Note: Descriptions are shown in the official language in which they were submitted.
CA 02299365 2000-02-04
WO 99/08102 PCT/GB98102354
The invention relates to a gas detection device, and method, which is suitable
for environmental
pollution monitoring, atmospheric tracer detection and monitoring gas or
vapour emitting
processes. In particular, the device is capable of detecting and
distinguishing between two
simultaneously emitted gases so that the individual contributions of each gas
to the total
concentration may be determined.
The detection and measurement of airborne pollutants is an ongoing and
increasing requirement
in a wide range of activities such as environmental pollution and
meteorological studies. In
particular, there is a need for fast response point sensors which are capable
of providing
information on the short term fluctuations in concentration which occur in the
vicinity of all
emission sources. For example, this is useful way of measuring the
concentration of hazardous
materials in the atmosphere. Furthermore, in addition to the need for a
measurement capability
for hazardous materials per se, a useful technique in the development of
control methods (e.g.
models) is to study the behaviour of a surrogate material, or tracer material,
under similar
conditions. This may be done by releasing a tracer compound i nto the
atmosphere and detecting
it at a distant point by suitable detection means.
Many tracer materials have been used in the past, for examplc fluorescent
particles, sulphur
dioxide and radioactive isotopes of gases such as krypton. Sulphur
hexafluoride has also been
used widely as a tracer although it is expensive. In particular. as it can be
detected down to very
low levels (typically 1 part in 10~°) it is useful for long range
studies. up to several hundred
kilometres. For detection in the region of 1-2 km downwind of a source, tracer
gases such as
propylene have been used successfully.
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WO 99/08102 PCT/GB98/02354
2
Any suitable detection system for measuring such tracer materials at a range
of several
kilometres from source must be capable of operating in field conditions. A
suitable detector with
this capability is the Ultra 'Violet Ionisation Chamber detector, or the UVIC~
detector, as
described in US patent 5 572 137. The UVICO detector is an ultra violet
exciter device
comprising an ultra violet Iamp for emitting ultra violet radiation into an
inlet tube into which the
tracer gas is introduced. The energy of the ultra violet radiation is such
that the tracer gas, for
example propylene, is ionised and electrons are collected further downstream
to give a measure
of the tracer gas concentration. The UVIC detector is capable of measuring
concentration
fluctuations arising from a single source.
The measurement of concentration fluctuations arising from a single source is
an important
element in quantifying the hazard or nuisance of a particular substance (that
is a reference to the
toxicity or flammability of a substance or a reference to the malodour).
However, in practice it is
not unusual for there to be several closely spaced sources emitting
simultaneously and this needs
to be accounted for when developing models and other tools to describe the
hazard. In the past
this has been done by summing the concentrations measured from each source at
the point in
question. However, analysis has shown that this approach is only acceptable if
the process
associated with the hazard is completely linear in its response to
concentration such as, for
example, exposure to airborne radionuclides. This is not the case, for
example, for the toxicity,
flammability or malodour of a substance as non linear effects are a
significant part of the process
and this technique tends to be inaccurate.
There is therefore a need to be able to examine, at a single point, the
behaviour of concentrations
in air of a pair of simultaneously emitted tracer gases. The statistics of the
individual
contributions to the total concentration can then be determined and an
accurate estimate of the
hazard developed. One known system for achieving this includes a UVIC detector
and a Flame
Ionisation Detector (FID) in co-location. The FID uses a controlled hydrogen
flame as an
ionising agent as opposed to the ultra violet lamp in the UVIC detector. The
ions produced by the
flame are advected both electrically and mechanically towards a small biased
electrode where
they are collected.
CA 02299365 2000-02-04
WO 99/08102 , PCT/GB98/02354
Although this system is capable of providing useful data on two tracer gases
emitted
simultaneously, the FID is not suited for operation in field conditions. For
example, the logistics
requirements posed by the-need for a hydrogen supply and the heavy electrical
power demands
of the FID are inconvenient for field use. Furthermore, the water vapour
generated by the
combustion of hydrogen tends to condense on some of the inner surfaces of the
instrument. This
compromises the integrity of the electrical insulation, leading to noisy and
unstable signals and
therefore low data quality.
The present invention overcomes the problems associated with the UVIC/FID
system in that it
removes the need for a hydrogen supply and does not have high electrical power
demands.
Furthermore, the device is convenient for use in field conditions, and
performs reliably even
under adverse conditions, for example, blowing dust, rain and mist. The device
is capable of
measuring the behaviour of a pair of simultaneously emitted tracer gases, at a
single point, so
that the individual contributions to the total concentration can be determined
and the
concentration fluctuations of the two gases may be measured. This is an
important aspect of the
development of models and other tools for describing a hazard or nuisance of a
substance. The
accuracy with which this measurement can be made provides a significant
improvement over
conventional single source summing techniques which do not allow for non-
linear processes. The
system has particular application in the field of environmental pollution
monitoring, atmospheric
tracer detection and in the monitoring of gas or vapour emitting processes.
*rB
CA 02299365 2000-02-04
WO 99/08102 PCTIGB98/02354
According to one aspect of the present invention, a gas detection device for
distinguishing two
different gases within a sample comprises;
two flow passages into which the sample is input, each flow passage having at
least one inlet and
at least one outlet and an exciter zone and each having a longitudinally
extending axis,
gas induction means for drawing the sample into and through the flow passages,
means for emitting ultra violet radiation into the two flow passages, wherein
the radiation
emitted into one of the flow passages is of sufficient energy to be capable of
ionising at least one
of the gases and the radiation emitted into the other flow passage is of
sufficient energy to be
capable of ionising both of the gases, such that upon irradiation by one or
more of the sources the
gases may be ionised and generate ions,
two electrode arrangements, each comprising at least one bias electrode and at
least one collector
electrode, having voltage supply means for supplying a voltage to the one or
more bias electrode
such that they may be differently charged to the collector electrode or
electrodes, wherein each of
the electrode arrangements is mounted within a different one of the flow
passages such that the
one or more collector electrode in each collects the ions produced in a
different one of the flow
passages, and
current measuring means, sensitive to the effects of the ions being
neutralised on the collector
electrodes, for providing an output from each of the electrode arrangements
dependent upon the
amount of gas or gases ionised within each of the flow passages.
In a preferred embodiment, the gas detection device comprises an input
passage, having an inner
wall, and means for segregating at least part of the input passage so as to
provide two flow
passages, each having an outer wall. The input passage and the flow passages
may have any one
of a substantially circular, elliptical, rectangular or hexagonal cross-
section. The device may also
comprise means for substantially preventing the flow of the sample along the
longitudinally
extending axis, between the inner wall of the input passage and the outer wall
of the flow
passages.
CA 02299365 2000-02-04
WO 99/08102 PCT/GB98/02354
The gas detection device may comprise a single source of ultra violet
radiation having means for
selectively transmitting radiation of selected energy into each of the flow
passages such that the
sample in each of the flow-passages is irradiated with radiation of different
energy.
Alternatively, the device may comprise two sources of ultra violet radiation,
each for emitting
radiation into a different one of the flow passages, such that the sample in
each of the flow
passages is irradiated with radiation of different energy.
The two sources of ultra violet radiation may have different emission spectra.
Alternatively, they
may have substantially the same emission spectra and each source may comprise
filtering means
for selectively transmitting radiation of the required energy into the flow
passages. For example,
the device may comprise any of a krypton lamp and a xenon lamp, an argon lamp
and a
deuterium lamp or a xenon lamp and a krypton lamp.
The device may comprise a krypton lamp for emitting radiation having energy of
less than 10.95
eV into one of the flow passages and an argon lamp for emitting radiation
having energy of
greater than 10.95 eV into the other flow passage. The argon lamp may comprise
a LiF window
through which ultra violet radiation is transmitted into one of the flow
passages and the krypton
lamp may comprise a MgF2 window through which ultra violet radiation is
transmitted into the
other flow passage. In this embodiment, the device is capable of
distinguishing propane and
propylene gases.
The distance between the exciter region in each of the flow passages and the
corresponding
electrode arrangement may be varied. The gas flow induction means may be a fan
which may be
operated at a variable speed such that the rate of flow of gas through the
input passage may be
varied.
Each of the electrode arrangements may comprise a substantially tubular outer
electrode
extending substantially along the longitudinal axis of at least part of the
length of the input
passage and a rod inner electrode, wherein the outer electrode is mounted
concentrically around
the rod inner electrode. Any one of the outer electrode and the rod electrode
in each of the
electrode arrangements may comprise one or more electrode sections.
CA 02299365 2000-02-04
WO 99/08102 PCT/GB98/02354
The outer electrode in each electrode arrangement may be biased by the voltage
supply means
such that it is differently charged to the corresponding rod electrode such
that ions generated as a
result of the ionisation of the tracer gas or gases are collected at the inner
electrode.
Alternatively, the rod electrode in each electrode arrangement may be biased
such that it is
differently charged to the corresponding outer electrode and ions generated as
a result of the
ionisation of the tracer gas or gases are collected at the outer electrode.
Typically, the voltage
applied to the bias electrode in each electrode arrangement is substantially
the same.
According to a second aspect of the invention, a method for distinguishing two
different gases in
a sample comprises the steps of;
(i) inputting the sample into two separated flow passages,
(ii) irradiating the sample in each of the two flow passages with radiation of
a different energy,
wherein the radiation emitted into both of the flow passages is of sufficient
energy to be capable
of ionising at least one of the gases and radiation emitted into just one of
the flow passages is of
sufficient energy to be capable of ionising both of the gases, such that one
of the gases is ionised
in both flow passages and the other gas is ionised in,just one flow passages
and ions are
generated as a result of the ionisation process,
(iii) passing the irradiated sample in each flow passage through a different
electrode arrangement
comprising at least one collector electrode and at least one bias electrode ,
(iv) applying a voltage to the one or more bias electrode in each electrode
arrangement such that
the bias electrode or electrodes may be differently charged to the collector
electrode or electrodes
and the ions generated by the ionisation process may be collected at the one
or more collector
electrode in each electrode arrangement, and
(v) measuring the current at each of the collector electrodes.
CA 02299365 2000-02-04
WO 99/08102 PCT/GB98/02354
In a preferred embodiment of this aspect of the invention, the method
comprises the further step
of;
(vi) deducing the individual concentrations of the gases in the sample from
the measured
currents.
CA 02299365 2000-02-04
WO 99/08102 PCT/GB98/02354
8 _
The invention will now be described by example only with reference to the
following figures in
which;
Figure 1 shows a diagram of a conventional UVIC device which may be used to
measure
concentration fluctuations of a gas arising from a single source,
Figure 2 shows a diagram of the gas detection device of the present invention,
Figure 3 shows an enlarged view of the electrode arrangement included in the
gas detection
device shown in Figure 2,
Figure 4 shows a black diagram of the circuitry which may be used in the gas
detection device
shown in Figure 2,
Figure 5 shows a diagram of the exterior of the gas detection device shown in
Figure 2 and
Figure 6 shows test results obtained using the gas detection device of the
present invention.
Figure 1 shows a diagrammatic cross sectional view of a conventional Ultra
Violet Ionisation
Chamber (UVIC) detector 1. The UVIC detector 1 comprises an input tube 2
having an open
inlet 3 at one end and a fan unit 4 at the opposite end so that. in operation,
the device draws air
through the inlet 3 and along the length of the tube 2. An ultra violet lamp 5
is mounted
externally of the tube so that ultra violet radiation 6 is emitted into the
tube 2 into an exciter zone
7 through an aperture 8 in the tube wall. An electrode unit 9 is situated
downstream of the exciter
zone comprising an outer electrode 10 coaxial along the length of the tube and
a rod inner
electrode 11.
The outer electrode 10 is biased by means of a DC power unit (not shown) by a
wire 12 passing
through as insulating plug 13 in the tube wall. The rod inner electrode 11
lies with its
longitudinal axis along the longitudinal axis of both the tube 2 and the outer
electrode 10 and is
connected to the other leg of the DC power supply unit via a wire 14.
CA 02299365 2000-02-04
WO 99/Q8102 PCT/GB98/02354
9
The ultra violet lamp is a krypton lamp which provides radiation 6 at 10.03 eV
and 10.65 eV and
is capable of ionising propylene (having an ionisation potential of 9.73 eV).
If, for example, a
propylene tracer gas enters through the inlet 3 to the tube it is irradiated
with ultra violet
radiation 6 and is ionised. The electrons generated as a result of the
ionisation process pass
downstream and are collected at the electrode unit 9. By measuring the current
collected by the
electrode, the concentration of propylene in the tube 2 may be determined.
Thus, the device
enables the concentrations fluctuations rising from a single source of
propylene tracer gas to be
measured. The UVIC detector 1 also has a strong response to ammonia (NH3)
since its ionisation
potential (10.16 eV) is close to and just below that of one of the two
principle emission bands of
the ultra violet krypton lamp 5.
Whilst the IJVIC detector 1 is capable of measuring gas concentration
fluctuations arising from a
single source, in practice it is common for there to be several closely spaced
gas sources emitting
simultaneously. The measurement of concentration fluctuations of two
simultaneously emitted
tracer gases at a single point has been achieved using a gas detection system
comprising the
UVIC device, as shown in Figure l, in co-location with a Flame Ionisation
Detector (FID), as
described previously. However, a problem with the FID detector is that its
behaviour in field
conditions can be unreliable. Also, it is not particularly convenient for use
in the field due to the
need for a hydrogen supply and the FID also requires a high power supply.
Referring to Figure 2, the gas detection device 15 of the present invention
comprises an input
passage 16, or tube, having an open inlet 17 at one end and a tan unit 18 at
the opposite end
having a power supply (not shown). The fan unit 18 may be anv means for
inducing gas flow in
the tube 16 such that in operation air is drawn into the tube 1 ( at the inlet
17, is drawn along a
length of the tube 16 and is output from an outlet 18a. In operation a
combination of any of air,
gas or vapour may be drawn through the tube 16. For the purpose of this
specification, the phrase
"gas" shall be taken to mean a gas or a vapour. The tube 16 may typically be a
length of tubing
which may have more than one inlet and outlet at the respective ends. The tube
may be rigid or,
in some application, it may be useful if the end portion is flexible to aid
probing.
CA 02299365 2000-02-04
WO 99/08102 PCT/GB98/02354
The device 1 S also comprises a flow separator 19 having its longitudinal axis
substantially
parallel with the longitudinal axis of the tube 16. The flow separator 19 is
situated substantially
at the centre of the tube 16 such that it divides the tube into two separate
flow passages 20a.20b.
The device also comprises two lamps 21,22, each lamp having an associated
power supply 23,24,
for emitting ultra violet radiation 23. The lamps 21,22 are mounted on
substantially opposite
sides of the tube 16 such that each emits radiation 23 into a different one of
the flow passages
20a,20b.
As shown Figure 2, the mounting of the~lamps 21,22 on the side of the tube 16
may be such that
the radiation 23 is emitted into the flow passages 20a,20b via apertures 25 in
the tube wall.
Alternatively, the mounting of the lamps 21,22 may be such that the surface of
the ultra violet
lamp through which radiation is emitted forms part of the wall of the tube 16.
It may be
preferable to mount the lamps 21,22 at a slight recess from the tube wall
(i.e. not protruding into
the flow passages 20a,20b) so as to maximise the flow of air and/or gas
through the respective
flow passages 20a,20b.
Each lamp has a dedicated coaxial ion collection electrode arrangement which
is mounted within
the tube and is situated downstream of the flow passages. An enlarged diagram
of the electrode
arrangements is shown in Figure 3 but, for clarity, is not drawn in full
detail in Figure 2.
Referring to Figure 3, the electrode arrangements 26a,26b are separated by a
small gap, the size
of the gap being dictated primarily by mechanical considerations (e.g. the
need for means for
supporting the electrode arrangements 26a,26b). Each electrode arrangement
26a,26b may
comprise a tubular outer electrode 27a,27b, which may be biased by means of a
bias supply 28
(i.e. a "bias" electrode 27a,27b), and an inner electrode 29a,29b (i.e. a
"collector" electrode). The
tubular outer electrode, 27a and 27b, may be concentrically mounted around the
inner electrode,
29a and 29b respectively, such that the inner electrode extends longitudinally
along the length of
the outer electrode 27a,27b. Each of the outer electrodes 27a,27b and the
inner electrodes
29a,29b may be a single electrode or, alternatively, each may comprise a
plurality of electrode
sections.
*rB
CA 02299365 2000-02-04
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11
The longitudinal axes of the electrode arrangements 26a,26b are preferably
substantially parallel
to the longitudinal axis of the tube 16 so as to maintain the passage of flow
between the inlet 17
and the fan unit 18. The bins supply 28 provides a suitable voltage to the
outer electrodes
27a,27b such that each may be differently charged from the associated inner
electrode 29a,29b
respectively. Each inner electrode 29a,29b is connected to subsequent
electronic components 30a
(not shown in detail in Figure 2), for example for reducing the drift and
noise and for measuring
the electrode currents at outputs S ~ and S2.
Referring back to Figure 2, in a preferred embodiment, the tube 16 may be of
substantially
circular cross-section, although it may also take other cross-sectional forms,
such as rectangular,
hexagonal or elliptical. Typically, the flow passages 20a,20b may have
substantially equal
diameters. For convenience, the outer electrodes 27a,27b may have
substantially the same cross-
sectional shape as the flow passages 20a,20b.
The inner wall of the tube 16 may conveniently be fabricated from a metal of
relatively high
electrical resistance and, more preferably, of reasonably high workfunction
with regard to the
emission of electrons upon irradiation with ultra violet radiation.
Furthermore, it is preferable if
the inner wall material does not retain traces of gases used upon its surface.
Suitable materials
from which the tube 16 may be made are, for example, aluminium and steel.
If the tube inner wall is of a non-insulating material, e.g. aluminium or
steel, the outer electrodes
27a,27b must be insulated from the inner wall of the tube wall. The insulation
of the outer
electrodes 27a,27b from the tube 16 may be achieved by mounting the outer
electrodes 27a,27b
on insulating rings or spacers 50 (as shown in Figure 3) which also serve as
support means for
the outer electrodes 27a,27b. The insulating rings 50 may be placed between
the outer electrodes
27a,27b and the inner sidewall of the tube 16 such that they circle the
circumference of the outer
electrodes 27a,27b. The insulating rings also prevent air and/or gas from
flowing around the
outside of the outer electrodes 29a,29b. This is important as it is desirable
for as much of the gas
input to the tube to pass between the electrode arrangements 26a,26b. This may
be achieved
using alternative means other than the insulating rings but it is convenient
to use the rings 50 for
both purposes.
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12
The inner electrodes 29a,29b may conveniently be rod electrodes which
preferably extend along
substantially the full length of the corresponding outer electrode, except
that a portion of each of
the inner electrodes 29a,29b may be bent to penetrate the wall of the tube 16
through an
insulating plug (not shown) for subsequent connection to the electronic
circuitry 30a. Connecting
the bent portion of the inner electrodes 27a,27b to the tube wall at the
insulating plug provides a
suitable means of supporting the inner electrodes 27a,27b.
The object of the invention is to provide a system which is capable of
detecting two tracer gases
emitted simultaneously from a single point. Therefore, typically in operation,
a sample of gas is
drawn into the tube 16, via the inlet I7, by the gas induction means I 8
wherein the gas sample
will typically comprise two such tracer gases in air. The tracer gases are
irradiated in the flow
passage regions 20a,20b by the ultra violet radiation 23 emitted from the
lamps 22,21
respectively. If upon irradiation the tracer gas becomes ionised, oppositely
charged ion pairs, or
an ion and an electron, are produced which are then drawn into the electrode
arrangements
26a,26b, by the gas flow induction means. In each electrode arrangement
26a,26b, the outer
electrode and the inner electrode are oppositely charged such that the inner
electrode in each case
is a "collector" electrode for positively charged ions produced by the ultra
violet ionisation
process.
The choice of the particular lamps and tracer gases used is determined by the
need for both of the
tracer gases to be ionisable by radiation emitted from one of the lamps and
for just one of the
tracer gases to be ionisable by radiation emitted from both the lamps. That
is, the lamp emitting
the higher energy radiation must be able to ionise both gases and the lamp
emitting the low
energy radiation must only be able to ionise one gas (i.e. the lower
ionisation potential gas).
In a preferred embodiment of the invention, the device 15 may comprise an
argon lamp (21 ) and
a krypton lamp (22). In operation, the krypton lamp 22 is emits radiation at
10.03 eV and 10.65
eV and the argon lamp 21 emits more energetic radiation at 11.60 eV and 11.80
eV. The argon
lamp comprises a LiF (lithium fluoride) coated window (not shown) which
permits radiation at
11.60 eV and 11.80 eV to be transmitted. The krypton lamp may typically
comprise a MgF2
(magnesium fluoride) coated window (not shown) which permits radiation at
10.03 eV and 10.65
eV to be transmitted.
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The device comprising a krypton lamp and an argon lamp may conveniently be
used to
simultaneously measure propane and propylene tracer gas concentrations. The
ionisation
potential of propylene (C3H6) is 9.73 eV and therefore propylene gas will be
ionised by radiation
23 emitted from both the krypton lamp 22 (at 10.03 eV and 10.65 eV) and the
argon 21 lamp (at
11.60 eV and 11.80 eV). Propane (C3H8) has an ionisation potential of 10.95 eV
and therefore a
propane tracer gas will only be ionised by the higher energy radiation emitted
from the argon
lamp 21.
The current collected at the inner electrode 29a associated with the krypton
lamp 22 will
therefore be in proportion to the concentration of propylene gas in the flow
passage 20a.
However, the current collected at the inner electrode 29b associated with the
argon lamp 21
arises as a result of the ionisation of both the propylene and propane tracer
gases in the flow
passage 20b and therefore this current is proportional to the total
concentration of both gases.
For example, the output signals, at S~ and S2, generated from the argon lamp
and krypton lamp
electrode arrangements 26b and 26a respectively, may be related to the
individual propylene
(C3H6) and propane (C3H8} tracer concentrations in the following way;
S, = A(C~ H6 ~+ B~C3 Ha
Equation 1
and SZ = C(C3H6~ Equation 2
where A,B and C are linear calibration constants.
The linear calibration constants may be determined in an exposure chamber
prior to
measurement. Knowing these values, it is then possible to extract the propane
and propylene
concentrations from the signals output from the device at S, and S2.
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14
The calibration process may conveniently be carried out in a sealed air-filled
chamber. The
chamber would typically comprise means of injecting the required quantities of
tracer gas into
the chamber and a mixing fan which would be required to ensure a uniform
concentration of the
gas was achieved quickly following injection into the chamber. Typically, for
the purposes of
calibration, the device 15 would first be exposed to a range of concentrations
of the individual
tracer gases and then to a mixture of the two. The concentrations of the
tracer gas in air would
typically be in the range of between 0-2000 ppm by volume for this purpose.
As well as using krypton and argon lamps in the device 15, several other lamp
and tracer gas
combinations may also be envisaged which satisfy the requirements for dual-gas
measurement.
For example, a krypton lamp may be used in combination with a xenon lamp, or
an argon lamp
may be used in combination with a deuterium lamp. As well as propane and
propylene tracer
gases, other tracer gas combinations may be used, such as ammonia and propane,
or ethylene and
propane. Alternatively, a xenon and krypton lamp combination may be used with
a benzene and
propylene tracer gas combination. In practice, environmental and economic
factors are likely to
limit the choice of tracer gas which may be used.
It may also be possible to use two sources of ultra violet radiation having
the same emission
spectra to irradiate both flow passages 20a,20b, for example two krypton
sources. The two
sources will have the same emission spectra (i.e. emit radiation comprising
the same wavelength
components and energies) but the window of each source may be fitted with a
suitable filter. so
that radiation of the required energy or energies may be selectively passed
into the flow passages
20a,20b.
In any practical device, such as that shown in Figure 2, ionic recombination
is likely to occur in
the region between the portion of the separate passages 20a,20b the ultra
violet radiation
irradiates and the point at which ions are collected further downstream in the
electrode
arrangements 26a,26b. This is an undesirable effect as it prejudices the
accuracy of the
measurement. These recombination processes may be minimised by reducing the
distance
through which the ionised gas passes prior to collection at the inner
electrodes 27a,27b and also
be increasing the flow speed.
CA 02299365 2000-02-04
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The use of the flow separator 19 ensures that two separate regions are created
in the tube 16
(passages 20a,20b). This is essential to the operation of the device as the
ionisation products
resulting from argon lamp'induced ionisation and the krypton lamp induced
ionisation processes
must not mix prior to ion collection further downstream at the inner
electrodes 27a.27b.
Furthermore, as the lamps are preferably mounted in substantially opposing
positions, the
illumination of the lamps with ultra violet radiation from the other may
interfere with lamp
performance. The flow separator I9 also prevents this occurring.
In a preferred embodiment of the invention, the flow passages ZOa,20b may be
formed within the
tube 16 by the presence of the flow separator I9, as shown in Figure 2).
Alternatively, the
separate flow passages 20a,20b may be formed from two separate tube sections,
within each of
which the associated electrode arrangement 26a,26b is situated downstream of
the ultra violet
lamp positions. In this embodiment, the common input section of the tube (i.e.
length 1 on Figure
2) is absent. Whilst this arrangement may be suitable for some applications, a
slight uncertainty
is introduced in that the two gases ionised in the separate flow passages are
not sampled at a
precise single point.
It may be preferable for the region of the separate flow passages which is
irradiated with the
radiation 23 to be as close to the electrodes as possible, so that the ionised
gas or gases have as
short a distance as possible to travel before they are collected. For some
applications, however, it
may be useful if this distance is variable. In any case, it is preferable to
avoid significant
ionisation of the gas or gases in the space between the electrodes. The amount
of radiation
impinging on the electrodes 27a,27b,29a,29b should be kept to a minimum or
avoided altogether.
If very high flow rates are to be used, the irradiation zone within the
separated flow passages
20a,20b may be further upstream than shown in Figure 2.
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16
It may be preferable if the tube 16 is moveable along its longitudinal axis
with respect to the
electrode arrangements 26a,26b such that the position of the in adiated
regions in the separated
flow passages 20a,20b may be varied by an operator depending on the intended
use of the device.
This may be achieved if the windows or apertures on both sides of the tube,
through which the
radiation 23 enters the tube 16, extend along a continuous length of the tube
16. Alternatively,
the tube 16 may comprise several apertures on each side of the tube 16. By
uncovering selected
apertures on each side of the tube and moving the tube along its longitudinal
axis, the selected
apertures may be aligned with the lamps 21,22, providing a variable distance
between the regions
irradiated by the lamps and the electrode arrangements 26a,26b. In either
case, slidable covers
would be required for placement over the apertures when not in use or the
portion of the aperture
not in use.
Figure 4 shows a block diagram of one circuit which may be used to control the
gas detection
device 15 shown in Figure 2. The operation of the circuit shown in the figure
would be
conventional to one familiar with the art. The features of the circuit are as
follows; EHTG; EHT
generator, MPS: main power supply, EM; electrometer, OS; off set control, FSC;
fan speed
controller, SGA; switched gain amplifier, RS; range switch, LPF; low pass
filter, INIA;
inverting/non-inverting amplifier, I(1); current corresponding to output S,,
I(2); current
corresponding to output S2. The currents I( 1 ) and I(2) measured at the
outputs S ~ , S2 may be
displayed on a visual display 31.
The operation of the circuit shown in Figure 4 would be conventional to one
familiar with
electrical circuits and the circuit is one of many circuits which may be used
to control the device
and measure the charge collected at the electrodes 29a,29b. The current
sensing part of the
circuitry is sensitive to the effects of ions being neutralised upon the
surfaces of the electrodes
collecting the ions, such that an output dependent upon the amount of each
tracer gas (or gases)
ionised within the flow passage is provided and the amount of each individual
tracer gas at the
measurement point may be determined.
*rB
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17
The lamp power supply units 23,24 and the bias supply 28 are connected to main
power supplies.
The fan unit 18 is also connected to a main power supply via a fan speed
controller (FSC) which
enables the rate of flow of gas through the tube to be adjusted. Preferably,
the gas flow induction
means 18 may be in the form of an electric fan. Flow rates of around 4 x 10'~
m3 s' may be
conveniently achieved using a radial fan, but for increased flow rates e.g. at
least 4 x 10-3 m3 s' it
may be convenient to use a centrifugal fan.
A schematic diagram of an example of the exterior of the complete gas
detection device 15 is
shown in Figure 5. The tube 16, preferably having the interior configuration
shown in Figures 2
and 3, is preferably mounted on a support 35 such that it may be moved along
its longitudinal
axis with respect to the support 35. This support 35 may also form an ultra
violet source box for
housing the ultra violet lamps 21,22. The ultra violet support box may be
mounted on separate
housing means 36 for housing the bias supply 28 and the circuitry for
measuring the electrode
currents and for reducing the drift and noise, as shown in Figure 4.
Alternatively, the housing
means 36 may be provided separately from the ultra violet source box 35 and
tube 16.
The gas flow induction means 18 are situated at the outlet of the tube 16 and
means for supplying
power to this unit 18 may be held within the ultra violet source box 35 or the
housing means 36.
The device may be a hand held device or a fixed device and a carrying strap or
handle may also
be provided (not shown).
The device 15 comprises a control 37 for activating the ultra violet lamps (or
controls for
activating both lamps independently) which may have an associated visual
display 38 for
indicating lamp operation. The device also comprises controls i9 for varying
the sensitivity of
the current sensing part of the circuitry, a control for backing off the zero
reading (i.e. the off set
controls) 40 and a control for activating the electrodes bias voltage 41. The
device 15 may also
comprise a visual indicator 31 (as indicated in Figure 4) to give an
indication of the current
measured at each electrode, from which the individual concentrations of the
tracer gases may be
deduced.
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Figure 6 shows the performance that can be achieved with the device of the
present invention
when tested under field conditions. The device under test comprised an argon
and a krypton
lamp. In this test continuous sources of propylene (0.3 L miri ~) and propane
(3 L miri ~) were
located 1 m above ground level at a crosswind separation of 5 m. The device
under test was
positioned 10 m downwind of the pair of sources. The ionisation currents,
following
amplification, were logged at a 10 Hz sampling rate on a digital computer. The
results obtained
were not atypical. The results were obtained using Equations l and 2, as
described earlier i.e. the
concentrations of propane and propylene were determined from the output
signals, S ~ and S2, of
the argon and the krypton lamps respectively. The results show excellent time
resolution and also
indicate the ability of the device to distinguish between two chemically
similar species.
For a hand held device, the typical dimensions of the tube may be between 1-5
cm. Fixed devices
may have larger diameter tubes in which case proportionally larger electrodes
will be required.
Typically, each of the outer electrodes 27a,27b has a length of several
centimetres, e.g. 3-5 cm
and the radial spacing of the inner and outer electrodes, 27a,29a and 27b,29b,
in each
arrangement 26a,26b may be between 0.2 and 1.0 cm. The voltage across each of
the outer and
inner electrodes, 27a,29a and 27b,29b, may typically be between 20-1000V with
currents of the
order of 10 nA generated by near-maximal ionised tracer gas levels contacting
the electrodes.
The voltage should be selected such that the device operates in the saturation
region, that is a
further increase in the bias voltage does not result in an increased
collection of ions. The bias
voltage applied to the outer electrodes 29a,29b may be either positive or
negative with respect to
the virtually earthed inner electrode. Typically, the current display 31 will
have a sensitivity of
between 300 pA to 30-100 nA.
Both of the outer electrodes 27a,27b may be biased by means of the same bias
supply, as shown
in Figure 2, and may be electrically at the same potential. However, it rnay
also be possible to
bias the outer electrodes 27a,27b with different voltages depending on the
intended application
of the device 15. If the bias voltages applied are substantially different, it
may be preferable to
mechanically re-arrange the two electrode arrangements 26a,26b.
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In an alternative embodiment to that shown in Figure 2, the inner electrodes
27a,27b may be
biased with respect to the outer electrodes 29a,29b such that the latter
operate as the "collector"
electrodes from which the'output signals may be taken.
The electrodes may be made from any suitably conductive metal, such as
stainless steel or copper
and, preferably, may be made from a relatively inert material such as gold
plated brass. The ring
seals or spacers for providing the insulation of the electrodes 27a,27b from
the wall of the tube
16 may be made of a suitably insulating material, such as polytetra-
fluoroethylene (PTFE) and
Darvic (IBM RTM).
The gas detection device is particularly suitable for operation in field
conditions, including
conditions of blowing dust, rain and mist. Furthermore, it may conveniently
take the form of a
hand held device. It may be used to examine the behaviour, at a single point,
of the
concentrations in air of simultaneously emitted tracer gases and is compatible
for operation with
gases are which are convenient and suitable for release into the open air.
