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Claims  |
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We claim:
1. The combination of a device for monitoring the electrical conductivity
of an electrically conductive fluid and an electronic circuit for
processing signals produced by the device, said device comprising at least
first and second electrical coils for immersion in the fluid, said first
coil constituting a coil to be energized with an alternating electrical
current and said second coil constituting a detection coil for providing a
signal representative of the electrical conductivity of the fluid, the
first and second coils being wound on a common magnetic core and being
arranged relative to each other so that, in use, said coils are mutually
inductively coupled and are inductively coupled by the fluid; said
electronic circuit comprising a source of electrical power for supplying
electrical current to the first coil, means for generating an electrical
reference signal with which the electrical current supplied to the first
coil is compared, a feed back circuit, which is operative in response to
variations from a predetermined relationship between the reference signal
and the electrical current supplied to the first coil, for stabilizing the
electrical current supplied to the first coil, and a comparator circuit
for comparing the stabilized electrical current supplied to the first coil
with the output signal of the second coil, the comparator circuit being
operable to produce an output signal indicative of the inductive coupling
between the first and second coils and hence the electrical conductivity
of the fluid.
2. The combination according to claim 1 wherein the device further
comprises a third coil wound on the magnetic core which is common to the
first and second coils, the third coil constituting a second detection
coil and the first coil being arranged co-axially with the second and
third coils and being positioned therebetween.
3. The combination according to claim 1 wherein the device further
comprises a third coil wound on the magnetic core which is common to the
first and second coils, the third coil constituting a second coil to be
energized, and the second coil being arranged co-axially with the first
and third coils and being positioned between the first and third coils.
4. The combination according to claim 1 wherein the device further
comprises third and fourth coils wound on to the magnetic core which is
common to the first and second coils, the fourth coil constituting a
second coil to be energized with an alternating current and the third coil
constituting a second detection coil, the second and third coils being
located side-by-side along the axis of said core and being positioned
between the first and fourth coils.
5. The combination according to claim 1 wherein the feed-back circuit
comprises means for deriving an alternating signal indicative of the
current supplied to the first coil, a first rectifying circuit for
rectifying the alternating signal which is indicative of the current
supplied to the first coil, a first integrating amplifier for generating
an output signal which is the integral of the output signal from the first
rectifying circuit with respect to time, and a variable gain controlled
amplifier which, in response to the output signal from the first
integrating amplifier, is operative to adjust the current supplied to the
first coil.
6. The combination according to claim 1 for monitoring the temperature of
the fluid wherein the coils are mounted on a core which comprises a
material the Curie point of which lies outside the range of temperatures
to be monitored.
7. The combination according to claim 1 for monitoring the temperature of
the fluid wherein the coils are mounted on a core which comprises a
material the Curie point of which lies within the range of temperatures to
be monitored so that the change of magnetic properties of the core when
the fluid heats the core to its Curie point can be monitored to provide an
indication that the fluid has reached a temperature sufficient to heat the
core to the Curie point of the core.
8. The combination according to claim 3 wherein the electronic circuit
includes two amplifier channels and the second and third coils are
connected together so as to provide a push-pull input to the two amplifier
channels, each of the amplifier channels comprising a variable gain
controlled amplifier and a rectifier circuit for rectifying the output
signal of the variable gain controlled amplifier, the second coil being
connected to an input of one of the variable gain controlled amplifiers,
the third coil being connected to an input of the variable gain controlled
amplifier, and the output of each of the channels being connected to an
input of a differential amplifier which is common to both channels and
which is operative to compare the output signal of the channels and
produce an output signal which is used to alter the gain of the variable
gain controlled amplifier of each channel in response to the output signal
of the differential amplifier.
9. The combination according to claim 2 wherein a second non-integrating
amplifier is connected in parallel with the second integrating amplifier.
10. The combination according to claim 5, wherein the comparator circuit
comprises, connected in electrical series circuit relationship, a second
rectifying circuit for rectifying the output signal of the second coil, a
second integrating amplifier for generating an output signal which is the
integral of the output signal from the second rectifier circuit with
respect to time, and a circuit for receiving the output signals from the
first and second integrating amplifiers and for producing an output signal
representative of the relationship between the current supplied to the
first coil and the voltage induced in the second coil.
11. The combination according to claim 5 wherein a first non-integrating
amplifier is connected in parallel with the first integrating amplifier.
12. The combination according to claim 11 wherein the said first
integrating amplifier is an inverting amplifier and the said first
non-integrating amplifier is a non-inverting amplifier.
13. The combination according to claim 11 wherein the said first
integrating amplifier is a non-inverting amplifier and the said first
non-integrating amplifier is an inverting amplifier.
14. The combination according to claim 9 wherein the said second
integrating amplifier is an inverting amplifier and the said second
non-integrating amplifier connected in parallel therewith is a
non-inverting amplifier.
15. The combination according to claim 9 wherein the said second
integrating amplifier is a non-inverting amplifier and the said second
non-integrating amplifier connected in parallel therewith is an inverting
amplifier.
16. The combination according to claim 9 wherein the electronic circuit
includes means for comparing the output signals of the said first
non-integrating amplifier and said first integrating amplifier with a
signal representative of the difference between the output signals of the
said second integrating amplifier and said second non-integrating
amplifier connected in parallel with the second integrating amplifier, to
produce an output signal representative of the relationship between
changes of the current supplied to the first coil and changes of the
signal produced by the second coil relative to predetermined mean values.
17. The combination according to claim 9 wherein the electronic circuit
includes means for comparing the output signal of the second integrating
amplifier with the output signal of the second non-integrating amplifier
which is connected in parallel with the second integrating amplifier for
providing an indication of the relationship between the mean level of the
voltage induced in the second coil and fluctuations of the signal
indicative of the voltage induced in the second coil.
18. The combination according to claim 9 wherein the electronic circuit
includes means for comparing the output signal of the second first
non-integrating amplifier with the output of the non-integrating amplifier
connected in parallel with the second integrating amplifier for providing
an indication of the relationship of variations of the current supplied to
the first coil and variations of the output signal of the second coil.
19. The combination according to claim 8 wherein means are provided for
generating a reference signal with which the output signal of the
differential amplifier is compared, and means are provided for producing a
signal, indicative of the difference between the reference signal and the
output signal of the differential amplifier, which is used to control the
gain of the variable gain controlled amplifier of each channel.
20. The combination according to claim 8 wherein the output signal of each
channel is fed to a potential dividing circuit so that a signal indicative
of the difference between the output signals of each channel is produced.
21. The combination according to claim 20 wherein the signal indicative of
the difference between the output signals of each channel is fed to the
input terminal of an integrating amplifier and a non-integrating
amplifier.
22. The combination according to claim 21 wherein the output signal of the
integrating amplifier and the output signal of the non-integrating
amplifier are fed to a potential dividing circuit so that a signal
representative of the difference between the output signals of the
integrating and non-integrating amplifiers is produced. |
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Claims |
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Description |
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This invention relates to sensors for detecting the conductivity of an
electrically conductive fluid in which the sensor is immersed. Such
sensors may be used to monitor the temperature of fluids the conductivity
of which varies as a result of temperature fluctuations and also for
detecting fluctuations in the flow rate of such fluids.
In known liquid-metal cooled nuclear reactors, fuel is arranged in
sub-assemblies which are disposed in the coolant flow path so that coolant
streams flow in parallel over the fuel to cool it. In a prototype fast
reactor there are as many as 450 sub-assemblies each of which contains up
to 64 fuel pins. A blockage in one of these coolant streams can cause
overheating of the fuel and it is desirable that the reactor operator
should receive early warning of such a blockage.
There is a need to monitor the temperature of the coolant, or at least
variations in temperature from a predetermined value, before the coolant
leaves the confines of the sub-assembly and mixes with the outlet streams
of the other sub-assemblies.
Random mixing of a number of coolant streams at slightly different
temperatures produces temperature fluctuations about a mean value and this
gives rise to a fluctuating signal, conveniently referred to as "fuel
temperature noise". It is believed that blockage of one coolant stream has
a greater relative effect on this fluctuating signal than on the mean
outlet temperature of the coolant. The time constant of the signal is
however, of the order of only a few milliseconds and in conventional
measurements of the temperature fluctuations using thermocouples all the
higher frequency components of temperature noise, due to mixing of coolant
streams of different temperatures, are lost. Furthermore the thermal
capacity of the thermocouples and their electrical insulation are
important limiting factors. Also, thermocouples detect the low frequency
components of "inlet temperature noise" and "reactor power temperature
noise" produced by variations in inlet temperature and fluctuations of
reactor power respectively.
British Pat. No. 1,252,257 discloses a conductivity sensor which is capable
of measuring the high frequency components of temperature noise and which
is suitable for detecting changes in the electrical conductivity of the
liquid metal coolant which result from temperature changes. The
conductivity sensor described in the above mentioned British Patent
employs two pairs of energised coils which are immersed in the conductive
liquid metal coolant. The coils of one pair are connected in a
series-aiding manner and the pair constitute a sensor-coil assembly, and
the coils of the other pair are connected in a series-opposing manner and
this pair constitutes a compensating coil assembly in the form of a
non-inductive bifilar coil which compensates for circuit impedance
changes. The sensor coil assembly is connected in electrical series with a
variable inductor and a unity ratio bridge circuit and the compensating
coil assembly forms the reference arm of the bridge circuit. A power
supply is provided to energise the bridge and means are provided to detect
when the bridge is in a null-condition. The sensor coil assembly and the
compensating coil assembly are not mutually inductively coupled. The
electronic detection circuit required for each sensor of the type
described in British Pat. No. 1,252,257 is, however, too complex to permit
the use of such sensors in large numbers as would be required for
individual monitoring of each fuel channel in a nuclear reactor.
According to the present invention there is provided a device for
monitoring the electrical conductivity of an electrically conductive fluid
comprising at least two electrical coils for immersion in the fluid, one
of the coils constituting a coil to be energised with an alternating
current and the other in use constituting a detection coil, the coils
being arranged relative to one another so that in use they are mutually
inductively coupled and are inductively coupled by the fluid, and an
electronic circuit which includes a power supply for supplying an
alternating current to the coil or coils to be energised, and a detector
for detecting the voltage induced in the detection coil or coils and for
providing an indication of changes in the inductive coupling between the
coil or coils to be energised and the detection coil or coils.
The device may comprise three coils positioned along a common axis one of
which constitutes a coil to be energised and the other two constituting
detection coils. In this case the coil to be energised is arranged
co-axially with the detection coils and is positioned between the
detection coils. Alternatively, a single detection coil may be arranged
between two coils to be energised.
The device may comprise four coils positioned along a common axis two of
which constitute coils to be energised and the other two constituting
detection coils. The two detection coils may be located side-by-side along
the axis and be positioned between the coils to be energised. In this
latter case the detection coils may be connected together in electrical
series, and if desired, in the case where there are two coils to be
energised they may be connected together in electrical series.
Preferably the electronic circuit includes means for generating a reference
signal with which the current supplied to the coil or coils to be
energised can be compared. The means for generating the reference signal
may comprise a feed back circuit which is operative in response to
variations from a predetermined relationship between the reference signal
and the electrical current supplied to the coil or coils to be energised
to tend to reduce variations in the electrical current supplied to the
coil or coils to be energised from the said predetermined relationship.
The feed-back circuit may further comprise means for deriving an
alternating signal indicative of the current supplied to the coil or coils
to be energised, a first rectifying circuit for rectifying the said
alternating signal, a first integrating amplifier for generating an output
signal which is the integral of the output signal from the first
rectifying circuit with respect to time, and a variable gain controlled
amplifier which, in response to the output signal from the first
integrating amplifier, is operative to adjust the current supplied to the
coil or coils to be energised.
Preferably a first non-integrating amplifier is provided for amplifying the
output signal of the first rectifying circuit, and the first
non-integrating amplifier is connected in parallel with the first
integrating amplifier. The said first integrating amplifier may be an
inverting amplifier and the said first non-integrating amplifier is a
non-inverting amplifier. Alternatively the said first integrating
amplifier is a non-inverting amplifier and the said first non-integrating
amplifier is an inverting amplifier.
The electronic circuit may further include a comparator circuit for
comparing the current supplied to the coil or coils to be energised with a
signal indicative of the voltage induced in the detection coil or coils,
and for producing an output signal indicative of the difference between
the current supplied to the coil or coils to be energised and the signal
produced by the detection coil or coils. The comparator circuit may
comprise in electrical series, means for receiving the signal produced by
the detection coil or coils, a second rectifying circuit for rectifying
the signal produced by the detection coil and a second integrating
amplifier for generating an output signal which is the integral of the
output signal from the first rectifier circuit with respect to time.
A second non-integrating amplifier may be connected in parallel with the
second integrating amplifier, in this case, either the said second
integrating amplifier is an inverting amplifier and the said second
non-integrating amplifier is a non-inverting amplifier, or the said second
integrating amplifier is a non-inverting amplifier and the said second
non-integrating amplifier is an inverting amplifier.
The electronic circuit may include means for comparing the output signals
of the said first non-integrating amplifier and said first integrating
amplifier with a signal representative of the difference between the
output signals of the said second non-integrating amplifier and said
second integrating amplifiers, to produce an output signal representative
of the relationship between changes of the current supplied to the coil or
coils to be energised and changes of the signal produced by the detection
coil or coils relative to predetermined mean values.
The electronic circuit may further include means for comparing the output
signal of the first integrating amplifier with the output signal of the
second integrating amplifier and for providing an indication of the
relationship between the mean levels of the current supplied to the
energised coil or coils and the mean signal produced by the detection coil
or coils.
The electronic circuit may include means for comparing the output signal of
the second integrating amplifier with the output signal of the second
non-integrating amplifier for providing an indication of the relationship
between the mean level of the signal indicative of the voltage induced in
the detection coil or coils and fluctuations of the signal indicative of
the voltage induced in the detection coil or coils.
The electronic circuit may include means for comparing the output signal of
the first non-integrating amplifier with the output of the second
non-integrating amplifier for providing an indication of the relationship
of variations of the current supplied to the coil or coils and variations
of the output signal of the detection coil or coils.
In the case where the device has two detection coils, the detection coils
may be arranged so as to provide a push-pull input to two amplifier
channels. In this case each of the amplifier channels comprises a variable
gain controlled amplifier, and a rectifier circuit for rectifying the
output signal of the variable gain controlled amplifier. Each of the
detection coils is connected to an input of one of the variable gain
controlled amplifiers, and the output of each of the channels is connected
to an input of a differential amplifier which is common to both channels.
The differential amplifier being operative to compare the output signal of
the channels and produce an output signal which is used to alter the gain
of the variable gain controlled amplifier of each channel in response to
the output signal of the differential amplifier.
Preferably means are provided for generating a reference signal with which
the output signal of the differential amplifier is compared, and means are
provided for producing a signal, indicative of the difference between the
reference signal and the output signal of the differential amplifier,
which is used to control the gain of the variable gain controlled
amplifier of each channel.
The output signal of each channel may be fed to a potential dividing
circuit so that a signal indicative of the difference between the output
signals of each channel is produced. The signal indicative of the
difference between the output signals of each channel is preferably fed to
the input terminal of an integrating amplifier and a non-integrating
amplifier.
The output signal of the integrating amplifier and the output signal of the
non-integrating amplifier are preferably fed to a potential dividing
circuit so that a signal representative of the difference between the
output signals of the integrating and non-integrating amplifiers is
produced.
Where a plurality of detection coils, or energised coils are provided, the
location of the energised coil or coils relative to the detection coil or
coils, and the inductive coupling between them may be arranged such that
the electrical conductivity of the fluid is monitored at two or more
regions.
One or more devices so constructed may be used to monitor the electrical
conductivity of a flowing stream at two regions, one of which is upstream
of the other, for example to measure the temperature of the fluid. In this
case, means may be provided for discriminating between changes in
electrical conductivity of the fluid due to temperature changes, and
magnetic flux distortions, affects of inductance, and reflected impedance
of the fluid, due to variations in the flow rate of the fluid.
The result of variations of flow rate will show up as an "apparant" change
in the electrical conductivity of the fluid but in reality the variation
in flow rate does not in fact, alter the electrical condictivity of the
fluid but rather it alters the inductive coupling between the fluid and
the detection coil or coils and distorts the magnetic flux generated in
the vicinity of the detection along the line of flow.
To discriminate between changes in detector coil voltage due either to
temperature changes or changes in flow rate of the fluid, the detection
coil or coils may be arranged relative to the energised coil or coils such
that the flow of the fluid serves to enhance the coupling between the
energised coil and a first detection coil but serves to diminish the
coupling between the energised coil and a second detection coil. In this
case, by adding the outputs of the two detection coils the effect of flow
rate of the fluid can be cancelled. On the other hand by subtracting the
output of one detection coil from the other detection coil the combined
changes in flow rate and temperature may be determined.
According to a further aspect of the present invention there is provided a
temperature sensor for monitoring the temperature of a liquid-metal
coolant flow in a nuclear reactor by monitoring changes in the electrical
conductivity of the coolant comprising a device constructed as aforesaid.
According to a further aspect of the present invention there is provided a
method of monitoring the electrical conductivity of an electrically
conductive fluid comprising the steps of locating two coils in the fluid
such that the coils are mutually inductively coupled and are inductively
coupled by the fluid, energising one of the coils with an alternating
current and detecting the output from in such a manner as to detect
variations in the fluid inductive coupling.
A number of embodiments of the invention will now be described by way of
example, with reference to the drawings filed with the provisional in
which,
FIG. 1 illustrates one form of device constructed in accordance with the
present invention for monitoring the electrical conductivity of an
electrically conductive fluid,
FIG. 2 is a circuit diagram of an electronic control system for use with
the device of FIG. 1 and for use with the devices of FIGS. 3 to 5 for
determining temperature only,
FIGS. 3 to 5 illustrate alternative forms of devices, constructed in
accordance with the present invention, for indicating the temperature, and
flow and temperature of an electrically conductive fluid,
FIG. 6 is a circuit diagram of additional electronic circuits which are
required for processing flow information from the detector coils of the
devices of FIGS. 3 to 5.
Referring to FIG. 1 there is shown a device 1 for monitoring the electrical
conductivity of an electrically conductive fluid 2 in which the device is
immersed. The former 3 is constructed from stainless iron and comprises a
tube 6 mms inside diameter, 7 mms outside diameter, 70 mms long (total
length covered by the windings being about 50 mm). The former 3 is
provided at its ends with flanges 4. On to the former 3 are wound coils 5
and 6 which are fabricated from ceramic or other high temperature
insulation wire. A mineral insulated feed cable 7 is brazed or welded into
the flanges 4 and their inner conductors are connected to the coils 5 and
6. The device so constructed, is covered with a thin stainless steel
sheath 8 and the complete sensor is designed to operate inside a stainless
steel thimble tube 9 which forms a physical barrier between the sensor and
the electrically conductive fluid 2 which is typically liquid sodium at
600.degree.C disposed around the outside of tube 9.
In operation of the device of FIG. 1, coil 5 is energised whilst coil 6
forms a detection coil; both coils 5, 6 are inductively coupled together
and are inductively coupled by the electrically conductive fluid 2.
It will be seen that the fluid 2 effectively forms a single turn tertiary
winding spanning the length of the former 3. Furthermore the voltage
appearing across the detection coil 6 will depend upon the coupling factor
between the coils 5 and 6 and the reflected effects of the fluid 2. When
the coupling factor is made high, then high levels of coil voltage are
obtained but the sensitivity to changes in the fluid 2 is low. Conversely,
by employing weak coupling, the amplitude of the coil voltage can be made
to change at a rate equal to the rate of change of resistivity of the
surrounding fluid 2. However, with weak coupling, signal levels are low
and better overall performance is obtained using a coupling which gives
approximately half the ultimate sensitivity to resistivity changes. This
optimum value of coupling appears to be in the range K = 0.1 to 0.3 where
K is the coupling factor.
The electrical control circuit for the device of FIG. 1 is shown in FIG. 2.
The device 1 is shown schematically, and the inductive and resistive
components due to the surrounding conductive fluid 2 are also indicated
schematically. The coil 5 is connected to a balanced secondary winding 10
of a transformer 11. Similarly, the detector coil 6 of the device 1 is
connected to a balanced primary winding 12 of a transformer 13 by way of
mineral insulated cables 7.
A stable source of alternating voltage (typically 1 kHz for sensors
operating inside a 13 mm outside diameter thimble tube 9 surrounded by
liquid sodium at 600.degree.C) is applied to the input terminal 15 of
variable gain controlled amplifier 16. The 1 kHz waveform is further
amplified by a 3 watt integrated circuit power amplifier 17. The power
level from the amplifier 17, typically 1 to 2 watts R.M.S., is applied to
one end of a primary winding 18 of the transformer 11. A resistor 19 is
connected in electrical series between the other end of the primary
winding 18 and an earth connection 20. Since virtually all of the current
flowing in the primary winding 18 of transformer 11 is due to driving the
coil 5 of the device 1 the voltage developed across resistor 19 is
proportional to the current flowing in the coil 5. The voltage appearing
across resistor 19 is amplified by an amplifier 21, rectified by a
rectifier circuit 22, and passed to the input of an inverting integrating
amplifier 23 to produce a DC signal (representative of the current flowing
in the coil 5) and to the input of a non-inverting non-integrating
amplifier 24. The output of the amplifier 23 is fed-back and compared with
a DC reference voltage, any errors between the voltage at the output of
amplifier 23 and the reference voltage are amplified by an amplifier 25
and used to control the amplification of the amplifier 16. By closing the
feed back loop, the voltage across resistor 19, and hence the current
flowing through coil 5 of the device 1, are stabilized. The stabilized
current level in the coil 5, typically 30 to 40 mA R.M.S., can be set by
various means such as by the choice of the reference voltage, the value of
resistance 19 or, for trimming purposes, by the adjustment of a variable
resistor 80 which is connected in the input line to amplifier 21 between
earth connection 20 and the junction between resistor 19 and the primary
winding 18 of the transformer 11.
The output signal of the amplifier 24 is a steady DC level on which is
superimposed signals indicative of short term fluctuations of the current
in the coil 5 and common mode noise. Since amplifier 23 is an inverting
amplifier, the output signal of amplifier 24 will be of the opposite sign
to the output signal of amplifier 23. Hence, if two equal resistors, 26
and 27, are connected in series between the outputs of the amplifiers 23,
24 then these output signals can be compared and the difference signal at
the junction between resistors 26, 27 can be amplified by an amplifier 28.
Since coil 6 is inductively coupled to coil 5 and to the electrically
conductive fluid 2, the current flowing through coil 5 induces a voltage
in coil 6. Coil 6 produces an output signal, typically 30 to 40 mV R.M.S.,
on the secondary winding 29 of the transformer 13. One side of the
secondary winding 29 is connected to earth 20 whilst the other side is
connected to an inverting amplifier 30 which amplifies the signal produced
at the secondary winding 29 of transformer 13. The amplified signal is
then rectified by the rectifier circuit 31 to produce an output DC level
of about 8 volts on which are superimposed signals indicative of short
term fluctuations in the electrical conductivity of the electrically
conductive fluid 2. The amplifiers 21 and 30 are provided with frequency
selective feed-back arranged to give maximum amplification at the driving
frequency of 1 kHz. Although the Q (about 3) of the amplifiers is low,
good rejection against both high and low frequency noise is obtained. The
voltage at the output of rectifier circuit 31 will be almost directly
proportional to the instantaneous temperature of the electrically
conducting fluid 2, and the time resolution is a function of the drive
frequency, physical size of the sensor, and the time constants of the
whole electronic circuit. A typical time resolution of the system
described may be of the order of 10 milliseconds. By increasing the
driving frequency (at input 15) to about 4 kHz and appropriately changing
the values of frequency selective elements of the circuits, resolution
times of approximately 2 ms can be obtained.
The output signal from the rectifier circuit 31 is fed to the input of an
inverting integrating amplifier 32 and a non-integrating, non-inverting
amplifier 33. In this way, small but rapid fluctuations in temperature can
be measured by subtracting the integrated and non-integrated signals and
then amplifying the difference using amplifier 34 in a similar way to that
described with reference to the amplifiers 23, 24 and 28.
The overall sensitivity of the transient temperature signals is such that
1% change in amplitude of the signal on the secondary winding 29 of the
transformer 13 produces 6 volts change at the output of an amplifier 35.
To ensure that variations of .+-. 0.1% in signal amplitude can be detected
with certainty, background noise has to be kept to a minimum. To achieve
this, the difference signal at the output of the amplifier 34 is fed to
the input 37 of the amplifier 35 by way of a resistor 36 connected in
electrical series with the resistor 38 and the output of amplifier 28. In
this way, variations in amplitude due to imperfections in the driving
current, or common mode noise, are cancelled at the input of the amplifier
35 by the signals from the amplifier 28. The signals from amplifier 34
will be of the opposite sign to those from the amplifier 28.
Across the output of the integrating, inverting amplifiers 23, 32 are
connected two equal resistors 39, 70 which are in series. The output
signals of the integrating inverting amplifiers 23, 32 can therefore be
compared and the difference signal at the junction between resistors 39,
70 can then be amplified by amplifier 71. The output signal of the
amplifier 71 is a steady DC voltage indicative of the actual temperature
of the fluid 2.
For convenience of calibration, the output of amplifier 71 may be set to be
zero DC output at a chosen reference temperature, typically 450.degree.C
for liquid sodium in a prototype fast reactor. The output of the amplifier
71 could be arranged to produce typically .+-. 12V DC when the sensor is
surrounded by liquid sodium at temperatures ranging from 300.degree. to
600.degree.C that is to say .+-. 150.degree.C about the reference
temperature.
The device of FIG. 1 is incapable of distinguishing between changes in the
electrical conductivity of the fluid 2 resulting from temperature, or flux
distortion due to the flow of the conducting fluid. (The moving fluid
distorts the magnetic flux pattern in the direction along the line of
flow). The device of FIGS. 3 to 5, however, can be used to distinguish
between temperature, or flow variations.
Referring to FIG. 3, the device 1 is constructed in a similar manner to
that of FIG. 1 except that the windings of coil 5 are split into two
sections 5A, 5B (from the electrical point of view the coil 5 is still a
single coil) and the coil 6 is located between each half of coil 5. Flux
distortions due to the movement of fluid 2 tends to assist the coupling
between one of the sections of coil 5 and the detection coil 6 whilst
simultaneously reducing the coupling between the other section of coil 5
and the detection coil 6. In this way the flux distortion effects due to
flow can be cancelled out automatically and the device becomes insensitive
to flow. The device 1 of FIG. 3 can be further modified to enable
temperature and flow information to be determined concurrently. To achieve
this, the coil 6 may be provided with a centre tapped output (not shown)
to provide effectively two coils 6A, 6B in a push-pull output
configuration. In this way the signals due to different coupling
conditions between coils 5 and 6 at each end of coil 5 may be used to
provide an indication of the flow rate of fluid 2.
The device of FIG. 4 can be used to indicate temperature or flow variations
of the fluid 2. The device of FIG. 4 is constructed in similar manner to
that of FIG. 1 except that the windings of the detection coil 6 is split
into two sections 6A and 6B instead of the coil 5. The flux distortions
due to the movement of the fluid 2 tends to assist the coupling between
the coil 5 and one of the sections 6A or 6B of coil 6 whilst
simultaneously reducing the coupling between the other section of coil 6
and the coil 5. If one adds the outputs of both sections 6A and 6B of coil
6, then flux distortion effects due to flow are cancelled and the combined
output is representative of temperature changes. On the other hand if the
outputs of one section of coil 6 is subtracted from the output of the
other section of coil 6 the difference signal is representative of the
change in coupling between coils 5 and 6 due to flow.
The device 1 may be constructed as shown in FIG. 5 to enable temperature or
flow variations to be determined concurrently. Referring in greater detail
to FIG. 5, the device 1 is constructed in a similar manner to the device
of FIG. 1 except that the windings of the coil to be energised, coil 5, is
split into two sections each of which is located at an end region of
former 3, whilst two detection coils 6A, 6B are provided between the
sections of coil 5. All the coils 5, 6A, 6B are mutually inductively
coupled and are also inductively coupled by the fluid 2.
An additional circuit diagram for use with the version of the device of
FIG. 3 that has a centre-tapped output on coil 6, or the devices of FIGS.
4 and 5, to enable flow and temperature information to be determined
concurrently is shown in FIG. 6. The coil 5 is energised in exactly the
same way as shown in FIG. 2 and the current supplied to coil 5 is
stabilised in exactly the same way using the same feed back loop (not
shown in FIG. 6 but shown in FIG. 2). Coil 6A is connected to a balanced
primary winding 42 of transformer 43 and coil 6B is connected to a
balanced primary winding 44 of a transformer 45. In the case of the device
of FIG. 3 with a centre tapped output on coil 6, the primary windings 42
and 44 of the transformers 43 and 45, instead of having centre tap outputs
connected to the earth point 40 would be connected in series with the
junction between them connected to the earth point 40.
Each transformer 43, 45 has two secondary windings 46, 47 and 48, 49
respectively. The secondary windings 46, 49 are connected in series and
arranged so that their outputs add. There combined output is then fed to
the detector circuits of FIG. 2 at terminals 50, 51. The transformer 13 of
FIG. 2 is redundant since the transformers 43, 45 perform the function of
transformer 13. The combined output signals of windings 46, 49 are
processed by the circuit of FIG. 2, in exactly the same way as described
above, to provide signals at the outputs of amplifiers 71 and 35 which are
respectively representative of actual temperature of the fluid 2 and
changes in temperature of the fluid about the mean temperature.
The secondary windings 47, 48 are also connected in series but their
junction is connected to earth point 40 thus providing a push-pull input
to two amplifier channels. The input signal to each channel is amplified
by a variable gain controlled amplifier 52, 53 and the output signal from
these amplifiers are further amplified by amplifiers 72, 73 and rectified
by rectifier circuits 54, 55. The amplitude of the voltages induced in
coils 6A, 6B will, by nature of the arrangement, vary proportional to the
temperature of the fluid 2. Therefore the magnitude of any direct
measurement of differences between the outputs of rectifier circuits 54,
55 for the purpose of flow determination will be affected by the
temperature response of the system. If, however, the sum of the voltages
induced in coils 6A, 6B (which is proportional to temperature ony), is
compared with a reference voltage and the difference between the reference
voltage and the sum of voltages from coils 6A, 6B used to control the gain
of the amplifiers 52, 53, the outputs of the rectifiers 54, 55 can be
normalised whilst still preserving the ratio between them. This
normalising procedure is performed by the output signal of each rectifier
circuit 54, 55 to a differential amplifier 56 where they are added. The
output signal of the differential amplifier 56 is then compared with a
reference voltage connected to terminal 57 and the difference signal,
after being amplified by the amplifier 58 is used to control the
amplification of amplifiers 52, 53. The reference voltage at terminal 57
is not necessarily the same as that connected to terminal 59 of the
circuit of FIG. 2. The output signals of the rectifier circuits 54, 55 can
be subtracted from each other. This is accomplished by connecting across
the outputs of the rectifier circuits 54, 55 two equal resistors 60, 61.
The difference signal, at the junction between resistances 60, 61, is
amplified by the amplifier 62. The output signal from amplifier 62 is fed
to the input of an inverting, integrating amplifier 63 and to the input of
a non-integrating, non-inverting amplifier 64.
The output signal of amplifier 63 is a steady DC voltage indicative of a
steady predetermined flow rate of fluid 2 whereas the output signal of
amplifier 64 is a DC voltage which has superimposed on it a signal
representative of short term fluctuations in the rate of flow of the fluid
2. By subtracting the signal from amplifier 63, from the output signal of
amplifier 64 one may obtain a signal indicative of the variation of flow
of the fluid 2. This is done by connecting two series connected resistors
65, 66 across the outputs of amplifiers 63, 64. The junction between the
resistors is connected to the input of an amplifier 67. The signal
appearing at the input of amplifier 67 is representative of the variation
in flow of the fluid 2.
Typically the amplification of amplifier 63 would be set to produce between
1 and 2 volts output per meter per second flow of conductive fluid. The
amplication of the flow variation channel (amplifier 67) would be
typically 10 times greater than thatof 63. This would give a sensitivity
of 10 to 20 volts per meter per second. Since in the signal processing,
two DC signals are subtracted, signs are preserved and reversal of flow
will produce a negative output voltage.
It has been found to be advantageous to employ a hollow core of Permendur
V(Trade Mark) alloy inside the stainless iron former 3. In this way it has
been possible to increase the range over which tempeprature can be
measured up to about 930.degree.C. The Permendur V alloy has a Curie
Temperature point of 980.degree.C.
During high temperature tests on a Permandur cored device, it was noticed
that during heating and cooling cycles, there was a momentary change in
the sensitivity of the device as the Curie point of the stainless iron
former 3 was transversed. The recorded output voltage trace contained a
small, but yet significant, blip at 705.degree.C (typically a drop of 10%)
on an otherwise smooth curve.
This effect may have use as a calibration marker for 705.degree.C.
Alternatively, if a material with a lower Curie point (about 400.degree.C)
were fitted inside the former 3 or the material from which the former 3 is
made is changed, then a marker could be provided at the lower limit of the
anticipated working range of temperature.
If the `marker` effect is not wanted, then only one magnetic material must
be used as the core of the device. The coil former could be made from
Permendur V. However, since it is difficult material to machine, an easier
solution is to fit a Permendur V liner inside the former 3 machined from
non-magnetic stainless steel.
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