Abstract: The invention concerns a testing procedure for detecting nonuniformities in the wall thickness of inaccessible metallic pipes in which electrical alternating currents of different frequencies are passed through the pipe and the magnetic fields generated by them are measured outside the pipe. The invention further concerns a device, particularly for carrying out the testing procedure, comprising a power source for passing an alternating current through the pipe, at least one magnetic field probe for measuring outside the pipe the magnetic field generated by the alternating current, and equipment for the evaluation which deduces the presence of a nonuniformity in wall thickness from the magnetic field signal of the magnetic field probe.

Abstract: A method for operating an eddy current sensor (10) with a measuring coil (2) and an evaluation circuit (4) for determining material or geometric parameters of a test object (5), in which the test object (5) is arranged at a distance (d) from the measuring coil (2). The impedance of the measuring coil (2) is evaluated, while the measuring coil (2) is being supplied with an alternating voltage of a predetermined frequency, and the evaluation circuit (4) determines the material and geometric parameters of the test object (5) based on the impedance of the measuring coil (2). The impedance of the measuring coil (2) is determined at an alternating voltage of a first frequency, and the impedance of the measuring coil (2) is determined at an alternating voltage of a second frequency, and the evaluation circuit (4) computes the material and geometric parameters of the test object (5) on the basis of the impedances of the measuring coil (2) at the first and the second frequencies.

Abstract: The invention relates to a method which allows to compensate errors due to offset voltages and external magnetic fields when using a magnetic sensor for coating thickness measurement. Also a method is described which allows compensation of a temperature dependent change of the output voltage of the magnetic sensor.

Abstract: The present invention relates to a device for measuring the value of a parameter relating to an electrically conductive object. Such a parameter can be the electric conductivity or the degree in which the material has changed under the influence of external conditions. Alternatively the parameter is the quality of a weld or the thickness of the object.

Abstract: Measuring the wall thickness of an electrically conductive object using a probe comprising a transmitter coil, a first receiver and a second receiver, which method comprises arranging the probe near the object; inducing transient eddy currents in the object and recording the signals of the receivers with time; measuring the wall thickness from a characteristic of one of the signals; calculating a characteristic value from a combination of the signals; and correcting the measured wall thickness for the distance between the probe and the object using a pre-determined relationship between the wall thickness and the characteristic value for different values of the distance between the probe and the object.

Abstract: A method is provided for creating a spectral EM frequency to calculate the thickness of a material with unknown permeability and conductivity using metallic transparencies. The method comprises the steps of testing empirically to approximate the conductivity, testing empirically to approximate the permeability, creating a first set of electromagnetic waves adjacent to the material to be measured of a relatively low frequency, impinging the first set of electromagnetic waves on the material for saturating the material, creating a second set of electromagnetic waves having specific constant amplitude of a higher frequency than the first set of electromagnetic waves, the second set of electromagnetic waves for engaging the material and generating a sensing signal having modified characteristics, and receiving the sensing signal through the saturated material such that the modified characteristics of the sensing signal are processed to determine the thickness of the material.

Abstract: This invention pertains to a method and apparatus for determining the thickness of a ferromagnetic or paramagnetic material when only one side of the material is accessible. In one embodiment, the invention provides a method for engaging a constant signal with the ferromagnetic material for inducing a changed signal, generating a stepped saturation signal over a range of currents for engagement with the ferromagnetic material, detecting the changed signal as the saturation signal is varied over the range of currents, determining the relationship between the changed signal and the stepped saturation signal, and evaluating the thickness of the material based upon the relationship between the changed signal and the stepped saturation signal.

Abstract: A method for measuring the thickness of a layer of crud containing ferromagnetic material deposited on nuclear fuel rods using eddy current lift-off (coil-to-conductor spacing) measurement by exciting a probe coil at predetermined frequency which penetrates only into the layer of crud containing the ferromagnetic material.

Abstract: The invention relates to a method and an apparatus for measuring properties of a moving web, a sensor (1) being arranged on a first side of the moving web (7), the sensor comprising a differential coil (3), which comprises at least two coils, and a reference plate (4). A sensor plate (6) is arranged on a second side of the moving web (7). Electric current is used to induce a voltage to the differential coil (3). The voltage of the differential coil (3) varies as the web (7) changes the effect caused by the sensor plate (6) to the differential coil. The properties of the web (7) are determined by measuring the voltage of the differential coil (3).

Abstract: A method for the nondestructive measurement of the thickness of thin layers having a probe, having a first coil device on an inner core, the geometrical center of which coil device and the geometrical center of at least one second coil device coincide, the at least one second coil device partially surrounding the first coil device, and an evaluation unit, to which signals of the coil devices are emitted during a measurement for ascertaining the layer thickness. A circuit is provided, by which the first and the at least one second coil devices are excited sequentially during a measurement.

Abstract: A method for operating an eddy current sensor (10) with a measuring coil (2) and an evaluation circuit (4) for determining material or geometric parameters of a test object (5), in which the test object (5) is arranged at a distance (d) from the measuring coil (2). The impedance of the measuring coil (2) is evaluated, while the measuring coil (2) is being supplied with an alternating voltage of a predetermined frequency, and the evaluation circuit (4) determines the material and geometric parameters of the test object (5) based on the impedance of the measuring coil (2). The impedance of the measuring coil (2) is determined at an alternating voltage of a first frequency, and the impedance of the measuring coil (2) is determined at an alternating voltage of a second frequency, and the evaluation circuit (4) computes the material and geometric parameters of the test object (5) on the basis of the impedances of the measuring coil (2) at the first and the second frequencies.

Abstract: A measuring apparatus has a magnetizing head for magnetizing a magnetic layer of a specimen, and a detecting sensor for detecting a magnetization of a magnetized area which has been magnetized by the magnetizing head. The magnetization detected by the detecting sensor is supplied via a sensor amplifier to a controller. Based on the supplied magnetization, the controller determines the thickness of the magnetic layer from a magnetization to magnetic layer thickness conversion table or formula. The thickness of the magnetic layer can therefore be measured accurately in a short period of time.

Abstract: An apparatus for detecting and measuring axially extending defects in ferrous tube includes a magnetizing coil for inducing a longitudinal magnetic field in the ferrous tube. Linear magnetic transducers are used to detect parallel non-linking flux leakage that occurs from axially extending defects. An analog to digital converter digitizes the measured signals representing the amount of parallel non-linking flux leakage detected. A processor subtracts signals typical of ferrous tube with no axially extending defects from the measured signals. The processor further processes the resulting signals after subtraction, by separating the alternating AC components from the constant DC components to remove the effects of perpendicular flux leakage, which may exist in the measured signals. The processor then calculates the percentage of missing material due to the axially extending defects by applying a proportionality equation to the remaining DC components of the measured signals.

Abstract: A measuring gage for determining the thickness of a non-metallic material utilizes an inductive sensor positioned on one side of the material to be measured and a metallic object placed on the opposite side. The output of the sensor is used to determine the distance between the sensor and the metallic object, and hence the thickness of the material. The movement of the sensor and the metallic object can be computer controlled to map the thickness of the material along a predetermined path and produce a cross-sectional thickness profile. The output of the sensor can be used in conjunction with a computer controller responsible for adjusting the size of a score line to prepare an airbag deployment section in a vehicle trim panel.

Abstract: A pair of copper coils are embedded in the foil strip. A first coil of the pair generates an electromagnetic field that induces eddy currents on the surface, and the second coil carries a current influenced by the eddy currents on the surface. The currents in the second coil are analyzed to obtain information on the surface eddy currents. An eddy current probe has a metal housing having a tip that is covered by a flexible conductive foil strip. The foil strip is mounted on a deformable nose at the probe tip so that the strip and coils will conform to the surface to which they are applied.

Abstract: A non-contacting thickness measurement apparatus is described, which is capable of on-line, highly accurate web thickness measurements. The apparatus is comprised of a first and second distance determining means which calculate a distance value from each distance determining means to the web surface. The distance determining means are on opposite sides of the web, so each measures the distance to a different side of the web. At the same time, the distance between the two distance determining means is measured to produce a z-sensor spacing value. The two distance values are thereafter subtracted from the z-sensor spacing value to get a web thickness value. An x-y position adjustment means is used to keep the measurement spot on the web for the two distance determining means at the same location on the web, and several techniques are combined to eliminate temperature-induced and/or other unknown variations or drifts in the web thickness value which are not caused by actual changes in the thickness of the web.

Abstract: A device that can sense small changes in distance between opposed surfaces includes a ferromagnetic material and a giant magneto resistance effect sensor. A current source applies a current to the sensor to operate the sensor and read the magnetic field that impinges on the sensor from the ferromagnetic material. The magnetic field changes when the distance between the opposed surfaces changes. The device can be used in imaging apparatus to sense multifeeds and other conditions.

Abstract: Eddy currents are generated in an object constructed of a conductive material by transmitting an electromagnetic signal to the object and detecting electromagnetic signals generated by eddy currents induced in the object, and an electromagnetic signal V(t) is described by a product of two factors F and G(t), wherein F is a function of the geometry and electrical and magnetic properties of the material and G(t) is a function of geometry of the material, the electrical and magnetic properties of the material, the thickness perpendicular to the surface of the material, and time.

Abstract: A nondestructive method for determining the thickness of a metallic protective layer (2) on a metallic base material (1), characterized in that the thickness of the metallic protective layer (2) is determined by detection of a different type of layer (3) which is situated between the metallic protective layer (2) and the metallic base material (1), by ultrasound, eddy-current or (pulsed) thermography. Consideration may be given to a nonmetallic layer (3) of oxides or carbides, a metallic layer (3) of pure nickel or a layer (3) with depleted Al and Ti content in that region of the base material which is close to the surface. This method can be used for turbine blades which are provided with MCrAlY in order to determine the layer thickness, for example during the test phase of coating, and to avoid the known destruction of the turbine blade, which is labor- and cost-intensive.

Abstract: The invention relates to a method and apparatus for measuring the caliper of paper substantially continuously during papermaking. A sensor (3a, 3b) is arranged above and/or under the paper (2), the sensor (3a, 3b) measuring the caliper of the paper (2) at least on one side of the paper (2) mainly without contacting the surface of the paper (2). At given intervals the sensor (3a, 3b) measuring mainly without contact is brought into momentary contact with the surface of the paper (2).

Abstract: The defect detection system of the present invention includes an encircling coil for providing a saturating DC magnetic fields to tubular goods. Also included is an encircling drive coil for applying a low level AC field using three-phase AC. Encircling pick up coils within the AC drive coils detect uniform, time varying magnetic fields to reveal defects within the tubular goods passing through the system.

Abstract: A method and a device for non-contacting measurement of the measures and position of objects of electrically conductive materials based on electromagnetic induction is provided. A measuring object, a transmitter coil and a receiver coil are placed in relation to each other such that a time-varying magnetic field generated by the transmitter coil at a measuring region of the measuring object is substantially parallel to the surface of the measuring region. The receiver coil is placed such that a conceived field line which touches the measuring region in its extension reaches the receiver coil. On the basis of the voltage induced in the receiver coil, the position and dimensions of the measuring region relative to a given reference point can be calculated.

Abstract: A dispensing device employs a mechanical touch sensor. The mechanical touch sensor includes a touching level with a magnet that generates a magnetic field and a Hall generator that interacts with the magnetic field to generate an output corresponding to a thickness of the sheet material for verifying a correct transport of the sheet material.

Abstract: A device for non-contacting measurement of a measuring object is provided. The device includes at least one pair of associated coils, including a transmitter coil and a receiver coil. The transmitter coil generates a time varying magnetic field. In the receiver coil, a voltage is induced. The voltage is dependent on the location of the transmitter coil, the receiver coil and a measuring region of a measuring object. The device comprises a U-shaped structure with two branches. A retaining part is arranged between these branches. The transmitter and receiver coils are fixed to the branches of the U-shaped structure and the measuring object is intended to be arranged centrally in the opening of the U-shaped structure.

Abstract: A pair of elastic strips are provided in order to hold sheet material to be measured in thickness. Each of the elastic strips includes of one or more elastic films. Ends of the elastic strips are supported by support portions, and the other ends of the elastic strips are provided with respective coils, opposed to push each other, and kept in contact with each other. When the sheet material is held between support portions, the inductances of the coils are changed and the thickness of the sheet material is detected as an electric signal representative of the changed inductance.

Abstract: A caliper gauge for moving sheet material such as paper which measures the thickness of the paper accommodates bumps or holes in the paper by providing at least one side of the caliper gauge with a long continuous smooth arm which because of its mounting in close proximity to the moving sheet, which is partially made possible by the use of air bearings for the upper and lower portions of the gauge, provides a gradual wedge-shaped opening which thus allows the transfer of the momentum of the bump or hole in the paper to gradually overcome the inertia of the measuring arm which is pushed away from the paper. The arm itself is made of a single piece of sapphire and is universally pivoted by the use of a polyimide material adhered to the top of the arm which gives the arm at least 2.degree. of dimensional freedom while still maintaining it aligned in the moving direction of the paper.

Abstract: A preform test fixture for receiving a blow molding preform includes a structural base, a measuring device mount, a support arm, and a bar member having preform bores. The bar member is adjustable so that it translates up and down on the support arm, and is removably fastened thereto by a knob assembly screwed into a hole in the bar member. The system includes utilizing several bar members to enable the fixture to measure wall thicknesses of a virtually unlimited number of unique preform designs. The fixture also has a pivot device that enables the bar member to pivot on the support arm. The measuring device mount is adjustable on the base. The adjustment capability of the fixture enables uniform and repeatable wall thickness measurements, concentricity measurements at various locations, and preform tip wall thickness measurement proximate the preform gate.

Abstract: A system can detect leading and trailing edges of documents, and document thickness with a single transducer. The transducer operates to measure the displacement of an idler pinch roller by a passing document.

Abstract: An automatic measuring apparatus is capable of measuring the thickness of a work piece with a high degree of precision. Distance data for carriers is removed from distance data representative of the distances between a pair of surface plates which are detected in a non-contact manner. An eddy current sensor mounted on a first surface plate detects a distance therefrom to a second surface plate and generates a corresponding analog voltage signal. When the eddy current sensor enters and exits a space between two adjacent carriers, voltage signals are generated. The voltage signals are processed to determine the voltage value of a digital voltage signal that has a maximum frequency or a maximum number of occurrences among the voltage signals being processed. The resultant determination is used to arrive at the work piece thickness.

Abstract: A method and apparatus for detecting a loss in cross-sectional area of metallic reinforcing members having magnetic properties of a conductor indicating corrosion effects on the conductor. The method comprises providing a motive force to move a data collection component and a detector component along a length of conductor. The detector includes a rotating magnetic source spaced apart from an electronic coil winding. In use, a conductor is interpositioned between the magnetic source and electronic coil winding. The amount of magnetic field passing through the conductor induces a voltage in the coil inversely proportional to the cross-sectional area of the steel reinforcing strands of a conductor. The described apparatus can be used to determine the rate of loss of cross-sectional area for a conductor over a period of in use by measuring the cross-sectional areas of the metallic reinforcing members over an adequate time interval.

Abstract: An openable ring assembly is externally centered around and moved parallel to the centerline of a stationary pipe. As a low-frequency EM source signal is applied to the ring assembly, the time and/or frequency shifts of a detected EM signal from the source signal can be correlated to pipe wall thickness without removal of external pipe insulation and metal cladding, if present. The ring assembly may be opened or expandably segmented to allow pipe thickness detection around obstructions or larger diameter pipe. Centering is preferably accomplished by using rollers circumferentially attached to the ring assembly.

Abstract: A modular coating thickness gauge includes a probe which generates a signal representative of coating thickness, a PCMCIA card connected to the probe for converting the signal into a standard PCMCIA output format, and a portable computing unit for receiving the signal via the PCMCIA card. The gauge allows the on-site user to alternately record coating thickness measurement data and descriptive textual or graphical data relating to each coating thickness measurement.

Abstract: A thickness gauging instrument uses a flux focusing eddy current probe and two-point nonlinear calibration algorithm. The instrument is small and portable due to the simple interpretation and operational characteristics of the probe. A nonlinear interpolation scheme incorporated into the instrument enables a user to make highly accurate thickness measurements over a fairly wide calibration range from a single side of nonferromagnetic conductive metals. The instrument is very easy to use and can be calibrated quickly.

Abstract: A magnetochemical sensor for continuous and in-situ sensing of a given chemical species/stimuli includes two magnetically-soft magnetic film layers and a chemical transduction layer that shrinks or swells in the presence of that given chemical species/stimuli. The magnetic switching characteristics of the sensor are dependent upon the thickness of the chemical transduction layer. A method for remotely interrogating the magnetic switching characteristics of the sensor is also provided. In the method, magnetic flux detecting coils are utilized to monitor the sensor.

Abstract: A measuring device scanning from edge to edge of a continuously fed sheet includes two sensing heads spaced apart from the sheet by air cushions whose thickness is not influenced by pressure of supplied air and is subjected to an invariable approaching gravimetric force. The air cushions are preferably formed of air jets from calibrated holes at the inside of both sensing heads, each of said holes being surrounded by annular barriers with circular rims. While the approaching force applied to the upper head is given by the own weight of the latter, the corresponding opposed force on the lower head is provided by a pressure regulator of internal air at the inside of the lower sensing head.

Abstract: A modular coating thickness gauge includes a probe which generates a signal representative of coating thickness, a PCMCIA card connected to the probe for converting the signal into a standard PCMCIA output format, and a portable computing unit for receiving the signal via the PCMCIA card. The gauge allows the on-site user to alternately record coating thickness measurement data and descriptive textual or graphical data relating to each coating thickness measurement.

Abstract: A method of electromagnetically measuring the distance between adjacent tube elements in a heat exchanger. A cylindrical, high magnetic permeability ferrite slug is placed in the tube adjacent the spacing to be measured. A bobbin or annular coil type probe operated in the absolute mode is inserted into a second tube adjacent the spacing to be measured. From prior calibrations on the response of the eddy current coil, the signals from the coil, when sensing the presence of the ferrite slug, are used to determine the spacing between the tubes.

Abstract: The device has a single strip having a first end, a second end, a length and a width. The first end of the strip is curved toward the second end of the strip to form a loop having a height. The length is approximately 10 mm, the width is approximately 5-8 mm, and the height is approximately 0.8-1.2 mm. The loop is preferably fabricated from copper. The loop is mounted directly to a test instrument such as a computer controlled impedance analyzer or network analyzer. The test instrument measures the inductance and resistance of the loop with no thin film sample placed therein, and then measures the inductance and resistance of the loop containing the sample under test. From these measurements, the device ultimately derives the permeability of the sample under test.

Abstract: A system and method for determining fiber-depth of at least one layer of reinforcing fibers within an electrically conductive workpiece is provided. The system comprises an eddy current probe having one or more eddy current elements for generating a respective electrical signal indicative of a spacing with respect to a predetermined section of the reinforcing fibers. A signal processor is connected to the eddy current probe to receive and to process each spacing-indication electrical signal so as to generate data indicative of fiber-depth of the predetermined section relative to a workpiece surface. A display device may be connected to the signal processor to provide visual indicia representative of the fiber depth. Further a controller may be connected to receive and store the fiber-depth data which can be used by a machining device for selectively removing material from the workpiece surface, in accordance with the stored fiber-depth data.

Abstract: The invention is a method and apparatus for calibrating a gap sensing probe (12) in a refiner having first (76) and second (80) grinding plates facing each other and supported by respective first and second plate holders, each grinding plate having a face (78, 82) defining a grinding gap (86). The gap measuring device, which is secured within one of the plates, includes a sensor probe (12) for measuring gap width data having a substantially cylindrical tip (14) with a flat frontal portion (16), an attached conical portion (18) and a longitudinal axis (20) therebetween. A cable (22) is electrically connected to the conical portion of the probe, for transmitting the gap width data. A protective sleeve (24) can be positioned around and contact the cylindrical tip, wherein the flat frontal portion of the tip is exposed through the sleeve.

Abstract: A method and apparatus for measuring an anodic capacity of a thermally sprayed coating are described. Eddy current techniques are used to probe the coating and resulting RMS voltages are translated into anodic capacity indications which can be standardized or absolute. Recalibration can be achieved using single point measurement.

Abstract: The invention relates to an improved system, method and apparatus for the external ultrasonic inspection of fluidized tubulars and tanks. The apparatus utilizes couplantless, contactless, computer controlled, Electromagnetic Acoustic Transducer (EMAT) probes for the external, longitudinal, scanning process. Thereby, detecting thin spots and other flaws along the circumferential belts of a pipe or tank without the need to encircle the pipe or tank with an array of transducers. The invention discloses a method of operation whereby the probe detects flaws in welds and the pipe walls by launching and receiving simultaneous ultrasonic signals, circumferentially around a pipe, from the same transducer.

Abstract: A sensor has a magnetic circuit and an electric circuit for scanning the passage of magnetizable and non-magnetizable material of relatively movable parts. The sensor has a housing made of first and second parts which define an interface between them. The first part of the housing can be mounted to a cylinder wall and includes a portion of a magnetic core. A remaining portion of the magnetic core is embedded in the second housing part and an electric circuit operatively coupled with the magnetic core is mounted entirely in the second housing part so that the interface between the housing parts is free of electrical contacts to thereby prevent signal degradation due to contamination of electrical contacts.

Abstract: A roll mandrel monitoring device in cold pilger mills with a coil which surrounds the rolled tube concentrically and is arranged on the delivery side after the rolling mill for detecting longitudinal movement of a coil core which is fastened inside the tube to a bar extending between the mandrel and coil coaxially to the tube. In order to achieve faster reaction times and accordingly to stop the rolling mill when the mandrel breaks, the bar is connected with the mandrel rod holding the mandrel so as to penetrate the mandrel concentrically and is constructed in the region of the mandrel as a tension bar which can be pretensioned against the end of the mandrel on the delivery side with the intermediary of an energy accumulator.

Abstract: Apparatus for measuring the sidewall thickness of cylindrical articles during in-process ram and die drawing, inclined axis multiple polar sensors induce eddy currents by means of the angular motion of a rotating magnetic field, the signal phase angle representing sidewall thickness, also ram and die centering signals generated. Inclined axis polar sensors also measure both cylindrical and flat article hardness by phase shift.

Abstract: This invention uses a single eddy current coil to measure multiple parameters of conductive target simultaneously using a single fixed frequency. For example, the system consisting of the sensor coil, connecting cable, and signal conditioning electronics, can measure the thickness of a target and the distance of the target from the coil. Alternatively, it could simultaneously measure the distance of the target to the coil, i.e. lift-off, and one of the electrical properties of the target, such as the resistivity. The present system is useful in material characterization of targets where the lift-off information can be used to correct for any lift-off induced error in the apparent resistivity. In general, it can determine any pair of two characteristics of the target/sensor relationship simultaneously.

Abstract: The present invention is a measuring method for detecting penetrant metal intruded in refractories having a conductive cover at a first wall and a nonconductive surface at a second wall and for measuring their thickness. At least one sensor head is used including a metal sensor with a first and second conductive coil for measuring distance between the sensor head and a conductive substance and a distance sensor for measuring distance between the sensor head and the refractories. The sensor head is disposed at a plurality of fixed positions close to the second wall to face the refractories and an alternating current is applied to the first coil to generate an eddy current in the conductive substance which faces the metal sensor. An induced voltage induced in the second coil by at least the eddy current is detected and a first distance is obtained between the sensor head and the conductive substance based on the detected induced voltage.

Abstract: Rolls of metallized strip material are changed automatically at a station incorporating a device by which the thickness of the strip material is sensed utilizing a resonant circuit driven at a frequency other than its own resonant frequency and consisting in a coil with a toroidal core affording an opening or gap through which the running strip material is directed; a voltage sensor connected to the terminals of the coil pilots the operation of two switched outputs by way of respective comparators such as can be preset and activated parametrically in response to selected voltage values registering at the terminals of the coil with variations in inductive reactance determined by and indicating the thickness and the electrical and physical properties of the passing strip.

Abstract: Apparatus (40) locates welds or other junctures (20) in sheet material (12). The apparatus (40) includes a sensor (16) that produces a varying signal that is a function of the distance (100) between a surface (26) of sheet material (12) and the sensor (16), a differentiator circuit (42) differentiates the varying signal as a function of time, a comparator (44) compares the differentiated signal with a calculated allowable and sends a signal to an input (90) of an electronic latch (46), the electronic latch (46) sends a signal to a reject mechanism (48), and the reject mechanism (48) sends a reject-accomplished signal to a reset terminal (98) of the electronic latch (46). The method of the invention includes transporting the sheet material (12), producing a rate of change signal that is a function of changes in thickness (18) of the sheet material (12), determining an allowable rate of change signal, and comparing the produced rate of change signal with the allowable rate of change signal.

Abstract: A method of calibrating a thickness measuring device having preferably two noncontacting or scanning displacement measuring sensors (2, 3), which allows to calibrate in a simple manner, at the location of measurement, thickness measuring devices operating by different measuring principles by means of a reference object (7).