Abstrict An ion drag air flow meter (10) including a gas ionizer (16), an
ion collector (14) and apparatus (30) for converting the collected
ions into a signal providing an indication of the flow of a gas
(40).
Claims What is claimed is:
1. An apparatus for measuring the flow of a fluid comprising:
a voltage source having a first and second terminal;
ion chamber means for collecting ions, said ion chamber means including
an outer conductor connected to a first terminal of the voltage
source and an inner conductor connected to a second terminal of
the voltage source, the outer conductor surrounding said inner conductor;
annular radioisotope means disposed at a first end of said chamber
means for ionizing the fluid, said radioisotope means consisting
essentially of an ionizing source disposed on a peripheral edge
of said outer conductor; and
circuit means electrically coupled to the ion chamber means and
the voltage source for converting a signal indicative of the collected
ions into a signal to provide an indication of the flow of the fluid.
2. The apparatus of claim 1 wherein said radioisotope means includes
a radioisotope for emitting beta particles.
3. The apparatus of claim 1 further comprising:
a second ion chamber means, said second ion chamber means including
a second outer conductor connected to said first terminal of said
voltage source and a second inner conductor connected to said second
terminal of said voltage source, said second outer conductor surrounding
said second inner conductor; and
second annular radioisotope means disposed at a second end of said
second chamber means to ionizing said fluid.
4. The apparatus of claim 3 said first radioisotope being disposed
at an intake end of said first outer conductor and said second radioisotope
being disposed at an exhaust end of said second outer conductor.
5. The apparatus of claim 1 wherein said circuit means includes
an operational amplifier coupled to said ion chamber means and said
voltage source to provide an indication of the direction and/or
velocity of the flow of the fluid.
6. The apparatus of claim 1 wherein the outer and inner conductors
are concentric cylinders.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to air flow meters. More specifically,
the present invention relates to ion drag air flow meters or ion
drag anemometers.
While the present invention is described herein with reference
to illustrative embodiments for particular applications, it should
be understood that the invention is not limited thereto. Those having
ordinary skill in the art and access to the teachings provided herein
will recognize additional modifications, applications, and embodiments
within the scope thereof and additional fields in which the present
invention would be of significant utility.
2. Description of the Related Art
High performance engines require precise measurement of air/fuel
ratio to achieve optimum performance and to meet current emission
standards. This function is performed by an air flow meter. The
harsh environment of the automobile engine places significant restrictions
on the design of conventional air flow meters. For example, the
sensors of the meter must be able to withstand the extreme temperatures,
humidities and pressures to perform adequately in a conventional
internal combustion engine.
Many current air flow meter sensors are based on the principle
of a heated wire or film placed in or adjacent to the air flow.
These devices sense heat removal as air passes the wire or film.
Those skilled in the art may appreciate that the limitations of
these sensors include a lack of ruggedness, susceptibility to fouling
and inability to sense flow direction. Further, the response of
these sensors is often limited by the mass and construction thereof.
Further, few conventional sensors can actually measure air mass
flow. This measure provides information on the amount of air mass
which is being burned by the engine and is of considerable utility
in the effort to achieve optimum engine performance.
Thus, there is a need in the art for an air flow meter for a rugged,
high performance air flow meter capable of measuring air mass flow.
SUMMARY OF THE INVENTION
The need in the art is addressed by the ion drag air flow meter
of the present invention which includes a gas ionizer, an ion collector
and apparatus for converting the collected ions into a signal providing
an indication of the flow of the gas.
In a specific embodiment, the invention utilizes an emitting isotope
to ionize the air mass flowing through an ion chamber. The ion chamber
consists of two cylinders, each having an inner conductor. A voltage
is applied between the cylinder and the inner conductor. The ions
are collected by the inner conductors. The current generated by
the inner conductor is converted to a voltage indicative of the
air mass flow.
BRIEF DESCRIPTION OF THE DRAWINGS
The Figure is a schematic diagram of an illustrative implementation
of an ion drag sensor constructed in accordance with the teachings
of the present invention.
DESCRIPTION OF THE INVENTION
Illustrative embodiments and exemplary applications will now be
described with reference to the accompanying drawings to disclose
the advantageous teachings of the present invention.
The Figure is a schematic diagram of an illustrative implementation
of an ion drag sensor constructed in accordance with the teachings
of the present invention. The sensor 10 includes a first cylinder
12 acting as a cathode and a first inner conductor 14 acting as
an anode. The first cylinder 12 and the first inner conductor 14
provide a first ion chamber. A first annular emitter source 16 is
provided at the top of the first cylinder 12. The first annular
emitter source 16 is a radioisotope emitting beta particles, i.e.,
Carbon-14. The first annular emitter source ionizes air flowing
through the first ion chamber. Ionization of air flowing into the
chamber is caused by the collision of the beta particles with one
or more of the orbital electrons of the gas atoms or molecules;
thus a positive ion and negative ion are formed (i.e. an ion pair).
A second cylinder 18 and a second inner conductor 20 make up a
second ion chamber. A second annular emitter source 22 identical
to the first, is provided at the bottom of the second cylinder 18.
In the preferred embodiment, the annular emitter source 22 is also
a radioisotope emitting beta particles, i.e., Carbon-14.
An electric field is set up between each cylinder 12 and 18 and
the associated inner conductor 14 and 20 by an applied voltage 24.
In the preferred embodiment, the negative terminal is connected
to each of the cylinders 12 and 18. The positive terminal is connected
to the first inner conductor 14 through a first resistor 26 and
to the second inner conductor 20 through a second resistor 28. Negative
ions are collected at the inner conductors 14 and 20. The first
inner conductor 14 is also connected to the negative terminal of
an operational amplifier 30 while the second inner conductor 20
is connected to the positive terminal of the operational amplifier
30.
The number of ion chambers and the dimensions, shape, and placement
of each may be determined by one of ordinary skill in the art, having
access to the teachings provided herein, as necessary for a given
application.
Air or other gas 40 flowing into the first cylinder 12 and out
the second cylinder 18 causes more ions to be collected in the first
inner conductor 14 than the second conductor 20. The amplitude of
the resulting output of the operational amplifier is indicative
of the air flow rate and the polarity thereof is indicative of the
air flow direction. If a positive ion radioactive source is used
(e.g. an alpha emitter), the polarity of the electrodes is reversed.
The output will have a transfer function such that
where: e(t) is the output signal voltage, Fr is the mass flow rate
and the constants A and B would be established as calibration parameters.
For a radioactive source of area A cm.sup.2 and of source strength
S curies, the current density of ionizing particles is:
If the ionizing particles are beta particles or electrons, the
range L in air is
where T is the temperature in degrees Kelvin, U is the energy in
eV, p is the pressure in mm Hg and M is the (average) molecular
weight of the air. On the other hand, if the ionizing particles
are alpha particles, the range in air is
where
The energy required to create an ion pair in air is
Thus, the number of electrons created by impact ionization per
second per cubic centimeter is
within the ionizing particle range of the radioactive source. The
recombination rate of the created ion pairs in air at standard temperature
and pressure is
where n is the density (electrons per cubic centimeter) and .alpha.=1.71.times.10.sup.-6.
Accordingly, at equilibrium
For a voltage V applied between the radioactive source location
and a collecting electrode at a distance d.apprxeq.L from the source,
the drift velocity of the electrons will be
where
and
where e is the electronic charge, m the electron mass, and the
collision frequency is expressed in terms of the molecular density
N of the air, the cross section of the air molecules .sigma. and
the thermal electron velocity v.sub..theta.. The resulting current
density is
For a sensitive air flow meter, v would be of the same magnitude
as the velocity of flow V.sub.Flow, and the mean-free path (N.sigma.).sup.-1
would be much less than d. To cover a wide range of velocities,
it may be necessary to use more than one source situated at different
positions along the flow direction relative to the collectors.
For typical .beta. and .alpha. emitters, use of these relations
indicates that for reasonable voltages (<100 volts), source strengths
(10-100 .mu.curies), dimensions (cm), it should be possible to measure
air flows in the range of interest for vehicle engines.
Alpha particles, beta particles and gamma radiation are three types
of ionizing radiation given off during the decay process of radioisotopes.
Gamma radiation is very penetrating. Air, being not very dense,
has a low stopping power (i.e., probability of collisions or absorption)
for this type of radiation. It is therefore not a viable candidate
for use as a short range ion generator.
The alpha particle, on the other hand, is a very large particle
(its mass is .apprxeq.7280 times larger than an electron) and is
a very good ion producer in air if it has sufficient energy. However,
due to the fact that it has a large mass, its path is somewhat deflected
by a high velocity gas.
Beta particle radiation of the appropriate energy range (velocity
range) also is a good ionizing source for gases. Several considerations
must be taken into account in the selection of the appropriate beta
emitting isotope including half-life of the material and the form
of which is currently available.
The half-life can be defined as the time in which there is one
half of the original quantity of radioactive material. The number
of unstable atoms, Q, at time t, is given in Equation [14] below:
Q.sub.o being the original number of unstable atoms at time t =0
and .gamma. is the value of the decay constant which is unique for
each radioisotope. From the definition of the half-life of a radioisotope,
it is evident that a longer half-life is desirable in the selection
of an appropriate material. The current collected at any fixed air
velocity is directly proportional to the amount of ions created,
which in turn is directly proportional to the amount of radiation.
Since this decreases with time, the source should have a half-life
much longer than the expected lifetime of the measuring device.
If the half-life is too short, either a recalibration of the instrument
will be required or some mechanism for compensating for this decay
must be incorporated.
Carbon-14 has a medium energy beta with a very long half-life (5568
years). Carbon-14 can be obtained in almost any chemical form. Carbon-14
can be prepared by irradiating carbon dioxide in a reactor and chemically
converting the irradiated carbon dioxide to methanol from which
almost any polymer can be made. This allows for almost any type
of molding, casting or machine process to be used which facilitates
packaging design.
Thus, the present invention has been described herein with reference
to a particular embodiment for a particular application. Those having
ordinary skill in the art and access to the present teachings will
recognize additional modifications applications and embodiments
within the scope thereof. For example, the invention is not limited
to the use of Carbon-14 as the beta emitting radioisotope. Nor is
the invention limited to the use of the inner conductors as the
anodes and the outer cylinders as the cathodes. The roles of the
inner conductors and the outer cylinders may be reversed. The source
can be connected to the inner conductors, while the outer cylinders
collect the ions and create a voltage difference at the input of
the operational amplifier. Furthermore, the ion chamber may be implemented
by the use of parallel plates in place of the cylinder and inner
conductor. In addition, the invention may be used to measure the
flow of fluids other than gases.
It is therefore intended by the appended claims to cover any and
all such applications, modifications and embodiments within the
scope of the present invention. |