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BACKGROUND OF THE INVENTION
This invention relates generally to the remote measurement of fluid
temperature and particularly to a temperature measurement system which
operates by analysis of backscattered Raman radiation.
Water, having high heat capacity and being the most abundant compound on
the surface of the earth, has been used extensively as a coolant and heat
exchange fluid in various kinds of experimental and practical equipment.
For example, in nuclear reactors it is desirable to operate at the highest
practicable temperature owing to the fact that at higher temperatures the
efficiency for conversion into useful power is greater. Since an increase
in temperature decreases the density of water and therefore the cooling
efficiency, pressure is usually applied to the cooling system. Under
extreme conditions of high pressure and high temperature, the temperature
of water is normally measured by thermocouples or, in the case of a
nuclear reactor, a boron thermopile. Inserting a thermocouple into the
point of interest has the effect of interfering with the flow pattern of
the water and, correspondingly, with the local temperature. Another
limitation in the nuclear reactor situation is that the temperature is
only indirectly measured by the density of neutron flux. Both
thermocouples and thermopiles are permanently engineered into the reactor
system and any replacement or redesigning is not easily undertaken
especially in a high pressure system.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a system
that accurately measures at a remote location the temperature of a
relatively small area within a fluid. Another object of the invention is
to provide temperature measurement of a fluid through the analysis of
backscattered Raman radiation. Yet another object of the present invention
is to provide for remote temperature measurement at various points within
the fluid. Still one other object of the invention is to provide for
remote temperature measurement of a fluid by a device which is noncomplex
in its nature and which can be used to determine fluid temperature in a
relatively minimum amount of time.
Briefly, these and other objects are accomplished by a pulse laser which is
tuned and focused at some point within the fluid. Backscattered Raman
radiation from the point is reflected by a mirror through a focusing lens
to a filter designed to screen out the tuned laser frequency and to pass
only the backscattered Raman radiation frequencies. The optical output
from the filter is divided by a beamsplitter and mirror combination which
respectively provides outputs to corresponding interference filters each
of which is tuned to a discrete predetermined Raman wavelength. Individual
detectors sense the respective outputs of the Raman wavelength filters
which are individually amplified and connected to the inputs of respective
pulse train encoders whose output signal lengths are proportional to the
amplitude of the signals from each of the detectors. Individual counters
respectively count the number of pulses from the respective encoder
outputs and the counts are connected to the input of a divider which
provides a quotient output to a function generator whose output causes an
indicator to show the measured temperature. A delay pulse generator
receives an input pulse synchronized in time with the output pulse of the
laser and produces a variable delay output pulse to each of the respective
encoders so that the effect of dark current noise from the detectors is
minimized and also to allow the measuring system to range over a variety
of measuring points.
For a better understanding of these and other aspects of the invention,
reference may be made to the following detailed description taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a model of the structure of water as acted upon by the present
invention;
FIG. 2 is a graph in wavelength vs. intensity of the Raman spectrum of the
fluid model shown in FIG. 1;
FIG. 3 is a block diagram of the present invention; and
FIG. 4 graphically illustrates the response function of a typical function
generator shown in FIG. 3.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1, there is shown a model molecule H--O--H, and a
hydrogen bonded polymer the structure of a fluid such as, for example,
water as a mixture of clusters and single molecules. Other fluids such as
hydrogen fluoride, which may be either a gas or liquid depending upon
pressure and temperature, and certain bioorganic molecules also exhibit a
similar mixture model. This model assumes the simultaneous existence of at
least two distinguishable species of water. Earlier sound absorption
measurements and many infrared, near infrared, and Raman spectroscopic
investigations support this mixture model. In the simplest version of this
model of liquid water, a kinetic equilibrium exists between two
structurally distinct species, a non-hydrogen-bonded monomer shown as the
smaller encircled molcule H--O--H, and a hydrogen bonded polymer
illustrated as the larger encircled cluster comprising a multitude of
hydrogen and oxygen elements. Since the water molecule cluster has a large
dipole moment and the monomer has a more compact structure than that of
the cluster, the population ratio of these two structural states is
controlled by the pressure of the water, the concentration of charged ions
in the water, and the temperature of the water.
Referring now to FIG. 2, there is shown a graph of the Raman spectrum of
liquid water plotted in wavelength vs. intensity. As noted, the two
continuous curves respectively represent the Raman spectrum of the
polymeric and monomeric species of liquid water with the polymeric species
producing a more pronounced effect at the low wavelength end of the Raman
spectrum and the monomeric species producing a pronounced effect at the
high wavelength end. The sum of the polymer and monomer curves are
illustrated in the discontinuous line shown in the graph. As earlier
noted, there exists a temperature dependence within the Raman spectra of
liquid water. This dependence causes a minor but detectable shifting
effect within the Raman spectra. Accordingly, as the temperature of the
liquid water is raised, the curves shown in FIG. 2 have a tendency to
shift to the right thereby incurring an increase in wavelength and a
corresponding decrease in intensity. Conversely, should the temperature of
the liquid water be lowered, the curves shown in FIG. 2 have a tendency to
shift to shorter wavelengths and a corresponding increase in intensity.
This phenomenom is discussed in further detail by G. E. Walrafen, Journal
of Chemical Physics, Volume 47, 1967. It should be noted that there are
two isosbestic points (not shown) situated at each side of the point of
maximum intensity that is shown in the sum curve of the Raman spectra. The
respective isosbestic points represent wavelengths between which the
curves are independent of temperature variation and, accordingly, no
shifting occurs. Thus the temperature information of the Raman spectra is
obtained from the shift of the relative intensities of the spectra
contours that lead away from the isosbestic wavelengths. By comparing the
relative spectral intensities in the two halves of the Raman band affected
respectively by the polymeric and monomeric mixture of the liquid water,
the temperature of the water can be calculated. The comparison of two
relative spectral intensities of differing Raman shifted wavelengths also
provides a cancellation effect to minimize any incremental change in
spectral intensity by the laser or other structural elements within the
measuring system.
Referring now to FIG. 3, there is shown a block diagram of the present
invention. A nitrogen laser 10 produces coherent light pulses which
sequentially pass through a dye cell 12, a hole in the center of a mirror
14, a first focusing lens 16 and an optically transparent window 18
attached to a cross-section 20 of a container of fluid such as water. The
plane of the mirror 14 is optically aligned at a 45.degree. angle with the
axis of the laser beam B and a second focusing lens 22 is optically
aligned at a 45.degree. angle with the plane of the mirror 12 and at right
angles to the axis of the laser beam. The lens 22 focuses incoming
radiation onto a filter 24 which passes filtered radiation to a radiation
detector network 26. Network 26 comprises a beamsplitter 28 which receives
the filtered radiation from filter 24 and splits the radiation into two
components, one of which is reflected onto a reflecting mirror 30
positioned at a 45.degree. angle to the incoming beam and the other
component of which is transmitted to a first Raman wavelength filter 32 a.
Mirror 30 reflects the incoming beam onto a second Raman wavelength filter
32b. Dual photomultiplier (PM) tubes 34a, 34b, receive respectively
optical outputs from filters 32a, 32b and provide corresponding electrical
outputs from the network 26. Charge sensitive amplifiers 36a, 36b are
connected to receive respective outputs from the photomultipliers 34a, 34b
and provide amplified outputs respectively to first inputs of encoders
38a, 38b. A delay pulse generator 40 is connected to receive a trigger
pulse from the laser 10 and provides an output signal that is commonly
connected to second inputs of the encoders 38a, 38b which provide
corresponding outputs to counters 42a, 42b. A divider 44 is connected to
receive respective outputs of the counters 42a, 42b and produces a
quotient output which is connected to the input of a function generator 46
whose output is connected to a temperature indicator 48.
Again referring to FIG. 3 the operation of the invention will now be
described. For the sake of simplicity and ease of illustrating the present
invention, specific operating parameters will be assigned to particular
structural elements within the invention and are offered by way of example
only inasmuch as the present invention is not to be restricted to these
exemplary operating parameters. Although other lasers may be used, the
laser in the embodiment of the present invention is a 3371A ultraviolet
pulsed nitrogen laser. The laser 10 output is received by the dye cell 12
which tunes the 3,371A beam to a 4600A beam. At 4600A, the water in the
container cross-section 20 appears more transparent to the laser beam than
at 3,371A. The output beam of the dye cell 12 is passed through the hole
in the mirror 14 and focused by the lens 16 through the window 18 of the
water container cross-section 20 onto a point P in the center of the
cross-section. The focal point of the lens 16 and other gating circuitry
which will later be explained determines the point P within the container
cross-section 20 at which the temperature is to be measured. Upon focusing
at the measurement point P within the water, there is a Raman
backscattering radiation effect covering wavelengths over a spectra of
approximately 5270A to 5600A as earlier illustrated in FIG. 2.
Correspondingly, the bandwidth of the Raman water spectra is approximately
50 to 100 angstroms. The backscattered radiation is collected and focused
by lens 16 upon the surface of the mirror 14 which is positioned at a
45.degree. angle to the laser beam B. Consequently, the mirror 14 causes
the focused backscattered radiation to be transmitted at a 90.degree.
angle to the laser beam and to be further focused by the second lens 22
which directs the backscattered radiation into the filter 24. Filter 24
blocks the remainder of the 4600A radiation reflected back from the water
but passes the Raman shifted rediation characteristic of the temperature
measurement point within the water. The filter 24 may be disposed
substantially anywhere within the reflected radiation optical path. One
suitable filter for this purpose is a liquid filter which exhibits
essentially complete isotropic volume bulk absorption of 4600A radiation
but is nearly completely transparent to longer wavelengths. The purpose of
the filter is to block the 4600A radiation from the dye cell which is
reflected from the target point P and which the Raman wavelength filters
32a, 32b, may not be able to reject fully. The optical output from filter
24 is transmitted to the input of the beamsplitter 28 which splits the
filtered output between a first component which is transmitted to a
reflecting mirror 30 and a second component which is transmitted to the
first Raman wavelength filter 32a. This first Raman wavelength filter is
designed to pass a predetermined wavelength, for example, of 5390A. This
wavelength and the succeeding wavelength to be used in the second Raman
filter 32b is predetermined by laboratory calibration to pass only the
Raman shifted wavelength which provides maximum sensitivity to temperature
changes within the water body under measurement. The predetermined filter
wavelengths are interrelated and are indicative of Raman induced shifts
caused by the dual species of the water molecules as earlier noted.
Accordingly, one Raman filter operates at a wavelength within the
monomer-induced portion of the Raman spectra and the other filter operates
at a wavelength within the polymerinduced portion of the Raman spectra.
The filters 32a, 32b have a band-width, for example, of 50 angstroms in
order to detect intensity changes over a relatively large spectrum of the
monomeric and polymeric species. Reflecting mirror 30 reflects the
incoming optical beam to the second Raman filter 32b tuned to a
wavelength, for example, of 5490A. The output from filter 32a is focused
on the field stop (not shown) of the first photomultiplier tube 34a and
the output from filter 32b is focused on the field stop (not shown) of the
second photomultiplier tube 34b. The photomultipliers 32 are sensitive to
the photoelectrons contained within the filtered Raman wavelengths and
provide an electrical output which is indicative of the intensities of the
photoelectrons at the respective wavelengths. The output of
photomultiplier 34a is connected to the input of a first charge sensitive
amplifier 36a and the output of photomultiplier 34b is connected to the
input of a second charge sensitive amplifier 36b. The respective amplified
outputs from amplifier 34 are connected to first inputs of the encoders
38a, 38b. The pulse delay generator 40 is connected to receive the trigger
pulse which indicates the occurrence of periodic light pulses from the
laser 10 provides a series of corresponding output pulses delayed in time
and which are commonly connected to the second or gating inputs of
encoders 38a, 38b. The purposes of the delay generator 40, which will
later be explained, is to minimize the dark current time produced by the
photomultipliers 32 and to provide ranging capability. Encoders 38a, 38b
are connected to receive the outputs from amplifiers 36 and respectively
provide high frequency pulse trains having a frequency for example, of 40
Megahertz, whose lengths are proportional to the amplitude of the signal
from the photomultipliers 34. Individual pulse trains are initiated by the
respective encoders upon the receipt of each delay pulse at the gating
inputs and continue to be produced for the duration of the individual
delay pulses. Accordingly, a new delay pulse will signal the start of a
new pulse train whose length is indicative of the signal amplitude to the
respective encoder at that time.
The variable length pusle train outputs from each of the encoders 38b, 38b
are respectively connected to individual inputs of the counters 42a, 42b
which provide an output count indicative of the sum of pulses within all
of the respective encoder pulse trains. Accordingly, the output from
counter 42a provides a count indicative of the spectral intensity of the
low wavelength end of the Raman spectra under observation and the count at
the output of counter 42b provides an output count indicative of the
spectral intensity at the high wavelength end of the Raman spectra under
observation. Divider 44 has a first input connected to receive the output
count from counter 42b and a second input connected to receive the output
count from counter 42b and provides a quotient output representative of
the division of the count provided at the second input divided by the
count provided at the first input. Obviously, since the present invention
processes a ratio or two spectral intensities, the divisor and dividend
signal levels may be interchanged. The function generator 48 is connected
to receive the quotient output from the divider 46 and generates an output
signal only according to a predetermined function as further described
hereinbelow.
Referring now to FIg. 4, a graph is illustrated in water temperature
(.degree.C.) vs. K (an equilibrium constant). The equilibrium constant K
is that value equal to the quotient output of the divider 44 which is
formed from the ratio of the two counter 42 outputs. In calibrating the
graph for the constant K, it is assumed that the Raman cross-section is
formed by equal contributions from each of the two species. More formally,
however, the equilibrium constant K is expressed as K = [M]/[B] where [M]
and [B] are mole fractions, respectively, of the non-hydrogen bonded
monomer and the hydrogen-bonded polymer. Accordingly, the constant K may
be adjusted to reflect differing contributions from each species. In this
example, the graph has been laboratory calibrated for equilibrium
constants ranging from 0.7 to 1.6 and the corresponding water temperature
ranges from 230.degree. to 300.degree.C. This exemplary graph depicts a
temperature sensitive curve which is calibrated at a particular fixed
pressure, for example, of 2500 psi and which minimizes the effects of
salinity. The plotted curve is substantially linear especially over short
temperature changes and indicates an increasing water temperature for an
increasing equilibrium constant. Obviously, other functions for differing
water temperatures, pressures and equilibrium constants may be plotted for
use in the present invention.
Referring again to FIG. 3, the function generator 46 is calibrated to
simulate the function shown in the exemplary curve of FIG. 4. Accordingly,
the generator 48 produces an output in any convenient form such as a
voltage level which is indicative of the water temperature under
observation. The temperature indicator which may be, for example, a meter
movement calibrated in degrees centigrade is connected to receive the
output from the function generator and provides an accurate indication of
the water temperature.
As earlier noted, the delay pulse generator 40 is utilized to minimize the
dark current count generated by the photomultiplier tubes 34. A typical
dark current counting rate is approximately 200 counts per second.
However, when the second inputs of the encoders 38a, 38b are gated with,
for example, 100 nanosecond gate pulses at the rate of 100 pulses per
second the dark current count is less than 2.times.10.sup.-.sup.3 per
second. One example of such a gated encoder is a model 100/N manufactured
by Ortek, Inc., Oak Ridge, Tenn. The foregoing example illustrates the
significant reduction in dark current count that can be achieved by
effectively gating the outputs of the photomultipliers so that the outputs
represent only the Raman scattered radiation from the measurement point P.
Such a gating effect also inherently provides ranging capability inasmuch
as the encoders only receive the photomultiplier outputs during intervals
which must relate to the laser pulse interval as a range interval. If the
gate pulse interval produced at the output of a generator 40 is coincident
with the laser pulse interval, the range is zero. Assuming, for example,
that the container cross-section 20 has a radius of 20 meters and the beam
distance from the laser to the point P is 10 meters, there would be
required an approximate delay of 90 nanoseconds between the receipt of a
trigger pulse from the laser and the output from the generator 40 for the
measuring system to focus on point P. Of course, the delay time that is
designed into the generator 40 could vary according to the optical range
between the laser and the target point.
Thus it may be seen that there has been provided a novel temperature
measuring system for remote measurements of fluid temperature without any
interference to the flow patterns of the fluid under observation.
Obviously, many modifications and variations of the invention are possible
in light of the above teachings. For example, a pulse counter may
conveniently be connected to the trigger pulse from the output of the
laser in order to count the number of pulses produced over a given period
of time in order to provide a predetermined statistical precision for
temperature measurement. It is therefore to be understood that within the
scope of the appended claims the invention may be practiced otherwise than
as specifically described.
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