EP0256080A1 - Ultrasonic thickness meter - Google Patents

Abstract

The measuring device described below (10) is used to measure the thickness of pipes used in the oil and / or gas industry, the thickness of the pipes generally being between 5mm and 100mm. The meter (10) uses a low voltage liquid crystal display (28) which is inherently secure and is controlled by a low voltage microprocessor (126). Power is supplied for a maximum of 4 hours by a 4.5-volt battery and low internal capacitance is ensured without the need for a barrier or a protective enclosure. An ultrasonic transmitter (22) and receiver (26) are contained in a two crystal probe (14), the reflected ultrasonic signals being sampled by an A / D converter (56) and stored in a digital memory (50) of which the reading is carried out periodically under the control of the microprocessor (126), the signals being transmitted to the low-voltage liquid crystal matrix display (28). The liquid crystal display (28) provides clear visual indications of thickness (25) on a graduated scale and gives a digital reading for where the thickness is measured. The measuring device (10) includes a calibration block (34) for internal calibration before and after the measurement.

Description

ULTRASONIC THICKNESS METER The present invention relates to an ultrasonic thickness meter particularly, but not exclusively, for measuring the thickness of pipelines used on the oil and gas industry. In the oil and gas industry pipelines contain gas-liquid mixtures which are very corrosive containing gases such as hydrogen sulphide, and these gases eat into the interior of the pipe with the result that it is possible for perforations to develop in the pipe with potential leakage of such toxic hydrocarbons. In addition, hydrogen sulphide is an extremely explosive and toxic gas and of course, it is highly desirable that any such leakage is prevented. Therefore, it is important to be able to assess the integrity of the pipeline from time to time so that the pipes can be maintained in a reasonable condition to minimise the likelihood of inadvertent leakage.

Thus in the oil, nuclear and gas industries measurement of wall thickness in pipework is used to provide an early indication of internal corrosion.

Conventional thickness testers will generally utilise an ultrasonic pulse echo principle, generally called an A-scan, whereby an acoustic stress wave of sufficient intensity is transmitted into the pipe in the appropriate location. A reflective echo from the back or internal wall of the pipe is then used to provide an indication of wall thickness from knowledge of the tansit time of the ultrasonic wave and the velocity propagation coefficient for the wave in the pipeline material. In general, such equipment is capable of high accuracy and coupled to a suitable display medium which is typically a cathode ray tube display for A-scan presentation, a test operator is provided with reliable diagnostic information. Such instrumentation normally requires high energy signals for ultrasonic transmission and cathod ray tube genera- tion and such instrumentation is unsuitable for use in the explosive environments encountered in the oil and gas industry. For example, many of the devices currently in use require power supplies up to 50 volts and this does not comply with the existing safety requirements. Fur- thermore, simply reducing the voltage renders the equip¬ ment unusable because there is insufficient power to pro¬ vide satisfactory signal transmission and cathod ray tube generation.

Attempts to_s overcome this problem have involved the removal of the cathode ray tube display and the provision of intrinsically safe barrier enclosures for the system electronics, such as encapsulating them in an insulating resin. However, such modified equipment has been found unsuitable because the diagnostic information provided by the A-scan display is lost and only the digital thickness read-out is retained. As a result, the true condition of the corroded or damaged surface is often obscured and it is often time consuming for an operator to review an area and record all of the numerical read-out data. Further¬ more, the systems electronics are generally not intrinsi¬ cally safe and some form of valid protection is always required. Consequently, some of the more stringent intrinsic safety codes cannot be satisfied and conse¬ quently such instrumentation is unusable in the field because of this.

An object of the present invention is to provide an improved ultrasonic thickness meter which obviates or mitigates disadvantages associated with the aforemen¬ tioned prior art systems.

This is achieved by providing an ultrasonic thick¬ ness meter which uses an intrinsically safe low voltage liquid crystal display under low voltage microprocessor control. The equipment operates from a 4.5. volts batt¬ ery supply and the system is designed to have low inter¬ nal capacitance levels to ensure that the complete elec¬ tronic system meets the most rigorous and intrinsically safe specifications without the need for barrier and enclosure protection. In particular, the reflected ultrasound signal is sampled and digitally stored in a memory which is then periodically read out under control of the microprocessor to a low voltage dot-matrix liquid crystal display which provides a clear visual presenta- tion of thickness on a scale and also gives a digital read-out at the point the thickness is measured at.

Accordingly, in one aspect of the present invention there is provided an ultrasonic thickness measurement device comprising, ultrasonic transmitter means for generating an ultrasonic signal, ultrasonic receiver means for receiving a reflected ultrasonic signal and for converting said received ultrasonic signal to an analogue electrical signal, digital sampling means coupled to said ultrasonic receiver means for • digitally sampling said analogue signal to provide digitised samples thereof, digital memory means coupled to said digital sampling means for storing said digitised samples therein, microprocessor control means coupled to said memory means and to a low voltage display means for controlling transfer of said digitised samples to said low voltage display means, said microprocessor being also coupled to said ultrasonic transmitter means for controlling the generation of said ultrasonic signal.

Preferably, said ultrasonic transmitter means and said ultrasonic receiver means are combined in a twin crystal singleprobe, one crystal generating an ultrasonic wave and the other crystal receiving the reflected echo of said ultrasonic wave. Alternatively, said ultrasonic transmitter and receiver means are provided by a singla crystal.

Preferably, said ultrasonic transmitter means and said ultrasonic receiver means include transmitt- ing and receiving circuitry having impedancies which are matched to maximise the signal to noise ratio and to minimise the voltage required in the generation and detection of the ultrasonic waveform. Preferably also, said low power display is a liquid crystal dot-matrix display. Conveniently, said low voltage liquid crystal dot-matrix display is modified to minimise capacitance therein to satisfy intrinsically safe requirements for said device.

Preferably also, said ultrasonic thickness measur ent device includes the calibration means for calibrating said device prior to beginning measirerient, said calibration means including a calibration sample of the same material as the material being measured. Conveniently, said device includes means for presenting indication on said display that said ultrasonic thickness measurement device has been calibrated.

Preferably also, said ultrasonic thickness measurement device includes means for displaying numerical thickness data and for providing an indication at which position said numerical thickness data has been measured from said displayed data.

Preferably also, said microprocessor control means includes input/output devices and is coupled to a random access memory (RAM) for receiving and storing said digitised samples, and is coupled to an erasable, prog¬ rammable, read only memory (EPROM) for containing programs for operating said device in accordance with a predetermined arrangement.

Accordingly, in another aspect of the invention there is provided a method for measuring the thickness of a mat¬ erial usiny an ultrasonic signal, said method comprising the steps of ; generating a transmitted .ultrasonic signal, receiving an echo from said transmittedultrasonic signal, conditioning said received echo to a first analogue signal representative of said echo, then sampling said analogue signal to provide a sampled digitised signal, storing said sampled digitised signal in a memory, reading said stored data from said memory in accor¬ dance with a predetermined format and displaying said read-out data on a low voltage display to provide a scaled indicationof the thickness of said material.

Preferably, said method further comprises the step of detecting a first echo and converting said first echo signal into another electrical signal for subsequent digitising.

Preferably also, said method includes the step of scaling said display including, comparing the first echo signal with the first measureable value and setting a display scale factor, scanning digitised samples stored in said memory and unpacking said stored data to provide an output to the screen proportional to the maximum value of data in the predetermined number of data bytes • Preferably also, said method includes the step of providing a pointer on said display pointing to a value on said scale corresponding to the point from where said first echo was taken. Conveniently, said display also displays the point at which the first echo was taken in numerical format.

Preferably also, said method includes the step of detecting a bad contact and low battery voltage and for displaying signal on said display representative of said bad contact and said low battery voltage.

These and other aspects of the invention will become apparent from the following description, when taken in combination with the accompanying drawings, in which:- Fig. 1 is a diagrammatic view of an ultrasonic wall thickness meter in accordance with an embodiment of the invention in which an ultrasonic transducer and receiver probe is shown in proximity to a cross-section through a pipe undergoing testing; Fig. 2 is a schematic block diagram showing the elements used in the apparatus illustrated in Fig. 1;

Fig. 3 is a circuit diagram of the transmit circuit illustrated in Fig. 2;

Fig. 4 is a circuit diagram of the receive circuit illustrated in Fig. 2;

Fig. 5 is a circuit diagram of the sampling and data conversion circuit illustrated in Fig. 2;

Fig. 6 is a circuit diagram of the microprocessor and I/O circuit, the memory circuit and the display circuit illustrated in Fig. 2;

Fig. 7 is a circuit diagram of the circuit used to detect low battery voltage, and

Fig. 8 is a schematic diagram of software module cells used to control operation of the circuitry shown in Figs. 2 through 8 for measuring wall thickness.

Reference is first made to Fig. 1 of the drawings which illustrates an ultrasonic wall thickness meter, generally indicated by reference numeral 10, in accord¬ ance with an embodiment of the invention which consists of a portable console display unit 12 coupled to an ult¬ rasonic transmitter and receiver probe 14 which is shown coupled to the outside surface 16 of a section of pipe 18 by an ultrasonic coupling medium 20. In response to control signals from the console 12 an ultrasonic beam is transmitted from a transmitter crystal 22 within the probe and this is reflected from the interior surface 24 of the pipe and the reflection is detected by a receiving transducer 26 and the receiyed signal is forwarded to the console 12 where the data is converted to digital form and presented on a dot-matrix liquid crystal display 28. This illustrates, on the X-axis, the position of the echo 30 from the defect 25 to permit the operator to compare the thickness of the pipe at that portion with echos 27, 29 from adjacent portions 31,33 of the pipe. A numerical read-out of the thickness is also presented and an arrow points to the position corresponding to the part of the pipe 25 which reflected the first echo and details of how this information is provided on the display will be later described in detail.

The console 12 includes a calibration block 34 which the probe 14 is located on prior to measurement for int¬ ernal calibration of the console to ensure that the system is functioning correctly for the intended measurement med¬ ium. In this regard, it should be noted that the cali¬ bration block is of the same material as the pipeline, which is generally mild steel. The console 12 also in- eludes an on/off switch 36 on the base of the unit and the console can conveniently be carried by hand or slung around the neck or the upper area by a strap 38 which is partly shown in broken outline.

The transmitter/receiver probe 14 is generally cylindrical in shape and includes a zener protection

*> circuit 40 for intrinsically safe operation and, as afore- described both transmission and receiver circuits are optimised by careful selection of components for maximum signal to noise ratio and acoustic impedance matching. The coupling gel 20 may be any suitable coupling gel for minimising acoustic losses and ensuring efficient trans¬ mission of acoustic energy between the transmitter and receiver crystals and the pipe surface 16.

Reference is now made to Fig.2 of the drawings which is a schematic block diagram of components used in the device illustrated in Fig.10. The device is based on a microprocessor and input/output circuit 42 which has con¬ trol lines 43,45 coupled to a transmit circuit 44 and a liquid crystal display control circuit 46. The micropro- cessor is also coupled via a bus 48 to a memory circuit 50 and to said display circuit 46. The display circuit 46 pro¬ vides information on thickness, A-scan, calibration and diag¬ nostic information and how the microprocessor controls the circuitry shown on the block diagram will be later des¬ cribed in detail. The transmit circuit 40 is coupled to the probe circuit 52 which includes the transmitter crystal 22 and receiver crystal 26 as shown in Fig. 1. The receiver portion of the probe circuit 52 is coupled to a receiver circuit 54 which processes the reflected ultrasound signal to provide a rectified analogue elec¬ trical signal as will be later described and which, in turn, is fed to the sampling data conversion circuit 56 which digitises the analogue electrical signal and feeds it via a bus 58 to random accessmemory inmemory circuit 50.

In the circuit that is illustrated, initiation of the transmission signal is provided from themicroprocessor via I/O control line 43. Aswillbe explained this causes the transmitter circuit to .discharge a capacitor and deposit charge on the transmitter crystal 22 which then moves and generates an ultrasonic waveform which passes through the coupling medium and through the wall of the pipe 18. The reflected signal is detectedvia the receiver crystal 26 which is amplified, rectified, and smoothedbefore being sampled by a six-bit flash A to D converter contained in sampling and data conversion circuit 56. Under the control of themicroprocessor the sampled data is stored in a RAM in the memory circuit 50. Timing for the data conversion circuit is supplied by separate 10 megahertz clock system, as will be explained, and the digital samples are then transferred to the system memory 50 under the microprocess control and this process is repeated until an entire A-scan has been digitised and stored.

The data stored in the random access memory is then processed in accordance with predetermined algor- ithims stored in an erasable programmable read only memory (EPROM) as will be later described before being transferred to the display circuit 46, along with any required diagnostic information, such as calibration state, and an indication of how well acoustic energy is being transmitted into the test specimen. Reference is now made to Figs. 3 and 4 of the draw¬ ings which illustrate respectively, a detailed circuit of the transmitter circuit 44 and the receiver circuit 54. Referring firstly to Fig. 3 it will be seen that the receiver circuit is based on a monostable multivibrator 60 and a transistor 62 which is type 2N222A. The trans¬ istor 62 has its base 64 coupled via a resistor 66 to the Q output on pin 6 of the monostable multivibrator 60 and the collector of the transistor 68 is coupled to a + 4.5 volts line via resistor 70 and to the probe transmitter transducer 22 via capacitor Cl. The probe

22 is coupled to the emitter 72 of the transistor 62 via a pulse shaping resistor 74.

As will be explained the transmitting transducer 22 is energised, by means of the switching action of trans- istor 62. Initially transistor 62 is off and capacitor Cl is charged to plus 4.5 volts because the voltage drop across resistor 70 is very small. When a suitable trigger pulse is received by the monostable multivibrator 60 from the microprocessor and I/O circuits 42 the monostable multi- virator 60 produces an output pulse from low to high on output pin 6. This switches transistor 62 "ON" and the capacitor Cl discharges through the transistor and through pulse shaping resistor 74 which results in a quantity of charge being deposited across the electrodes of the transmitting transducer. Selection of the value of resistor 74 is important because this acts as a pulse shaping component to optimise the energising wave¬ form. When the charge is deposited on the transmitting transducer the piezoelectric effect causes a displacement of the transducer at ultrasonic frequency which results in ultrasonic acoustic waves being produced and this is the acoustic transmission wave. The transmission wave passes through the coupling medium 20 and into the wall of the pipeline 18. As the wave front hits the surface of the pipe there will be a reflection and there will also be reflections from various discontinuities and the interior of the pipe surface, such as pitting cracks and the pipe wall itself. Reflections fromeach of these dis- continuities will be collected by the receiver circuit, however, as will be later described, the receiver cir¬ cuit is tuned to detect the process data from the first echo corresponding to the minimal thickness of the pipe¬ line. Other reflections are also processed and displayed, however it is the first echo which is of most importance because this is representative of the minimum thickness of the wall because of the minimum transit time between the transmitted wave and the detected signal. Referring now to Fig. 4 of the drawings, a reflected wave impinges on the receiving transducer, and displaces it causing an induced polarisation across an input capaci¬ tor 76. The inputvoltage thus generated is amplifiedvia R.F. amps IC1,IC2 and IC3 but separated by DC decoupling capacitors 78 and 80 respectively. Gain of IC1,2 and 3 are controlledby adjustment of a ten kilohmpotentiometer 82. The amplified radio frequency signals are rectified via diode 84 and are simultaneously filtered via the R-C network and passed to an output buffer stage generally indicated by reference numeral 86. The buffer stage 86 has an output 88 which is coupled to the input of an ana¬ logue to digital converter in said sampling and data con¬ version circuit 56 in which data is digitised and stored to provide the A-scan display as will be later described. Reference is now made to Fig. 5 of the drawings which is a circuit diagram of the sampling and data conversion unit 56.illustrated in Fig. 2. For completeness, the transmitter circuit Fig. 3 is also shown in the diagram and the operation of this circuit has already been descri¬ bed with reference to Fig. 3 of the drawings. The circuit 46 is based on a 6-bit flash Ato D converter 90, type RCA 3300, and which has an input 92 coupled to the output 88 of the receiver circuit 54. A resistor R4 and diode Dl are coupled to pin 9 of the A to D converter 90 for sett¬ ing a reference voltage for digitisation. Digital data is obtained on output lines, generally indicatedby reference numeral 94, coupled to pins 1,15,17 and 18. That is, only the four most significant bits of data are read out to the RAM. Pin 7 is coupled to the Q output of a Nand gate located in a 4-input 2 Nand gate low power circuit device 96 type 74HCOO which provides a start convert,or digitise, signal to the A to D converter 90 at an appropriate point in the A-scan cycle, which will be later described.

The circuit 56 includes 3 counters 98,100 and 102, type Texas Instruments 54HC193,which are set to count to a particular value via load input lines, generally indicated by reference numeral 104, from the processor circuit 42 as best seen in Fig. 6 of the drawings.

The digital sampling procedure will now be described in detail. Initially, in the absence of a transmit signal from the microprocessor there is no sampling. In this condition the counting of counters 98,100 and 102 is inhibited by means of J-K flip-flop 106 and Nand gate 96, which prevent the counters from receiving clock data from a ten megahertz clock 108. When the monostable multivibrator 60 receives an instruction to fire pulse on line 43 from the microprocessor it provides, as previously described, a fire pulse to the transistor 62 of the transmitter circuit from its Q output 6 and simultaneously provides a complimentary pulse from its Q output 1 on control line 110 to J-K flip-flop 106.

This pulse causes the flip-flop to toggle and the Nand gate then passes the clock pulses from the clock 108 to the counters which then commence counting. When the counters reach a preset level, determined by the load input values from the microprocessor, a pulse is produced at pin 4 of the counter 102 which is fed by control line 112 to pin 3 of Nand gate 96. A counter inhibit signal is provided from pin 3 of the Nand gate and this is fed on control line 114 of pin 1 of a monostable multivibrator 116 which is type 54HC123. The monostable produces an output pulse from the Q output on line 118 which is fed to an input pin 4 of Nand gate 96 and simultaneously to pin 3 of a dual D-type flip-flop 120 of type 54HC74. On receipt of the pulse on line 4 the Nand gate provides an output pulse from Q output and pin 6 via control line 122 which is fed to input pin 7 of the A to D converter 90 to initiate conversion of each analogue value sampled to the 4-MSB digital code. And, as will be described, the sample data is transferred via output lines 94 and bus 58 to a random access memory stored in memory circuit 50.

The D-type flip-flop 120 has an input on pins 1 and 3 coupled via control lines 119 and 121 to pins 11 of the counters 98,100 and 102. When the counters have finished counting to the predetermined value, a signal is sent to the D-type flip-flop 120 and this signal is combined with the start convert signal fed to pin 3 via control line 117 so that the flip-flop 120 provides an output pulse from pin 6 which indicates that data has been converted. This pulse is fed by control line 123 to pin 22 of the micro¬ processor which permits the microprocessor to load the 4 most significant bits of the converted data into the RAM before initiating digitising the next sample.

The pulse on control line 118 to input pin 4 of the Nand gate 96 is also fed via control line 124 to the I/O module and to the microprocessor which causes the micro¬ processor to reset the counters and reload themto a preset counting value after the completion of the predetermined count and the entire sampling process is repeated. The load inputs are updated by 1 and the number of times the load inputs are so updated is monitored by the micropro¬ cessor and when a value of 512 has been reached, that is corresponding to 512 samples taken from the echo, the microprocessor sends a further transmit pulse to the transmitter circuit 44 to again fire the transmitter and repeat the process. The Nand gate 96 also provides an output pulse to J-K flip-flop 106 to reset the flip-flop and inhibit countinguntil it is again toggled in response to a pulse in themicroprocessor as aforedescribed. Because a ten megahertz clock 108 is used, the A-scan is divided into sub-sections of 100 nano seconds. This time interval corresponds to an accuracy of approximately 0.5 mm. It will also be appreciated that the clock circuit 108 is used to generate a five megahertz clock formicroprocessor timing as will be described with reference to Fig. 6.

Reference is now made to Fig. 6 of the drawings which illustrates inmore detail the microprocessor and I/O cir¬ cuits 42 and display circuit 46 illustrated inFig.2 of the drawings. The circuit 42 is based on an NSC800 micropro¬ cessor 126 which is coupled via bus 48 to a random access memory 128 type MM82PC and to an erasable, progammable read only memory (EPROM) 130 type 27C16. The random access memory 128 contains bytes of the digitised data received from the A to D converter 90 and the EPROM 130 contains the operations programs for operating the various desired algorithims that the device performs, for example, calibrate, low battery check and data processing. The microprocessor is interfaced to the sampling and data conversion circuit through I/O module 132 which is type NSC810 and is also interfaced to the display module 46, which is type EGY84320AT, through an I/O module 134 which is also type NSC810. Data for the system display is provided via the I/O module 134 and the I/O module 132 provides the count delay, the trans¬ ducer fire pulse and also receives the converted data and other diagnostic information such as, the battery low detection, and bad coupling information.

The microprocessor controls re-routing of the data stored in the RAM and to the display module and a con¬ figuration of this data to present a display such as shown in liquid crystal display 28 of Fig. 1 providing details of the minimum wall thickness, the point at which the measurement is taken and the numerical thickness at the position the arrow is pointing to on the displayed echo.

Reference is now made to Fig. 7 of the drawings which illustrates an exemplary form of battery-low detection cir¬ cuit generally indicated by reference numeral 136 which is based on a differential amplifier 138 type LM339 arranged as a comparator configuration. The Zener diode Dl provides a fixed voltage to the non-inverting input of the amplifier and if the voltage across the ^:voltage divider provided by resistors 140 and 142 also drops, then this is sensed by the amplifier and provides an output on line 144 which is fed to the microprocessor 126 via the I/O module 132. In response to this signal, the microprocessor controls the display module to provide a "Bat. low" message to be sent to the display. A similar message can be sent to the display to indicate "B.C^1* for bad coupling between the probe and the surface of the pipe being inspected.

Reference is now made to Fig. 8 of the drawings which is a schematic diagram of the software modules used to provide a user-friendly interface to the instrument operator as well as providing overall control of the operation of the instrument and providing a potential for future expansion.

The software program modules are stored in the EPROM 130 and the function of each of the modules illustrated in Fig. 8 is as follows; the system initialise module SYSINIT 144 initialises the hardware of the system and sets the top of the processor stack, initialises the I/O ports and the liquid crystal display. It also set timers; one to give a pulse rate frequency clock of 1 kilohertz and the other being used as a count down timer to remove power from the system after ten minutes of non-use, if so desired. In addition,the data stored in the RAM is clear¬ ed and the screen scale flag (SCF) is set to 50mmor 100mm depending on the scale required. Finally, the probe delay counter offset is set to a nominal value. The sys¬ tem initialise program then is passed to a system clear module 146 which clears the liquid crystal display of all data and then the main program module 148 is called.

The main program 148 is coupled to a number of sub- modules and the main programcycles through the sub-modules in a direction left to right as shown in Fig. 8 in the direction of the arrow and it continues to loop around these modules as will be described. The main program firstly calls to the calibrationmodule 150 before it enters the main program loop. The calibration routine is contained in the module. The probe is located on top of the cali¬ bration block 34 and a calibrate message is sent to the screen and remains until a first echo (FEC) is received. This firstecho is modified to FEC and is tested to see whether it corresponds to 20 mm plus or minus range error. If the range error is greater than the preset limit the calibrate message "calibrate" remains on the screen, otherwise the error from 20 mm is calibrated and then used to compensate the probe-delay offset. Hence, after the calibration, the echo is equal to zero and corresponds to the steel surface.

The main program then enters the main program loop and here the battery low indicator is firstly assessed for lowbattery condition. A message "BAT. LOW-" is displayed on the screen if a low battery condition is present as determined from the low battery condition circuit shown in Fig. 7.

The program then calls a module RSCFE 152 (receive first echo) which returns the delay count to the first echo, FEC. If the hexadecimal code, FF, is returned then a message indicating poor coupling is displayed and this is "B.C" for bad coupling. The power counter is reloaded indicating that the instrument is still in use and the A-scan display is generated, before the first echo is modified according to a threshold criteria in module MODFEC 154. For the modified first echo, FEC, the required thickness is calculated and displayed as will be explained.

The first echo is modified to FED' by the MODFEC module 154. This operates as follows, starting from the first echo, the data RAM 128- is inspected for 8 locations upwards and the highest data value found, less a preset value, is then used to set a new threshold. The data point corresponding to this new threshold gives the modi¬ fied first echo, FEC. The effect is to give a decreas- ing threshold with decreasing echo height.

The program then calls the A-scan module 156 which generates the A-scan scale on the bottom two lines of the display. The A-scan module firstly sets the screen scale factor for 50. to 100mm depending on the first echo, FEC, being less than 45mm or greater than 55mm. If the first echo is between 45mm and 55mm the old scale factor (SCF) is used. The data RAM 128 is scanned and the data unpacked for display by module BACCUM 158 for display on the LCD 28. For SCF equal to 5mm the greater of two 4 bit bytes at a given RAM location is displayed as a single dark column and this selection is achieved by calling module DTSC 162 which provides the maximum screen data byte so that the greatest of the two 4 bit bytes at a given RAM location is displayed on the LCD 28 as a single dark column. The height of the column is proportional to the maximum value data byte. For SCF equal to 100mm, the greatest of 4 data bytes contained within two successive RAM locations is also displayed as a column and this is also achieved by calling the DTSC module 162.

The A-scan display then calls module DPFSE 164, which is a module controlling the data pointer to first screen echo, and this mdoule generates and the arrow 32 shown in Fig. 1 at the position of the first screen echo on the liquid crystal display. The position of the arrow 32 corr¬ esponds to the modified first echo, FEC'on the A-scan display. The main program then invokes the multiply module 166 which receives the modified first echo count FEC and which contains data corresponding to the velocity of propagation of the wave in the material under test and converts the modified first echo signal to a binary number equal to the material thickness, which is RBIN. In particular, this is simply achieved by multiplying the velocity propagation constant for the material, for example, the velocity for mild steel is 5.96mm per second, by the time for the first echo count to provide RBIN. The main program then calls the RBCD module 168 which converts the binary thickness to a binary coded decimal thickness, RBCD for use by display on the liquid crystal display screen 28. Each BCD digit is character coded with a particular character from a look-up table using the SCMD module 151. The SCMD module stores a series of character -codes in memory and uses these to generate a line of characters on the screen. Two such lines of up to ten characters can be shown on the top two lines of the display.

The main program then calls the data capture module 170 which captures the RAM data from the A-scan hardware. This module is interrupt driven by the 1 kilohertz pulse rate frequency timer and the module offsets the counter delay by the probe delay, and then loads the delay into the A-scan counters. It then activates these counts and waits for.an.end of convert signal from the microprocessor response to a data convert signal from the D-type flip- flop 120. The received data is then transferred via the A to D converter 90 I/O module 132 to the RAM 128 and is then packed as a 4-bit byte onto the data RAM; two such bytes making 1 RAM byte. The main program continues to loop as aforedescribed and to call the modules as desired to continue to present the data to the display to illustrate the display as shown in Fig. 1. The entire operation may take up to 200 milli¬ seconds to complete the cycling through the modules and present the sample data to the screen, however, this time is sufficiently short so that the operator perceives the display as continuous and a flicker of the display is generally unnoticable. In operation, the instrument is firstly calibrated which consists of placing the probe on the 20 mm test block 34 and switching on. As described above, a cali¬ brate message is sent to the screen and the processor validates the resulting thickness error to check it is within limits, before compensating the instrument to give a true reading with the probe in use. This is because the response to the transducer is non-linear and a calibration curve is stored in the EPROM which overcomes the non- linearity problem.

For normal operation,^" the material thickness is presented in a digital read-out on the screen together with the full, software generated A-scan display as des¬ cribed above. Thickness readings are taken against vari- able threshold depending on -echo height and the dynamic threshold means that if a small amplitude of first pulse is detected, the threshold is reduced. For high amplitude signals the threshold is high to adjust the scale in order to provide the required accuracy of plus or minus ten percent between 5 and 100 mm thickness of sample being measured. The provision of the arrow to indicate in the A-scan trace the point at which the thickness reading is taken minimises the errors due to misreading by the operator and internal features such as slag incisions are readily detected. Other useful diagnostic information is presented to the operator, namely that contact between the probe and the material is poor in whichcase the message "BC" is displayed and message "BAT.LOW." is also displayed when the power supply battery is low on voltage.

Thus the system aforesdescribed permits an operational thickness range of 5 mm to 100 mm with an accuracy of plus or minus 10% of the nominal wall thickness to be measured and operation is achieved simply using two 4.5. disposable batteries and in use, battery life is approximately 4 hours. The unit is internally calibrated via fixed calibration block situations on the housing for mild carbon steel and the housing itself is spray proof.

It will be appreciated that various modifications may be made to the apparatus- and method hereinbefore described without departing from the scope of the invention, for example, the .probe used in this invention could be an angled,shear and longitudinal wave, probe and such equipment could be utilised as a general purpose flaw detector which is also intrinsically safe. In addition, the apparatus hereinbefore described may also be extended to phased array systems so that information in two or even three dimensions can be displayed and other information may be provided in the read-out, for example a absolute minimal thickness or sending an alarm should a thickness be recorded which is less than a safe value. A hard copy facility of test data may be readily incor- porated which would permit an operator to record a choice of data and to evaluate results at a later stage. This may be provided by including an additional software module for hard copy capability, and in addition, other software modules may be extended and added as required to permit further functions to be achieved by the devices, for example a communications module could be included for transmitting data to an external computer. Furthermore, the device has application in areas other than in measuring the thickness of pipelines, for example, it could be used in medical imaging or in any other field where it is desirable to obtain imaging of an object beneath the surface and to present this information clearly and unambiguously on a display which is intrinsically safe. Furthermore, information measured and recorded can be stored in a memory which could be removed at the end of a test and sent to a remote location where the memory could be unloaded and a hard copy retained to provide a permanent record of the inspected surface.

The components generally described herein are low power components such as low power Schottley devices but may be CMOS or TTl devices which have the requirement that they operate at low voltages to comply with intrinsically safe specifications such as BS5501 and EN50020 safety standards. In addition, a single crystal could be used for both transmitting and receiving data or any number of crystals could be used depending upon the imaging requirement. Also, the device is adaptable for thickness measurement depending upon the data in the EPROM and the device could be used with different materials by changing the calibration block to change the propagation constant and this can update the calibration information in the EPROM to provide a flexible system which could be used to measure the thickness of different materials to high accuracy.

Advantages of the invention are that an intrinsi- cally safe device is presented which can be used to measure the thickness of pipelines in the oil and gas industry to a higher accuracy of a range of thicknesses varying from 5 to 100 mm. Smaller thicknesses, that is less than 5 mm may be accurately measured using a smaller probe. A clear unambiguous display is presen¬ ted to the operator and diagnostic information is also presented to the operator to ensure that accurate results are obtained and the system wholly portable and has application in different fields.

Claims

CLAIMS:

1. An ultrasonic thickness measurement device com¬ prising, ultrasonic transmitter means for generating an ultrasonic signal, ultrasonic receiver means for receiving a reflected ultrasonic signal and for con- verting said received ultrasonic signal to an analogue electrical signal, digital sampling means coupled to said ultrasonic receiver means for digitally sampling said analogue signal to p^'rovide digitised samples thereof, digital memory means coupled to said digital sampling means for storing said digitised samples therein, microprocessor control means coupled to said memory means and to a low voltage display means for controlling transfer of said digitised samples to said low voltage display means, said microprocessor being also coupled to said ultrasonic transmitter means for controlling the generation of said ultrasonic signal.

2. An ultrasonic thickness measurement device as claimed in claim 1 wherein said ultrasonic transmitter means and said ultrasonic receiver means are combined in a twin cyrstal single probe, one crystal generating an ultra¬ sonic wave and the other crystal receiving the reflected echo of said ultrasonic wave.

3. An ultrasonic thickness measurement device as claimed in claim 1 wherein said ultrasonic transmitter and receiver means are provided by a single crystal.

4. An ultrasonic thickness measurement device as claimed in any preceding claim wherein said ultrasonic trans¬ mitter means and said ultrasonic receiver means include transmitting and receiving circuitry having impedancies which are matched to maximise the signal to noise ratio and to minimise the voltage required in the generation and detection of the ultrasonic waveform.

5. An ultrasonic thickness measurement device as claimed in any preceding claim wherein said low power display is a liquid crystal dot-matrix display.

6. An ultrasonic thickness measurement device as claimed in claim 5 wherein said low voltage liquid crystal dot- matrix display is modified to minimise capacitance therein to satisfy intrinsically safe requirements for said device.

7. An ultrasonic thickness measurement device as claimed in any preceding claim wherein said ultrasonic thickness measurement device includes the calibration means for calibrating said device prior to beginning measurement, said calibration means including a calibration sample of the same material as the material being measured.

8. An ultrasonic thickness measurement device as claimed in any preceding claim wherein said device includes means for presenting indication on said display that said ultrasonic thickness measurement device has been calibrated.

9. An ultrasonic thickness measurement device as claimed in any preceding claim wherein said ultrasonic thickness measurement device includes means for displaying numerical thickness data and for providing an indication at which position said numerical thickness data has been measured from said display data.

10. An ultrasonic thickness measurement device as claimed in any preceding claim wherein said micro¬ processor control means includes input-output devices and is coupled to a random access memory (RAM) for receiving and storing said digitised samples, and is coupled to an erasable, programmable, read only memory (EPROM) for containing programs for operating said device in accordance with a predetermined arrangement.

11. An ultrasonic thickness measurement device as claimed in any preceding claim wherein a method for measuring the thickness of a material using an ultra¬ sonic signal, said method comprising the steps of: generating a transmitted ultrasonic signal, receiving an echo from said transmitted ultrasonic signal, conditioning said received echo to a first analogue signal representative of said echo, then sampling said analogue signal to provide a sampled digitised signal, storing said sampled digitised signal in a memory, reading said stored data from said memory in accor¬ dance with a predetermined format and displaying said read-out data on a low voltage display to provide a scaled indication of the thickness of said material.

12. A method as claimed in claim 11 wherein said method further comrpises the step of detecting a first echo and converting said first echo signal into another electrical signal for subsequent digitising.

13. A method as claimed in claim 11 or 12 wherein said method includes the step of scaling said display including, comparing the first echo signal with the first measurable value and setting a display scale factor, scanning digitised samples stored in said memory and unpacking said stored data to provide an output to the screen proportional to the maximum value of data in the predetermined ^"number of data bytes.

14. A method as claimed in any one of claims 11, 12 or 13 wherein said method includes the step of providing a pointer on said display pointing to a value on said scale corresponding to the point from where said first echo was taken.

15. A method as claimed in any one of claims 11 to 14 wherein said display also displays the point at which the first echo was taken in numerical format.

16. A method as claimed in any one of claims 11 to 15 wherein said method includes the step of detecting a bad contact and low battery voltage and for displaying signal on said display representative of said bad contact and said low battery voltage.