This invention relates to microwave moisture sensing arrangements.
In Y. Miyai "A New Microwave Moisture Meter for Grains", J. Microwave
Power 13 (2), 1978 is described a microwave moisture meter for grains using a hybrid
microwave integrated circuit (MIC) and a strip line sensor which measures the
microwave attenuation due to the measure of the moisture content of grain passing
over the sensor.
The device is comprised in a casing adapted for installation in a
window made in the wall of a hot-air dryer in which rice grains are circulated
on a conveyor. A sensor plate incorporating the strip line sensor is on one wall
of the casing and is placed against the window so that the rice grains flow over
Such an arrangement is useful only where flowing materials are being
monitored, and was devised in particular for automatic operation of the rice-grain
There are many instances, however, where moisture content in static,
penetrable materials needs to be measured. The Miyai instrument is quite unsuited
to such measurements.
The present invention provides a microwave moisture sensing arrangement
which is especially adapted to such measurements and is useful in a wide variety
of circumstances and for measuring moisture in a wide variety of materials as well
as being adapted for use as a level sensor.
The invention comprises a microwave moisture sensing arrangement
comprising an oscillator and a detector and a microwave strip line sensor connected
between said oscillator and said detector and adapted for insertion of said strip
line sensor into bulk, penetrable materials whereby the detector detects a change
in received signal due to absorption of radiation from the sensor, in which the
oscillator and detector are contained in a housing and the strip line sensor is
carried on a probe projecting from said housing and in which said detector detects
attenuation and phase change of microwave signals through said sensor.
The probe may carry a graduation.
The line sensor may be connected to the oscillator and detector by
waveguide means or coaxial link.
The said detector detects attenuation of microwave signals through
said sensor and phase change of such signals, and may comprise a microwave mixer,
which may be a balanced mixer.
One of the problems of microwave moisture measurement, which is observed
by Miyai but not dealt with in the construction of his meter, is that the change
in microwave signal is affected by the density as well as the moisture content
of the material under observation.
This is dealt with in the present instance by detecting phase change,
which is affected by density differently to attenuation. Phase change is now detectable
within the scope of a portable, inexpensive microwave moisture meter by using a
microwave mixer, particularly a double balanced mixer such as is available from
Plessey Microwave Limited of Towcester, Northants, NN12 NJS, England.
Clearly the density in a bulk container of grain, for example, will
depend upon the depth in the container as well as on the extent to which the load
has settled due to vibration e.g. from transportation. Measurements of moisture
content will be affected by such density variations and also by an local packing
or loosening occasioned by the insertion of a probe, and this will lead to spurious
The facility to measure both attenuation and phase change, however,
is not required for some measurements. The line sensor for example is mounted on
the end of a graduated probe adapted for insertion into a tank containing layers
of different penetrable materials to detect the interface level or levels by observing
the change in signal as the sensor passes between the layers. Such an arrangement
can of course be used to check the homogeneity of a mixture of materials, settlement
out of suspensions or of immiscible liquids and so on.
A line sensor comprises essentially a conductive plate with a dielectric
substrate carrying a thin (eg printed) strip of conductor. The conductor may be
e.g. copper or gold. The line may have a configuration other than a straight line,
for example, it may have wavy or convoluted configuration, to increase the length
of line in a given area or to produce a radiation field of a particular shape adapted
to a specific measurement. It may be desired, for example, in level sensing applications,
to have a small, sharply-defined field to increase the accuracy with which a level
can be located. It may, on the other hand, be desired to have a uniform, extended
field in order to integrate the attenuation or phase change effect over a region
to get an average reading in a not necessarily homogeneous material. In general,
the longer the line, the more sensitive will be the measurement since the attenuation
will depend on the line length, but this, too, is moderated by line configuration.
A tank level sensor according to the invention may comprise an elongate
loop line sensor depending into the tank from a unit containing the MIC and detector,
the analogue attenuation signal varying with the level of liquid or moist material
in the tank. Such an arrangement may have a uniformly constructed loop to give
a linear attenuation/depth relationship. However, even a non-linear arrangement
can be calibrated.
Another tank level sensor according to the invention, however, may
have a small sensor drivable up and down the tank, possibly in a servo arrangement
detecting only a sharp change in attenuation and phase and the level of such sharp
change being determined from the position of the sensor.
Embodiments of microwave moisture sensing arrangements according to
the invention will now be described with reference to the accompanying drawings
- Figure 1
- is an elevation of a hand-held moisture sensing arrangement;
- Figure 2
- is a view of a microwave strip line sensor head arrangement suitable for use
with the arrangement illustrated in Figure 1;
- Figure 3
- is a cross section of the head of Figure 2;
- Figure 4
- is a section through a liquid-storage tank having one kind of arrangement according
to the invention used as a level sensor.
The drawings illustrate microwave moisture sensing arrangements.
Figure 1 illustrates such an arrangement comprising an oscillator
11 and a detector 12 and a microwave line sensor 13 connected between said oscillator
and adapted for insertion of said line sensor into bulk, penetrable materials,
whereby the detector 12 detects a change of received signal due to absorption of
radiation from the sensor.
The oscillator 11, which may comprise a hybrid microwave integrated
circuit (MIC) as described by Miyai, or a Gunn diode or FET oscillator arrangement
as from a domestic burglar alarm and the detector 12 (which may comprise for example
a crystal or low-cost satellite dish amplifier or LNB) are contained in a housing
14, while the line sensor 13 is carried on a probe 15, which carries a graduation
15a in centimetres for example, projecting from the said housing 14.
The line sensor is shown in greater detail in Figures 2 and 3. It
comprises a ground plate 21 e.g. of copper or gold with substrate 22 of dielectric
such as fluor-glass or ferrite, with a printed-on strip 23 of copper or gold. The
line 23 is covered with a protective film 24, which is, however, thin so as not
substantially to reduce the field propagated by the strip line.
In Figure 2, the line 23 is shown as a wavy line, which increases
the propagated field, as would even more a convoluted or labryrinthine configuration,
within a given spatial compass, thereby increasing the microwave power available
for absorption by the moisture and hence the sensitivity of the arrangement.
The line 23 is connected to the oscillator 11 and the detector 12
by waveguide or other transmission line means 25.
The housing 14 has a handle 16 and includes a power source 17 and
a double balanced microwave mixer 18 of the type sold by Plessey Microwave Limited
aforementioned, used a phase detector.
Analogue signals representative of attenuation and phase change can
be digitised and displayed on a convenient display e.g. and LCD panel 19.
For a general purpose portable probe, it will suffice to give "raw"
values of attenuation and phase change so that the arrangement can be calibrated
for any particular function. If the arrangement is to be used for example with
textile fibres, a series of measurements can be made using fibres with different
known values of moisture regain, or different values measured by conventional means
as by drying and weighting. The values of attenuation and phase change for a particular
instrument corresponding to these known or conventionally measured values are then
noted, so that when these values are observed in a sample for evaluation of moisture
content, the look-up table can be consulted to find the actual moisture content.
Where the same probe is then to be used with a different material,
it can be calibrated again and a new look-up table generated.
Usually, linear relationships can be assumed for interpolation purposes.
Where an arrangement is dedicated to a single end use, a microprocessor
can convert the raw attenuation and phase change data into units representative
of moisture content or level appropriate to the circumstances.
Figure 4 illustrates such a dedicated arrangement for measuring water
level in a tank 41 in which the housing 14 is located atop the tank and the line
sensor comprises an elongate loop 42 with parallel, closely spaced sides 42a, 42b
extending from near the housing 14 to the bottom of the tank 41.
The measurement of attenuation alone will usually provide a sufficiently
good value of tank contents when the tank contains liquid water, but when the tank
contents comprise sand or grain, a combination of attenuation and phase measurement
is likely to give improved accuracy taking density into account.