Abstract:
A probe for measuring the hardness of concrete includes a body adapted to receive a retainer such that the combined body and retainer can be held in an aperture extending through a wall of a mould for the hardenable material, with the body extending from the inside of the wall. A sensor circuit includes detector elements supported by the body and disposed when in use in the concrete, and circuit terminals which face the exterior of the mould wall when in use, and are accessible through the retainer for connection to terminals of an external instrument. Preferably, the detector elements are conductive portions of a printed circuit board forming a resonator. The instrument can operate a switch in the probe which interconnects the terminals to indicate a good connection and measures resonator impedance at multiple frequencies.

Description:
RELATED APPLICATION 
       [0001]    This application claims priority of European Patent Application No. EP 06251199.3 filed Mar. 7, 2006, entitled MEASUREMENT OF HARDENABLE MATERIAL CHARACTERISTICS, which is incorporated herein in its entirety by this reference. 
       FIELD OF THE INVENTION 
       [0002]    This invention relates to a method and apparatus for detecting a characteristic of a hardenable material. The invention is particularly, but not exclusively, applicable to detecting the hardness of a cementitious material such as concrete, but is also applicable to detecting other characteristics of a range of construction materials. 
       BACKGROUND OF THE INVENTION 
       [0003]    It is known to provide a probe for measuring characteristics of a hardenable construction material. For example, one known system for measuring the hardness of concrete (see U.S. Pat. No. 6,023,170) uses at least two electrodes which are placed in the concrete for the purpose of measuring the dielectric characteristics of the concrete, which are related to the hardness of the concrete. 
         [0004]    It is known to provide a probe structure as shown in  FIG. 1 . The probe  2  comprises a base  4  and two hollow cylindrical metal tubes  6 , and is supported in wooden formwork  8 . The probe  4  is first attached to a holder  10  by sliding the cylindrical metal tubes  6  on to two prongs  12  of the holder  10 . The probe  2  is then supported on the inside of the formwork  8  by disposing a central cylindrical section  14  of the holder within a hole drilled in the formwork  8 . Flanges  16  of the holder  10  have holes extending therethrough to permit the holder  10  to be fastened to the formwork using screws  18 . After the concrete has set, the holder  10  can be removed, thus permitting the terminals of a connector to be fitted into the cylindrical metal tubes  6 , thereby enabling a measurement of dielectric characteristics of the concrete surrounding the tubes  6 . 
         [0005]    This arrangement suffers from a number of disadvantages. First, the dielectric measurements made using the probe  2  do not distinguish clearly between materials of different characteristics, particularly when the characteristics are similar. Second, the holder  10  can only be removed to permit connection of an instrument after the material within which the probe  2  is embedded has hardened. Accordingly, the probe cannot be used during the initial stages of hardening. A further disadvantage is that the construction of the holder  10  is such that it can only be used with wooden formwork or shuttering  8  in which the screws  18  can be fastened, and only with shuttering  8  of a limited range of thicknesses. Also, there is a significant chance of leakage of the hardening material through the hole drilled in the formwork  8 , around the circumference of the holder  10 . 
         [0006]    It would be desirable to provide an arrangement which overcomes or mitigates at least some of these disadvantages. It would also be desirable to provide a measurement system which can be used throughout a very wide range of situations. 
         [0007]    It would be desirable to provide a measurement probe which can be used for hardness testing within an experimental set-up, as well for “in-situ” strength measurements of concrete in a variety of different physical locations, which vary in their accessibility as well as in the size and shape of the concrete masses. It would also be desirable to achieve this while using a low-cost, consumable, embedded measurement probe. 
       SUMMARY OF THE INVENTION 
       [0008]    Aspects of the present invention are set out in the accompanying claims. 
         [0009]    The invention will be described primarily in the context of a probe for measuring the hardness of concrete which is poured into a mould defined by formwork, but the invention is equally applicable for measuring other characteristics and for use with other hardenable construction materials (such as gypsum). 
         [0010]    According to a further aspect of the invention, a measurement probe for measuring a characteristic of hardenable construction material (e.g. for measuring characteristics indicative of the hardness of concrete) comprises a body supporting components of a sensor circuit. The body has means for attachment to a retainer so that the body can be held in position within a mass of hardenable material before the hardenable material has set. The attachment means is designed so as to permit access to terminals of the sensor circuit while the body is held in position by its attachment to the retainer. The attachment means of the body is preferably an inner cylindrical surface of a hollow portion of the body, the surface preferably being threaded. In this way, the outer cylindrical surface of the retainer can be fitted through a hole in a mould wall and attached within the body of the probe. 
         [0011]    The retainer may engage the outer surface of a mould wall and the body engage the inner surface of the mould wall, to hold the probe and retainer in position. Such an arrangement allows for use of the probe in a variety of different situations. Different retainers can be used for different thicknesses of the mould wall. 
         [0012]    Preferably, there is also or alternatively provided a retainer having a different configuration which is intended for permitting the probe to be mounted entirely from the exterior of the mould wall. The retainer is fitted to the probe, and is provided with an outer annular resilient ring member. The mould wall has a hole sized to permit the probe to be inserted through the wall. However, the resilient annular member has a portion which extends into the hole, between the wall of the hole and the retainer. After insertion of the probe through the hole in the mould wall, a tightening member is used to compress the ring between the tightening member and the probe. This expands the portion of the ring between the wall of the hole and the main body of the retainer so as to hold the retainer, and thus the probe, in position. The annular member may be used as seal, and can form a significantly more effective one than in the prior art arrangements. The member may also have a thicker portion forming a flange larger than the hole to assist in sealing the hole and/or locating the retainer in position. 
         [0013]    Preferably, the tightening member is a cylindrical sleeve threadably mounted on the main body of the retainer. Preferably, interengaging threads on the probe body and the retainer are threaded in the opposite sense to the threads between the tightening member and the main body of the retainer, so that tightening of the tightening member does not transmit, via the resilient member, a force tending to unfasten the probe. Preferably the resilient member is engaged under a lip of the body of the probe, so as to prevent the retaining member from being displaced outwardly, over the end of the probe body. A measuring instrument is preferably also provided, the instrument having terminals for connecting to the terminals of the sensor circuit carried by the measuring probe. Preferably, the probe and instrument have interengageable formations to ensure that the instrument and probe are in a predetermined relative orientation to facilitate interconnection of the terminals. 
         [0014]    In the harsh surroundings often encountered when using construction materials, there is a danger that terminals of a probe may not make proper connections with terminals of an external instrument. This may be because deposits of hardenable material prevents proper fitting of the instrument to the probe, or because the material is deposited on the terminals themselves, preventing proper electrical connection. One possible solution to this problem would be to detect whether the measurement produced using the probe is indicative of a poor connection. However, it can be difficult to distinguish between such measurements and genuine measurements encountered in particular states of the hardenable material. This is especially the case when the measurement probe is designed to produce a wide variety of outputs depending upon the characteristics of the material, which is generally desirable to facilitate distinguishing between different conditions of the material. 
         [0015]    According to a further aspect of the invention, the sensor circuit of a probe for measuring the characteristics of a hardenable material has terminals for connection to an external instrument. Two terminals of the sensor circuit are interconnected by a switch which can be operated by the external instrument when it is connected to the probe. In this way, the external instrument can detect that it is correctly coupled to the terminals of the sensor circuit. Preferably, the switch is operated by applying a signal to one of the terminals across which it is connected, although if desired a third terminal could be provided to carry a signal controlling the switch operation. 
         [0016]    Preferably, the two terminals across which the switch is connected are also connected to a sensor device with characteristics dependent upon the characteristics of hardenable material in proximity to the sensor device. Preferably, the sensor device carries a high frequency signal which is influenced by the characteristics of the hardenable material. Preferably, at least one of the terminals is connected to the sensor device by a capacitor which conducts the high frequency signal, but which blocks a DC signal used to operate the switch. The switch may for example be a PIN diode which is closed in response to a DC current applied therethrough, and which has response characteristics such that it cannot be operated in response to the high frequency signal used to operate the sensor device. 
         [0017]    In accordance with a further aspect of the invention, the sensor circuit of a probe for measuring the characteristics of a hardenable material comprises a resonator which, in use of the probe, is embedded in the hardenable material, and which is formed by conductive portions of a printed circuit board. 
         [0018]    Although resonators have been used in the past for measuring characteristics of concrete being mixed in a mixer, the resonators have been bulky and expensive devices and therefore unsuitable for use as a consumable probe embedded in concrete. However, by forming the resonator out of the conductive portions of a printed circuit board, it has been found that a resonator of adequate characteristics can be made sufficiently inexpensively to make it commercially attractive to use as an embedded, consumable sensor probe. Appropriate operation of the resonator can produce material measurements which are significantly better, particularly in terms of the signal-to-noise ratio, than non-resonator embedded sensors. 
         [0019]    The resonator characteristics can be enhanced by forming gaps in the printed circuit board within which, in use, the hardenable material is situated. For a given area of the conductive portions, the gaps produce a larger region over which the hardenable material is in intimate contact with the edges of the conductive portion. It is believed that this reduces the dependency of the sensor on the distribution of the components (e.g. the aggregate masses) of the surrounding material, and thus reduces the variability of the measurements. The gaps may be of any desired shape, such as circular, but better results are achieved if the gaps are in the form of elongate slots. 
         [0020]    Preferably, the resonator is formed by conductive portions each disposed on a respective side of the printed circuit board. 
         [0021]    In accordance with a still further aspect of the invention, there is provided a method of measuring characteristics of hardenable material using a resonator comprising detector elements embedded in the material, in which the resonator circuit is driven by a predetermined frequency which is known to differ from the resonant frequency of the resonator. The drive signal is applied to the resonator to allow the taking of a reading dependent on the impedance of the resonator at the predetermined frequency. 
         [0022]    The resonant frequency of the resonator will depend on the characteristics of the surrounding material, which influence the complex impedance of the resonator circuit. The drive signal is influenced in a measurable manner dependent upon the resonator impedance. The non-linearity of the frequency response curve of the resonator means that relatively small changes in material characteristics can produce a significant difference in the measured effect of the impedance on the drive signal. 
         [0023]    Preferably, the measurement technique involves taking two readings, one of them a measurement reading influenced by the characteristics of the resonator, and the other being a reference reading taken with the resonator switched out of circuit and indicative of the power level (e.g. the amplitude) of the drive signal. Both readings are used to derive a material characteristic measurement value. This reduces the influence of component tolerances, measurement conditions, etc. on the resulting derived value. Instead of requiring a power-level reading, with sufficiently close-tolerance components it may be possible to generate drive signals of predetermined power without the need for reference measurements. 
         [0024]    In a more preferred arrangement, the measurement-taking technique involves taking two measurements of the influence of the resonator on the drive signal, each measurement being taken when the drive signal has a respective different power level. The derived output value is dependent upon the relationship between these two measurements. It has been found that such an arrangement can significantly reduce the variability of the output value. 
         [0025]    In a technique according to a preferred embodiment of the invention, measurements are taken at multiple different predetermined frequencies, rather than a single frequency. All the frequencies are selected to lie on one side (i.e. above or below) of the resonant frequency, and the measurements are then combined (e.g. by summing or averaging). By spreading the measurements over a band of frequencies it is possible to mitigate the effect of erroneous measurements due to interference at a particular frequency, and it becomes easier to meet regulatory requirements regarding emission characteristics. 
         [0026]    Although each of the aspects mentioned above is independently advantageous and can be used separately, certain additional advantages can be achieved by combining some or all of these aspects. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    An arrangement embodying the invention will now be described by way of example with reference to the accompanying drawings. 
           [0028]      FIG. 1  is a schematic section view of a prior art probe for measuring the hardness of concrete. 
           [0029]      FIG. 2  is a perspective view of a measurement probe according to the present invention, supported on formwork and coupled to a measurement instrument. 
           [0030]      FIG. 3  is a further perspective view, showing the measurement probe from the proximal end thereof. 
           [0031]      FIG. 4  comprises  FIGS. 4A and 4B , which are respectively a side elevation of the probe and a retainer attached thereto, and a cross section through line A-A of the side elevation. 
           [0032]      FIG. 5  is a perspective view of the measuring instrument of  FIG. 2 . 
           [0033]      FIG. 6  is a cross section through the measurement probe and a different retainer. 
           [0034]      FIG. 7  is an enlarged view of part of the measurement probe and retainer shown in  FIG. 6 . 
           [0035]      FIG. 8  schematically shows the circuit of the measurement instrument and measurement probe. 
           [0036]      FIG. 9  is a graph illustrating the response characteristics of the measurement probe. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0037]    Referring to  FIGS. 2 to 4 , a measurement probe  20  comprises a hollow, generally cylindrical cup-shaped body  22  which is open at its proximal end and which has a closed, distal end  23  from which a sensor device  24  projects. 
         [0038]    The probe  20  is intended to be embedded into concrete and can be supported in a number of different ways (as will be described below). In the mode of operation shown in  FIGS. 2 and 4 , the probe  20  is held on the inside of a mould wall formed by formwork  8 , using a retainer  26  mounted on the outside of the formwork  8 , but extending through an aperture in the formwork into the body  22  of the probe  20 . 
         [0039]    In the illustrated preferred embodiment, the body  22  of the probe  20  has a threaded inner cylindrical surface  28 , for cooperating with the threaded outer cylindrical surface of an extension  30  of the retainer  26 . At the proximal end of the cylindrical surface  28 , the inner surface has a enlarged-diameter portion forming a recess surrounded by a lip  31  for accommodating a resilient O-ring  32  (see  FIG. 4B ). 
         [0040]    The probe is mounted by fitting the O-ring  32  into its proximal end, holding the proximal end against a hole formed in the formwork  8  and then inserting the extension  30  of the retainer  26  through the hole and screwing it into the body  22  of the probe  20 . The threaded extension  30  of the retainer  26  ends in a shoulder  34  of an enlarged portion  36  of the retainer  26 . When the retainer  26  has been fully screwed into position, the assembly comprising the retainer  26  and the probe  20  is held firmly in position within the hole in the formwork, with the shoulder  34  abutting an annular region around the hole on the outside of the formwork  8  and the proximal end of the body  22  engaging an annular region around the hole on the inside of the formwork. 
         [0041]    Concrete can then be poured into the formwork in order to embed the probe  20 . Although significant internal pressures can be created, particularly if the probe  20  is mounted at the bottom of a large volume of concrete, the sealing ring  32  effectively prevents egress of the material through the hole in the formwork  8 . Also, after removal of the formwork  8 , the lip  31  will be disposed beneath the level of the surface of the concrete, so that the probe is completely submerged, as is often desirable. 
         [0042]    The extension  30  of the retainer  26  and the threaded inner cylindrical bore  28  of the probe  20  extend over substantially large axial distances so as to accommodate formwork  8  of a large range of wall thicknesses. Other retainers  26 , of different dimensions, could be provided if desired to accommodate still further thicknesses. 
         [0043]    The sensor device  24  comprises a printed circuit board  33  which is held by a support formation  38  formed by moulded parts of the base  23  of the body  22  of the probe  20 . The printed circuit board  33  extends inwardly of the base and has conductive portions (not shown) formed at a proximal end  39  to constitute terminals of the sensor device  24 . The board also supports circuit components (not shown) as will be described in more detail below. The base  23  of the body  20  has, on the interior of the body, a locating structure  41  which extends proximally from the base  23  to a distance slightly greater than the end  39  of the printed circuit board. 
         [0044]    A portable measurement instrument  42  (see also  FIG. 5 ) can be fitted to the probe by inserting it through the retainer  26  and the body  22  of the probe. A locating recess  44  can be fitted on to locating projection  41  by twisting the generally-cylindrical measurement instrument  42  about its axis until the locating projection  41  interengages with the locating recess  44 . (Obviously the projection and recess could instead be carried by the instrument  42  and probe  20 , respectively.) Further axial movement of the measurement instrument towards the probe causes the proximal end  39  of the printed circuit board to be inserted into a circuit board connector  46  of the measurement instrument in order to connect terminals (not shown) of the measurement instrument to the terminals of the sensor device. 
         [0045]    It is not always easy to access the outside of the formwork  8  at positions at which probes are mounted. If desired, the measurement instrument  42  may be mounted on the distal end of a tubular extension by means of which it can be held at a position relatively distant from the formwork but nevertheless inserted through the retainer  26  into cooperation with the probe. To aid this, the enlarged part  36  of the retainer  26  is preferably provided with an inner cylindrical threaded surface  48  by means of which a supporting tube can be threadably engaged with the retainer  26 . The supporting tube thus guides the measurement instrument  42  as it is pushed towards and through the retainer  26 . 
         [0046]    The arrangement described above requires the probe  20  to be held on the inside of the formwork  8  in order to fix the probe into position. However, there may be circumstances in which there is no ready access to the interior of the formwork at the location where the probe is to be mounted. In this case, a different retainer, as shown in  FIG. 6 , is used. 
         [0047]    In the arrangement of  FIG. 6 , the formwork  8  is provided with a hole which is sufficiently large that the probe  20  can pass through the hole. Prior to this, the probe is mounted on a retainer  50  having a main tubular portion  52  which is threaded on its outer distal end  53  for cooperation with the inner threaded surface  28  of the probe  20 . The retainer  50  also carries an annular resilient member  54  around its distal end, and a locking sleeve  56  around the main body portion  52 , and located proximally with respect to the member  54 . The sleeve  56  has an inner threaded surface engaging an outer threaded surface of the main portion  52  of the retainer  50 , so that rotation of the sleeve  56  with respect to the main body portion  52  causes the sleeve  56  and body  52  to move axially relative to each other. 
         [0048]    The annular member  54  has an enlarged-diameter portion  58  with an outer diameter larger than the diameter of the hole in the formwork  8 . The member  54  also has a small diameter portion  60  which extends axially from the enlarged diameter portion  58  towards the distal end of the retainer  50 . As shown more clearly in  FIG. 7 , the distal end of the small diameter portion  60  has a reduced diameter annular projection  62  which locates under the lip  31  of the probe body  22 , which in the arrangement of  FIG. 4  accommodates the O-ring  32 . An intermediate portion of the small-diameter extension  60  has an enlarged inner circumference to form a thin wall  64 . 
         [0049]    In operation, the distal end of the retainer  50  with the probe  20  attached thereto is inserted through the hole in the formwork  8  until the enlarged portion  58  of the member  54  engages the formwork. The sleeve  56  is then rotated relative to the main body  52  so that the member  54  is compressed between the sleeve  56  and the probe body  22 . The compressive forces applied to the small diameter extension  60  of the member  54  will cause the extension  60  to buckle outwardly in the area of the thin wall  64  so that the inner circumference of the hole in the formwork  8  is gripped by the outward expansion of the extension  60 . The retainer  50  and attached probe  20  are thus firmly held in position in the hole. The engagement of the annular projection  62  under the lip  31  of the body  22  ensures that the member  54  does not slip over the outside of the body  22  during this operation. It will be noted also that the member  54  has a proximally-extending extension  66  located under a lip  68  of the sleeve  56 , to ensure that the member  54  does not slip over the outside of the sleeve  56 . 
         [0050]    In this embodiment the annular member  54  used for holding the probe and retainer in position serves also the additional function of sealing the hole in the formwork  8 . 
         [0051]    The retainer can also be used to advantage when the interior of the formwork  8  is accessible. In this case, the orientation of the member  54  may be reversed, so that the enlarged diameter portion  54  is on the inside of the formwork  8 . This provides an even more effective seal against egress of concrete through the hole in the formwork. 
         [0052]    It will be appreciated that the frictional engagement of the member  54  with, on the one hand, the sleeve  56  and, on the other hand, the probe body  22  could cause the rotational forces applied to the sleeve  56  to be transmitted to the body  22 . It is desirable to prevent the possibility that this will cause the body  22  to be unscrewed from the end of the retainer  50 . Accordingly, it is preferred that the cooperating threads on the inner surface  28  of the body  22  and the outer cylindrical surface of the main retainer portion  52  be threaded in the opposite sense from the cooperating threads on the inner cylindrical surface of the sleeve  56  and the outer cylindrical surface of the main retainer portion  52 . For example, the former cooperating threads may be right-hand threads, and the latter left-hand threads. Accordingly, any rotational movement transmitted from the sleeve  56  as the sleeve tightens against the member  54  would, if transmitted to the body  22 , cause additional tightening of the body on to the end of the retainer  50 . 
         [0053]    The retainers mentioned above, and the probe body, may for example be made from moulded plastics material or rubber. 
         [0054]    The sensor device  24  of the probe  20  includes a resonator formed by a conductive portion  72  on one side of the printed circuit board at its distal end and a conductive portion  74  on the other side (see  FIGS. 2 to 4 ). In this embodiment, the portions are of the same shape and extend over the same area. Two elongate gaps  76  extend through the printed circuit board to permit more intimate contact of the concrete with the resonator element, and to extend the regions adjacent edges of the conductive portions. It is desirable to use gaps dimensioned to have a significant effect; for example the gaps should preferably occupy an area not less than 10% of the area bounded by the outer edges of the detector elements. 
         [0055]    Each conductive portion  72 ,  74  is connected via conductive traces on the printed circuit board to a respective terminal at the proximal end  39  of the printed circuit board. The circuit board also carries additional components connected to the terminals, these components being mounted on the portion of the printed circuit board held by the supporting structure  38 . This portion and the supporting structure may be sealed, for example using heat shrinkable plastics material or resin. 
         [0056]      FIG. 8  is a schematic circuit diagram of the measurement instrument  80  and the sensor circuit  90 , the resonator forming part of the sensor circuit  90  having an equivalent circuit as shown at  100 . The equivalent circuit comprises a pair of resistors  102 ,  104  in series, with a series-connected inductance  106  and capacitance  108  connected across the resistor  104 , thus forming a resonant circuit. The various component values of the equivalent circuit will vary depending upon the characteristics of the material surrounding the resonator. 
         [0057]    The terminals of the sensor circuit include a power supply terminal (not shown), and a pair of input/output terminals  110 ,  112  connected to the resonator  100  by a series capacitor  114 , although only a single input/output terminal could be used, if desired. There are also provided one or more further terminals  116  connected to a memory device  118  supported by the printed circuit board, and one or more ground terminals  120 . 
         [0058]    The sensor circuit  90  also includes a PIN diode  122  connected between the input/output terminals  110 ,  112  and the ground terminal  120 . The PIN diode  122  can be operated to function as a switch, effectively shorting out the resonator  100 . 
         [0059]    The sensor circuit  80  comprises a microprocessor  140  controlling measurement operations. One output  141  of the microprocessor controls the frequency at which a voltage controlled oscillator (VCO)  142  operates. The output of the VCO  142  is delivered to a microprocessor input terminal  143 , and, via an adjustable attenuator  144  and a switch  146 , to the input terminal  110  of the sensor device  90 . This enables a drive signal of predetermined frequency and amplitude to be applied via the capacitor  114  to the resonator  100 . The resonator will influence the amplitude of the drive signal. This can be sensed via the output terminal  112 , which is coupled via a switch  148  to the input of a peak detector (or envelope detector)  150 . The output of the peak detector  150  is delivered to an input  151  of the microprocessor  140 , which has internal circuits for determining the amplitude of the signal input thereto. 
         [0060]    With the switches  146  and  148  open, the output of the attenuator  144  can be presented, by closing a switch  152 , to the input of the peak detector  150 . In this way, the sensor circuit  90  can be disconnected and a reference measurement made by the microprocessor  140 . 
         [0061]    The microprocessor controls the switches  146 ,  148  and  152  using control signals (not shown) in a manner well known per se. The microprocessor can also deliver a variable amplitude signal from output  153  via a resistor  154  and an inductance  156  to the attenuator  144 , in order to control the level of the signal at the output of the attenuator. The attenuator comprises a series resistor  158 , the output of which is connected to ground via a circuit comprising a inductor  160  in parallel with a series-connected PIN diode and capacitance  162  and  164 , respectively. The conductivity of the PIN diode  162  is controlled by the DC level of the signal received from the microprocessor via the resistor  154  and inductor  156 , which is coupled to the junction between the PIN diode  162  and the capacitor  164 . The inductor  156  blocks the high frequency output of the VCO, so as to avoid influencing the high-frequency measurements made using the peak detector  150 . 
         [0062]    The microprocessor can also, using an output terminal  165 , apply a DC voltage via a resistor  166  and an inductance  168  to the input terminal  110  and thus the PIN diode  122 . This causes the PIN diode  122  to form an effective short between the input terminal  110  and the ground terminal  120 . The inductance  168  prevents this arrangement from having any significant effect on the high frequency signal received by the peak detector  150 . 
         [0063]    The microprocessor also has one or more input/output terminals  167  connected to the sensor terminal(s)  116  and thus the memory device  118  to permit data to be read from and/or written to the memory device  118 . 
         [0064]    The operation of the circuit will be described below. First, however, reference is directed to  FIG. 9 , which is a graph illustrating the effect of frequency on the amplitude of the drive signal as measured using the peak detector  150 . The broken line shows the amplitude/frequency response curve for the sensor circuit when the resonator is surrounded by air. It will be seen that the minimum level of the amplitude (which occurs when the impedance of the resonator is at its minimum value) is at approximately 100 MHz corresponding to the resonant frequency of the resonator in those conditions. However, in wet concrete, the resonant frequency decreases to around 50 MHz. Because of the use of a resonator, there is a significant variation in amplitude with frequency. Furthermore, there is a significant change in the response curve depending upon the characteristics of the surrounding materials. Accordingly, there is a large difference between the amplitude values found, for example, in a range R of 28 to 40 MHz, as shown in  FIG. 9 , depending upon the material characteristics. Therefore, an indication of the material characteristics can be obtained by taking an amplitude measurement within this frequency range. 
         [0065]    Apparatus of the present invention can be used to measure various characteristics, including the hydration state of the material, which is an important indicator of concrete hardness. The measurement of the amplitude of the drive signal within the frequency range R mentioned above would give an indication of the hydration state. The amplitude varies as a function of the strength of the signal applied thereto, but this variability can be taken into account by using a reference value corresponding to the strength of the signal. In practice, it is preferred to derive this value by measuring the output of the attenuator  144 , although in theory, with sufficiently close-tolerance components, it would instead be possible for the microprocessor to control the attenuator to generate a predetermined output without such measurement. 
         [0066]    However, the measurement would also be influenced by a number of other factors including component values, temperature, etc. It has, however, been found that although a given measurement could represent different hydration states, the extent to which the amplitude varies with respect to the strength of the signal applied to the resonant circuit is a relatively reliable indicator of the hydration state. Thus, it would be desirable to derive a characteristic-representing value measurement which depends on the relationship between the measured amplitude of the signal at different power levels. 
         [0067]    As indicated above, it is desirable to take a reading at a frequency within, for example, the frequency range R indicated in  FIG. 9 . In a preferred embodiment of the invention, however, readings are taken at multiple different frequencies to derive a measurement value. In this way, it is not necessary to concentrate the power applied to the resonator at a particular frequency, thus rendering it easier to avoid problems due to regulatory emission requirements and also avoiding potential errors if particular frequencies are subject to interference or otherwise give anomalous results. 
         [0068]    Taking into consideration the above factors, the measurement instrument of  FIG. 8  is arranged to operate in the following way in order to (a) take into account the level of the applied drive signal when reading the effect of the resonator; (b) take multiple measurements at different frequencies; and (c) take (at least) two measurements for each frequency at different levels of the drive signal, in order to provide a reliable value indicative of characteristics of the construction material. 
         [0069]    First, the microprocessor  140  is operable to ensure that the switch  152  is opened and that the switches  146  and  148  are closed. Then, the microprocessor applies a DC voltage to the PIN diode  122  via the resistor  166  and the inductor  168 . At that point, a measurement is taken of the output of the peak detector  150 . Because the DC voltage acts to drive the PIN diode  122  into conduction, the output of the attenuator  144  is effectively shorted. The capacitor  114  prevents the resonator  100  from adversely influencing this operation. Accordingly, the output of the peak detector  150  should be measured to be substantially zero (ground level). Assuming that the measured level differs significantly from that level, then it is determined that there may be a fault in the connections between the measurement instrument  80  and the sensor circuit  90 , and the microprocessor generates an error signal. 
         [0070]    Otherwise, the DC voltage is removed, to open the switch formed by the PIN diode  122 . Then, the switches  146  and  148  are opened, and the switch  152  closed. The microprocessor controls the voltage controlled oscillator  142  to generate a first frequency, of for example 28 MHz. This can be achieved by measuring the frequency of the oscillator at input  143  and varying the voltage at output terminal  141  until the correct frequency is obtained. The output of the attenuator is measured using the peak detector  150 , and varied by altering the voltage applied to the attenuator  144  via the resistor  154  and inductor  156  until the attenuator output reaches a first predetermined voltage. 
         [0071]    At that time, the switch  152  is opened and the switches  146  and  148  are closed. This will cause a drive signal of the predetermined voltage and the desired frequency to be applied via the input terminal  110  and the capacitor  114  to the resonator  100 . The resonator  100  will influence the amplitude of the drive signal to an extent dependent on the characteristics of the surrounding material. The output voltage is presented via output terminal  112  and switch  148  to the input of the peak detector  150 , and is thereby measured by the microprocessor  140 . 
         [0072]    This operation is repeated after controlling the VCO to adopt a second frequency, thereby obtaining a second measurement with the attenuator output set to the first predetermined level. The operation continues until 32 measurements have been made, each measurement being made at a respective different frequency within the range 28 to 40 MHz. 
         [0073]    These measurements are then combined. In the preferred embodiment, this is achieved by summing the measurements, although other techniques could be used. For example, a weighted sum could be used, with higher-frequencies being given a greater weight, in view of the fact that (as shown in  FIG. 9 ) frequencies closer to the resonant frequency exhibit bigger changes as a result of changes in material characteristics. 
         [0074]    The microprocessor then opens the switches  146  and  148  and closes the switch  152 , and thereafter controls the voltage control oscillator  142  and the attenuator  144  until the output of the oscillator adopts the first predetermined frequency again, and the output of the attenuator adopts a second predetermined level which is greater than the first predetermined level. The operation proceeds as above in order to derive  32  more measurements, at the higher output level of the attenuator, these measurements then being combined as before. 
         [0075]    Accordingly, at the end of this operation, the microprocessor derives two material measurements, M 1  and M 2 , each formed by combining 32 individual measurements at different frequencies, and each corresponding to a respective different reference level R 1  and R 2 . 
         [0076]    The microprocessor then derives an output value V=(M 1 −M 2 )/(R 1 −R 2 ). If desired, an additional temperature compensation factor can be applied to the resulting value. 
         [0077]    The output value is representative of characteristics related to the hardness of the material in which the probe is embedded. These readings can be repeated whenever desired, for example to determine when the formwork  8  can be removed, and/or to predict the ultimate strength of the concrete. 
         [0078]    The measurements may be output via a port, for example using a USB cable and plug  190  (see  FIGS. 2 and 5 ). Additionally, the measurements could be stored in the memory device  118  carried by the probe  20 . The memory device  118  may also or alternatively store a unique identification number associated with the probe. 
         [0079]    Various changes can be made to the arrangement described above. For example, although the preferred embodiment takes readings at two different levels of the attenuator, more than two levels could be used if desired. The level of the attenuator could be adjusted each time the frequency is changed. The multiple frequencies could instead be applied concurrently, so separate readings of different frequencies are not needed. Instead of using multiple frequencies, a single frequency could be used. The frequency or frequencies used for measurement could be all located either below or above the range of resonant frequencies likely to be encountered in practice. 
         [0080]    Different measurement techniques could be used. For example, the attenuator could be a constant current source or sink capable of operation at different current levels, under the control of the microprocessor. Alternatively, the attenuator could be replaced by a circuit which is adjusted, with the resonator  100  in circuit, until the voltage across the resonator reaches a predetermined level; in this case, a reading is taken either by detecting the level of current flowing through the resonator or by measuring the level of the control signal applied to the attenuator. 
         [0081]    The switch formed by the PIN diode  122  could additionally or alternatively be used in place of the switches  146 ,  148  and  152  for permitting reference readings to be taken while the resonator  100  is switched out of circuit. 
         [0082]    Various techniques have been described for mounting the probe  20  into a mould wall, such as formwork used to cast concrete. In some circumstances, it may be desirable to measure the properties of the concrete at a top surface thereof. In that case, there may be no formwork in the required location. In that event, the probe  20  could be fixed below a large-area sheet of, for example, wood, which would float on the surface of the concrete to enable the probe to be held in the right location.