Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
BACKGROVND OF THE INVENTION
This invention relates -to the detection of oil in
water and in particular to arrangem~nts for detec-ting and
measuring the concentrations of different -types of oils in
water. Typically, these arranyemen-ts are used in ba]last and
bilge monitoring operations on ocean golng -tankers and/or
other vessels.
One of the problems in oil-in-water detection and
measurement by a light scattering technique results from
widely differing responses from different oils that must be
measured~ Not only should the detector employed be able to
distinguish between suspended solid particles and oil droplets,
but it also should be able to compensate for the response
speeds of different types of oils. Hitherto this has not been
possible~
The intensity of light scattered by a suspension of
oil-in-water is a function of the scattering angle and has a
maximum value at an angle de-termined primarily by the averaye
size of the oil droplets. The droplet size is, in turn, det-
ermined by the oil viscosity. The viscosity range of both
cr~d,e and refined oils is, of course, very wide. They are
also temperature dependerlt. This makes it necessary to recal-
ibrate when a single scatter angle cell is employed to measure
different types of oil or to measure the same oil a~ widely
differing temperatures.
G.D. Pitt 24, U.S. Patent No. 4,265,535 issued
May 5, 1981, describes a dual angle scatter cell in which the
light scattering ang].es are such that the effects of suspended
solid scatter can be reduced from the oil reading.
..
G. D. Pitt et al. 3~-S-lX
SUMMARY OF THE PRESENT INVENTIO~
In accordance with ~he o.il detector of the present
invention, there is provided an oil-in-water detector
arrangement compr.ising: a scatter cell; a pulsed light
source, scattering of light from said pulsed light source
being e~ected; drive means for said pulsed l.ight source; a
first photodetector disposed in alignment w;th said pulsed
light source so as to receive light ~ransmitted directly
through ~he oil and water mixture: second and thircl
photodetector6 dispos0d respectively at first and second
angles to said incident liyht beam~ said incident light beam
being generated by said pulsed light source so as to receive
light scattered by the oil and water mixture; first, second
and third amplifier channels having first, second and third
synchronou6 detecto.rs, respectively, associated with said
first, second and third pAotodetectors, respectively, and,
in use, enabled when said pulsed li.ght source is pulsed by
said drive means; gain control means including a feedback
loop from said first synchronous datector via an associated
amplifier channel to said drive means, control of said
pulsed light source intensity being provided by said
feedback loop; and means responsive to the difference
between the outputs oE said second and third amplifier
channels for producing an indication of oil concentration.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings which illustrate
exemplary embodiments of the present invention:
Fig. 1 is a diagrammatic view of an oil-in-water
detector constructed in accordance with the presen~
invention;
Fig. 2 is a diagrammatic view of circuitry for the
detector of Fig. 1:
Fig. 3 is a diagrammatic view alternative to that
of Fig. 2; and
Figs. 4 and 5 are graphs of the responses of the
circuits of Figs. 2 and 3, respectively, ~o various types of
oil.
-3- G.D. Pitt: et aL. 3~-5-1
DESCRIPTION OF THE PREFERRED EMBODIMBNTS
. . .
Referring to Fig. 1, the scatter cell of the present
invention comprises a housing lL through which water c~rrying
suspended oil may flow. Housing 11 is provided with an inlet
window 12 for incident light and a plurali-ty of outlet
windows 13, 14 and 118 for receivinq light scattered from the
suspended oil. Typically, the incident liyht bearn is provided
via an infra~red solid state laser (not shown), but a ZED or
other light source may also be used.
A baffle 15a may be mounted adjacent the window 12 to
ensure that only scattered light reaches the windows 13 and 14.
In some applications the baffle may be mounted perpendicular to
the incident ligh~ beam as indicated by the reference 15b.
Scattered light reaching the windows 13 and 14 is fed to
respective photodetectors 16 and 17, preferably via optical
fibers 18. Advantageously the scatter cell also includes a
further output window 118 whereby the intensity of the incident
beam may be monitored. Compensation for changes in that
intensity may then ~e made.
The outputs of the photodetectors 16 and 17 are coupled
to electronic circuitry~ e.g., a microprocessor 19, which is
programmed to compute an oil concentration level from the
absolute and relative values of the outputs of photodetectors 16
and 17.
In accordance with the present invention, it has been
found that the angle at which light is scattered from oil
droplets suspended in water and the scattered light intensity
depend primarily on the average droplet diameter and refractive
index. This, in turn, is determined by the oil viscosity.
Thus, for exampler diesel oil which is relatively light produces
small droplets which scatter light through a relatively large
angle, whereas a more viscous medium fuel oil produces larger
droplets which scatter light predominantly through a relatively
small angle. Thus, each oil produces its own characteristic
scattering intensity at each of the photodetectors 16 and 17,
-~- G.D. Pitt et al. 34~5~LX
and by comparing these two intensities, a measure oE the oil
concentration can be obtained~ This technique also provides
compensation for the lowee intensity of llght scattered by more
viscous oil~.
The difference in scattering response characteristics of
different types oE oils is illustrated in the fo]lowing example~
Water containing an injected 100 parts per million of
diesel oil or medium fuel oil (MFO) was passed throuyh a ce]l of
the type shown with windows 14 and 13 disposed, respectively,
lQ at 17, 30 and 38 to an incident gallium arsenide
infra-red light source. Photodetector outputs monitored for
each type of oil might result as follows (in arbitrary units).
Detector Response
Oil Type/Angle
17 30o 38
-
Diesel 118118 17
MFO 77 81 2
From the above it will be clear that by suitable
programming of the circuitry coupled to the photodetectors
(photocells), an absolute concentration value for each type of
oil can be obtained without individual calibration of the system.
By performing subtractions or ratioing of the light
scatter signals at the various scatter angles, the concentration
of each individual oil can be obtained. Thus, the detector
system requires only a single initial calibration and can then
be used on all types of oil without further adjustment.
Cells with perpendicularly mounted baffle l5a can be
constructed having scattering angles al and ~2 of 22.5
and 45, respectively, and will be found to give good results
with a wide range of oils. Typicallyl scattering angles ~1
and a2 of 20 to 25 and 40 to 50, respectively, are
preferred. These scattering angles are given by way o example
and are not to be regarded as limiting.
-5- G.D. Pitt et al. 34-5-LX
A square or rectangular ceLl could be used. Such
geometries allow for an array of detectors mounted opposite the
source on a flat printed circuit board. Other cell cross
sections can also be used.
Referring now to Fig. 2, this shows a typical clrcuit
arrangement for calculating oil concentration levels from the
outputs of the dual angle cell arrangement of E'ig. 1. The cell
outputs for scattering angles of zero, ~1 and a2 are fed via
photodiode detectors D0, ~1 and D2 to preamplifiers PA0, PAl and
PA2. These preamplifiers may each include a Eield efEect
transistor or an operational amplifier.
The outputs of the preampliflers PA0, P~l and PA2 are
fed via amplifiers AMP0, AMPl and AMP2 to respective synchronous
detectors SD0, SDl and SD2.
Light is injected into the scatter cell via a laser or
an LED 21 driven in a pulsed mode by driver circuit LDl.
Typically, the light source 21 is operated at a low duty cycle,
for example 2%, thus ensuring that the source has an extended
lifetime. The driver circuit LDl is coupled to the synchronous
detectors SD0, SDl and 5D2 such that the detectors are enabled
only when the light source 21 is pulsed on.
The output from the synchronous detector SD0 associated
with the direct light path through the cell is fed back to the
drive circuit LDl so as to provide an automatic gain control
feedback loop whereby compensation is provided both from aging
or drift of the light source and from oil fouling of the cell.
This technique ensures that continuous calibration of the
detector arrangement is effected.
The outputs of synchronous detectors SDl and 5D2
associated with the scattered light signals are fed to the
respective inputs of a differential amplifier DAl whose output
comprises an analog signal corresponding to the difference in
intehsity between the two scattered light signals. Although oil
viscosity has a significant effect on the scatter profile of an
incident light beam, we have found that the difference signal
obtained from scatter signals received at two suitable angles to
an incident light beam is substantially independent of oiL types.
-- 5 --
-6 G.D. Pitt et al. 34-5-lX
Typically, the differential amplifier output signal is
fed via a buffer stage BSl to a control and displa~i~ arrangement
CDl which may include means for recording measured oil levels
and for generating a warning signal when a predetermlned oil
leve] is exceeded.
The circuit arrangement shown in Fig. 3 is some~hat
similar to that of Fig. 2 but employs digital processing
techniques. The input circuit stages comprising preamplifiers
PA0, PAl and PA2, amplifiers AMP0, AMP1 and AMP2 and synchronous
detectors SD0, SDl and SD2 operate in a similar manner to the
arrangement of Fig. 2 and need not be further described.
The outputs of the synchronous detectors SD0~ SDl
and SD2 are fed via an analog multiplexer to an analog to
digital converter A/~l and a microprocessor MPUl programmed to
perform the computation of oil concentrations from the digitized
detector output signals. The microprocessor output may be used
to drive a variety of operating functions including cell
flushing and water sampling. The microprocessor can also drive
an output recorder and an excess oil alarm syste~.
Typically, the microprocessor is programmed with an
algorithm constructed~ e.g., to compensate for droplet-size
variations in the fluid flow through the cell or to provide
outputs giving an indication of droplet size distribution.
In an alternative embodiment a single synchronous
detector preceded by a multiplexer may be employed. The single
detector then feeds into an analog to digital converter.
In further applications light scattering may be effected
at three or more angles to the incident beam to provide further
accuracy in the measurement process.
Fig. 4 illustrates the results obtained from the
measurement of various types of oil using the circuits of
Figs. 2 and 3. Fig. 5~ which is included for comparison
purposes, illustrates the corresponding measurements obtained
from a conventional single angle scatter cell. In each case
measured quantities of each type of oil were injected into a
~7- G.D~ Pitt et a]. 34-5-lX
water stream flowing through the cell and the correspondlng
detector output response was determined. As can be seen from
Figs. 4 and 5, the response spread from the various types of
oils at an injected level oE 100 parts per million is ~ 27% for
a single angle scatter cell ~Fig. S) but this spread is reduced
to within _ 20% (Fig. 4) by using the double angle scatter
techniques described herein. This represents a s:igniEicant
improvement in accuracy over conventional techniques described
herein suitable for bilge water monitoring applications without
the need for recalibration for different oilsu
The algcrithms necessary for the computation process can
also be partially dealt with using set gains at different angles
on the amplifiers, the ratios/values of these present gains
being optimized in prior experiments and tests. This opens up
the use of such equipment for generalized measurements on
3 phase systems - e.g. for measuring oil/particles/water etc.,
and filtering checking systems.
Typically, the algorithm for a particular cell geometry
is determined for measurements on known injected oil levels, the
~0 necessary techniques being known to those skilled in the art.
Once a particular scatter cell has been calibrated in this way
no further calibration is necessary.
