GB2256489A - Capacitative measuring apparatus - Google Patents

Abstract

An electric measuring apparatus (e.g. for measuring the water content of a heterogeneous mixture of soil and air) includes an oscillator 2 and a sensor capacitor in the tuning circuit of the oscillator, the sensor capacitor has spaced electrodes 1 that extend from the apparatus in such a way that they are able to be placed in direct contact with the medium and produce an electric field that traverses the medium between the electrodes. The oscillator output is transmitted optically, either by optical cable or in free space to a receiver 5 with display. The electrodes are preferably in the form of rods and may be detachable so that they can remain in the medium under test. The oscillator frequency may be 100-160MHz and this frequency is reduced by division before optical transmission. Use of optical transmission minimises interference from capacitative effects associated with an operator. <IMAGE>

Description

Electric Measuring Apparatus and Svstems This invention relates to electric measuring apparatus and particularly though not exclusively to systems for measuring the distributed capacitance of heterogeneous systems at high frequency in the range 50 NHz to 200 MHz.

In one application the apparatus is used for the measurement of the water content of soil.

In an embodiment of the invention to be described moisture measuring apparatus comprises a sensor unit which is positioned at the point of measurement and which is linked by purely optical means employing no electrical conductors to a remote display or recording system.

Electrical properties, such as capacitance, can be employed as a measure or monitor of many physical variables.

Capacitance when so employed may be localised and well defined, or it may be distributed. The dielectric material associated with electrodes providing a capacitor may be homogeneous or heterogeneous and, if the latter, it may have a spuriously high value of dielectric constant if measured at frequencies less than a few tens of megahertz.

In order to avoid this problem, it is necessary to use higher frequencies, but at higher frequencies radiation effects are accentuated and the presence near to the circuit of any electrical conductor, such as an operator's hand, can disturb and affect the result. Hence if the sensor is remote from the readout, as is usually the case, any movement near to a connecting cable is likely to result in an inaccurate operation of the apparatus.

In the specification of UK Patent No. 2,180,937, there is described a measuring apparatus which has a pair of cylindrical sensor electrode plates spaced apart along their common longitudinal axis. In use the apparatus is lowered into an access tube made of a plastics material.

The present invention provides arrangements which are simpler and more easy to use than the previously proposed arrangement.

In an arrangement to be described below by way of example, the effect of radiation upon the connection between the sensor and the display is minimised. In the preferred embodiment, the connection is made by optical means so that the sensivitivy of the apparatus may be maintained while the possibility of interference by radiation is minimised. In the particular embodiment to be described, the technique is applied to the in-situ measurement of soil moisture.

Apart from the use of electrical capacitance, soil moisture may be measured by employing one of a number of techniques including gravimetric, neutron scattering, gamma ray attenuation, time domain reflectometry and electrical conductivity.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which Figure la, Figure lb, and Figure lc, each show schematically one particular basic arrangement, Figure 2 is a block schematic diagram in more detail of a preferred embodiment, Figure 3 is a longitudinal cross-section of a sensor, and Figure 4 is an electrical circuit diagram.

Referring to Figure la, a sensor A is shown with fixed rods B inserted into the ground C. An output is fed via a cable D to a display unit E from the sensor A. The fixed rods B of the sensor A are easily inserted into the ground C and enable measurements to be made rapidly at any of a number of sites. Figure lb shows an arrangement for carrying out a measurement at the same place over a period of time. Rods Bl are inserted into the left in the ground C. A sensor Al is coupled to rods Bl at will in order to make a measurement and subsequently removed. This operation is repeated as required. In Figure lc, a sensor A2 has flat plates B2 which are placed on the surface of the ground C and is connected to a display unit F.

Refering to Figure 2, there are shown rod electrodes 1 which form the plates of a sensor capacitor. The electrodes 1 are coupled to form a part of a high frequency oscillator 2 and from the electrodes 1 a varying electric field corresponding to the oscillations of the oscillator 2 spreads out into the surrounding media provided by any moist soil, for example, into which the electrodes 1 may be inserted and which under study. The dielectric constant of the soil media determines the capacitance to be measured. The capacitance forms a part of the circuit of the high frequency oscillator 2, which is similar to a Clapp oscillator, and, together with the fixed components of the oscillator, determines the frequency of oscillation.

The output of the oscillator 2 is connected to a frequency divider 3, which reduces the frequency sufficiently for it to come within the range of an optical convertor 4, to whose input the output from the frequency divider 3 is connected.

The components 2, 3 and 4 are included in the housing of a sensor, such as that shown at A in Figure la and are linked by means of a fibre optic cable 5a to a hand held display 5, or to a data acquisition (logging) system.

Figure 3 illustrates an arrangement of the parts of the sensor A. A moisture proof housing is provided by a tube 6 of a dielectric material sealed by means of a bottom cap 7 and a top cap 8. Rod electrodes 11 of the sensor capacitor are shown supported directly by the bottom cap 7. The rods 11 each make electrical contact, via wires indicated at 13a, with two respective sockets indicated at 14.

The top cap 8 holds batteries which power the sensor and its suports a printed circuit board 18 which carries all of the electronic components, apart from an optical emitter which is housed in the top cap 8. The printed circuit board 18 also carries a coupling plate 16 with a guide pin 17 which ensures the correct alignment of electrical pins 15 with the sockets 14, thereby making electrical contact between the sensor capacitor electrodes 11 and the electronic circuitry on the printed circuit board 18.

Referring to Figure 4, the electrical circuit, a part of which is accommodated on the board 18, is seen to include the sensor capacitance C1 between the rods 11 connected in series with a capacitance C5, an inductance L1 a capacitance C2 and a temperature conpensating network D1, D, C3 and C4 which, overall, are the major factors determining the operating frequency of an oscillator, which also includes a transistor TR1, Typically the operating frequency is in the range 100-160 MHz.

The output of the oscillator transistor TR1 is taken via an emitter follower TR2 to a frequency divider IC1, (indicated at 3 in Figure 2), which reduces the frequency by a factor of 1/81 to wit;hin the capability of an optical transmitting system which comprises an emitter diode D3, a fibre optic cable, a detector and an optical communication receiver (not shown in Figure 4, but indicated in Figure 2).

The output from the optical communication receiver, indicated at 5 in Figure 2, can be displayed as a frequency on a hand held meter or processed with appropriate microprocessor circuitry to give a direct reading in a soil moisture unit. Alternatively, the information in either form can be stored in a data logger to provide a continuous record of soil moisture and soil moisture changes.

The oscillator circuit described has high inherent frequency stability but a small residual temperature dependence remains. This small error may be acceptable to the user, but to exploit the full sensitivity of the system a compensating circuit is included. A temperature dependent generator TG1 and amplifiers IC2aand IC2b feed a temperature dependent compensating voltage to voltage controlled diode capacitances D1 and D2, which in turn compensate for the temperature dependent effects in the oscillator circuit.

The circuit is brought into operation by operating the switch shown in Figure 4, thereby causing an electric field to extend between the rods of the sensor electrodes 1 (Figure 2); C1 (Figure 4) and to traverse heterogeneous mixture of soil, air and water C between the rods.

The overall dielectric constant of the heterogeneous mixture of soil, air and water traversed by the electric field determines the frequency of the oscillator as measured by the apparatus. The frequency will not be simply related to the soil moisture content and calibration will be necessary in using the apparatus.

In the arrangement shown in Figure lc, asensor has a flat system of sensor electrodes which are placed in use directly onto the surface of the soil.

It will be appreciated that, although the invention has been described with reference to particular embodiments, by way of example, variations and modifications may be made within the scope of the invention.

For example, although in the particular embodiments the outputs from the sensors are coupled via cables that transmit signals in the optical band, it is possible to transmit the signals optically without the aid of a cable, while still minimising the effect of interference due to the presence of electrical conductors.

The electrodes need not be rods; they may, for example, be in the form of spades, or plates with curved cross-sections. In one embodiment, more than two sockets are provided on the sensor A, so that it is possible to vary easily the spacing between the electrodes, as required.

Claims (8)

1. An electric measuring apparatus including an oscillator, a sensor capacitor arranged in the tuning circuit of the oscillator, the sensor capacitor having spaced electrodes which are arranged to produce an electric field that is able to extend through a surrounding medium, means to transmit a signal which is at a frequency in the optical frequency band, means to enable information related to the output of the oscillator circuit to be conveyed by a signal transmitted in the optical frequency band by the transmitter means and means to receive the signal which is transmitted in the optical frequency band, in which the electrodes extend from the apparatus in such a way that in measuring a characteristic of the medium they are placed in direct contact with the medium either in or on the medium.

2. Apparatus as claimed in claim 1 in which the electrodes are designed to remain in the position in or on the medium in which a measurement is to be made and are attached to the remainder of the apparatus only during the making of a measurement.

3. Apparatus as claimed in claim 1 in which the electrodes are fixed to the remainder of the apparatus and are removed, in operation, from the medium upon the completion of each measurement.

4. Apparatus as claimed in any one of the preceding claims in which the electrodes are in the form of rods.

5. Apparatus as claimed in any one of claims 1 to 3 in which the electrodes are in the form of flat plates.

6. Apparatus as claimed in any one of the preceding claims in which the tuning circuit of the oscillator includes an inductor and a further capacitor.

7. Apparatus as claimed in any one of the preceding claims including means to transmit the signal in the optical band optically without a cable.

8. Apparatus as claimed in claim 1 substantially as described herein with reference to any one of Figs. la, lb, lc, 2, or 3 and Fig. 4 of the accompanying drawings.