Mw2001 is a measuring system designed to quantify the moisture of tobacco in processing lines. It is designed to be easily installed on conveyors and makes use of microwave EMF to precisely measure the percentage of product water.
Online Moisture Measuring System MW2001
- Up to 4 reading units per CPU
- Easy to operate
- Easy to install
- Easy to calibrate
- Microprocessed measuring system
- Numeric display
- Adjustable to many different materials
Moisture occurs in nearly all substances and can greatly affect the properties of the host material as well as contribute to improve critical aspects of cost and product quality. There is a large number of techniques for the measurement of moisture. These have mainly grown up in discrete industries, as a consequence there is a good deal of confusion about measurement units, calibration problems and moisture interaction with host materials. These three subjects can be dealt briefly as follows.
Essentially the existing techniques can be broken down into three major areas as described below.
Standard gravimetric methods, in which a known weight sample is heated to drive off the free water by evaporation and the remaining dry weight measured. This now well-established and universal method is often the only way to provide the basic calibration required for on-line processing measurement methods. However, it involves destructive and intrusive sampling, with samples having to be removed from the process area, taken or sent to a laboratory or similar facility with balance and oven drying equipment in order to be analyzed there. There are thus disadvantages both in time and variability in obtaining gravimetric results, not to mention laboratory costs.
Electrical impedance techniques, which make use of the huge difference in the dielectric constant of water compared to most common host materials. By applying a potential to sample measurements of the current flow, the resistance to flow, or the charge capability of being put into the field of view, a reading of the relative permissiveness can be made and a moisture level gauged. This technique makes use of the very high relative permissiveness of water compared to any other host substrate whether solid or gas. When the gap between the plates of a capacitor is filled with a dielectric material the capacitance is increased. The effect of the dielectric is to reduce the potential difference across the plates so that the external power supply can then pump more current round the circuit until the extra charge on the plates is restored. It will be deduced that measurements of the capacitance, or resistivity or conductivity are all basically the same technique.
Spectroscopic methods, largely using the Near Infra Red part of the spectrum, or more exactly one of the three vibrational energy levels of the hydrogen/oxygen bands. For gases this Infra Red technique works well but for measurement in solids it is used as a surface reflectance technique, where a multi-frequency signature beam is reflected from the surface of the material and the strength of the water molecule compared with the remaining part of the signature. This reading can then, once processed and calibrated, give a read-out of the moisture level.
There are several other techniques, mainly in the measurement of gaseous moisture or relative humidity, in which long-established methods exist. The purpose here is to clear up misunderstandings about the applicability of these techniques to on-line moisture measurement in industrial processing, with particular reference to tobacco moisture measurement, where existing techniques do not apply and where microwave technology provides a better solution.
Microwave moisture measurement. This technique uses an entirely different method to determine bulk moisture levels. At high frequencies water molecules can be made to rotate or spin. This spin energy level is specific to water due to its size and uniqueness as a triatomic, polar molecule with a single symmetrical rotational dipole.
This specific excitation approach is very similar to those used in Near Infra Red techniques, which make use of the hydrogen-oxygen vibrational energy level where the bond acts as a spring. In water, however, the molecular spin/rotation by microwave technology is very specific and offers an accurate means of measuring water content in solids, powders and granulates which other methods do not provide due to their surface, contact or intrusive nature.
In practice, the equipment used transmits a low energy microwave which is focussed or shaped by means of horns or lenses and uses signal processing circuits to measure beam changes. The properties of microwaves allow both transmission and reflection techniques to be used. This means that measurement can take place by passing the beam through the bulk to be measured and receiving it on the other side or by passing it through transmitter/receiver probes inserted into the moving host material.
The equipment can thus be used for remote, non-contact measurement in chutes, across conveyors, through webs, rolls or bales, in pipework, and above metal rollers in web or packaging processes. It can also be used by contact wave guides of probe form in hoppers, silos and fluid pipework.
Variations in bulk density due to flaws, air pockets, bubbles, etc, will cause fluctuations in the beam attenuation therefore the method can be used for flaw or moisture detection as much as for measurement and process control. This detection facility can provide quality control on thickness of substrate in continuous board and packaging production but it is also applicable to many other industrial processes.
The advantages of microwave technology over the other methods include:
Bulk Measurement: the beam passes right through the material to be measured, not just the surface. It can be remote as well as of a contact nature depending on the application.
Accuracy: microwave energy is absorbed by unbound water molecules only and is unaffected by colour, emissivity, texture or speed of passage of the host material to be measured.
Calibration: microwave equipment is easy to calibrate, either using periodic gravimetric checks or a series of calibrated cells provided by the manufacturer.
Operation: microwave equipment is easy to operate and to locate at the correct processing point. Modern data processing software enables full process control and statistical recording to be carried out at low cost.
Examples of the particular applications for which microwave moisture measurement provides solutions, which other existing methods do not cover, include:
- tobacco processing lines
- board production of all kinds
- paper and pulp manufacture
- packaging laminates
- timber processing
- sand and ceramics
- animal feed, cereals, powders and pelletised product processes
- grain drying and agricultural harvesting
- frozen food, milk processing and many other individual applications
Mw2001 block diagram, as shown below, is composed of a Transmitter which generates the microwave energy, a Receiver which detects and amplifies the microwave signal and a Central Processing Unit responsible for the mathematical conversion of the electrical signal into a moisture read-out.
The Transmitter Unit irradiates the microwave energy continuously while the product passes between transmitter and receiver. The product under measure absorbs the energy, dropping the level of signal detected by the Receiver. The signal detected is continuously amplified and delivered to a data acquisition system embedded in the Central Processing Unit.
The controlling software sweeps the acquisition board 50 times per second to make an accurate image of the moisture profile inside the product. Mathematics algorithm thus calculate the total amount of water and therefore the moisture contents.
For each measuring unit ( Transmitter + Receiver ), the CPU outputs a 4 to 20 mA signal proportional to the moisture detected.
- Measuring range: Adjustable within 1% to 50%
- Repeating accuracy: Better than 0,1%
- Measuring accuracy: Better than 0,3%
- Measuring sampling: Up to 50 samples / second
- Environment temperature: 0º to 40º C
- Interface: four analog 4 to 20mA outputs
- Mains supply: 220V
The transmitter unit is composed of:
- 1 Power Supply Microwave 2035
- 1 Microwave Generator 2032
- 1 Antenna and Backplane
The receiver unit is composed of:
- 1 Power Supply Microwave 2035
- 1 Conditioning Board 2008B
- 1 Antenna and Backplane
- Voltage to Current Converter 2026B
The Central Processing Unit is composed of:
- 1 Processing Electronics
- Wiring connections
The picture below shows an example of mechanical solution to position the receiver above the conveyor and the transmitter under it. The fixture must allow the user to adjust distances from the units to the conveyor belt.
Once positioned, the units will be connected to the CPU through cables wired to each connection box. Electrical schemes showing all wiring are presented further on.
Transmitter Wiring – The transmitter requires only a 220 VAC connection. This is obtained from the CPU wiring connectors. The picture below shows the AC connector inside the opened box.
Receiver Wiring – The receiver unit connection box is shown opened below. This unit is powered with 220VAC and delivers a 4 to 20mA signal that must be conveyed to the CPU through a shielded cable.
Central Processing Unit Calibration
The CPU is calibrated on the Processing Electronics by navigating the screens and adjusting the parameters. The Processing Electronics Panel features a two-line LCD and four function keys.
F1 – activates the previous screen
F2 – activates the next screen
F3 – increases the parameter on the current screen
F4 – decreases the parameter on the current screen
When turned on, the main screen is activated. This screen presents the four moisture read-outs as in the picture that follows.
The following screens allow the user to visualize and adjust the parameters for each reading unit. The manual presents the procedure to calibrate the first channel. The remaining channels are calibrated the same way through their own screens.
Channel 1 Calibration
Zero Adjust – If the current screen is the main screen, press F2 twice to switch to the ZERO SCREEN. Press F3 or F4 to adjust the moisture indication at 4 mA to the minimum expected moisture.
Span Adjust – Press F1 or F2 to move to the SPAN SCREEN. Press F3 or F4 to adjust the moisture indication at 20 mA to the maximum expected moisture.
Gain and Offset Adjust – A dry sample and a wet one are required for the gain and offset adjust. Both samples must have their moisture contents previously measured through laboratory procedures. Calibration consists of:
- Place the dry sample between units and adjust the OFFSET so that the moisture read-out displays the value already known.
- Place the wet sample between units and adjust the GAIN so that the moisture read-out displays the value already known.
- Repeat steps 1 and 2 as long as no further adjusts are required.
OBS: Press F1 or F2 to switch to the desired screen. Press F3 or F4 to adjust the moisture indication to reflect the actual values.
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