Storing and manufacturing food materials like grain can be extremely challenging. Even the slightest bit of extra humidity can create mould growth, causing the grain to deteriorate. This leads to economic losses for manufacturers in the commercial food industry.

1. Digital Re-print September | October 2013
High-precision sensors: the ideal solution for
measuring grain humidity
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2. FEATURE
Figure 1:
Microwave
resonator with
inner lens tube
High-precision sensors:
the ideal solution for measuring grain humidity
by Hinrich Römhild, sensors and measurement senior
R&D engineer, WORK Microwave GmbH, Germany
S
toring and manufacturing food
materials like grain can be extremely challenging. Even the slightest bit of
extra humidity can create mould growth,
causing the grain to deteriorate. This leads
to economic losses for manufacturers
in the commercial food industry.
Microwave resonator-based sensors offer
a solution to this problem by enabling manufacturers to obtain a precise measurement
of the weight, moisture, or water content
of grain. Moreover, microwave sensors can
also be used to identify foreign particles
or substances that have come into contact
with grain.
This article explains the benefits of relying
on microwave frequencies and RF resonators to accurately measure the humidity of
grain.
38 | September - october 2013
Measuring grain humidity
In the food manufacturing business, if
grain materials are exposed to too much
humidity, they can develop mould and would
then need to be discarded. This can be prevented by controlling the humidity within the
Key for Figure 2:
¢ Oats
¢ Triticale
¢ Barley
¢ Wheat
Figure 2: Resonance curve for different samples of grain
&feed millinG technoloGy
Grain

3. FEATURE
production facility using microwave frequencies and RF resonators.
Resonators are high-quality factor (Q factor) structures that resonate at specific frequencies. When a sample of the grain material is inside of the cavity region, it affects the
cavity’s centre frequency and Q factor. The
material’s electrical permittivity and permeability are determined using the frequency
shift between the resonant frequency of the
unloaded resonator (f Res unloaded) and
the loaded resonator (f Res loaded). The Q
factor is calculated based on the frequencies
from 3 dB to the magnitude at resonance.
Measuring the shift in resonance frequency
and Q factor helps to determine two corresponding physical qualities, such as weight
and humidity. In many cases, only one physical measurement is required, necessitating
the resonator to only measure the shift in
resonance frequency or the Q factor.
A typical resonator measurement system
using the resonant cavity method includes
a resonator cavity, signal processing part,
and software control by a computer. For
this specific application, the resonator is
aluminum-based and has a diameter of
about 22 cm and a height of approximately
32 cm (Figure 1).
The electrically active part is supplemented by two flanges on the top and the
bottom. They enable integration in a tube
system and avoid any discharge of electromagnetic radiation. The aluminum resonator
also features a tube made of PEEK. PEEK,
or PolyEtherEtherKetone, is a type of plastic
approved for appliance in food technology. It
is extremely resistant to chemicals and offers
a heat resistance to 480°F. Thus, a PEEK
tube does not degrade after being exposed
to water or steam and is flame- and radiation-resistant. Unfilled PEEK received FDA
approval in 1998 and may be used in food
contact and in processing equipment without danger of contamination or degradation.
In an aluminum resonator with a PEEK tube,
the grain only encounters the PEEK tube,
ensuring the materials are not compromised.
The plastic tube of the sample resonator has
an inner diameter of 62 mm.
For measurement of electrical characteristics, the resonator includes two small
antennas. One is used for sending microwave signals, the other for receiving. In order
to analyse the data from the antennas, the
absorption of the microwaves on their way
from the transmitting antenna to the receiving antenna needs to be contemplated. Thus,
the resonator is connected to an evaluation
unit. A laptop shows the transmittance
measured.
By applying the microwave transmittance
over the frequency, users can determine a
maximum with a frequency of 1050 MHz
in an empty test resonator. If the user fills
grain in the resonator, this maximum will
be shifted to lower frequencies. This makes
it possible to distinguish between different
kinds of grain (Figure 2).
&feed millinG technoloGy
Grain
Making
the
same adjustment,
it is also possible
to
distinguish
between samples of the same
grain with diverse
humidity. Figure
3 shows the differences between
various
wheat
samples.
The curves
received
are
easily reproducible. The analysis
shows that changes in humidity
are detectable in
a one-tenth percent range without any problems.
Examination
of the complete
resonator curve is
not feasible in the
field. Therefore,
the output of
the sensors has
been limited to
the transmittance
of one frequency.
In this instance,
only one value
corresponds with
the humidity of
grain. In order to
demonstrate the
measurement
process an adjustment was made
(Figure 4).
Figure 4 shows
a tube of acrylic
glass with three
samples of grain
with
different
humidity.
The
samples are physically fastened by
foam bucklers.
The
measurement reading is
the transmittance
on a working
frequency while
the test tube is
pushed through
the resonator.
Figure 5 shows
the measurement
result over time.
The samples of
grain can be distinguished clearly.
The minima arise
if the foam is in
the active area of
the resonator.
Analog & Digital RF-Solutions
Precision Sensors
for the production environment
Typical
Applications
Mass / Weight
Optimize your
Production Process with
Microwave Sensors
»» Microwave technology
measuring principle
»» Ideal for demanding in-line
applications
»» Suitable for solid, granular and
Moisture
powdered materials
»» Measures samples as small as
1 mm³
»» Up to 10,000 samples per second
Foreign Particles
»» Rugged housing for harsh
environments
»» Contactless and maintenance
free measurement
»»Contact us
Address:
Tel.:
Fax:
E-Mail:
Web:
WORK Microwave GmbH
Raiffeisenstraße 12
83607 Holzkirchen – Germany
+49 8024 6408 0
+49 8024 6408 40
sales@work-microwave.de
www.work-microwave.de
September - october 2013 | 39

4. FEATURE
Figure 4: Measurement
adjustment in humidity
Key for Figure 3:
¢ Wheat
humidity 14%
¢ Wheat
humidity 18%
¢ Wheat
humidity 20%
Figure 3: Resonance curve for wheat with diverse humidity
The sensor (including local electronic)
can be easily integrated in a transport system of a grain dryer. Analysis can then be
performed on a local laptop or on a PC at
a central location via ethernet connection.
Of course, a custom-built adaptation on the
type of dryer in question is necessary.
Benefits of microwave resonator
technology
In addition to helping food manufacturers accurately measure the humidity of
grain, microwave resonator sensors offer a
number of other benefits. One is the ability
to measure extremely small samples (e.g. as
small as 1 mm3) with precision accuracy at
high speeds of 10,000 samples per second.
Given the small size of grain, this is important
on a manufacturing line.
Microwave resonators are also highly
sensitive, making it possible to detect small
differences of the unloaded resonator while
the probe is on. This is critical to measuring the humidity of grain with the utmost
accuracy. The repeatability of a microwave
resonator is about 0.1 percent, indicating
that the variation in measurements taken
by a single sensor is quite low and that an
accurate measurement is being taken.
The microwave resonator technique is
also much safer and more cost-effective
than using nuclear-based sensors. In the
food industry, consumer safety is of utmost
concern. Nuclear sensors contain radioactive
material, requiring manufacturers to train
their employees about the proper protocols
involved with handling radioactive materials.
Nuclear sensors also require a strict waste
disposal process that is heavily regulated by
the government.
Conclusion
Figure 5: Sensor signals for wheat with different
humidity (14, 18 and 20 %)
40 | September - october 2013
Food manufacturers have a responsibility
to deliver the freshest quality product possible to consumers. A microwave resonatorbased sensor allows them to achieve this
by performing real-time, accurate measurements of the weight, moisture, and water
content of grain. Microwave resonator sensors can easily be integrated into any manufacturing facility to increase productivity and
profits.
While this article specifically addressed
the use of microwave resonator technology for grain, RF sensors based on the
microwave resonator technique can also
be used to measure other food materials.
They can also be applied in other industries,
such as tobacco, pharmaceutical, automotive, recycling, and chemical to measure
moisture, mass, and density, as well as to
identify foreign particles and measure dielectric properties.
&feed millinG technoloGy
Grain

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