GB2156084A - A resistivity meter - Google Patents

Abstract

A meter for measuring resistivity of concrete, for example, using a four probe method, comprises a current generation and measuring circuit (2) which generates and measures an alternating current between two of the probes (14,15). A voltage measuring circuit (7) measures the voltage between the other two probes (21,22). Outputs from the circuits (2,7) are periodically sampled and held (4,4) for division by an analogue divider (5) to give an output indicative of the resistivity to be measured. If the alternating current waveform is a square wave, values of voltage and currents can be sampled at a central portion of each pulse where they will remain substantially constant. <IMAGE>

Description

SPECIFICATION A resistivity meter This invention relates to a resistivity meter for measuring the resistivity of a substance such as, for example, concrete.

A known method of measuring the resistivity of earth is disclosed in a paper by Frank Wenner (published in the Bulletin of U.S.

Bureau of Standards, Science Paper 1 2 (1916) No. 3)-the so-called ' "4 probe method". Until recently, a method and instrumentation such as those described by Wenner, but slightly modified have been used to measure the resistivity of concrete. A more modern piece of equipment which uses digital electronics, the "'NORMA', Digital Earth Resistance Meter", has also been used to measure concrete resistance, and hence concrete resistivity. Such prior art instruments have been specifically designed for earth resistance and resistivity measurements and, it is now appreciated, are somewhat limited in their application to measurements of concrete resistance and resistivity.

As is known, direct current techniques cannot be used because of polarization effects between the probes used in the measuring apparatus and the substance whose resistivity is to be measured, e.g. earth or concrete.

Apparatus for measuring the resistance of earth therefore use alternating current techniques. However, the use of alternating currents necessarily produces a value of resistivity containing reactive components. The frequency of the alternating current of such apparatus must generally therefore be low (-100Hz) to minimise the effect of the reactive components on the measured value of resistivity. Ideally, a measure of the true d.c.

resistivity component is usually required.

In general, the resistance of earth is relatively low and the magnitude of the current flowing therethrough may be of the order of mA. The actual current flowing is not only dependent upon the resistance of the earth but also on the contact resistance between the earth and the probes. These contact resistances may be of the order of ten times the value of the resistance of the earth. In contrast, the resistance of concrete may be much higher than that of earth and, more importantly, the contact resistance between a probe and concrete can be more than one hundred times the value of the resistance of the concrete.Thus, the current flowing during measurement of resistance of concrete may be of the order of yA. When the resistivity measuring apparatus mentioned hereinabove, which have been specifically designed for measuring the resistivity of earth, are used to measure the resistivity of concrete, the very small currents flowing have the effect of desensitizing the measuring apparatus, resulting in poor resolution and unreliable results. In addition, with very low currents, the input impedance to a voltage stage of the measuring apparatus becomes very important and must be very high, at least one hundred times the contact resistance. Under exceptional circumstances, contact resistances greater than 10 Mohms have been observed between probes and concrete.

A severe problem can exist when contact resistances are dissimilar and can be illustrated in the following manner. Suppose that the total contact resistance of both current probes is of the order of one hundred times the concrete resistance between the voltage probes of a voltage measuring means. The voltage drive to the 'current' probes must then be one hundred times the value of V, the measured voltage. If the contact resistances of the current probes are different then a large common mode voltage will exist on the voltage probes: for example, if the contact resistance of one current probe is half that of the other, the common mode voltage will be a third of the voltage drive; that is, thirty-three times the values of V. Provided the voltage measuring means has differential inputs with a good common mode rejection ratio then no inaccuracy in the measurement of V will result.However, large differences between the contact resistances of the volage probes introduce a phase shift between the two voltage inputs that effectively reduces the common mode rejection ratio drastically. The resulting measurement of V is thus swamped by an indeterminate measurement of the phaseshifted common mode voltage. Differences in the contact resistances of the current probes can be tolerated if both voltage probe contact resistances are identical, and vice-versa. However, if contact resistances are high, they are generally not identical.

According to the present invention there is provided a resistivity meter, for measuring the resistivity of a substance using four probes spaced apart and in contact with the substance, the meter comprising: current generating means for causing an alternating current with a periodic waveform to flow through the substance between two of the probes by generaing at a first one of the probes a known voltage and at a second one of the probes a voltage of equal magnitude and opposite sense to the known voltage; current measuring means for measuring the current; voltage measuring means for measuring the voltage between the other two of the probes, said other two probes being disposed between the first and second probes; and computing means which accept periodically simultaneous values of the current and of the voltage is response to signals emitted from the current generating means and which generates from the values an output value respre sentative of the resistivity of the substance.

The alternating current waveform is prefera bly a squarewave.

In the meter of the present invention, use is made of the 4-probe system for measuring resistivity described by Wenner. In this system with a linear probe array and equal spacing between the probes, the measured value of p is given by the expression, p = 27ra (V/I) where 'a' is the spacing between the probes in centimetres, I is the current flowing between the two outer, 'current' probes, and V is the voltage measured across the two inner, 'voltage' probes. The units of p are thus ohm.cms. The meter of the invention generates and measures the value of I and also measures the value of V, the value of V/I being computed, preferably by an analogue divider circuit. A variable gain stage with a ten-turn variable resistor, calibrated in terms of the probe spacing, may be incorporated to enable the operator to 'dial-in' the value of 'a'.The factor of 27r is conveniently built-in to other gain stages of the voltage measuring means. The resulting output voltage is thus directly related to resistivity. Provision can be made for three ranges to be selected to give a full scale deflection of 1OKohm.cm, 100 Kohm.cm and 1 Mohm.cm, respectively.

In one embodiment of the invention in order to allow for high probe/concrete contact resistances, the voltage drive to the 'current' probes is made as high as is possible by using + 1 5 volt supply rails to power standard operational amplifier IC's (op. amps.). The effective voltage drive is doubled by using two operational amplifiers in "push-pull". One of the two 'current' probes is connected to the output of a respective one of these op.amps.

A resistor in series with the output of one op.amp and its associated 'current' probe has a voltage developed across it which is proportional to the current, I, flowing through it.

This is used as a measure of the value of I, the arrangement allowing for accurate measurements of current as low as liuA (on the 1 Mohm.cm range). The maximum voltage drive is about 50 volts (peak to peak), or 25 volts (rms) using a square-wave current source: with this voltage, contact resistances as high as 12 M ohms per 'current' probe can be accommodated. For a typical probe spacing of 5 cm, 1 Mohm.cm is equivalent to a concrete resistance between the 'voltage' probes of 31.8 Kohms. This represents a ratio of contact resistance per probe to concrete resistance of nearly 400:1. To enable the meter to cope with such high contact resistances on the voltage probes, the input impedance of the input stage to the voltage measuring means is preferably of the order of 1012 ohms.The current drawn by the voltage measuring means is then less than 1 pA at full-scale on the 1 Mohm.cm range and 5 cm spacing. When the external load on the current probes is of low resistance, the output current is limited to a maximum, constant value. When the load is of high resistance the voltage drive is limited to a maximum value.

The analogue divider circuit used to compute the value of V/I can handle variations in its denominator value of 100:1; however in this embodiment of the meter, the range of variations is limited to 25:1 at which a 'low current warning' indication is given. On the two lowest resistivity ranges (10 and 100 Kohm.cm) the 'constant' value of current is 250 ,uA and the low current warning is given when the current falls to 10 ,uA; on the highest range (10 Mohm.cm) the current is 25 A, and the warning is given at 1 juA.

These warnings are intended to indicate to the operator that contact resistances are extremely high and problems may be experienced in obtaining good results.

The use of a wholly sinusoidal waveform for the current source is not suitable, as any phase shift between the two probes has an adverse effect on the complete waveform.

However, with a square wave or truncated sine wave for example, any phase shift is only apparent on the leading and trailing edges of the resultant voltage waveform; the central portion of each half-cycle is unaffected. Consequently this central portion is sampled, well away from the leading and trailing edges, the sampled value being thus representative of the complete half-cycle. Also, by sampling both positive and negative half-cycles, and differentially adding the sampled values, a useful voltage doubling can be achieved, whilst eliminating any d.c. component in the waveform.

There is another advantage to be gained from sampling square waveforms: since parallel reactive components affect the rise time of square waves, sampling the central portion of the waveform excludes these reactive components from the measurement. Series reactive components affect the so-called "droop" of the central portion of a square wave. If, therefore, both current and voltage waveforms are sampled at the same instant the resultant value of V/I will be independent of any series reactive component. Thus with sampling carried out in this manner, a purely resistive measure of the concrete's resistivity, without reactive components, is provided.

The frequency of the alternating current waveform is advantageously as low as possible subject to the use of reasonably valued capacitors for a.c. coupling between components of the meter. Such a.c. coupling is desirable to block unwanted d.c. levels from causing probe polarization effects and op.amp. offsets. With low levels of current flowing between the current probes (e.g. < 250 Zap and a very high input impedance stage for the voltage measuring means, there is a possibility of measurements being affected by mains frequency interference. To minimise this, screened leads are provided on the current and the voltage probes.Since screening to a ground potential can cause stray leakage currents which may affect the accuracy of measurement, each lead may be screened, using, for example unity gain buffer amplifiers, with a low impedance source at its own voltage level. This is similar to 'guardring' techniques used on printed circuit boards to protect a high impedance input connection. Preferably, a filter is provided to attenuate any mains frequency (50 Hz) interference which is nevertheless picked up by the meter. This filter may be a 3-element, low pass, Butterworth filter having a pass band of 3 Hz and an attenuation to 50 Hz signals of approximately 76 dB.

For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a block circuit diagram of a resistivity meter in accordance with one embodiment of the present invention; Figure 2 is a circuit diagram of the oscillator 1, and the divide-by ten circruit 3 in Fig. 1; Figure 3 is a circuit diagram of the current generator 2 in Fig. 1; Figure 4 is a circuit diagram of the voltage measuring stage of the meter 7 in Fig. 1; Figure 5 is a circuit diagram of the sample/hold circuitry 4 and analogue divider 5 in Fig. 1; Figure 6 is a circuit diagram of the low current warning circuit 6 in Fig. 1; Figure 7 is a circuit diagram of a low pass Butterworth filter; Figure 8 illustrates the connections to a digital display panel;; Figure 9 shows the circuit details of a battery operated power supply; and Figures 10a to I0fillustrate the relationship between output signals of various components of the meter of Fig. 1.

A block circuit diagram of the components of a resistivity meter is shown in Fig. 1. A 260 Hz oscillator 1 generates a square wave of for example 0 to 1 5 V peak to peak which is passed to a divide-by-ten and sample/hold pulse generation circuit 3 wherefrom control pulses are emitted to control the operation of sample/hold circuits 4 and wherein the frequency of the square wave is divided by 1 0.

A current generation circuit 2 (to be described in more detail hereinafter) is connected to current probes 14, 15, and a voltage measuring circuit 7 is connected to voltage probes 21, 22. A range switch 8 controls the range over which the current generation circuit 2 and voltage measuring circuit 7 function, while the spacing between the probes (14-21, 21-22, 22-15) can be dialled into the circuit 7 by means of a dial 9. Outputs from the current generation and voltage measuring circuits 2, 7 are sampled in sample/ hold circuits 4 in accordance with the pulses emitted from the divide-by 10 circuit 3 and are divided in an analogue divider 5. The output from the analogue divider is filtered by a low-pass filter 5a, the output of which is representative of the resistivity of the concrete C.A low current warning circuit 6 enables an operator to be informed that the current has reached an unacceptably low level and consequently subsequent results of the value for resistivity may be in error.

Fig. 2 is a circuit diagram of the oscillator used in the meter shown in Fig. 1, i.e. a 260 Hz square-wave oscillator 1 and of the "divide-by-ten" IC 3. The 260 Hz oscillator 1 comprises a CMOS 4047 IC 10 connected as an astable multivibrator. A CMOS 401 7 IC 11 divides the output from the oscillator 1 by ten to produce from pin 1 2 a symmetrical squarewave output, at a frequency of 26 Hz. Outputs from other pins of the IC 11 produce pulses that occur for 1 /10to of one cycle of the signal outputted at pin 1 2. The pulse outputted on pin 7 occurs during the fourth fifth of the positive half-cycle and that on pin 9 during the fourth fifth of the negative halfcycle. Figs. 1 Oa to 10c show the relationships between the input and output pulses of the 4017 IC 11.Fig. 1 Oa shows the output of pin 12, Fig. 1 Ob that of pin 7 and Fig. 10c that of pin 9. The outputs from pins 7 and 9, control signals SH1, and SH2, are used to control the sampling of the 'current' and 'voltage' waveforms in other components of the meter to be described hereinafter. The two CMOS ICs 10, 11 are connected for their power supply between ground and a + 1 5 volt supply rail.

Fig. 3 is a circuit diagram of the current generator 2 of Fig. 1. The 0 to 1 5 V square wave outputted at pin 12 of IC 11 in Fig. 2 is decoupled by a capacitor C1, e.g. of 10 yF, to produce a square wave of + 7.5 V which is fed via resistor R1, having a value of, for example, 1 50 Kohms, to the negative input of op.amp. 1 3a. Op.amp. 1 3a is connected as an inverter having negative feedback applied to its negative input by means of op.amp 1 3b and via resistor R2 (e.g. 100 kohm).The output from op.amp. 1 3a forms the voltage drive to a first current probe 14, via a current measuring resistor selected from R3 (200 kohm) and R4 (20 kohm) by means of a three pole rotary switch SW(a), SW(b). Op.amp.

1 3b measures the voltage developed across the current measuring resistor R3 or R4. It is connected as a differential input amplifier of unity gain. The positive input is connected to one side of the current measuring resistor and to the output of op.amp 13a, and the nega tive input is connected to the other side via op.amp. 1 3c connected as a unity gain buffer amplifier. Op.amp. 1 3c is used to reduce the current drawn from the current measuring resistor R3 or R4 to an insignificant amount.

The output of op.amp. 1 3c is also used to drive the unity gain inverting stage of a further op.amp. 13d. The output of op.amp. 1 3d thus drives a second current probe 1 5 with a voltage that is equal in magnitude but of opposite phase to the voltage output on the first current probe 14. The current probes thus "see" a voltage drive that is balanced about the zero volt (ground) line.

The four op. amps. 1 3a to 1 3d are conveniently contained in a quad IC package such as TL074CN. For a low power supply e.g. a battery, a TL064CN package may be used.

Any suitable high performance general purpose op.amp. could be used.

When the resistance load on the current probes 14, 1 5 is low, the current flowing will be limited to a maximum value in the following manner. The voltage output from op.amp.

1 3b is given by Vo = I.Rc, where I is the current flowing through the current measuring resistor, Rc (R3 or R4). This voltage, Vo, is fed back to the input of op.amp. 1 3a via the resistor R2. The feedback loop controlling the output of the op.amp. 1 3a will be closed when the condition: Vi/150K= - Vo/100 k is met. Vi is the input voltage to the resistor R1 (i.e. j 7.5 V).

Thus, (j7.5 V)/150 k= -(I.Rc)/100 k and therefore I = 100 k (i 7.5V)/(150 k.

Rc)= i5/Rc: when Rc = 20 k, I = + 250pA; and when Re = 200 k, 1= ~ 25,uA.

The output from op.amp. 1 3b, Vo, is always a measure of the current flowing and is therefore used as the denominator input to the subsequent analogue divider circuit 5.

When the current output is constant, the voltage output from op.amp. 13b, Vo, is j 5V. If, due to a high resistive load across the current probes, the current falls below its 'constant' value, the output from op.amp.

1 3b will be less than i 5 V. In an attempt to increase the current to its constant value, the output from op.amp. 1 3a will tend to increase as a result of the feedback loop; however it will be prevented from doing so by two 1 2 V zener diodes D1, D2 connected between its output and its negative (inverting) input. The maximum output voltage swing of op.amp.

1 3a is thus limited to a maximum level of about i 1 2.6 V (the zener voltage of one diode plus the forward diode voltage drop of the other).

Op.amp. 1 3b measures the differential voltage across the current measuring resistor R3, R4. When the current is low and the voltage drive of op.amp. 1 3a is at a maximum, there will be a large common mode voltage on the inputs to op.amp. 13b. A 1 Kohm variable resistor R5 in series with a 19.5 Kohm resistor R6 allows the common mode rejection ratio of op.amp. 1 3b to be adjusted to a maximum. In practice it is trimmed for zero volts output from op.amp. 1 3b when the current probes 14, 1 5 are on open circuit and I is thus zero.

Various capacitors are used in the circuit for high frequency stability of the op.amps. operating under varying conditions. Two resistors R7, R8 having a low value, for example 2.7 Kohms, are connected in series with the out- .

puts to the current probes 14, 1 5 for this reason of high frequency stability. The accuracy and operation of the circuit is otherwise unaffected by their inclusion. The lead of current probe 14 is screened by the output of op.amp. 13c. The lead of current probe 1 5 does not need to be screened because it is driven from a low output impedance source. It is screened, however, as shown so that identical cables and connctors may be used for the two probes 14, 15.

Fig. 4 is the circuit diagram of a high input impedance voltage measuring stage of the meter. Op.amps, 16a, 1 6b simply act as high input impedance, unity gain buffers capable of providing a low output impedance screen for the input leads at a similar potential. As d.c. coupling is used for simplicity, 100 Kohm resistors R9, R10 are used in the input lines, to afford some protection to the op.amp. inputs. Capacitors C2a, C2b, of 47 nF, connected in parallel with resistors R 11 a and R 11 b attenuate the gain of the differential amplifier stage at high frequencies, reducing the subsequent noise level of the 'voltage' signal.

The effect of a large common mode voltage signal on the voltage probes 21, 22, together with a phase shift between the signals on the input to the differential voltage measuring stage is shown in Figs. 1 Od to 10f, exaggarated for the sake of clarity. Fig. 1 Od shows the voltage waveform on the positive input to the differential amplifier, Fig. 1 Oe shows the voltage waveform on the negative input of the differential amplifier and Fig. 1 Of show the resultant output from the differential amplifier.

Op.amps. 16c, 16d, and 1 7a provide the main differential input stage of the meter. This is a standard circuit and has unity gain to d.c.

and common mode signals, and a differential gain of 1 + 2 (180 k/130 k)=1.2. The common mode rejection ratio may be adjusted to a maximum using a 1 Kohm variable resistor (not shown) in series with a fixed 19.5 Kohm resistor R11. In practice, this is achieved by shorting the two voltage inputs together and connecting them to the current probe 1 5 output with zero current flowing.

The 1 Kohm variable resistor is then trimmed until there is zero volts on the output of op.amp. 17a.

The illustrated embodiment of the meter has three ranges of resistivity: 10 Kohm.cm.

100Kohm.cm. and 1Mohm.cm. The highest range of 1 Mohm.cm. is obtained by reducing the current to 1/10th of that which flows on the 100 Kohm.cm range. The lowest range of 10 Kohm.cm. is obtained by increasing the voltage sensitivity by 1 0. For this purpose the gain stage comprising an op.amp. 1 7b may be switched to either unity or x10. The final gain stage, op.amp. 1 7c has a nominal gain of a/3, where 'a' is the probe spacing. The value of 'a' is determined by a variable 100 k resistor R12, which is a 10-turn potentiometer; each turn thus represents a spacing of 1 cm. The output of op.amp. 1 7c is fed, via the sample/hold circuits 4, (Fig. 1) to the numerator input of the analogue divider circuit 5, (Fig. 1).

The main differential input stage can handle a maximum differential input voltage equivalent to that obtained when maximum current is flowing through the current probes 14, 1 5 and the probe spacing is not less than 1.5 cms:- i.e. on the 100 Kohm.cm. range where 'a' is 1.5 cm and I is 250yA, the input voltage (for a full scale reading) is given by: V = (p.l)/(2so.a) = (100 k X 0.25 mA)/(37r) = 2.65. When amplified by the differential input stage, with a gain of 1.27r, this becomes 10 V, (actually + 10V).

Fig. 5 is the circuit diagram of the sample/ hold circuits 4 (Fig. 1) and the analogue divider IC 5. One pair of sample/hold circuits is used for each of the 'voltage' and 'current' waveforms; one sample/hold circuit samples the positive half-cycles and the other the negative half-cycles of each waveform. Each sample/hold circuit comprises an analogue switch 18a-d, a capacitor C3a-d and a high input impedance, unity gain buffer amplifier 19a-d. The four analogue switches 18a-d are conveniently contained in a quad IC package e.g. a DG308CJ, and the four amplifiers 19a-d are conveniently contained in a quad op.amp. IC, e.g. TL064CN. The 'opening' and 'closing' of the analogue switches 18a-d is controlled by the two control signals, SH 1 and SH2, generated by the divide-by-ten circuit shown in Fig. 2.

When an analogue switch e.g. 18a is closed the associated capacitor e.g. C3a is charged to the level of the input signal. When the switch 1 8a is opened the capacitor 3a retains the level of the signal at that instant.

As a consequence of the bias current drawn by the buffer amplifier e.g. 1 9a being very small (-30 pA) the capacitor will discharge only very slowly and, during the period between samples (-38 mS for a signal frequency of 26 Hz) will not discharge to any signficant degree. The sample/hold circuits 4 are not required to have any significant dynamic response since, apart from at the instant of switch-on, or probe contact, the signal levels will change only very, very slowly.

The fact that the analogue switches have a finite 'on' resistance of about 100 ohms and that the sampling takes a finite time (the time to charge a capacitor to within 0.1 % of its final level is 6.9 (R.C.), where R is the resistance in the charging circuit and C the capacitance of the capacitor, which equals 69yS in this case) to establish the voltage level on the capacitor is thus of no consequence.

Since square wave of a steady level are being sampled, the outputs from the sample/ hold circuits 4 are effectively d.c. If any mains frequency, or other interference signal, is imposed on the square wave signals, the resultant output from a sample/hold circuit will be a steady d.c. signal modulated by the interference.

The outputs of the sample/hold circuits 4 of each 'current' and 'voltage' pair are summed by differential inputs on the analogue divider circuit 5. The sampled values of the positive and negative half-cycles are thus added together; any common d.c. signal on the input to the pair of sample/hold circuits is thus cancelled.

The analogue divider circuit performs the function: Vo = 10(Z2-Z1)/(X,-X2) + Y1, where X1, X2, Yt, Z, and Z2 are indicated in Fig. 5 as inputs to the divider circuit 5. The outputs from the 'voltage' sample/hold circuits are thus summed on the 'Z' inputs; the outputs from the 'current' sample/hold circuits are summed on the 'X' inputs; and Y1 is connected to ground. The phase relationship between the sample/hold pulses, SH1 and SH2, and the 'current' waveform is fixed internally such that the output of the op.amp.

1 9a is more positive than that from op.amp.

1 9b. Thus the value of (X1-X2) is always positive. There is no such fixed relationship between the sample/hold pulses and the 'voltage' waveform since it is possible to swap over the lead connections to the two 'voltage' probes 21, 22 (Fig. 4). Consequently it is possible to obtain a negative voltage on the output from the analogue divider. This is of no consequence since the magnitude of the positive and negative readings should be identical.

As mentioned earlier, d.c. components on the inputs to the 'voltage' and 'current' sample/hold circuit pairs are cancelled by the differential input summing of the analogue divider. There is thus not need to trim the offsets of the preceding op.amp. stages (Figs.

3 and 4). There are however d.c. offsets associated with the 'X', 'Y' and 'Z' inputs to the analogue divider and it is essential that these be zeroed. Offset controls are thus included for this purpose. Any offsets from the preceding op.amps. 19a-d of the sample/ hold circuits will only add to or subtract from the divider's offsets and will therefore be taken care of by these controls.

Fig. 6 is a circuit diagram of the 'low current warning' circuit 6 (Fig. 1). The output from the current measuring circuit 2 is a + 5 V (peak) signal when the output current is a 'constant'. It falls to + 0.2 V (peak) when the current falls to 1 /25th of its 'constant' value.

Op.amps. 20a and 20b are comparators which give a high, positive output voltage when the associated positive and negative values of the current measuring circuits output are greater than 0.2 V in magnitude. Op.amp.

20c is connected as an astable multivibrator which has an 'on' (positive o/p) 'off' (negative o/p) ratio of 1:5. It is prevented from oscillating when the outputs of either of the comparators-op.amps. 20a and 20b are 'high'. When both outputs are 'low' (when the positive and negative values of the output from the voltage measuring circuit are below 0.2 V in magnitude) the multivibrator 20c is able to oscillate, giving a 'low current warning' via the flashing LED. The multivibrator 20c draws most current when the LED is lit.

The on/off ratio of 1:5 thus conserves power for battery operation of the resistivity meter.

Fig. 7 is the circuit diagram of a 3-element, low-pass, Butterworth filter with a low pass bandwidth of ~3 Hz. It is designed to attenuate 50 Hz signals by '-76 dB., and is used to filter the output from the analogue divider 5.

In the illustrated embodiment 3 op.amps. of a quad TL074 package are used: a low power version could use a quad TL064 package.

Fig. 8 shows details of connections to a commercially available digital panel meter, with annunciators, for displaying the resistivity reading.

Fig. 9 shows the circuit details of a battery operated power supply using Nickel-Cadmium (Ni Cd) cells. A d.c. to d.c. converter 22 converts the 5 V battery supply to nominal supply rails of + 1 5 V. All the circuitry described hereinabove has been designed for minimal power consumption, drawing approximately + 13 mA from the i 1 5 V supply rails. Consumption from the 5 V suply is thus about 100 ma. There is also provided a series regulator IC 23 that acts as a constant current limiter to the recharging of the Ni Cd cells.

The resistivity meter described above with reference to the drawings has been designed specifically for the measurement of concrete resistivities, and makes allowances for extremely high contact resistances up to about 10 Mohms/probe, and, further, eliminates the problem arising from dissimilar probe contact resistances occurring with prior art meters; that is, when the contact resistance of one current probe and one volage probe is very much higher than the others, a meaningless measurement could result with the operator being unaware of the problem. Further, the meter has two advantageous features not present in the prior art meters, namely: the provision of an analogue output signal to enable continuous monitoring of concrete resistivities using embedded probes to be carried out; and the ability for the probe spacing to be set and 'dialled-in' to the meter so that a direct measure of resistivity, not resistance may be made.

Thus, using the meter described herein, a true measure of the d.c. component of concrete resistivity may be obtained.

Claims (9)

1. A resistivity meter, for measuring the resitivity of a substance using four probes spaced apart and in contact with the substance, the meter comprising: current generating means for causing an alternating current with a periodic waveform to flow through the substance between two of the probes; current measuring means for measuring the current; voltage measuring means for measuring the voltage between the other two of the probes, said other two probes being disposed between the first mentioned two probes; and computing means which is capable of accepting periodically corresponding values of the current and of the voltage and of generating from the values an output value representative of the resistivity of the substance.

2. A meter as claimed in Claim 1, in which the current generating means comprises a voltage source capable of generating an alternating voltage with a periodic waveform, and means for generating at a first one of the first-mentioned probes a known voltage and at a second one of the first-mentioned probes a voltage of equal magnitude and opposite sense to the known voltage.

3. A meter as claimed in Claim 1 or 2, wherein said current generating means comprises means for generating periodically signals to control said accepting of said values by the computing means.

4. A meter as claimed in Claim 3, in which the computing means comprises: sample and hold circuitry responsive to said signals periodically to sample and hold corresponding values of the current and of the voltage; and dividing circuitry for generating as said output value a value proportional to a voltage value divided by the corresponding current value.

5. A meter as claimed in any preceding claim, wherein the periodic waveform of the alternating current has periods during which its value is constant.

6. A meter as claimed in Claim 5, in which the periodic waveform of the alternating current is a squarewave.

7. A meter as claimed in Claim 5, in which the periodic waveform of the alternat ing current is a truncated sinewave.

8. A meter as claimed in Claim 5, 6 or 7 in which said periodic accepting is carried out during said periods having a constant value.

9. A meter substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.