GB2198238A - Method and apparatus for measuring a property of material - Google Patents

Abstract

The moisture content of soil is measured by inserting a probe in the soil, the probe having an electric heater element 20, which establishes a temperature gradient extending into the soil from the heater. The power of the supply to the heater is controlled by feedback to maintain a temperature difference of around 3 DEG C between two temperature sensors, e.g. platinum resistance thermometers PRT-1, PRT-2, bonded to the inside of the stainless steel probe body 10 and separated by polystyrene insulation 24. The heater 20 is carried by a steel insert 16 overlying the thermometer PRT-1. The current fed to the heater is converted to a corresponding power signal representing soil moisture content by a conditioning stage including a linearising network. The output is continuous and is independent of the temperature of the soil. The method is applicable to measuring thermal conductivity or other related properties of soil or other materials. <IMAGE>

Description

METHOD & APPARATUS FOR MEASURING A PROPERTY OF A MATERIAL The invention relates to methods and apparatus for measuring a property of material, particularly though not exclusively the moisture content of soil.

It is proposed in US patent specification No. 3813927 (Furgason) to pass grain over a plate and to heat the plate using an electric heater the current to which is controlled so as to maintain the plate at a predetermined temperature. A temperature sensor in the plate gives a signal used to control the current. The moisture content of the grain is said to be detected as a voltage across the heater, the voltage varying as the moisture content changes causing differing rates of heat transfer from the plate to the grain.

It is proposed in US patent specification No. 4197866 (Neal) to control the supply of irrigation water by using a ground probe in which there is an electric heater and two thermistors which are connected in a Wheatstone's Bridge circuit. One thermistor is for ground temperature compensation. The other senses a transient temperature which occurs in response to energisation of the heater for a predetermined period. The temperature reached depends on the moisture content of a porous rod in which the thermistors are embedded and which absorbs moisture from the soil when the probe is inserted in the soil. The temperature depends on the soil moisture content.

A probe is proposed in a paper by H J Goldsmid, K E Davies and V Papazian in Journal of Physics 'E' Scientific Instrumentation, Volume 14, 1981, pages 1149 to 1152 for use in making transient observations of soil moisture content. The probe has a copper body directly engaging the soil and contains a heater resistor and a thermocouple junction. Temperature is monitored as a function of time after switching on the heater. The current supply to the heater is constant from a regulated low-voltage DC source. Moisture content is derived from graphs plotted in terms of temperature and time, and by calculation.

The object of the invention is to provide a method and apparatus by which a property of soil or other material, being the thermal conductivity or a property related to thermal conductivity, can be measured contInuous lv.

A method of measuring a property of material being the thermal conductivity or another property related thereto according to the invention comprises energising an electric heater of negligible thermal capacity at an input power representing a first parameter to supply heat from the heater to the material to establish a temperature gradient extending from the heater into the material and around the probe, sensing the temperatures at two locations on the gradient using respective transducers of negligible thermal capacity the difference between the temperatures representing a second parameter, keeping one of said parameters constant, and deriving the value of said property from the value of the other of said parameters.

Apparatus for measuring a property of material being the thermal conductivity or a property related thereto comprises a probe means engageable with the material and an electric circuit system, said electrical supply said heater and said transducers forming part of said circuit system, and said heater and said transducers being carried by said probe means.

An example of how to perform the method and an embodiment of apparatus for performing the method will now be described with reference to the accompanying drawings in which: Figure 1 is a side elevation with part removed to show the interior of a probe forming part of the apparatus; Figure 2 is an enlarged longitudinal section through the main body part of the probe shown in Figure 1; Figure 3 is an enlarged transverse section through the body part shown in Figure 2; Figure 4 is an enlarged plan of an insert assembly shown in the probe shown in Figure 1; Figure 5 is an enlarged end elevation of part of the insert assembly shown in Figure 4; and Figure 5 is a diagram of the electric circuit of the apparatus including components of the probe shown in Figures 1 to 5.

The apparatus shown in the drawings is particularly intended for use in measuring the moisture content of soil, which is a property related to thermal conductivity. However, the method and apparatus, or modifications of the same are also applicable to the monitoring or measurement of other properties related to thermal conductivity, or of thermal conductivity itself whether in soil or in other materials.

The probe shown in Figures 1 to 5 consists of the following main components:- a cylindrical hollow stainless steel body 10; a stainless steel end cap 12; a cable gland 14; an insert assembly made up of a stainless steel insert body 16 in which a platinum resistance thermometer PRT1 is mounted and which carries an electrical resistance heater ; a second platinum resistance thermometer PRT2; expanded polystyrene chippings 24 occupying the remaining space in the body 10; and a ble 26 passing through the gland 14 and containing conductors connecting the thermometers PRT1 & PRT2 and the heater array.The platinum resistance thermometer characteristics are matched to within plus or minus 0.02 degree Celsius over the range -10 C to +30"C. The absolute accuracy is not important.

The cap 12 and the gland 14 are bonded to the body 10 with adhesive. The thermometer PRT1 is bonded to the insert body 16 and the thermometer PRT2 is bonded to the inside of the body 10 with epoxy resin adhesive. The insert assembly is bonded to the inside of the probe body 10 with adhesive.

The electrical resistance heater 20 is preferably one or more strain gauge elements, in this case two elements SG1 and SG2 (Figure 6) connected in parallel.

The polystyrene 24 effectively insulates the platinum resistance thermometer PRT2 from the heater 20 (Figure 1).

In figure 6 the circuit components have the values or specifications assigned to them below: RESISTORS R1 5K ohms Wire Wound 0.1% R2 5K ohms Wire Wound 0.1% R3 1.0 ohm +1% 0.5W 100 ppm/OC R5 5K ohms Wire Wound 0.1% R6 100 ohms Wire Wound 0.1: 27 47 ohms* R8 47 ohms* R9 1 Meg ohms* R10 1 Meg ohms* R11 10K ohms R12 10K ohms R13 10K ohms R14 1 Meg ohms* (Cable Resistance Compensation T1/T2) R15 1 Meg ohms* R16 5.6K ohms R17 5.6K ohms R18 100 Meg ohms R19 100 Meg ohms R20 22 Meg ohms R21 1K ohms R22 20 ohms + 1% 0.5W 100ppm/OC R23 8.2K ohms R24 1 Meg ohms* R25 1 Meg ohms* R26 10K ohms R27 4.3K ohms* R28 3.3K ohms* R29 2K ohms* R30 2K ohms* R31 2K ohms* R32 1.8K ohms* R33 300 ohms R34 10K ohms * = 0.25 Watt Metal Film + 1% SOppm/OC.

VARIABLE RESISTORS VR1 20 ohms Cermet multi-turn VR2 2K ohms VR3 200K ohms Cermet multi-turn CAPACITORS C1 0.1 Microfarad C2 0.1 Microfarad C3 0.1 Microfarad C4 2.2 Microfarad C5 10 Microfarad polycarbonate C6 1 Nanofarad C7 0.47 Microfarad C8 1 Nanofarad STRAIN GAUGE HEATER RESISTANCES 5G1 120 ohms SG2 120 ohms TRANSISTORS TR1 TIP121 On heatsink TR2 BC 182 TR3 BC 182 ZENER DIODES Z1 9491 Z2 9491 Z3 9491 Z4 9491 DIODES D1 IN4148 D2 IN4148 PLATINUM RESISTANCE THERMOMETERS PRT1 PT100 ) matched to within 0.02 C.

PRT2 PT100 ) Absolute accuracy # 0.5 C.

SWITCHES SW1a ) (GOLD CONTACTS )ganged ( SWlb ) (SHOWN IN NORMAL OPERATING POSITION SW2 PUSH TO MAKE METER Ml 0 to 1 volt scaled as 0 to 0.5 grammes per cubic centimetre of water.

INTEGRATED CIRCUITS IC1 OP07 IC2 OP07 IC3 OPO7 IC4 OP07 IC5 071 IC6 072a IC6 072b IC7 741 POWER SUPPLY PS-1 STABILISED + 15 volts + 1% 200 milliamperes maximum 240 Volts AC input Constant Current Bridge Circuit This supplies a virtually constant current to both the thermometers PRT1 and PRT2 and the zero temperature reference resistor R5. The switch SW1 switched to the ZERO position allows the bridge balance to be set using the zero set potentiometer VR1, to set the voltage at the terminal TP1 between OV and -0.1V. The zero-set switch SW1 is switched to this position before power-up to avoid current passing to the heater array 20 otherwise the system would have to be left to stablise at a constant temperature. The resistors R3 and R4 set the internal offset temperature which the circuit will balance to, in this case 30C, for example.The external temperature difference is 1.5"C for 30C internal offset. When the switch SW1 is switched to 'normal, the resistors R3 and R4 are switched in and the output of the high-gain amplifier stage is connected back to the output drive-stage via the filter stage.

The circuit is zeroed with the probe in a thermally conducting material at constant temperature, such as soil or water, for example.

Cable Resistance Compensation This circuit compensates for the voltage drop across the connecting cable resistance between the terminals T1 and T2 and the thermometers PRT1 and PRT2. This allows temperature to be measured between the OOC reference terminal TPO and the thermometer PRT1 or PRT2 to an accuracy of +j- 10C over the range -10 C to +30"C. The temperature at the thermometer PRT2 is the soil temperature.

Filter Stage This controls the settling time of the closed-loop feedback system.

The settling time for a O.1g/cc change in soil water density (approx. 6% moisture) is about 10 minutes. This time constant is for long-term installation moisture monitoring, which is what this circuit was designed for. When the probe is moved from one sample to another, the speed-up circuit button can be pressed to bring the output indicator M1 on-scale to the approximate value of moisture content before allowing the system to settle. It will then settle in approximately 10 minutes.

When the system was used in laboratory tests a chart recorder was always used to indicate when the system had settled. A chart recorder is not necessary when the system is installed as a long-term measurement system because moisture levels are expected to change slowly, but such a recorder is useful in measurements on different samples.

Differential Amplifier Stage This is effectively a high-gain, error-amplifying stage. The error is the difference in voltage between the resistors R12 and R13. This voltage is reduced as the resistance of the thermometer PRT1 increases and as it is heated by the heater 20 (SG1 and SG2). When the system is at balance, VIN is reduced to a very small level given by: VIN = Vo 21000 Power Output Stage This is a simple current drive class "A" emitter - follower stage using a Darlington pair package TR1, in a high-input resistance configuration with integrated circuits.

Current-to-Power Output - Signal Conditioning Stage This stage gives an output signal proportional to the amount of power needed to balance the feedback system. The power is dissipated in the heater resistance array (SG1 in parallel with SG2) causing the thermometer PRT1 to heat up and causing a thermal gradient to be set up around and away from the probe. For a 30C internal tenperature offset, a 1.50C temperature gradient exists around the probe.

The current is first measured by measuring the voltage across the resistor R22 in series with the output drive current.

The integrated circuits IC6a and IC6b form an output signal zero and scaling circuit up to the terminal TP2.

The integrated circuit IC7 in combination with the linearising network forms a linearising circuit which converts the current signal into a power-related signal.

The values in the linearisation network shown in Figure 6 are for a square law. In combination with the output "zero set" VR2 this gives a power-related output voltage Vo which is proportional to moisture content calibrated in g/cc of moisture in soil. The voltage Vo is given by: kVo = (Vdry - Vwet)2 where Vdry is the voltage across R22 representing dry soil i.e. having zero moisture content; Vwet is the corresponding voltage for a wet sample of the same soil; k is a scaling factor determined by VR3.

The linearisation network shown has given good results for the soil type used in tests. Different soil types have different thermal conductivities with varying linearities. The linearisation network can be tailored to accommodate those variations. A number of networks for different soil types could be switched or plugged into the circuit, for example. Alternatively, the system can be calibrated to indicate thermal conductivity, and moisture content can be determined from tables of soil moisture contents corresponding to conductivities for various soil types.

Power Supply At balance and full-scale the total current consumption is about 70mA for the circuit shown. The circuit could alternatively be configured to operate from batteries. For example, using batteries, the system can be operated using only 35mA and can provide a stabilised voltage for the constant-current bridge.

Calibration This calibration procedure applies where a linerisation network has been configured for a particular soil type and the meter is calibrated in g/cc of water with a full-scale deflection of O.5g/cc.

The switch SW1 is set to zero and the circuit is powered up and allowed to stablise for 15 minutes. Then the zero is set to be O to -0.1V at the terminal TP1 using the variable resistor VR1. Two samples of the soil type to be measured are prepared. One sample has 0 moisture, the other has about 2!3 the saturated level for the soil. Both samples have densities as close as possible to the dry density of the soil to be measured (within about O.lg/cc). Using volume measurement, weighing and small sample drying, the actual dry density and the moisture content in g/cc of water are evaluated.

The probe is placed in the dry sample and the latter is compacted to the correct density. The output on the meter M1 is zeroed using VR2.

Next the probe is placed in a wet sample which is then compacted to the correct density. The meter M1 is set to read the moisture content of the sample using the variable resistor VR3.

The zero and span set are repeated until no change occurs. In practice only one extra zero and span set have been found necessary.

About 15 minutes should be allowed after inserting the probe in the sample before adjustment and ideally a chart-recorder and voltmeter are used in the output terminal O/P.

Next, the linearising circuit is set such that -3.0V at TP2 gives 1V at the output and OV in gives OV out.

In tests done in the laboratory with grey boulder clay the meter was set to 0.37 g/cc moisture content corresponding to a moisture content of 22% by weight at a dry density of 1.668 g/cc, and to zero with a dry sample of dry density 1.628. The saturation level of this type of clay is about 30% moisture content by weight.

Another possible calibration method is to set the zero using a dry sample as above and set the scale reading in water. By coincidence, in the above example, 22% moisture at a dry density of 1.668 g/cc reads the same as water at 210C and could be set this way. This output level could be determined for a fixed moisture content in various soil types, making an easy calibration method corresponding to approximately half-scale.

However, the thermal conductivity of water does vary with temperature and this would have to be taken into account. Using the calibration methods set out above, with a suitable linearisation network, accuracies of plus or minus 2% moisture content can be obtained.

The apparatus and method are intended for use in the field particularly, though not exclusively, where long-term monitoring of soil moisture content is required. The electric circuit can be readily adapted for powering from a battery supply instead of AC mains supply.

The moisture content of the soil is proportional to its thermal conductivity. The rate of heat flow from the heater 20 into the soil is proportional to its thermal conductivity. Accordingly, the moisture content is proportional to the power delivered by the heater 20. The method is therefore based on maintaining a constant temperature difference between two locations on the temperature gradient, which extends from the heater 20 into the soil. The two locations are at the two thermometers PRT1 and PRT2. The temperature difference is maintained by controlling the power of the supply to the heater 20.

The internal temperature difference is preferably maintained at a value less than 5"C, for example at 3.0 C. The external temperature difference around the probe is approximately half the internal difference.

The method and apparatus provide a continuous analogue output which is an accurate representation of soil moisture content. This is quite different from the transient results available from methods such as that proposed in US patent specification No. 4197866 (Neal).

In a modification, (not shown) more than one probe is used. For example, the heater 20 and the thermometers PRT1 and PRT2 are each carried by a respective probe. In another modification (not shown) there are two probes one carrying the heater and one thermometer and the other carrying the second thermometer. Instead of thermometers it is possible to use thermocouples, semiconductor devices thermistors, or other temperature-sensitive components.

The method or apparatus is applicable to measuring a property other than moisture content.

For example, the thermal conductivity can be measured. The method or apparatus is applicable to the measurement of moisture content, thermal conductivity or other related property of material other than soil. For example, such a property can be measured in grain.

Whatever the application, the method has the advantage compared with the method proposed in US patent specification No. 3813927 (Furgason) that the output is not affected by the temperature of the material whose property is being measured because the temperature difference maintained between the two thermometers PRT1 and PRT2 is entirely independent of the temperature of the soil or other material. This enhances the accuracy of the method of the invention.

Claims (15)

1. A method of measuring a property of material being the thermal conductivity or another property related thereto comprising energising an electric heater of negligible thermal capacity at an input power representing a first parameter to supply heat from the heater to the material to establish a temperature gradient extending from the heater into the material and around the probe, sensing the temperatures at two locations on the gradient using respective transducers of negligible thermal capacity the difference between the temperatures representing a second parameter, keeping one of said parameters constant, and deriving the value of said property from the value of the other of said parameters.

2. A method according to claim 1, the output of the electrical supply being variable and said output being controlled using the error between a predetermined difference and the actual difference between said temperatures, and the value of the property being derived from the value of the power of the input to the heater at which said error is negligible or from an electrical parameter related to said value of the power.

3. A method according to claim 1 or claim 2, in which said predetermined temperature difference is not more than 5 degrees Celsius.

4. A method according to any preceding claim, in which the heater and transducers are part of a probe insertable in the material.

5. A method according to any preceding claim, in which each transducer is a platinum resistance thermometer.

6. Apparatus for performing the method according to claim 1 comprising a probe means engageable with the material and an electric circuit system, said electrical supply said heater and said transducers forming part of said circuit system, and said heater and said transducers being carried by said probe means.

7. Apparatus according to claim 6, in which said probe means comprise two or more probes.

8. Apparatus according to claim 6, in which said probe means comprises a probe which carries said heater and said transducers, one transducer being closer to the heater than the other.

9. Apparatus according to claim 8, in which said probe comprises a thermally conductive hollow body which carries said transducers on its inside surface at spaced-apart locations.

10. Apparatus according to claim 9, in which one transducer is located between a conductive insert body and the inside surface of the hollow body of the probe, said heater being carried on the insert body at a surface thereof facing towards the interior of the probe body.

11. Apparatus according to any claim of claims 6 to 10, in which said transducers are in an electric bridge circuit and said electrical supply comprises an amplifier the output of which is dependent on electric imbalance in said bridge circuit.

12. Apparatus according to any claim of claims 6 to 11, in which said transducers comprise platinum resistance thermometers, thermistors, or semiconductor or thermocouple devices.

13. Apparatus according to any claim of claims 6 to 12, in which said heater comprises a strain gauge element.

14. A method according to claim 1 substantially as herein des.r bec with reference to the drawings.

15. Apparatus according to claim 6, substantially as herein described with reference to the drawings.