the present invention provides a method for measuring any parameter of interest in a particular sample using an eddy current probe . for example , the probe may be used to measure hydrogen concentration in a piece or section of metal , such as the hydrogen concentration in the cladding of a nuclear fuel rod . generally , the eddy current probe is calibrated using a calibration standard that provides varying amounts of the parameter of interest in a material or substrate that represents the sample to be measured . for example , the calibration standard may be a piece of metal having varying hydrogen concentrations similar to the piece of metal or sample to be measured . the calibrated probe is then used to measure the parameter of interest in a given sample . fig1 is a process flow diagram according to one embodiment of the present invention . fig1 provides an overview of one embodiment of a process that may be followed to obtain a measurement of hydrogen concentration in metal . the process is described in the context of measurement of hydrogen in nuclear fuel rod claddings . in this embodiment , the fuel rods will preferably be located in a fuel storage pool , which is filled with water . however , the present invention is not limited to either the measurement of hydrogen concentration or to nuclear fuel rods , nor is it limited to use in aqueous systems or any other type of environment . the invention is also discussed in a certain order of operation . the order of operation is not critical , and the invention is not limited to the order presented . at step 110 , a probe attached to an eddy current measurement device is brought into contact with a calibration standard . the eddy current probe used in this invention is preferably a surface probe . the probe should contact the surface in a reproducible manner to prevent a decrease in sensitivity due to inadequate contact with the surface . preferably , contacting the surface in a reproducible manner is accomplished through the use of spring - loading , which provides constant pressure sensor - to - surface contact . additionally , the probe will preferably be oriented perpendicular to the surface of the calibration standard with little or no variation from the perpendicular direction . also , the area ( s ) selected for measurement should be as similar as possible in geometry to each other . differences in geometry such as curves , edges or other types of discontinuities may affect eddy current response and lead to errors in concentration measurement . in addition to geometric or positioning sources of error , there are other typical material conditions that may lead to variable eddy current response that may obscure the response from the hydrogen concentration . typical sources of background include : 1 ) variation in zirconium oxide thickness , ( 2 ) ferromagnetic and non ferromagnetic crud thickness , ( 3 ) variation in the crud magnetic permeability , ( 4 ) variation in hydrogen concentration across the cladding wall , and ( 5 ) variation in cladding temperature . these material conditions should be accounted for when preparing the calibration samples . these factors may be accounted for by using a calibration standard that replicates as closely as possible whichever of these factors will be present in the actual sample to be measured . at step 115 , the calibration standard is measured at a predetermined number of frequencies and locations , where each location contains a known level of the parameter of interest and representative values of the background variables . as discussed in more detail below , the calibration standard would have varying levels of the parameter of interest extending beyond the range expected in the in - situ measurements , as well as having representative levels of background variables that may affect the measurement of the sample . when measured , the eddy current probe response is dependent upon the electrical conductivity of the calibration standard , which , in turn , is dependent upon the level of the parameter of interest in the standard , such as hydrogen concentration . fig2 is a schematic diagram of a movable probe mount and a calibration standard according to an embodiment of the invention . as shown in fig2 , to perform the calibration in this embodiment the eddy current probe 200 is brought into contact with the surface of the calibration standard 230 . the probe 200 is held using a mount 210 that allows the probe 200 to traverse the length of the calibration standard 230 . the mount 210 further includes a mode of automatically traversing the calibration standard , such as wheels 220 powered by an electric motor ( not shown ). the purpose of the mount 210 is to allow reproducible sample - to - probe contact pressure , as well as to allow the probe 200 to be moved along the length of the sample and calibration standard 230 at a reproducible rate . the probe response from the calibration standard 230 is measured as the probe 200 moves along the standard . in connection with the present embodiment , calibration standards are , in general , fabricated to duplicate the size , shape , and construction material of the fuel rod measured as closely as possible . in the present embodiment , the calibration standards should contain varying amounts of hydrogen , preferably encompassing the concentration range of hydrogen expected , with the exception that the standard contains no fissionable material . for instance , if the highest concentration of hydrogen expected is approximately 500 parts per million ( ppm ), the highest concentration standard should contain at least this concentration . preferably , the concentration of the highest standard should be at least 25 percent more than the highest concentration expected . the distribution of the varying parameter levels in each calibration standard point is determined by routine destructive and nondestructive techniques capable of measuring hydrogen concentration , e . g ., hot vacuum extraction , and imaging hydride platelets within the cladding wall , e . g ., metallography . fuel rod cladding is often constructed of zircaloy ™- 4 in a tubular shape . therefore , in a preferred embodiment , the calibration standard should be constructed of zircaloy ™- 4 material , in a tubular configuration , of approximately the same dimension as the fuel rods . typically the cladding wall thickness is the order of 0 . 6 mm . hydrogen - charged samples are prepared by any of a number of methods known to the art , such as electrochemical deposition or elevated temperature exposure of the metal standard to flowing hydrogen gas . in a preferred embodiment , the samples are prepared by flowing hydrogen gas in combination with thermal cycling . the distribution of hydrogen across the calibration standard is controlled through the application of differing temperature gradients across the standard . alternatively , the standards may be prepared by flowing hydrogen across the standard , then subsequently applying heat in the manner described above . the calibration standard may also be used for temperature calibration . such a calibration standard would include a source of heat to produce the same temperature and possibly temperature gradient across the calibration standard . in an embodiment , the heat source is an internal electrical heater rod . however , any other type of suitable heat source may be used , such as a heat exchange coil or circulating water bath or heating tape attached to the calibration standard . in practice , however , it is often not necessary to perform a rigorous temperature calibration for each set of measurements . often , it is possible to quantify the relationship between cladding temperature and eddy current response in a separate activity , such as a laboratory environment . any temperature gradient through the cladding wall can often be accounted for analytically , or , if sufficiently small , ignored . often , only one temperature calibration measurement is performed at the od of the fuel rod cladding . one method of measuring temperature at the od would be using a thermocouple , for instance . other variables may be generated in the calibration standards using a number of different methods known to the art . for example , zro 2 overlayers may be produced by flowing a mixture of helium and oxygen over the od of the calibration standard while holding the standard at elevated temperatures , and magnetic shims or magnetic tapes of varying thicknesses may be used to simulate the magnetic crud overlayers . the minimum number of frequencies to be measured at each point along the calibration standard is dependent upon the number of significant independent background variables . each data point collected at each frequency is composed of two components , a resistance component and a reactance component ( in some measurement instruments the in - phase component and the quadrature component , or the amplitude and phase angle of the response ). therefore , when n different frequencies are measured , 2n sets of data result . generally the method can suppress the effects of 2n − 1 background variables . thus if there are three background variables , two frequencies are required . if there are five background variables , three frequencies are required . the greater the number of background variables to be suppressed the higher the data precision required . in one embodiment , three background variables are assumed to have significant effects on the eddy current response : zirconium oxide thickness , effective crud permeability and fuel rod temperature . thus the test must be performed at two frequencies or more , and variation in these parameters must be included in calibration data . with two frequencies there are five first order constants . very high test frequencies are required to reduce the sensitivity to cladding wall thickness and to increase the sensitivity to the high hydrogen concentrated near the od . the penetration depth at a given frequency is given by the following relationship : here δ is the electromagnetic depth of penetration ( skin depth ) in mm , ρ is the electrical resistivity in micro ohm cm , f is the electromagnetic frequency in hz , and μ r is the relative magnetic permeability . in one embodiment , one frequency may be 25 mhz , and a second frequency may be 10 mhz . at these frequencies , the depths of penetration are the order of 0 . 085 mm and 0 . 135 mm , respectively , and only approximately 15 % and 25 % of the zircaloy ™ cladding wall is penetrated . an example calibration test matrix may include 10 or more test points . test matrices may be derived using design of experiment techniques or through other methods known in the art . below is a representative 15 point test matrix ( in this example , “ magnetic crud layers ” refers to a relative value proportional to the eddy current response obtained from measuring a crud layer of non - specified thickness and composition , i . e ., a value of “ 2 ” represents twice the measured eddy current probe response that a value of “ 1 ” does ): test point hydrogen zro 2 magnetic temperature number ( ppm ) ( microns ) crud layers (° c .) 1 0 0 0 30 2 300 0 0 30 3 600 0 0 30 4 0 0 0 40 5 300 100 0 40 6 600 0 0 40 7 0 50 0 30 8 300 0 1 30 9 600 100 0 30 10 0 100 2 40 11 300 0 2 40 12 600 50 1 40 13 0 0 0 30 14 300 50 1 30 15 600 0 2 30 returning to fig1 , at step 120 calibration constants are calculated based on the eddy current responses measured for the calibration standards . it should be appreciated that no mathematical model of the instrument , sensor , or the interaction of the sensor with the fuel rod and environment are assumed , nor is an iterative procedure used to determine the free parameters of a mathematical model . it is assumed that the physical parameter of interest ( for example , hydrogen concentration ) is dependent upon eddy - current response . to determine the calibration constants , a taylor series expansion around a nominal value of the parameter of interest is performed . a description of this approach is given in suresh yagnik et al ., eddy current measurements of corrosion and metal loss in zircalloy cladding with ferromagnetic crud , nuclear technology , vol . 147 , august 2004 , p 1 , which is incorporated by reference herein in its entirety . if only the first order expansion terms our included then the physical parameter of interest ( pp ) can be expressed as : pp = a + b 1 * x 1 + c 1 * r 1 + b 2 * x 2 + c 2 * r 2 + . . . + b n * x n + c n * r n , where a , b 1 , c 1 , . . . b n and c n are constants , x 1 and r 1 are the reactance and resistance at frequency 1 , x 2 and r 2 are the reactance and resistance at frequency 2 , and x n and r n are the reactance and resistance at frequency n . the constants can be determined if the detailed relationship between the physical parameter and the eddy current response parameters are known , or , as in a preferred embodiment , if a set of at least 2n + 1 measurements are made on the calibration standard . better results may be obtained if 4n or more measurements are made with a best fit used to determine the 2n + 1 calibration constants . this approach is rigorously correct for a small range of the physical parameter measured . for a larger range , higher order terms can be included in the expansion , or the expansion can be repeated at different nominal values . however , either of these approaches requires more calibration constants and calibration measurements . the calculation may be done remotely , using a desktop computer and commercially - available software packages . spreadsheet - based software packages such as microsoft excel ™, for instance , may be adequate for many applications . alternatively , the code may be resident in the data - handling system of the eddy current measurement device . at step 125 , once the calibration standards have been measured and the calibration constants have been calculated , the fuel rod sample is measured . in one embodiment , the outside surface temperatures of the cladding are measured using a thermocouple in contact with the outside of the fuel rod . the same sensor , eddy current instrument , cabling , test frequencies , voltage levels input to sensor , and measurement procedures as used in the calibration phase may be used . at step 130 , the parameter of interest , hydrogen concentration in this embodiment , is calculated using the calibration constants determined as described above . with the calculated constants and the values of r i and x i obtained from the sample measurements , the hydrogen concentration is calculated at each point of interest . the calculation may be done remotely using a desktop computer and commercially - available software packages . spreadsheet - based software packages , for instance , may be adequate for many applications . alternatively , the software code necessary to perform this calculation may be resident in the data - handling system of the eddy current measurement device and the hydrogen concentration calculated in real time . at step 135 , the calculated parameters of interest , in this embodiment hydrogen concentrations , are displayed . the display may take any form but typically comprises displaying the results on a computer screen or as a hard - copy printout . fig3 is a schematic diagram of a calibration standard 320 and a fuel rod 330 according to one embodiment of the invention . in this embodiment , the calibration standard 320 is immersed in a fuel storage tank ( not shown ) either adjacent to the tank used for the storage of nuclear fuel rods or preferably in the same tank as the one used for the storage of the nuclear fuel rods . the calibration standard 320 is held in a mechanical mount 325 . the mechanical mount 325 is further configured so that it can hold a nuclear fuel rod 330 to be measured . when a measurement is made , the nuclear fuel rod 330 is transferred using a transfer means ( not shown ) from its storage position into the mechanical mount 325 . the fuel rod 330 is positioned adjacent to and substantially in - line with the calibration standard 320 as shown in fig2 . an eddy current probe 310 is held in a movable probe mount 305 . the eddy current probe 310 is brought into contact with the calibration standard 320 . after the probe 310 is brought into contact with the surface of the calibration standard 320 , the probe 310 begins traversing down the length of the calibration standard 320 while simultaneously monitoring the eddy current response at the requisite number of frequencies . the translation rate along the calibration standard 320 , and ultimately the nuclear fuel rod 330 , in one embodiment is approximately 160 inches / minute , although any practical rate can be used depending on the nature of probe used and the configuration of the standard and sample . it should be noted that while calibration of the present invention is performed in - situ , the present method is not restricted to in - situ calibration . in the current embodiment , the outer surface temperature is read from a thermocouple 315 attached to the calibration standard 320 . if desired or necessary , temperature calibration measurements may be done alternatively in a laboratory setting . in yet another embodiment , after a sufficient number of calibrations have been performed on a given fuel rod type , a large enough database may exist to allow for calculation of the temperature effect , thus eliminating the need for a separate temperature measurement . once a sufficient number of measurements have been made on the calibration standard 320 , the eddy current probe 310 continues on to traverse the nuclear fuel rod 330 . less preferably , the calibration standard 320 may be removed using a remotely - controlled transfer means ( not shown ), and a nuclear fuel rod substituted for the calibration standard 320 in the rod mount 325 . once the nuclear fuel rod 330 is in place , the eddy current probe 310 is brought into contact with the surface of the nuclear fuel rod 330 , and the probe 310 is driven along the surface of the nuclear fuel rod 330 at the same rate used for the calibration . after completion of calibration and nuclear fuel rod data collection , calibration constants are calculated from the calibration measurements , and hydrogen concentrations for each sample point are calculated using the calibration constants and nuclear fuel rod data . after calculating the hydrogen concentrations , the results are displayed using whatever means are convenient and appropriate . possible means include digital meter readout , analog meter readout , computer display , or operator notation . typical units for display will be ppm hydrogen . although the present invention has been described with particular reference to its preferred embodiments , it should be understood that these embodiments are illustrative and that the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein . it is therefore evident that the particular embodiments disclosed above may be altered or modified in ways that such variations are considered within the scope and spirit of the invention . for example , the present invention may be utilized to measure other parameters that influence metal conductivity , such as temperature and microstructural damage from irradiation alone or in combination with parameters that influence the separation of the sensor from the metal surface , such as oxide and crud thickness . in these cases , the same methodology may be used by first calibrating the eddy current probe using a calibration standard that has known amounts of the parameter of interest and is also configured to similar the material or substrate against which the eddy current probe will be used . therefore , the scope of the invention should not be limited by the specific disclosure herein .