GB2102128A - Apparatus for and a method of monitoring liquid content of a material - Google Patents

Abstract

The material is contained in a trough (20) which is coupled via conductive means with a pick-up coil (L3) which picks up radio frequency flux from an oscillator (not shown) to expose the material to a radio frequency field. A signal is provided dependent on the radio frequency conductance of the material and on a meter (M) there is displayed the percentage by weight of moisture in the material using that signal. The radio-frequency field may be modulated by means of triangular waveform generator (A) and variable capacity diodes (VC1), (VC2) whereby a tuned circuit including coil (L3), and the material is brought into resonance this being detected by an oscilloscope connected to (MP). In addition the output across coil (L3) is peak detected, amplified at (C) (D), (F) and fed to a recorder (M). Temperature compensation is provided by amplifier (E) to which a temperature sensor is connected. <IMAGE>

Description

SPECIFICATION Apparatus for and a method of monitoring liquid content of a material The present invention relates to apparatus for and a method of monitoring liquid content of a material, for example the measurement of percentage by weight of moisture in material such as tobacco, smoking articles, grain, lignous materials, and ores.

It is known to measure the moisture content of a material by measuring the dielectric constant of the material, the dielectric constant being known to vary in accordance with the moisture content of the material.

Apparatus hitherto available for measuring moisture content of a material by measurement of the dielectric constant is expensive and not as accurate as one would wish for precise control of moisture content of a material such as tobacco or grain. The organo-leptic properties of smoking tobacco, for instance, depend in part on its moisture content. Further, the behaviour in rough seas of a cargo vessel loaded with grain or ore will be affected by the moisture content of the grain or ore.

It is an object of the present invention to provide a relatively cheap, reliable, and easily used method of an apparatus for determining moisture content of a wide variety of materials in situ by measuring a physical parameter of the material.

According to a first aspect of the present invention there is provided apparatus for monitoring the liquid content of a material, comprising first means, for providing a radio frequency field to which such material is exposed in use of the apparatus, and second means, for providing a signal dependent on the radio frequency conductance of such material exposed to that field.

According to a second aspect of the present invention, there is provided a method of monitoring the liquid content of a material, comprising exposing the material to a radio frequency field and producing a signal dependent on the radio frequency conductance of the material.

The first means may comprise a radio frequency oscillator and a container for such material so that such material in the container is exposed to the radio frequency field from the oscillator. The radio frequency oscillator may further comprise a pick-up coil responsive to flux from the oscillator, the container being coupled with the coil via electrically conductive means for establishing the radio frequency field.

The apparatus may further comprise modulating means for modulating the radio frequency field for achieving resonance of a circuit including such material with the radio frequency field. For example, the modulating means may comprise a triangular wave-form generator. In the case of the use of such a pick-up coil, the generator could be coupled with the coil, for example via at least one variable capacitance diode.

Moreover, in the case of the use of such a pick-up coil, the second means may comprise detector means coupled with the coil for producing the signal. The second means may then further comprise indicator means coupled with the detector means for displaying an indication dependent on the amount of a liquid in such material.

The invention will now be described by way of example with reference to the accompanying drawings in which, Figures 1-4 are circuit diagrams of the four principal parts of an electrical circuit of an example of apparatus according to the present invention, for measuring the percentage by weight of moisture in a material; and, Figure 5 shows an example of a trough for use in apparatus, for containing material whose moisture content is to be measured.

Referring to Figure 1 there is shown the circuit diagram of a radio frequency oscillator 10 freerunning at approximately 1.75 MHz. In particular the oscillator 10 comprises a Hartley oscillator which is controlled in amplitude by limiting supply voltages, further stabilisation being obtained through negative feed-back occurring across a 1.8 kQ resistor R1 which also raises the input impedance of the base of a type TIP47 transistor T1 to an order above the value of a 14 kQ resistor R2.In the oscillator 10, coils L1 and L2 are respectively 47 ,uH and 10 ,uH, a resistor R3 is a 100 kQ resistor and capacitors Cl, C2 and C3 are respectively 0.1 ,uF, 0.01 ,uF and 68 juF capacitors.

Referring to Figure 2, a triangular wave-form generator, free-running at approximately 50 Hz, is provided by an operational amplifier A, a 10 kQ resistor R4, a 22 kQ resistor R5, a 10 kQ resistor R6 and 1 ,uF capacitor C4. The triangular wave-form is fed via a 47 kQ resistor R7 to a buffer amplifier B whose gain is variable by means of a 200 kQ maximum value variable resistance resistor VR1,the value of a resistor R8 being 100 kQ. Operational amplifiers A and B are type348 operational amplifiers of a first integrated circuit.

A variably tunable tank circuit capable of being controlled by the output of the buffer amplifier B is provided by a pair of variable capacitance diodes VC1 and VC2, the value of a resistor R9 being 1 MQ and the value of a capacitor C5 being 0.001 ,rF.

Material, the percentage by weight of whose moisture content is to be measured, is received in a trough 20 in which a radio frequency field is developed as a result of its connection with a 47 ,uH coil L3 which picks up the constantly changing flux iinkage from coil L1 so that a voltage is developed across it. The tuning of coil L3 is continually being changed at a lower frequency by variable capacitance diodes VC1 and VC2 which, at radio frequency, are in parallel with coil L3.The coil L3 is arranged, together with a variable capacitance capacitor VC3 of a maximum value of 30 pF and the loaded trough 20 in use of the apparatus (or a variable resistance resistor VR2 of a maximum value of 100 kQ and a variable capacitance capacitor VC4 of a maximum value of 30 pF; or a variable resistance resistor VR3 of a maximum value of 100 kQ and a variable capacitance capacitor VC5 of a maximum value of 30 pF-see below) to resonate once per voltage sweep from a point TP 1, which is a test point. Thus, there is a resonance twice per cycle, once on the leading edge of the triangular waveform and once on the trailing edge. The resulting modulation may be observed by connecting an oscilloscope to an oscilloscope monitor point MP and will be seen to take the form of a mirror-image wave-form.The peak value of this wave-form is a measure of the radio frequency conductance of the circuit connected across the coil L3 and, when the trough 20 is loaded with material whose moisture content is to be measured, is used to provide a reading of the percentage by weight of the moisture content of the materials. It should be noted that changes of capacitance in the tank circuit do not materially affect the reading since the only criterion is that the circuit is taken through resonance by means of the buffer amplifier B driving variable capacitance diodes VC1 and VC2. This results in a double-humped mirror-image as mentioned above. A single hump would mean that the resonance peak has been approached but not necessarily reached.

The circuit of Figure 2 further comprises, connected to the coil L3 and the circuit including the trough connected across it, a peak amplitude detector comprising a diode Dl, a 1 MF capacitor C6 and a 0.1 ,uF capacitor C7. The peak amplitude detector is connected via a series connection of two 1 resistors R10 and R1 1 to an operational amplifier C provided with "zero" and "sensitivity" controls and having a "virtual earth" input. The output of the amplifier C is connected via a 10 kS2 resistor R12 to an operational amplifier D which has a "virtual earth" input with the facility (if required) to include temperature compensation from an operational amplifier E. An anti-phase control output AP is available from the amplifier D.The output of the amplifier D is connected via a 10 kQ resistor Rl 3 to an operational amplifier F, which is an output amplifier whose output, in use, is in the range from 0 volts (dry end of the scale) to 10 volts (wet end of the scale) and is fed to a moving coil meter M calibrated to read in terms of percentage by weight of moisture in material being examined. An additional output from the amplifier F is taken via a 1 kQ resistor R14 to a jack plug in order to feed a recorder, the resistor R1 4 enabling the use of a 10 volts or 10 mA recorder. This output via resistor R14 may also be used for control purposes.

As mentioned above, an operational amplifier E is provided for the purposes of temperature compensation if required. This is done by a temperature sensor connected to pin 12 of the amplifier E and a temperature display instrument i connected via a 1 kQ resistor R1 5 to pin 14 of the amplifier E.

The instrument I may be a -10 volts to + 10 volts or a -1 mA to + 1 mA instrument scaled to read from --1 OOC to + 00 C. If temperature compensation is not required, then instrument I would be omitted and pin 12 of amplifier E connected to 0 volts. The feed-back resistor R1 6 from pin 14 to pin 13 of amplifier E is a 1 M resistor.

Amplifiers C, D, E and F are type 348 operational amplifiers of a second integrated circuit.

The circuit shown in Figure 3 included in the apparatus switch 5 having a first position, called REF. 1; a second position, called REF. 2; and a third position, called MEASURE.

In the first position, the switch S causes energisation via a 1 kQ resistor R17 of a coil RR1 of a first read relay, to close a contact CCl (Figure 2) of that relay and connect into the tank circuit for calibration purposes the variable resistance resistor VR2 (Figure 2) and the variable capacitance capacitor VC4 (Figure 2); in the second position of the switch S, it causes energisation of a coil RR2 of a second reed relay, to close a contact CC2 (Figure 2) of resistance resistor VR3 (Figure 2) for test purposes; and in the third position of the switch S, it causes energisation of a coil RR3 of a third reed relay, to close a contact CC3 (Figure 2) of that relay to connect the trough 20, for moisture content measurement purposes, to the tank circuit.

Figure 4 shows a power supply circuit included in the apparatus, for providing the necessary supply voltages from a 120 volts alternating supply. A transformer T provides a voltage of amplitude 1 4 volts across each half of its centre-tapped secondary winding, across which is connected a bridge rectifier R to the positive output of which is connected on a +15 volts regulator X of type 7815 or similar, and the negative output of which is connected a -l 5 volts regulator Y of type 781 5 or simiiar.

Resistor R18 is a 110 Q resistor, capacitors C8 and C9 are 1000 MF capacitors and capacitors C10, Cl 1, Cl 2 and C13 are 0.22 ,uF capacitors.

Other component values in the circuit shown in Figure 2 are as follows. A resistor R19 is 3.3 MQ; a eapacitor C14 is 0;1 MF; resistors R20 and R21 are 10 kQ; a resistor R22 is 47 k n; resistors R23 and R24 are 10 kQ; a resistor R25 is 1 5 kQ; a resistor R26 is 10 kQ; a resistor R27 is 1 kS2; a resistor R28 is 1 MQ; a capacitor C15 is 0.001 yF; a resistor R29 is 220 Q; a capacitor C16 is 10 nF; and potentiometers P1 and P2, called respectively "zero" and "sensitivity" potentiometers, are 5 kQ, ten turn potentiometers.

Figure 5 shows a trough 20 suitable for use in measuring the percentage by weight of the moisture content of cigars stacked to fill the troughs, references 21, 22 denoting metal plates on the outside of the otherwise non-conductive trough which are connected to respective sides of the coit L3 (Figure 2). In use, care should be taken to ensure that the mass undergoing measurement remains sensibly constant. This is best arranged by designing the trough 20 so that the radio frequency field is contained in the lower part of the trough and the material extends beyond the field.

The nature of design of the trough 20 will depend on specific applications, in particular the particular material whose percentage by weight of moisture content is being measured. For example such material may include tobacco, cigars, cigarettes, grain, rice, ores (non-conductive), other granular bulk solids, bulk paper, and wood. It is expected that the invention will be suitable for measuring moisture contents in the range from about 9% to more than 30% by weight, but it will be understood that the invention is not intended for determination of quantities of standing water, e.g. a film of water on the outside of a package.

Before use, the apparatus is tested and calibrated as follows. For testing, first the + 1 5 volts and -15 volts supply voltages are checked to ensure a voltage tolerance of +0.6 volts, and it should be ensured that any ripple is no more than of the order of 10 mV. An oscilloscope is then connected to test point TP 1 and the variable resistånce resistor VR1 e adjusted so that the triangular wave-form is not quite clipped at its positive and negative excursions. This will normally result in a peak-to-peak voltage in the range from 20 to 25 volts, the frequency not being critical.The variable resistance resistor VR2 should be set to 50 kQ and the variable resistance resistor VR3 to 14 kQ. The oscilloscope is then connected to the monitor point MP and, with the switch S in its first position, namely the position called REF 1 variable capacitance capacitors VC3 and VC4 are adjusted to give a typical double-humped mirror-image display on the screen of the oscilloscope. The coil L3 is adjusted to produce a maximum peak of about +4 volts. On switching the switch S to its second position, namely the position called REF. 2, the voltage at point MP should be about 3 volts.The potentiometers P 1 and P2 are then set to produce indications on the meter M of about 12% with the switch S in its REF. 1 position and 1 6% with switch S in its REF.'? ppsition. The trough 20 is then connected, depending on the type of material whose percentage by weight of moisture content is being measure, and it is then to be checked that, with the switch S in its third position, called MEASURE, and with the trough 20 normally loaded, the oscilloscope still shows the typical double-humped mirror-image display The apparatus is then to be calibrated according to the following calibration procedure.

(1) Obtain a sample of the material whose moisture content is known to. be near the lower end of the range of the apparatus (Sample 1).

(2) Load the trough 20 correctly.

(3) Adjust the "Sensitivity" potentiometer P2 for a best estimate, say 11%.

(4) Obtain a sample of the material whose moisture content is known to be near the.top end of the range of the apparatus (Sample 2).

(5) Load the trough 20 with Sample 2.

(6) Adjust the "Zero" potentiometer P 1 for a best estimate, say 15%.

(7) Reload Sample 1 in the trough 20.

(8) If the result still fairly close to 11% go to Step 9, otherwise repeat Steps 3 to 8. Record the results from Samples 1 and 2.

(9) Set switch S to its position REF. 1 and record the scale reading of meter M.

(10) Set switch S to its position REF. 2 and record the scale reading of meter M.

(11) Obtain oven test results for Sample 1 and Sample 2.

(12)-Construct a graph with 10% to 18% Sample moisture on the vertical axis and 10% and 18% scale reading of meter M on the horizontal axis.

Moisture % from Example Scale reading oven test Given:- Sample 1 11 10.6 Sample 2 15 13.9 Switch S in position REF. 1 1 5.5 Switch S in position REF. 2 11.7 (13) Plot Sample 1 and Sample 2 locations and join these points with a straight line.

(14) Draw verticals from REF. 1(1 5.5) and REF. 2 (11.7) to intersect this line.

(15) These intersections now represent two moistures.

(16) Draw the "desired" calibration line from 10, 10 to 18, 1 8.

(17) Lines drawn horizontally from REF.1 and REF. 2 intersections meet the "desired" line at REF.

1' and REF. 2'-.

REF. 1' represents 14.3% REF. 2'represents 11.7% (18) Adjust the "Zero" and "Sensitivity" potentiometers (P 1 and P2) until the switch S in position REF. 1 there is produced a reading on meter M of 14.3% and with switch S in position REF.2 there is produced a reading on meter M of 11.7 /O.

To set the "REF" points to particular values as taken from the graph: (a) use the "Zero" potentiometer (P 1) to set a value nearer the top end of the scale; (b) use the "Sensitivity" potentiometer (P2) to set a value nearer the bottom end of the scale; (c) switch alternately from the higher value ("Zero") to the value value ("Sensitivity") and back as many times as necessary to ensure that the values with switch S in its positions REF. I and REF. 2 are achieved.

(19) Check that Sample 1 now indicates 10.6% Check that Sample 2 now indicates 13.9%.

In one example, the above described circuitry for use with a trough 20 is mounted within a cast aluminium box with the water M located on the front face of the box. A hinged plastics cover provides lockable access to the potentiometers P 1 and P2.

Located on one side of the box are a UHF plug for connection to the trough 20 and the jack plug, for connection to an ink recorder or control system.

The apparatus may be modified for use with a 1 2 volt battery supply.

The apparatus is suitable for use with either static or moving material, the trough 20 being designed accordingly. In the latter case, in particular with a vibratory conveyor providing trough 20, it should be noted that the parameters of the material as measured by the apparatus will be different with the conveyor running than when it is stopped. This is to be expected since whilst running, the material will spend the majority of this time in the air rather than in contact with the conveyor.

Calibration should therefore take place whilst running.

Any trough 20 to be used with a vibratory conveyor should be equipped with a means of adjustment. This is to enable it to be tilted in a longitudinal plane relative to the main conveyor. The purpose of this is to ensure that the trough is adequately filled and continuously replenished.

In an alternative embodiment, especially when samples of stationary friable or granular material are being tested, the trough may take the form of an upright cylinder.

In yet another embodiment (not illustrated) the apparatus may be modified to test for damp wood in buildings.

Values of circuit comonents may be altered to suit specific applications.

Claims (20)

Claims

1. Apparatus for monitoring liquid content of a material, comprising first means, for providing a radio frequency field to which such material is exposed in use of the apparatus, and second means, for providing a signal dependent on the radio frequency conductance of such material exposed to that field.

2. Apparatus according to claim 1 , wherein the said first means comprises a radio frequency oscillator and a container for such material so that such material in the container is exposed to the said radio frequency field, from the oscillator, in use of the apparatus.

3. Apparatus according to Claim 2, wherein the said first means further comprises a pick-up coil responsive to flux from the said oscillator, the container being coupled with the coil via electrically conductive means for establishing the said radio frequency field.

4. Apparatus according to any preceding claim, further comprising modulating means for modulating the said radio frequency field for achieving resonance of a circuit including such material with the said radio frequency field in use of the apparatus.

5. Apparatus according to claim 4, wherein the said modulating means comprises a triangular wave-form generator.

6. Apparatus according to claims 3 and 5, wherein the said generator is coupled with the said coil.

7. Apparatus according to claim 6, wherein the said generator is coupled with the said coil via at least one variable capacitance diode.

8. Apparatus according to claim 3 or any of claims 4 to 7 as dependent upon claim 3 wherein the said second means comprises detector means coupled with the said coil for producing the said signal.

9. Apparatus according to claim 8, wherein the said second means further comprises indicator means coupled with the said detector means for displaying an indication dependent on the amount of a liquid in such material in use of the apparatus.

10. Apparatus for monitoring liquid content of a material, substantially as herein described with reference to Figure 1 of the accompanying drawings.

11. A method of monitoring liquid content of a material, comprising exposing the material to a radio frequency fieldand producing a signal dependent on the radio frequency conductance of the said material.

12. A method according to claim 11, wherein the said radio frequency field is provided from a radio frequency oscillator, the said material being in a container.

13. A method according to claim 12, wherein a pick-up coil responsive to flux from the said oscillator is coupled with the container via electrically conductive means for establishing the radio frequency field to which the material is exposed.

14. A method according to any of claims 11 to 13, wherein the radio frequency field is modulated for achieving resonance of a circuit including the said material with the said radio frequency field.

1 5. A method according to claim 14, wherein the modulation is achieved using a triangular wave-form generator.

1 6. A method according to claims 1 3 and 15, wherein the said generator is coupled with the said coil.

1 7. A method according to claim 16, wherein the said generator is coupled with the said coil via at least one variable capacitance diode.

1 8. A method according to claim 13 or any of claims 14 to 1 7 as dependent upon claim 13, wherein detector means is coupled with the said coil for producing the said signal.

1 9. A method according to claim 18, wherein indicator means is coupled with the said detector means for displaying an indication dependent on the amount of a liquid in the said material.

20. A method for monitoring liquid content of a material, substantially as herein described with reference to Figures 1 4 of the accompanying drawings.