
halves 1 and 2 are identical thus necessitating, for example, additional 
recesses to be formed in the right side of the nephelometer shown in FIG. 
2. Thus, for example, the groove 41 is adapted to hold aperture ring 26, 
and groove 42 and 43, lenses 22 and 24, respectively. However, complete 
symmetry cannot be preserved; vent holes 44 and 45 must be drilled, since 
their presence in the area of the diffusion screen 30 would cause 
additional light to fall on it and give misleading readings. 
Referring now to FIG. 3, which is taken along 3--3 of FIG. 1 and of FIG. 2, 
the orthogonal cross-sectional view of the nephelometer according to the 
invention is shown. Water to be sampled arrives from the right side of the 
figure via a tube 5 shown in phantom and passes through a helix 50. This 
helix may desirably be formed by injection molding out of a suitable 
plastic material and comprises essentially planes at angles to one 
another. A second identical helix 48 is provided in the exit 4 of the 
nephelometer. The use of the second helix 48, while not strictly necessary 
to cause the helical flow pattern, does tend to keep it regular. Both 
helices 48 and 50 comprise more than a full 360.degree. spiral. In this 
way the water is caused to travel in a spiral path and therefore tends to 
continue to travel in that same spiral path as it traverses the sample 
tube 28. The effect of the helical flow pattern is to scrub the inside 
walls of the sample tube 28, thus keeping the readings picked up by the 
two photoelectric cells 32 and 38 relatively constant over time. Other 
methods of cleaning the sample tube 28 are possible, such as ultrasonic 
vibration thereof, but this has so far not proven to be required. Another 
possibility is to rotate the sample tube at a high enough speed so that 
the readings tend to average out over time. This introduces some 
additional complexity in the apparatus, but serves the purpose well. After 
having been sampled, the water passes through exit helix 48 and a second 
tube 6 shown in phantom. The helixes may desirably be contained in molded 
adapters 3 and 4 which can be made identically if carefully designed and 
adapted to mate with tubes 5 and 6. In a particularly preferred embodiment 
these are made to mate with a quick connect fastener. The helix-containing 
adapters 3 and 4 may be secured to the main halves of the nephelometer 1 
and 2 by means of screws or other retaining means. O-rings 52 and 54 may 
be provide to seal the glass tube, the two halves of the nephelometer 1 
and 2 and the helix adapters 3 and 4. Also shown in FIG. 3 due to the view 
chosen are the lenses 34 and 36 which focus the scattered light on 
photocell 38, and the join line 15 at which the two halves of the 
nephelometer body 1 and 2 are joined. The caps 8 and 12 are shown in FIG. 
3 and illustrate how the two halves 1 and 2 of the nephelometer are held 
together along join line 15. Also shown in FIG. 3 is aperture plate 26 
which serves to delimit the size of the incident beam of light passed onto 
the fluid sample held in sample tube 28 as will be evident from an 
examination of the optical paths shown in FIG. 2. 
Referring now to FIG. 4, a cross-section is taken along line 4--4 of FIG. 2 
and shows how the two halves 1 and 2 of the nephelometer body fit together 
by means of tongues 56 fitting into grooves 58. By this construction, the 
two halves can be made identical and yet can mate without slippage or any 
necessity of tolerance for fit and can be adapted to seal very well 
without the necessity of gaskets, glue or other sealing means. A groove 60 
is also provided which runs essentially around part line 15 and which is 
used to carry the wires from the photocells 32 and 38 and bulb 20 to the 
power source and other parts of the system within which the nephelometer 
is used. Cap 10 is also shown in FIG. 4 and shows how it holds the two 
halves 1 and 2 of the nephelometer together along part line 15. The cap 
may be secured by screws but this is not essential. Reviewing FIGS. 1, 2, 
3, and 4, one can gain an understanding of the overall construction of the 
nephelometer according to the invention and of the several inventive 
features thereof. In particular, its design is such that the two halves 1 
and 2 of the nephelometer not only can be molded identically at vast 
savings of labor and mold requirements but can also be made to hold the 
various components of the nephelometer in fixed positions without the 
necessity of bracketry, riveting, glues, screws, mastics, gaskets and the 
like. In a preferred embodiment, the nephelometer body halves 1 and 2 are 
molded of ABS plastic, 20% glass filaments. In particular, note how the 
provision of the groove into which the lamp socket 21 fits not only 
locates the lamp 20 at a predetermined distance from lenses 22 and 24, 
thus providing a pre-focused beam of light at aperture partition 26, but 
also serves to locate the panel on which photosensor 32 is located. 
Similarly, the provision of the grooves 41, 42, and 43 which hold the 
aperture 26 and the lenses 22 and 24, respectively, means that these parts 
are located precisely to provide a properly sized focussed light beam and 
do not require mounting on a separate bracket but can be simply assembled 
within the two halves of the nephelometer body. The cap 8 may also be 
adapted to contain a portion of light absorbing material 40 such a black 
felt to absorb any stray light which may scatter in that direction from 
the incidence on the fluid contained in the sample tube 28. In this 
connection, it will be noted that it is desirable throughout the interior 
of the nephelometer to provide a surface finish to its parts which does 
not tend to reflect light so that all the light in the nephelometer is 
that in the optical path shown in FIG. 2 so as to retain optical accuracy; 
the surface finish may be specified to be between 1,000 and 3,000 
microinches average peak to valley distance. In this way, any light which 
is incident on the inside surface of the nephelometer will not be 
reflected. 
The overall operation of the system therefore is substantially as follows. 
In a particular system embodiment such as that described in the Selby 
patent referred to above, water flows through the system and through the 
sample tube 28 at a rate of approximately 0.8 gallons per minute. The lamp 
20 provides a high intensity, focused beam of light which travels through 
the sample tube 28 and illuminates the diffusion screen 30 on its other 
side. The photosensor 32 placed on the other side of diffusion screen 30 
is operated in a feedback mode to maintain the intensity of illumination 
provided by lamp 20 at a constant level. This through-sample lamp 
regulation method thus provides for color rejection, as the amount of 
light exiting the sample tube is held constant regardless of sample color. 
The diffusion screen 30 is used to effectively integrate the light exiting 
the tube thereby reducing erroneous readings due to particulate matter 
that may temporarily adhere to the inside surface of the sample tube 28. 
Diffusion screen 30 is desirably tilted at approximately 45.degree. in 
order to reduce any background illumination around the tube produced by 
reflections off the smooth surface of the screen. The inside walls as 
discussed above of the nephelometer are sandblasted or otherwise formed to 
a dull black color, thus attenuating any reflected light. 
The small amount of light scattered from the sample in the sample tube 28 
due to suspended particles therein is then focused, by means of lenses 
34-36, on the second photosensor 38. The signal produced by photosensor 38 
is then amplified and processed through bubble rejection circuitry and 
output as a signal desirably between 0 and 10 volts. The aperture and lens 
system constrains the sensor to see only the center of the sample tube 28 
at which the incident beam is focused. This reduces the background 
illumination due to inherent imperfections in the glass tube 28 at where 
the incident beam enters and exits. Since the sample tube 28 is 
cylindrical and the beam is focused in its center, all the light paths are 
incident perpendicular to the surface of the tube 28 and no refraction of 
the light occurs due to passage through the transparent glass or plastic 
sample tube. It will be appreciated by those skilled in the art that the 
resolution of the nephelometer of the invention will be maximized if the 
orientation of the filament of the bulb 20 and that of the photocell 38 
are, as shown in FIG. 2, such that the image of the filament as focused by 
lenses 22, 24, 34 and 36 substantially coincides with the sensitized area 
of the photocell. Bubble rejection circuitry is incorporated in the signal 
processing circuitry of the invention and it operates by limiting the 
upward slew rate of the circuitry to about 0.5 volts per second. This 
allows the differentiation of bubbles from turbidity since the bubbles 
tend to move through the tube faster, tending to float upward, than the 
average flow rate. Typical sample flow rate, as discussed above, is 0.8 
gallons per minute representing a bubble velocity of about 5 inches per 
second for a total optical reflection time of about 40 milliseconds for 
one bubble flowing past the sensor area. Thus, a single bubble produces a 
momentary output reading of only 20 millivolts. This effectively cancels 
erroneous readings due to bursts of bubbles flowing through the sample 
tube 28. Such circiutry is known in the prior art. 
As discussed above, it as been a traditional problem in the design of 
optical instruments for measurements on fluid samples to prevent the 
viewing ports from becoming fouled over time due to the presence of a 
stable laminar layer of the sample fluid on the inside surfaces of the 
ports. In addition to a gradual build-up of small particulate matter, 
bubbles tend to adhere to these surfaces and algae growth may occur. In 
the nephelometer according to the invention, the problem is alleviated 
first by turning the lamp 20 off except when readings are desired, to 
reduce the opportunity for algae growth, and second by utlizing in-stream 
helices 48 and 50. The adhesion of particles and bubbles to the inside 
surface of the sample tube 28 is due largely to the formation of a static 
laminar layer a few thousandths of an inch thick at the water glass 
interface. This static layer due to the viscosity and surface tension of 
the water provides a surface for particles and bubbles to adhere to. The 
in-stream helices 48 and 50 swirl the incoming water centrifugally to 
centrally break down the laminar layer and wash the surface of the sample 
tube 28 clean. The velocity of the water at the water/glass interface is 
relatively high and prevents any algae growth or particle build-up to 
occur. The cleaning action is much the same as water jets constantly 
washing the surface clean. Furthermore, the helixes which are slightly 
over one complete revolution serve as light traps preventing any light 
from entering via the hoses. It will be understood by those skilled in the 
art that helices disposed in the flow path will be useful to keep viewing 
ports clean in a wide variety of instrument systems. 
Referring now to FIGS. 5 and 6, the specific electronic circuitry used will 
be described. FIG. 5 shows the circuitry of the nephelometer amplifier. As 
illustrated in FIG. 2, the photocell 38 detects the light scattered from 
the turbidity in the sample tube 28 varying the voltage on the positive 
input of an operational amplifier 62. This is then amplified, clipped by a 
diode 64, fed through an integrator stage comprising amplifier 66 and fed 
back to the input of op-amp 62. Initially both inputs and the output of 
amp 64 are at 0 volts. A diode 68 and a resistor 70 provide a stable 
voltage reference for the photocell 38. As the light strikes the 
photocell, its resistance goes down causing the output of the op-amp 62 to 
go down. This is then fed through a resistor 72, clamped by diode 64 to 
approximately -0.2 volts. Diode 64 and resistor 74 cause a constant 
current to charge capacitor 76 at approximately 0.5 volts per second. As 
this capacitor 76 charges, the output of amplifier 66 ramps up and is fed 
back by means of trim pot 78 and resistor 80 to the input of amplifier 62, 
thus stabilizing the output at a voltage proportional to the light 
striking the photocell 38. The trim pot 78 is also used to adjust the gain 
to trim out component tolerances which is done by inserting a sample of 
known turbidity in the sample tube at installation of the device in a 
system and adjusting the trim pot 78 until the output is appropriate. 
Referring now to FIG. 6, the lamp regulating circuitry, by which the light 
detected by the second photocell 32 is fed back in a loop to adjust the 
power delivered to lamp 20 and therefore its intensity, will be described. 
The photocell 32, which is preferably a cadmium sulfide photocell, is used 
as a feedback element. This photocell 32, a resistor 82, a trim pot 84, 
diodes 85 and 86 form a resistive light dependent voltage divider sourcing 
current to a transistor 88. This transistor 88 in turn feeds Darlington 
transistor pair 90 which control the intensity of power delivered from an 
8 volt source to the lamp 20. Trim pot 84 adjusts the closed loop gain to 
trim the lamp intensity for component variation. As discussed above, it is 
desirable to turn the lamp 20 on and off slowly so as to preserve its bulb 
life. This is done by a network comprising transistors 92 and 94. When a 
bulb-on input is applied to the base of transistor 92 indicating that the 
bulb is required, transistor 92 saturates and causes capacitor 96 to 
discharge through a resistor 98. As the gate voltage to transistor 94 is 
ramped down, the base of transistor 88 is allowed to ramp up thus causing 
bulb 20 to gradually light and the loop to close. When the bulb-on input 
to the base of transistor 92 is pulled low, the transistor 92 opens and 
capacitor 96 charges through resistors 98 and 99 thus causing the bulb 20 
to ramp back down. Diode 100 and resistor 101 work in conjunction with the 
Darlington pair 90 to serve two functions; first, to keep a slight voltage 
on bulb 20 when it is off and second, to turn on transistor 102 when the 
bulb filament fails. When the bulb 20 is nominally off, diode 100 is 
forward-biased and the Darlington pair 90 is adjusted to provide 
approximately 0.5 volts on the bulb so as to provide a "keep-alive" 
current. A voltage drop of approximately 0.35 volts is across resistor 101 
keeping transistor 102 off. When the filament fails, the voltage at the 
base of transistor pair 90 rises from approximately 6 volts to 
approximately 14 volts raising the voltage across the resistor 101 to 
about 0.7 volts turning on transistor 102 which may be used to provide a 
signal that the bulb 20 has burned out. By providing a keep alive function 
as discussed above, a small current is continuously passed through the 
bulb keeping the tungsten filament from passing through the 
250-350.degree. C. tungsten ductile-brittle transition region which causes 
bulb failure. As described above, the circuit ramps on and off over 
approximately 5 seconds. This too allows the bulb to warm up slowly and 
extends its life substantially. Moreover, this use of intermittent and low 
current operation of the bulb 20 allows the total heat output by the 
nephelometer to be reduced, which extends component lifetime in general. 
To this end, a heat sink 7 as shown in FIG. 1 may be mounted on Darlington 
pair 90 which contro the power supply to the lamp 20. 
It will be understood by those skilled in the art that the circuits of 
FIGS. 5 and 6, while sophisticated in operation require relatively few 
components; indeed, it is possible to mount all the components of these 
circuits inside the caps 10, 11, and 12 of the nephelometer unit, thus 
simplifying its hook-up remarkably. It will be understood that numerous 
changes and modifications may be made to the nephelometer of the invention 
without departing from its essential spirit and scope which is defined in 
the following claims.