Linear Output Moisture Meter With Temperature Compensator

1. Field of the Invention

This invention relates to a moisture meter for measuring the water content in a specimen through measuring the electric resistance of the specimen, and more particularly to such a moisture meter provided with temperature compensation.

Description of the Prior Art

In grains such as rice or wheat, the water content was a correlation with the electric resistivity and hence it can be discovered by measuring the electric resistivity.

The electric resistivity of grain with a constant water content, however, shows a temperature dependence corresponding to the water content of .+-. 1.0% for the temperature change of .+-. 10.degree.C. Thus, temperature compensation should be done in measuring the water content of a grain specimen from the electric resistivity. It has been done by compensating the measured result with a temperature compensation table, or with a compensating quantity which is arranged to be directly read from a thermometer. "Grain moisture meter, model SP-1" (since April, 1967) of Kett Electric Laboratory in Japan and "moisture meter, model TW73" (trade name "Protimeter Grain-master") (Since 1973) of Protimeter Ltd. are examples of these types.

According to such methods, however, temperature compensation is troublesome and time-consuming.

It has been known that at a constant temperature the electric resistivity of grain varies with the water content as is shown in FIG. 1, i.e. it varies exponentially in the water content range of 12 to 20% and relatively linearly in the water content range of 20 30%. However, in performing such a method with one range of reading, there occur problems of making the circuit structure extremely complicated and the adjustment of the circuit extremely difficult, thereby bringing it into practice being difficult. Thus, it can be considered to divide the whole range into a low water content range of 12 to 20% and a high water content range of 20 to 30%.

As an attempt in linearizing the indication scale in a moisture meter, Japanese Patent Publication No. 35039/1973 discloses such an electric resistance measuring apparatus in that an electric signal is amplified in an amplifying transistor in the low water content range in which it was difficult to measure fine differences in the water content because of its high resistivity, an electric signal is by-passed through a compensating transistor used in diode-like manner in the high water content range to decrease the rate of change, and the water content is preliminarily converted into the current flowing through a reference resistance and indicated in a meter to enable direct reading. In this apparatus, however, no consideration is made of the temperature compensation which is indispensable in the water content measurement of grain specimens.

This invention is made on review of these conventional techniques and intended to eliminate the drawbacks thereof.

An object of this invention is to provide a moisture meter capable of automatic temperature compensation in the measurement of the water content in specimens with a simple circuit structure, of easy and highly accurate measurement of the water content in specimens, and of being easily brought into practical use.

According to this invention, the electric resistivity of a specimen is measured to know the water content in the specimen which has a correlation with the electric resistivity. Measurement of the water content in a specimen in the low water content range in which the electric resistivity changes nonlinearly with the water content is done through a linearizer. Temperature compensation of the measurement is done by a common temperature compensator in both the low and the high water content ranges. Thus, the temperature compensation for the water content in specimens can be done automatically with a simple circuit structure, and measurement of the water content in specimens can be done easily and accurately. Further, the moisture meter according to this invention can be easily brought into practical use.

Other objects, features and advantages of this invention will become apparent from the following detailed description when taken in conjunction with the accompanying drawings.

FIG. 1 is a graph showing the relation of the water content and the electric resistivity in grain at a constant temperature.

FIG. 2 is a circuit diagram of an embodiment of the moisture meter according to this invention.

FIG. 3 is a circuit diagram of another embodiment of the moisture meter according to this invention.

An embodiment of the moisture meter according to this invention is shown in FIG. 2. A dc power source E, e.g. 6V, such as a battery is connected through a power switch PS to an inverter circuit IN including a first transistor Tr.sub.1 of NPN type and an inverter transformer T. The output of the inverter circuit IN is rectified and smoothed in a rectifying diode D and a smoothing circuit FC. Thus, a low dc voltage, e.g. 6V, from said dc power source E is converted to a relatively high dc voltage, e.g. 20V or above. Lines l.sub.1 and l.sub.2 are connected to the positive and the negative terminals of said smoothing circuit FC. Between the lines l.sub.1 and l.sub.2 are connected a voltage dividing series circuit formed of resistors R.sub.1 and R.sub.2 and a first field effect transistor FET.sub.1 of N-channel type having a drain connected to the line l.sub.1 through a series circuit of a resistor R.sub.3 and a first variable resistor VR.sub.1 and a source to the line l.sub.2 through a series circuit of a second variable resistor VR.sub.2 and a resistor R.sub.4, a linearizer LC comprising a second and a third transistors Tr.sub.2 and Tr.sub.3 of PNP type, and a fourth transistor Tr.sub.4 of NPN type, in which the transistor Tr.sub.3 and Tr.sub.4 are complementally connected. In said linearizer LC, the second transistor Tr.sub.2 has an emitter connected to a base of the fourth transistor Tr.sub.4, a collector to the line l.sub.2, and a base to the line l.sub.2 through a parallel circuit of a resistor R.sub.6 and a potentiometer PM and to the line l.sub.1 through the movable terminal of the potentiometer PM and a resistor R.sub.7 in series connection. The fourth transistor Tr.sub.4 has a collector connected to the base of the third transistor Tr.sub.3, an emitter to the collector of the third transistor Tr.sub.3 and the line l.sub.2 through a resistor R.sub.8. The third transistor Tr.sub.3 has an emitter connected to the line l.sub.1 through a resistor R.sub.9. A resistor R.sub.5 is connected between the gate of the first field effect transistor FET.sub.1 and the line l.sub.2. Further, between the lines l.sub.1 and l.sub. 2 are connected a second field effect transistor FET.sub.2 of N-channel having a source connected to the line l.sub.2 through a resistor R.sub.10, and a temperature compensating circuit TCC including a third and a fourth field effect transistor FET.sub.3 and FET.sub.4 of N-channel and a temperature detecting circuit TC having a thermistor TH of negative characteristic (its resistance decreases with an increase in temperature) for detecting the temperature of a grain specimen to be described later. In the temperature detecting circuit TC, the thermistor TH has one end connected to the line l.sub.1 through a resistor R.sub.11 and the other end to the line l.sub.2 through resistors R.sub.12 and R.sub.13 and a third variable resistor VR.sub.3 connected in series, and a resistor R.sub.14 is connected in parallel with the series connection of the thermistor TH and the resistor R.sub.12. In the temperature compensating circuit TCC, the third field effect transistor FET.sub.3 has a drain connected to the line l.sub.1, a source to the line l.sub.2 through a resistor R.sub.15, and a gate to the drain of the fourth field effect transistor FET.sub.4. Further, the fourth field effect transistor FET.sub.4 has a drain connected to the line l.sub. 1 through a resistor R.sub.16, a source to the line l.sub.2 through a series connection of a resistor R.sub.17 and a fourth variable resistor VR.sub.4, and a gate to the interconnection point of the thermistor TH and the resistor R.sub.12. The second field effect transistor FET.sub.2 has a gate connected to the line l.sub.2 through a resistor R.sub.21 and to a change-over switch CS.sub.3, a drain connected to the line l.sub.1, and a source connected to the line l.sub.2 through resistor R.sub.10. Between the sources of the second and the third field effect transistors FET.sub.2 and FET.sub.3 is connected a series connection of a fifth variable resistor VR.sub.5, a resistor R.sub.18 and a water content meter M. A pair of electrodes P.sub.1 and P.sub.2 are provided. One electrode P.sub.1 is connected to the line l.sub.1 through a "dry" contact D.sub.1 of a first changeover switch CS.sub.1 and to the interconnection point of the resistors R.sub.1 and R.sub.2 through a "wet" contact W.sub.1 of the change-over switch CS.sub.1, and the other electrode P.sub.2 to the emitter of the second transistor Tr.sub.2 through a series connection of a "dry" contact D.sub.2 of a second change-over switch CS.sub.2 and a resistor R.sub.19 and to the gate of the first field effect transistor FET.sub.1 through a "wet" contact W.sub.2 of the second change-over switch CS.sub.2. Further, the gate of the second field effect transistor FET.sub.2 is connected to the emitter of the third transistor Tr.sub.3 through a series connection of a dry contact D.sub.3 of a third change-over switch CS.sub.3 and a resistor R.sub.20, and to the drain of the first field effect transistor FET.sub.1 through a wet contact W.sub.3 of the third change-over switch CS.sub.3. Here, the respective change-over switches CS.sub.1, CS.sub.2 and CS.sub.3 are mutually interlocked so that the on-off operation of the dry contacts D.sub.1, D.sub.2 and D.sub.3 and the wet contacts W.sub. 1, W.sub.2 and W.sub.3 are respectively synchronous. A grain specimen G such as rice or wheat is disposed between the electrodes P.sub.1 and P.sub.2. Here, an ammeter is used as the water content measuring meter M, in which a current scale is replaced with a corresponding water content scale.

In the above circuit, when the water content of a grain specimen G is in the low water content range of 12 to 20%, the wet contacts W.sub.1, W.sub.2 and W.sub.3 of the respective change-over switch CS.sub.1, CS.sub.2 and CS.sub.3 are opened and the dry contact D.sub.1, D.sub.2 and D.sub.3 are closed to interpose the linearizer LC between the electrode P.sub.2 and the gate of the second field effect transistor FET.sub.2. In this state, when the power switch PS is thrown in, the emitter potential of the second transistor Tr.sub.2, i.e. the base potential of the fourth transistor Tr.sub.4 of the linearizer is established according to the electric resistance of the grain specimen G between the two electrodes P.sub.1 and P.sub.2 and those of the transistors Tr.sub.2, Tr.sub.3 and Tr.sub.4. In a transistor, the collector current varies exponentially with respect to the base-emitter voltage. Thus, the operation characteristics of the transistors Tr.sub.2, Tr.sub.3 and Tr.sub.4 can be predetermined to generate an electric signal output changing linearly with the water content in the grain specimen G from the exponential variation of the resistivity of the grain specimen G with the water content. Thus, the linearizer LC generates an output signal changing linearly with the water content. The output of the linearizer LC, i.e. the emitter output of the third transistor Tr.sub.3 is supplied to the gate of the second field effect transistor FET.sub.2 which has a linear gate-source voltage vs. drain current characteristic. Hence, the second field effect transistor FET.sub.2 controls the drain current in proportion to the water content of the grain specimen G and changes the source potential linearly with the water content of the grain specimen G. As a result, the current flowing through the meter M varies linearly with the water content of the grain specimen G, and the needle of the meter M moves linearly thereby.

When the water content of a grain specimen is in the high water content range of 20 to 30%, the dry contacts D.sub.1, D.sub.2 and D.sub.3 of the respective change-over switches Cs.sub.1, CS.sub.2 and CS.sub.3 are opened and the wet contacts W.sub.1, W.sub.2 and W.sub.3 thereof are closed to interpose the first field effect transistor FET.sub.1 between the electrode P.sub.2 and the gate of the second field effect transistor FET.sub.2. When the power switch PS is thrown in this state, the gate potential of the first field effect transistor FET.sub.1 is established in accordance with the electric resistance of the grain specimen G between the electrodes P.sub.1 and P.sub.2 to activate the transistor FET.sub.1 accordingly. Here, the field effect transistor changes the drain current linearly with the gate-source voltage. Thus, the linear change of the electric resistivity with the water content in the grain specimen produces a corresponding linear change of the drain output of the first field effect transistor FET.sub.1. As the drain output of the first field effect transistor FET.sub.1 is applied to the gate of the second field effect transistor FET.sub.2, the second field effect transistor FET.sub.2 controls the drain current in proportion to the water content of the grain G and changes the source potential linearly therewith, similar to the case of the low water content range. In this case also, the current flowing through the meter M varies linearly with the water content of the grain specimen G and the needle in the meter M moves linearly thereby.

Therefore, it is possible by the adjustment of the first, the second and the fifth variable resistors VR.sub.1, VR.sub.2 and VR.sub.5 and potentiometer PM to arrange the linear movement of the needle in the meter G in both the low and the high water content ranges and the water content scale with equal gaps. Thus, reading of the meter G is made easy.

On the other hand, in the temperature compensator TCC, the resistance of the thermistor TH varies according to the temperature of the grain specimen G and the gate potential of the fourth field effect transistor FET.sub.4 is changed thereby. The drain output of the fourth field effect transistor FET.sub.4 controlled by the resistance of the thermistor settles the gate potential of the third field effect transistor FET.sub.3 to control it accordingly. Here, the resistance of the thermistor TH does not necessarily change linearly with the temperature, but the gate potential of the fourth field effect transistor FET.sub.4 can be predetermined to change substantially linearly with the temperature of the grain G. The field effect transistors FET.sub.3 and FET.sub.4 provide drain currents changing linearly with the gate-source voltages. Thus, the source voltage of the third field effect transistor FET.sub.3 changes linearly with the temperature of the grain G. Namely, the temperature compensator TCC linearly controls the current flowing through the water content meter M with respect to the temperature of the grain specimen G.

Thus, when the circuit is so arranged that the changes in the output of the temperature compensator TCC (the source voltage of the third field effect transistor FET.sub.3) compensates the changes in the source voltage of the second field effect transistor FET.sub.2 due to the temperature variation, the current flowing through the water content meter M does not change with the temperature of the grain specimen G but only with the water content of the grain specimen G. Therefore, the meter G can indicate the true water content of the grain G.

As is described above, the water content of a grain specimen is measured by measuring the electric resistance of the grain specimen through a linearizer in the low water content range and directly in the high water content range, and thereby the water content of the grain can be read out with temperature-compensation throughout the low and the high water content ranges by a common temperature compensator.

Therefore, the temperature compensation for the water content of grain specimens can be done automatically with a simple circuit structure and the water content of grain specimens can be measured easily and highly accurately.

A simplified embodiment of the moisture meter according to this invention is shown in FIG. 3, in which similar parts with those of FIG. 2 are indicated by similar reference marks. In this circuit, a changeover switch CS.sub.5 is provided at the interconnection point of the emitter of a fourth transistor Tr.sub.4 and the collector of a third transistor Tr.sub.3. A wet contact W.sub.5 of the switch CS.sub.5 is connected to a line l.sub. 2 through a biasing series connection of a resistor R.sub.4 and a variable resistor VR.sub.2, and a dry contact D.sub.5 thereof to the line l.sub.2 through a resistor R.sub.8. The emitter of the third transistor Tr.sub.3 is connected to the line l.sub.1 through a potentiometer VR.sub.6 and a resistor R.sub.9 and to the line l.sub. 2 through a resistor R.sub.21. The moving contact of the potentiometer VR.sub.6 is connected to a meter M through a series connection of a resistor R.sub.18 and a variable resistor VR.sub.5. The base of the fourth transistor Tr.sub.4 is connected to an electrode P.sub.2 through a resistor R.sub.19. The other electrode P.sub.1 is connected to the line l.sub. 1. The interconnection point of a voltage dividing circuit consisting of resistors R.sub.6 and R.sub.7 is connected to the base of a second transistor Tr.sub.2 of PNP type. The collector of the transistor Tr.sub.2 is connected to the line l.sub.2. A change-over switch CS.sub.4 is provided at the interconnection point of the resistor R.sub.19 and the base of the fourth transistor Tr.sub.4, and has a dry contact D.sub. 4 connected to the emitter of the second transistor Tr.sub.2 and a wet contact connected to the line l.sub.2 through a parallel circuit of a capacitor and a resistor R.sub.5. The change-over switches CS.sub.4 and CS.sub.5 are interlocked in relation with the selection of the water content range of the specimen. In this embodiment, for the measurement in the high water content range the transistor Tr.sub.2 is isolated by the change-over switch CS.sub.4 and the emitter resistance for the transistor Tr.sub.4 is changed to high by the change-over switch CS.sub.5 so as to increase the input impedance for the transistor Tr.sub.4 and to change the base potential of the transistor Tr.sub.4 linearly with the water content of the grain specimen G. It is apparently possible to provide an appropriate amplifier to linearly amplify the output signal for improving the sensitivity.

In the above embodiments, description of the linearizer was limited to ones including the second, the third, and the fourth transistors. It will be apparent, however, that the linearizer is not limited to the described ones. Further, although the specimen was grain in the above embodiments, it is also apparent that the specimen is not limited to grain. Yet further, it is apparent that the circuit structure of the temperature compensator is not limited to those of the above embodiments.