GB2210693A - An electronic moisture measuring instrument - Google Patents

Abstract

A portable electronic moisture measuring instrument e.g. for bulk grain or cotton comprising an electronic circuit 10, a display 30 and a capacitive probe. The electronic circuit comprises an oscillator 12, providing an oscillating signal which drives a reference channel 14 and a measurement channel 18. The reference channel 14 provides a reference signal based on the oscillating signal and the measurement channel provides a measurement signal based on the oscillating signal and influenced by moisture content related electrical properties, e.g. the dielectric coefficient, of a material under test. A comparator 24 compares the measurement and reference signals and produces an output signal representative of the moisture content of material into which the probe has been inserted. The output signal drives the display 30. A temperature measuring circuit 26, e.g. using a thermistor, provides a temperature compensating signal to the comparator 24. The display may be set to read the temperature of the material. Zero adjustment and calibration adjustment for different materials are provided. <IMAGE>

Description

AN ELECTRONIC MOISTURE MEASURING INSTRUMENT This invention relates to moisture measuring instruments and particularly though not exclusively to a portable electronic moisture measuring instrument.

The moisture content of many bulk materials, for example grain or cotton, often needs to be measured by interested parties such as farmers and raw material importers. The moisture content of stored grain is of particular interest to farmers since deviations from ideal storage conditions can quickly lead to a serious deterioration in the quality of the grain and thus to a reduction in the market value of the grain or even to deterioration of the grain to the point where it becomes unfit for sale. The moisture content of cotton is of particular interest to cotton traders since cotton is usually bought and sold by weight. Similar considerations apply in respect of many other materials.

There are a certain number of water molecules bound up within the complex molecular structure of many fibrous materials, such as cotton, and granular materials, such as corn. Indeed most natural materials and many man-made materials include bound water molecules. Such bound water molecules are an inherent part of the material and their presence is not ofdirect concern to those who are interested to know the percentage moisture content of the bulk material. Thus moisture content determining instruments should desirably be arranged to be non-responsiye to the presence of these bound water molecules.Therefore, if a moisture detection instrument is to be readily adaptable for use with a wide range of materials, calibration and zero offsetting facilities should desirably be provided in order to adjust the behaviour of the instrument in accordance with the bound water characteristics of the different materials.

It is well known that the moisture content of a material influences the electrical properties of the material and more particularly influences the electrical resistivity and dielectric coefficient of the material.

As will be described hereinafter the moisture measuring instruments of the present invention are arranged to be responsive to the capacitive properties of the material being measured, and are, in practice, arranged to provide a calibrated display of detected percentage moisture content as a function of the dielectric capacity of the bulk material.

Both the resistivity and the dielectric capacity of non-dry materials, that is materials with a percentage moisture content, are dependent upon the temperature of the material and it has been found that for each Celsius degree increase in temperature the measured percentage moisture content would conventionally display an increase of typically 0.04%. This is in part due to the fact that the dielectric coefficient of water has a negative temperature dependance so that at 40C the dielectric coefficient is approximately 91, whilst at 980C the dielectric coefficient is 47. Therefore, in order to provide a moisture content reading which is independent of the temperature of the material, so that a consistent measurement is obtained which will be invariable with temperature changes in the measured material, and also is independent of the water molecules bound up within the material, the moisture measuring apparatus of the invention advantageously provides automatic compensation for different measuring temperatures and advantageously also provides facilities enabling compensation for different materials to be effected.

It can be seen from the foregoing that the detection of the moisture content of for example a particular or fibrous bulk material is frustrated by several conflicting factors. The present invention aims to provide a moisture measuring instrument which accommodates these conflicting factors thereby yielding percentage moisture content measurements (and preferably also temperature measurements) which can be utilized with a high degree of confidence.

It is well known that the moisture content of materials can be measured as a function of the electrical properties of the material and, more particularly, that there is a correlation between the moisture content and the dielectric coefficient of the material. An example of a prior moisture measuring instrument can be found in US 4,174,498 (Preikschat) and in US 4,181,881 (Preikschat). In the Preikschat instruments, a sample of material to be tested is placed in a sample box which forms part of a bridge circuit. When energised, the bridge circuit outputs a signal whose phase and amplitude is related to the admittance of the sample.The Preikschat disclosure, which is directed towards measuring the moisture content of wood chips, recognises that there is a poor correlation between the dielectric coefficient of the wood chips and their true moisture content on account of other influencing factors and Preikschat applies various compensations to his measured dielectric coefficient including a temperature related compensation. Thus, the Preikschat instrument described in US 4,174,498 includes a temperature sensor and associated circuitry which provides compensation for temperature related changes to the admittance of the sample material.

The Preikschat instruments described in US 4,174,498 and US 4,181,881 are essentially sophisticated bench top instruments designed for use in a laboratory or other quality assurance workshop. In use of such instruments a sample of the material in question must be removed from the batch that is to be tested and taken to the laboratory for analysis of its moisture content, which is a time consuming and inconvenient procedure.

Moreover, this procedure tends to produce results with limited confidence since there is a degree of uncertainty as to whether the tested sample is truly representative of the batch as a whole. The Preikschat instruments are not suitable for field testing of bulk materials as is frequently required.

For the field testing of the moisture content of bulk materials a portable instrument is required. Any such portable instrument must be easily transportable and must be of rugged construction in order to be able to withstand rough handling and general maltreatment outside of a laboratory or workshop environment. Moreover, to be suitable for testing materials in bulk the instrument should ideally include a rigid elongate probe member that can be readily inserted into the material to be tested, thereby eliminating the need to remove a sample of the material for subsequent analysis.

Portable moisture measuring instruments are known and one such is described in GB 1,419,235 (Ontario Research Foundation). The instrument disclosed therein comprises a capacitive probe and an electronic detection circuit responsive to the probe capacitance as determined by the electrical properties of the material within the immediate environment of the probe.Whilst the instrument disclosed in GB 1,419,235 is adapted for use in the field, there is no appreciation in GB 1,419,235 that in order to provide a meaningful, reliable moisture content indication account should be taken of temperature related changes in the electrical properties of the material under test, and there is likewise no described provision for adjustment of the instrument to take account of the different intrinsic and non-moisture content related electrical properties of different materials. The sophistication of laboratory type moisture measuring instruments thus has not hitherto been imported into the portable market and it is considered that a significant requirement exists for a portable capacitive probe type moisture measuring instrument providing a reliable and temperature-invariable percentage moisture content indication.

According to one aspect of the present invention there is provided a portable electronic moisture measuring instrument comprising a capacitive probe and associated circuitry responsive to the dielectric coefficient of a probed material for deriving a corresponding moisture content signal, and wherein the probe further comprises a temperature sensor responsive to the temperature of the probed material and the circuitry is arranged to apply a temperature compensation to the derived moisture content signal.

Advantageously, the moisture measuring instrument according to the present invention also comprises means enabling adjustment of the instrument to take account of the different inherent electrical properties of different probed materials. The instrument preferably further comprises zero setting and calibration facilities, and may also advantageously include means whereby a temperature measurement is also provided in addition to the moisture content measurement.

According to another aspect of the invention there is provided an electronic circuit for use with for example a portable moisture measuring instrument, the electronic circuit comprising: an oscillator circuit providing an oscillating signal, a reference channel for providing a reference signal based on the oscillating signal, a measurement channel for providing a measurement signal based on the oscillating signal and influenced by moisture content related electrical properties of a material under test, comparing means for comparing the measurement signal and the reference signal and producing an output signal representative of the said electrical property and hence of the moisture content of the material, and a display for displaying a reading related to said output signal from the comparing means.

Other features and advantages of the invention may be well understood from consideration of the following detailed description of exemplary embodiments which are illustrated in the accompanying drawings, in which: Figure 1 shows a schematic circuit diagram of an exemplary moisture sensing instrument in accordance with the invention; Figure 2 shows a detailed circuit diagram of a first embodiment circuit; Figure 3 shows a schematic overall view of a moisture sensing instrument; Figure 4 shows an exploded perspective view of a probe; Figure 5 shows a section through the line V-V in Figure 4; Figure 6 shows a detailed circuit diagram of the moisture measuring instrument in accordance with a second embodiment of the invention; Figure 7 shows a power supply circuit; Figure 8 shows an optional additional temperature correction circuit; and Figure 9 shows the structure of a twin capacitive element probe tip.

Before referring to the operation of the circuits in detail, a generalised schematic representation of the circuits will be described with reference to Figure 1 in order to facilitate a general understanding of the invention which will provide a basis for a greater understanding of the detailed explanation of the preferred embodiments that will follow.

As is shown in Figure 1, the moisture detection circuit generally indicated at 10 comprises an oscillator 12 a reference channel 14, a reference channel filter 16, a measurement, or detector, channel 18 and a measurement channel filter 20. The oscillator 12 drives the reference channel 14 which, in turn, drives the reference channel filter 16. As will be described in more detail hereinafter, the reference channel filter 16 converts the output from the reference channel 14 into a constant DC voltage which is used to drive a non-inverting, unity gain operational amplifier 22. The output from the operational amplifier, at point C in the diagram, is a constant voltage which is used as a reference value as will be described in greater detail hereinbelow.

The oscillator 12 also drives the measurement channel 18 which in turn drives the measurement channel filter 20. A capacitive moisture probe forms part of the measurement channel circuit and is arranged so that the output from the measurement channel 18 is a function of the probe capacitance. The output from the measurement, or detector channel 18 is converted into a DC voltage by the measurement filter 20. Since- the operation of the measurement channel 18 is influenced by the capacitance of the probe which is itself proportional to the moisture content of the probed material, therefore the output DC voltage from the measurement filter 20 is proportional to the moisture content.

The reference voltage from the operational amplifier output C is applied to the non-inverting (+) input of a differential operational amplifier 24 and the output voltage from the measurement filter 20 is applied to the inverting input (-) of the same.

Because the dielectric coefficient of a bulk material having a moisture content is affected by changes in temperature, therefore a temperature measuring circuit 26 is also included. The temperature measuring circuit 26 provides a temperature compensating output which is applied to the inverting input of the amplifier 24 in order to compensate for temperature related changes in the voltage output from the measurement filter 20.

A display mode selection switch 32 is provided to selectively connect either the moisture related output from the amplifier 24 or a temperature related output from the temperature measuring circuit 26 to a digital display panel 30.

The moisture probe circuit 10 of Figure 1 will now be described in more detail with reference first of all to Figure 2 of the drawings. The oscillator circuit 12 provides a high frequency square wave signal, typically a 1.2 million pulses per second (Mpps) signal, derived from the charge and discharge of a capacitor through a resistor, controlled and buffered by Schmitt type inverters.

The output from the oscillator 12 is used to stimulate both the reference channel 14 and the measurement channel 18 which both operate in a substantially similar manner. In the reference channel 14 a Schmitt inverter controls the charge and discharge of a capacitor through a resistor, the resulting output of which is buffered by a further Schmitt inverter thus providing a reference square wave which is input into the reference filter network 16 in order to smooth the reference square wave thereby converting it into a constant DC voltage.

Similarly, the measurement channel 18 includes a Schmitt inverter which controls the charge and discharge of a capacitor network. The capacitor network comprises the capacitance of the probe in parallel with a padding capacitor of similar capacitance which, in combination, makes the overall capacitance of the network approximately the same as the capacitance of the reference channel. The buffered and inverted output from the measurement channel, a square wave with characteristics dependant upon the probe capacitance and therefore representative in use of the detected moisture content of the material under test, is input to the measurement filter 20 which outputs a corresponding DC voltage level.

It has been found that, in general, as the percentage moisture content of a material increases, so the dielectric capacity of the material also increases in a substantially linear relationship. However, an increase in moisture content also tends to reduce the resistivity of the material in a substantially logarithmic relationship which would significantly influence the operation of a capacitive measuring probe. As a result of these conflicting parameters, the range of reliable measurement is limited to Xc c R, where Xc is measured capacitive reactance and R is measured resistance of the material. This limitation arises on account of the fact that as Xc increases so the amount of current flowing in the resistor R will increase accordingly, and when Xc becomes greater than R the resistive current will be greater than the capacitive current by virtue of which the moisture content of the material is derived. Thus, any further changes in capacitance will have a lesser determinable effect since the changes in capacitive current will be relatively insignificant in comparison to the resistive current. In practice it is therefore preferable to limit the detection range to Xc < 2R in order to ensure that this problem does not arise. This is achieved by virtue of the probe construction and of the setting up of the electrical currents of the instrument.

It should be appreciated that although the Schmitt type inverter characteristics, ie. the threshold voltages, are sensitive to changes in ambient temperature, any temperature related drift between the measurement channel 18 and the reference channel 14 is substantially eliminated when the outputs are differenced by the differential amplifier 24. Moreover, by using a single integrated circuit package which provides all six of the Schmitt inverters used in the circuit 10 stability can be increased further, since any change in ambient operating conditions is likely to affect all inverters in the same way. In this way any changes in the operation of the inverters will tend to be cancelled out. A suitable integrated circuit which includes six Schmitt inverters in a single package is the readily available 4584 device.

As with the Schmitt inverters, providing the operational amplifiers within a single integrated circuit package will help to reduce drift caused by changes in ambient temperature conditions. One such integrated circuit which is suitable in this respect is an LM324N which contains four operational amplifiers in a single package.

As has already been mentioned hereinabove, the outputs from the two filters are connected to respective amplifiers 22,224. The operational amplifier 22 associated with the reference channel is arranged to provide unity gain and is driven through its non-inverting input (+) by the reference filter 16. The output voltage, at the point C in Figure 2, is a constant DC voltage which is used as a reference voltage for other stages within the system.

Two band gap reference diodes 42,144 connect the point C to the positive supply rail and the ground rail of the regulated circuit power supply 40 through resistors 43,45 and provide two further reference voltages at C+1.26 volts and C-1.26 volts. The three reference voltages are used as bias and driving voltages by the temperature measuring circuit 26, the display 30, and other parts of the circuit as will be described hereinafter.

The differential amplifier 24 is also arranged to provide unity gain. The DC voltage output from the measurement filter 20 is applied to the inverting (-) input of the differential amplifier 24 and the noninverting input (+) is held at the reference voltage C.

The output from the differential amplifier 24 is therefore the difference between the reference voltage C and the measurement voltage which is, of course, proportional to the moisture content being measured. As will be described hereinafter the inverting terminal of the measurement differential amplifier 24 is additionally biassed by a temperature correction voltage from the temperature measuring circuit 26 and by calibration and zero correction voltages from two preset zero correction potentiometers 48,49 which are driven by the reference voltages C volts and C-1.26 volts.

The temperature measuring circuit 26 measures changes in thermistor resistance thereby determining changes in the temperature of the material under test.

The temperature circuit 26 also provides a voltage output which is proportional to the temperature of the material and a voltage output which is used to compensate for changes caused by temperature change in the detected moisture reading. A negative temperature coefficient (NTC) thermistor 50 provides a substantially inverse logarithmic change in resistance with an increase in temperature. The thermistor 50 provides a negative feed back path for the output from an operational amplifier 52. A resistor 51 is provided in parallel with the NTC thermistor 50 in order to compensate for the non-linear characteristics of the thermistor 50. Careful choice of thermistor and resistor values results in a substantially linear response of acceptable accuracy over the expected normal range of operation of the circuit.For example, a 50k ohm z 250C thermistor in parallel with a 33k ohm resistor can provide a linear response to within plus or minus 1 Celsius degree over an operating range of O to 55 degrees Celsius.

As described hereinbefore, three temperature related correction voltages are summed with the DC voltage output proportional to moisture content at the inverting input of the differential amplifier 24. The three correction voltages are: the temperature correction voltage derived from the temperature related voltage from the operational amplifier 54, the cal voltage derived from the reference voltages C and C plus or minus 1.26 volts, and the zeros voltage derived from the same.

It will be appreciated that, because water molecules are inherent within many of the materials which can be tested by the instrument, different materials will have different zero moisture content values according to their molecular structure. For example, the molecular structure of a first material may comprise 2% water molecules, whilst the structure of a second material may comprise 5% water molecules. Clearly if the probe was adjusted to detect moisture in the first material but was instead being used to detect moisture in the second material an error of 3% would result.

In order to overcome this problem a zeros voltage is adjustably summed onto the inverting input of the differ ential amplifier 24. The zeros voltage is derived from the voltage at the output of the operational amplifer 52 and referenced to the reference voltage C-1.26 volts by way of the offset zero potentiometer 148. The potentiometer 248 can be set with the probe in free space, that is to say with the probe on a work bench and not inserted into a moisture containing material, by adjusting the displayed value in accordance with tabulated responses.

The zeros voltage can be further'adjusted by way of the true zero potentiometer 249 which is optionally included to offset any drift within the temperature measuring circuit caused by ageing of the electronic components therein.

The output from a unity gain driver amplifier 54 is used to provide the temperature correction bias voltage via a temperature correction potentiometer 56 preset in a manner described hereinbelow which is summed with the measurement voltage output from the measurement filter 20 at the inverting input of the differential amplifier 24, thereby compensating for temperature effects on the detected moisture.

In order to correctly calibrate the output from the temperature measuring circuit 26, a process which is preferably carried out as part of the production stage, two methods are proposed. The first method involves placing the probe member, or at least the thermistor contained therein, into an environment of known temperature and adjusting the potentiometer 56 until the displayed temperature is in accordance with the temperature of the known environment. This method is however, difficult and time consuming to perform in practice because of the need to adjust the potentiometer 56 over the range of operating temperatures in order to ensure good responses over the range.

The second method, which is the preferred method since it avoids the empirical aspects of the first method, involves switching the thermistor out of circuit and replacing it with high stability resistors of known resistance and then adjusting the potentiometer 56 in order to achieve the required response. For example a 50k ohm resistance should result in a temperature of 25 0C being displayed. Once the required response has been achieved the thermistor can be switched back into circuit.

The output from the operational amplifier 52 is a DC voltage proportional to temperature, and is used to drive the inverting input of the driver amplifier 54. The driver amplifier output can be selectably connected by way of a switch 32, via resistors to the display 30 which is preferably a Lascar DPM 2000 display panel meter with a 2 digit display arranged to receive a maximum input of 199mV. With the output from the driver amplifier 514 connected to the display 30, the display shows the sensed temperature of the material under test.

The display 30 can be driven by the differential amplifier 24 in order to display moisture content or it an be switched over so that it is driven by the driver amplifier 54 in the temperature measuring circuit in order to display temperature. It should be noted that the display is referenced to the reference voltage C so that, in practice, the display will show the difference between the reference voltage and either the moisture related voltage or the temperature related voltage. The difference voltage is directly related to the percentage moisture content detected when driven by the amplifier 24 and to the detected temperature in degrees Celsius when driven by the amplifier 54.

The display additionally includes: the character "k" which is arranged to be displayed when switches within the circuit are set for circuit calibration; the character "%" which is displayed when the switches are set so that the display shows a moisture content reading; and the character Or which is displayed when the switches are set so that the display shows a temperature reading. The switching arrangement which controls the display of these characters is shown at 58 in Figure 2.

A further optional "battery low" indication can be provided along with appropriate low voltage detection circuitry to indicated when the batteries, which may typically be PP3 type cells or equivalent, need to be replaced.

The exterior form-of the moisture measuring instrument generally indicated at 1 and the construction of the moisture probe indicated at 70 is shown in detail in Figures 3,4 and 5. As is shown in Figure 3, the instrument 1 comprises a case 60 which contains some, but not all, of the components of the circuit described herein with reference to Figures 1 and 2, and a probe 70 which comprises, inter-alia, a capsule 71 which contains the remainder of the Figure 2 circuit components. The case 60 is preferrably of suitable dimension and form to enable the user to hold the instrument 1 firmly and comfortably in his hand whilst inserting the probe 70 into the material being tested for its moisture content, and whilst measurements are being taken.The display 30 and the switches shown in Figure 2 for switching the instrument on, selecting temperature or moisture content display, etc., are located at convenient locations upon the case whereat, in use, the display may be easily read and the switches easily operated by the user.

The case 60 is preferably of a robust material and it is preferably shrouded by a guard (not shown) which serves to protect the case 60 and the parts of the electronic circuit 10 contained therein from accidental damage in the event that, say, the instrument is dropped by the user. The construction of the probe 70 can be seen best in Figure 4 which shows that the probe 70 comprises a support tube 72 secured to a bush 73 which is connected to the capsule 71 containing the measurement and reference channels and filters and the thermistor from the temperature measuring circuit 26 of the Figure 2 circuit.The capsule may be secured either directly to the case 60 by way of a connector arrangement which unites the probe member and the case and which connects the electronic sub-circuits contained within the capsule 71 to the appropriate parts of the remaining electronic sub-circuits within the case 60 or indirectly by way of an extension coupling sleeve 74 (as shown in Figure 3) containing an extension lead (not shown) as will be described in greater detail hereinbelow.

The probe further comprises a first insulator 75, an electrode 76, a second insulator 77 and a tip electrode 78. When assembled, the first insulator 75, the electrode 76 and the second insulator 77 fit slidingly over the support tube 72 with the electrode 76 supported by annular flange portions 75a, 77a extending respectively from the ends of the insulators 75, 77 and the tip electrode is screwed onto the end of the support tube 72 where it is partially supported by a further annular flange portion 78 on the second insulator 77.

As mentioned above, the bush 73 engages with the cylindrical case 71 containing electronic components making up the oscillator 12, the measurement channel and filter 18,20 and the reference channel and filter 14,16.

These stages of the circuit are provided within the probe so that any potential interference from external sources, for example radio frequency electromagnetic interference (EMI), does not significantly affect the performance of the measurement channel 18 by altering the signal received by the measurement circuit from the probe capacitance. The thermistor 50 is also provided within the probe in order to facilitate temperature measurements of the material. Connecting wires from the outputs of the measurement and reference filters 20,16 connect to a socket 79 on the capsule 71 which engages with a complimentary plug (not shown) either on the case 60 or on the receiving end of the extension sleeve 74. The receiving socket is accordingly connected to the inverting input of the differential amplifier 24 and the non-inverting input of the operational amplifier 22 respectively and the thermistor 50 is connected to the temperature measuring circuit 26. In this way the two remote circuit parts are united to form the complete circuit 10 shown in detail in Figure 2.

Referring to Figure 5, the probe components, when assembled, form a single probe unit 70 which can be inserted by the user into the material to be tested. The tip electrode 78 is screwed onto the end of the support tube 72 and is secured thereto by a grubscrew 80.

Similarly, the support tube 72 is secured to the bush 73 by a grubscrew 81. The tip electrode 78 in particular, and preferrably the electrode 76, are made from an electrically conductive material which is also mechanically hard. In this respect stainless steel is an ideal material although spun brass may alternatively be used.

The capacitance in the probe is realised by the capacity which is created between the electrode 76 and the tip electrode 78, with the dielectric between the two electrodes 76, 78 being formed partially by the second insulator 77 and partially by the material under test when the probe 70 is inserted into the material.

Consequently, the relative dimensions of the insulator and electrodes will influence the overall capacity of the probe. Preferrably the length of the electrode 76 should be between 12 and 2 times the length of the insulator 77.

The first insulator 75 can be substantially identical to the second insulator 77.

The two electrodes 76,78 which form the probe capacitor are connected electrically to the measurement circuit contained within the capsule 71 by way of fine insulated wires (not shown) one of which extends within the length of a void 82 defined by the support tube 72 from the measurement circuit to the tip electrode 78 to which the wire is secured by soldering, brazing or any other suitable method, and the other of which wires extends from the measurement circuit partially within the void 82 through a small aperture 83 drilled in the wall of the support tube to the first electrode 76 to which the wire is similarly secured.

Hereinafter described is a modified form of the portable electronic moisture measuring instrument. The structure and general operation of this modified instrument is in many respects very similar to that of the first embodiment described hereinabove and hereinafter only the differences between the two will be described.

Referring to Figure 6 it can be seen that the circuit 110 comprises an oscillator circuit 112 which drives a reference channel 114 and a measurement channel 118 as in the first embodiment described hereinabove. In this instrument the measurement channel 118 is optionally provided with a loading resistor R45 in parallel with the probe capacitance, the resistor R45 having a resistance value chosen to correct any phase shift introduced into the signal by the capacitance of the probe. The reference channel is similarly optionally provided with a loading resistor R46 in parallel with capacitor C14 in order to balance the two channels 114,118.

The signals output from the channels 114,118 are square waves having a mark to space ratio proportional to the capacitance in the channels, and these signals are passed through respective filters 116,120 which convert the square wave signals into DC voltages proportional to the capacitances of the channels. As compared to the embodiment described previously, in the present instrument the filters 116,120 are LC filters. The outputs from the filters 116,120 are connected to respective amplifiers 122,124 as in the previously described embodiment.

Referring to the temperature measuring circuit 126, a negative temperature coefficient (NTC) thermistor 150 is connected in parallel with resistor R27 and capacitor C11 in a negative feedback path for the operational amplifier 152 in order to provide amplifier 152 with a substantially linear temperature response over a predetermined range of temperature variation. Amplifier 154 is provided to invert the slope of the temperature response and resistors R23 and R28 which are referenced to a reference voltage C, i.e. the output from the reference amplifier 122, serve to set the zero point of the response curve as required.

In order to facilitate calibration of the temperature measuring circuit 126 precision resistors R36, R37 are provided which can be switched in parallel with the thermistor 150. During calibration, the resistors R36, R37 are switched into - the circuit in place of the thyristor 150 and the thyristor 150 is switched out of the circuit. Potentiometer 156 can then be adjusted to calibrate the circuit.

Turning now to the differential amplifier 124, the output of this amplifier is used to drive a display meter DPM 1. The inverting (-) input of the amplifier 124 is driven by the output from the reference filter 120 and by five additional feedback-type signals,- namely: zero, calibration, temperature correction, a temperature derived signal via resistors R43, R44 and a reference signal via resistors R39, R40. The zero, calibration and temperature correction signals serve the same purpose as in the embodiment described in our earlier application namely: to adjust the zero offset of the amplifier 124 to compensate for the inherent moisture content of the material to be measured by the instrument, to correct for temperature changes in the measured material and to control the gain of the amplifier 124.The reference signal, via resistors R39, R40, is provided in this second instrument to enable a greater degree of control to be realised over the zero offset provided by the zero signal. By connecting pin 2 of link LK1 to either pin 1 or pin 3 the user, when calibrating the zero offset of the amplifier 124, can optionally select to offset the zero point of the amplifier circuit from the reference voltage C, or from the voltage C+1.22v, or from the voltage C-1.22v, thus providing a greater degree of control by way of fine adjustments to the zero offset made by altering the resistance of the potentiometer array VR5-VR7.

The non-inverting (-) input of the amplifier 124 is connected via resistor R10 to the reference voltage output from the reference amplifier 122. The reference voltage output is connected to the supply line (6V2) via high value resistors R41, R42 to compensate for any minor switch-on drift which can arise if there is a temperature difference between the head circuit and the amplifier circuit.

Referring now to Figure 7 it can be seen that the power supply circuit of this instrument is provided with a band-gap reference diode RF3 on the common line COM of voltage regulator REG1. The reference diode RF3 is provided to increase the output voltage on pin 3 of the regulator from 5 volts to 6.22 volts (i.e. 5 volts + 1.22 volts the voltage drop across reference diode RF3). This increase in output voltage from the regulator enables the head circuit to be operated in the range 1.22 volts to 6.22 volts, as opposed to the range 0-5 volts, thus enabling any non-linearity in the lower end of the head circuit characteristics to be avoided in the operation of the instrument.

In Figure 8, an optional additional stage, which can be added to the temperature correction circuit, is shown.

In some circumstances it may be desirable to have additional control over the temperature correcting aspects of the circuit, and for this purpose the circuit of Figure 8 may be inserted into the circuit of Figure 6 with either point A3 or A24 connected to the circuit at link LK2. The Figure 8 circuit comprises a current source TS1 whose current output is proportional to temperature. The current source TS1 drives the inverting input of operational amplifier to provide a temperature related voltage at A3 and at A4 which can be used to further bias the temperature related voltage signal input to amplifier 124 via R43, R44.

In order to obtain a high speed response to conditions when the instrument is inserted into a material it may be advantageous to provide a probe with two capacitive elements formed therein. Figure 9 shows one possible arrangement by which this may be achieved.

In Figure 9 the probe comprises a steel tip 202 and steel rings 204-210 separated by glass loaded PTFE bushes 212220 all amounted on a brass support tube 222. The steel tip 202 is of hollow construction and is machined to remove as much metal as possible without unduly reducing the strength of the tip, thus increasing the responsiveness of the tip to temperature.

The thermistor 150 shown in Figure 6 can be secured within the steel tip 202 by means of a suitable thermally conductive adhesive. In order to provide adequate support for the first ring 204, the two bushes 212 and 214 may be formed as a single structure with the ring 204 being pushed thereover and secured thereto by a grubscrew 224.

The two capacitive elements formed by the rings are arranged electrically in parallel and this doubles the sensitivity of the probe as compared to a single capacitive element arrangement of similar physical size.

To summarise, the invention provides an electronic moisture and temperature sensing instrument in which electrical stability of meter readings are achieved by the use of a reference voltage to which measurements are compared. Moreover, high stability of performance can be realised by using high stability components and by the use of off-the-shelf integrated circuits which provide all Schmitt inverters within a single package and all operational amplifiers within a single package. Further measurement accuracy is achieved by arranging for the dielectric capacity and displayed readings to have a linear relationship, and by arranging the temperature sensing circuit to have a linear response to within plus or minus one Celsius degree or better over the operating range of the device.

The invention having thus been described, it should be apparent to those skilled in the art that many modifications to the circuit and physical arrangement of the device are possible without departing from the scope of the invention.

For example, in particular the probe may be modified to enable its use in measuring the moisture content of materials other than bulk materials such as grain or cotton. It is envisaged that a probe could be designed for use with margarine, butter and similar edible fats.

Such a probe would be provided with a flat or slightly curved end which would be received into said edible fats for analysis. Also, a probe could be designed for use with fine compressible powders, in which case the probe could be provided in the form of say a cup and pressure plate arrangement.

Claims (10)

CLAIMS:

1. A portable electronic moisture measuring instrument comprising a capacitive probe and associated circuitry responsive to the dielectric coefficient of a probed material for deriving a corresponding moisture content signal, and wherein the probe further comprises a temperature sensor responsive to theçtemperature of the probed material and the circuitry is arranged to provide for temperature compensation to the derived moisture content signal.

2. A portable electronic moisture measuring instrument according to claim 1, further comprising means enabling adjustment of the instrument to take account of the different inherent electrical properties of different probed materials.

3. A portable electronic moisture measuring instrument according to claim 1 or claim 2, further comprising zero setting means for setting the zero level of said moisture content signal.

4. A portable electronic moisture measuring instrument according to any of the preceding claims, further comprising calibrating means for enabling calibration of the instrument according to the inherent electrical properties of a material to be probed.

5. A portable electronic moisture measuring instrument according to any of the preceding claims, further comprising displaying means for displaying a reading representative of the moisture content or the detected temperature of the probed material.

6. An electronic circuit for use with for example a portable moisture measuring instrument, the electronic circuit comprising an oscillator circuit providing an oscillating signal, a reference channel for providing a reference signal based on the oscillating signal, a measurement channel for providing a measurement signal based on the oscillating signal and influenced by moisture content related electrical properties of a material under test, comparing means for comparing the measurement signal and the reference signal and producing an output signal representative of the said electrical property and hence of the moisture content of the material, and a display for displaying a reading related to said output signal from the comparing means.

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7. An electronic circuit according to claim 6, further comprising a temperature measuring circuit which provides a temperature compensating signal to the comparing means to compensate for temperature related changes in the measurement signal.

8. An electronic circuit according to claim 7, in which the temperature measuring circuit further provides a signal representative of the temperature of the material under test and the display means is adapted to display a reading related to said signal.

9. A portable electronic moisture measuring instrument according to any of claims l to 5 in combination with an electronic circuit according to any of claims 6 to 9.

10. An electronic moisture measuring instrument having any or each of the novel features disclosed herein.