| The
RFM-1000 Radio Frequency
Moisture Sensor generates a Radio Frequency Fringe Field at
less than 10 MHz. This field penetrates into the product being
measured. As the total water content of the measured product
changes, the RFM-1000 quantifies
the resultant shift in frequency. This frequency shift is
converted into a high resolution digital count value, directly
proportional to the change in percentage total moisture. This
operation is performed continuously at 200 measurements per
second. The RFM-1000 consists
of an electrode, an oscillator circuit and a digital circuit.
The sensor can be connected to a PSC processor with electronics
display or to a host computer.
How
does the sensor work?
Water has a very high dielectric constant,
usually 20 times greater than the dielectric of the solid
material being measured. We use a radio frequency fringe field
to measure the dielectric in the product/water combination.
The sensor with the product is set to a specific frequency,
its ‘resonant frequency’. As the water level changes,
the dielectric value in the product changes. Therefore the
‘resonant frequency’ tuning changes up or down
depending on the concentration of water. The sensor works
by identifying and measuring this change in frequency and
displaying the change as a moisture percentage.
Does the sensor need to
touch the product?
No but it has to be in close proximity, Less than 1/4 inch
(6mm) away.
Can the sensor look through
a window?
Yes, however it should be non metallic, be stable in moisture
and have an overall thickness of not more than
1/8 inch (3mm).
Does temperature affect
the measurement?
It can but over a +/- 5 C not generally a problem.
Does change in color affect
the measurement?
Not normally. However the presence of carbon black can create
a problem. The high dielectric value of TiO2 can also create
a problem.
Can a shifting component
matrix affect the measurement?
Only if it affects the density. The change of raw ingredient
normally will not as the dielectric will probably be similar.
Does density change affect
the measurement?
Yes, it can. It is best to stick to constant volume products,
if possible, for best accuracy and repeatability. It is possible
to measure density and bring an input to the sensor for compensation.
What are typical applications?
Board products, wood such as lumber after a dryer, non homogeneous
products like beans and grains, cooked products before processing
like rice, corn, coffee beans, cereals.
What is the life of the
sensor?
There are no moving parts so the sensor is very robust and
reliable. You can expect many years of routine service.
What is involved in setting
up a calibration?
The sensor needs to be compared to known moisture samples
determined by a standard primary method. The
relationship to moisture is linear so most products are already
well understood and have standard calibration values that
are inserted at the factory.
Do I have to vary the
process to confirm a calibration?
Yes. This is always a good idea. To vary the moisture content
of the product on line will easily confirm that the sensor
is tracking moisture appropriately.
What sort of routine maintenance
is required?
None other than to check that the sensor is not fouled in
any way or coated on the measurement surface. It is also important
to make certain that there is no build up of free water on
the sensor.
CONFIGURATION OF THE RFM SERIES
Stand Alone Smart
Sensor
The RFM-1000 sensor can operate with no proprietary processing
electronics. The sensor makes the measurement and creates
analog and digital outputs that can communicate directly with
a PC, Controller, Recorder, Data Acquisition System or PLC.
Single Sensor
and Processor Combination
The RFM-1000 sensor can be attached by a proprietary
cable to a single PSC Processor. The Processor displays the
moisture value and produces analog and digital outputs that
can communicate directly with a PC, Controller, Recorder,
DAS or PLC. Bus communication such as Profibus, Devicenet,
Ethernet, Wireless, etc reside in the Processor.
Multiple Sensor
and Processor Combination
Up to 4 RFM-1000 sensors can be attached to a single
LCD based, touch screen PSC Processor. The Processor screen
will display up to 4 moisture values simultaneously. All analog,
digital and bus communications are generated at the PSC Processor. |
