Abstract:
An ultrasonic system for determining the quality of concrete under water  hout inaccuracies caused by electromagnetic interference from the ultrasonic generator. An ultrasonic generator applies pulses to the concrete. An ultrasonic detector detects the ultrasonic pulses and produces corresponding signals that are indicative of ultrasonic pulses that have passed through the material. Signal processing circuitry processes the signals to determine the transit time of the ultrasonic pulses through the material. The signal processing circuitry is disabled for a predetermined time after application of each ultrasonic pulse to the material to prevent noise produced by the means for applying ultrasonic pulses to the material from entering the signal processing circuitry and causing spurious measurements.

Description:
BACKGROUND OF THE INVENTION 
     This application is a continuation in part of U.S. patent application Ser. No. 07/860,744, filed Mar. 25, 1992, now abandoned. 
    
    
     1. FIELD OF THE INVENTION 
     This invention relates generally to a system for non-destructive testing of a material such as concrete. In particular, this invention relates to an ultrasonic system for testing the quality of concrete in underwater structures. 
     2. DESCRIPTION OF THE PRIOR ART 
     The velocity of ultrasonic pulses traveling in a solid material depends on the density and elastic properties of the material. The quality of some materials is sometimes related to their elastic stiffness so that measurement of ultrasonic pulse velocity in such materials can often be used to determine the elastic properties of the materials. Materials that can be assessed in this manner include concrete and timber. Underwater concrete structures include pier, retaining walls and encasements. Proper maintenance of these and similar structures requires above-water and below-water inspections of the concrete. Visual inspection is the most common underwater inspection technique. Visual inspection has many limitations, particularly in evaluating the internal condition of the structure. 
     The transit time of high-frequency sound waves through concrete can be used to assess its condition. Ultrasonic test equipment is commercially available for in-air testing. Measuring sound velocity in a non-homogeneous material, such as concrete, requires using separate transmit and receive transducers to avoid energy scattering problems. Sound velocity through the concrete is calculated by measuring the time required to transmit over a known path length. The measurement of average sound velocity through concrete is recommended as a means to establish the uniformity of the concrete being tested. 
     There are three approaches to measuring sound velocity in concrete. These three approaches are generally known as direct transmission, indirect transmission and semidirect transmission. The most common method is direct transmission where the transducers are positioned on opposite sides of a test specimen. Longitudinal waves propagated directly toward the receiver. For indirect transmission both transducers are placed on the same side of the concrete, and the receive transducer detects energy scattered from discontinuities within the concrete. Semidirect transmission involves placing the transducers on perpendicular surfaces of the structure. Semidirect transmission is not normally used because of difficulties in maintaining a known path length. 
     Direct transmission of the ultrasonic pulse is the preferred approach for measuring average sound velocity in concrete because direct transmission provides the maximum sensitivity and a well-defined path length. Indirect, or surface, transmission is used only when one surface of the concrete is accessible. A retaining wall is a typical example of a concrete structure where indirect transmission is used. The indirect transmission does not provide a well-defined path length and indicates the quality of the concrete primarily near the concrete surface. The average pulse strength detected in an indirect transmission system is generally less than 5% of the average pulse strength detected for the same path length when direct transmission is used for measuring the quality of the structure&#39;s concrete. 
     A problem often arises in using ultrasonic waves to determine the quality of concrete in that ultrasonic generators produce considerable electromagnetic interference. This electromagnetic interference causes false counts in the counting circuitry that determines the transit time of pulses in the material. 
     SUMMARY OF THE INVENTION 
     With the disadvantages inherent in prior art apparatus for measuring the velocity of ultrasonic pulses traveling in a solid concrete structure and thus accessing the quality of the concrete, the present invention was conceived and one of its objects is to provide a reliable and highly accurate measuring apparatus for assessing the quality of concrete in an underwater structure. 
     Another object of the present invention is to provide an ultrasonic test system for determining the quality of concrete without inaccuracies caused by electromagnetic interference from an ultrasonic generator. 
     The ultrasonic test system according to the present invention is a battery powered instrument designed to measure the velocity of sound through concrete and provide a general condition assessment rating of the concrete. The ultrasonic test system of the present invention provides information about an underwater concrete structure by allowing a diver to quantify the internal condition of the concrete through the use of ultrasonic sound waves. Ultrasonic sound velocity information is generated when the diver places the faces of transducers on the surfaces of the underwater structure being tested. This information is correlated with the internal condition of the concrete to provide a general condition assessment rating. 
     An ultrasonic test system for testing the quality of a solid material such as concrete in an underwater structure comprises means for applying ultrasonic pulses to the material and means for detecting the ultrasonic pulses and producing corresponding signals that are indicative of ultrasonic pulses that have passed through the material. The invention further comprises signal processing means for processing the signals to determine the transit time of the ultrasonic pulses through the material and means for disabling the signal processing means for a predetermined time after application of each ultrasonic pulse to the material to prevent noise produced by the means for applying ultrasonic pulses to the material from entering the signal processing means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates placement of a transmitter and a receiver for direct transmission of ultrasonic waves through a concrete structure; 
     FIG. 2 illustrates placement of a transmitter and a receiver for indirect transmission of ultrasonic waves through a concrete structure; 
     FIG. 3 is a block diagram of an ultrasonic test system according to the present invention that uses direct transmission of ultrasonic waves through a concrete structure; 
     FIG. 4 is a block diagram of an ultrasonic test system according to the present invention that uses indirect transmission of ultrasonic waves through a concrete structure; 
     FIGS. 5A-5H comprise a schematic circuit diagram of a system that provides for direct transmission of ultrasonic waves through a concrete structure; and 
     FIG. 6 is a graph illustrating the three charge states of the battery charger of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, direct transmission of ultrasonic waves through a concrete structure 20 includes an ultrasonic transmitter 22 placed adjacent to a surface 24 of the concrete structure 20. An ultrasonic receiver 26 is mounted to a surface 28 of the concrete structure 20. Ordinarily in the direct transmission method the surfaces 24 and 28 are parallel. 
     Referring to FIG. 2, indirect transmission of ultrasonic waves uses an ultrasonic transmitter 30 and an ultrasonic receiver 32 mounted to a generally planar surface 34 of a concrete structure 36. 
     The velocity of a pulse of longitudinal ultrasonic vibrations in an elastic solid is given by the following expression: ##EQU1## where 
     E is the dynamic elastic modulus; 
     P is the density; and 
     J is Poisson&#39;s ratio for the material. 
     The above equation generally applies to longitudinal pulse transmission in a solid of any shape or size provided that the smallest dimension measured perpendicular to the path of the pulse is not less than the wavelength of the vibrations that comprise the pulse. The table below gives the condition rating of concrete based on sound velocity. 
     
         ______________________________________Condition Rating          Sound Velocity (ft/sec)______________________________________Excellent      &gt;15,000Good           12,000-15,000Questionable   10,000-12,000Poor            7,000-10,000Very Poor       &lt;7,000______________________________________ 
    
     The block diagrams of FIG. 3 and FIG. 4 illustrate respectively the main components of a direct transmission system 40 and an indirect transmission system 42. 
     Referring first to FIG. 3, direct transmission system 40 comprises a transmit transducer 50 which produces an ultrasonic wave front that is directed toward a receive transducer 52. The transmit transducer 50 and the receive transducer 52 are connected to a separation distance transducer 54. The separation distance transducer 54 produces a signal indicative of the distance between the transmit transducer 50 and the receive transducer 52. The output of the separation distance transducer 54 is input to an analog to digital converter 56 that is included as a component of a microcomputer 58. 
     The microcomputer 58 also includes an interrupt controller 60, a memory 62, a power supply 64, parallel input/output ports 66 and an RS-232 serial input/output port 68. A keypad 67 is connected to a decoder 69, which provides its output to the parallel input/output ports 66 and to the interrupt controller 60. 
     The analog to digital converter 56 also receives pressure signals from a pressure transducer 70. The pressure transducer 70 measures the water depth. 
     An ultrasonic transmitter 72 provides ultrasonic sound signals to the transmit transducer 50. Signals indicative of the start time of the ultrasonic sound signals are input to a blanking circuit 74. The output of the blanking circuit 74 is connected to a V-meter electronics module 76. The V-meter electronics module 76 receives an input from a set reference adjustment circuit 78 and provides an output to a liquid crystal display 80. The liquid crystal display 80 preferably is a liquid crystal device that provides a display of the time of transmission of ultrasonic pulses from the transmit transducer 50 to the receive transducer 52. Signals output from the ultrasonic transmitter 72 activate the blanking circuit 74 and start the counter circuit within the V-meter electronics module 76. 
     Signals output from the receive transducer 52 are input to a preamp 82. Signals from the preamp 82 are then input to the blanking circuit 74. Signals output from the blanking circuit 74 are input to the V-meter electronics module 76 with the signals stopping the counter within the V-meter electronics module 76. Signals output form the V-meter electronics module 76 are input to a parallel-to-serial converter 88. The parallel-to--serial converter 88 exchanges input and output signals with the microcomputer 58 via a pair of the parallel input/output ports 66. 
     One of the parallel input/output ports 66 of microcomputer 58 provides output signals to a serial-to-parallel converter 90. The serial-to-parallel converter 90 provides signals to a 4-line liquid crystal display 92, which also receives signals from the microcomputer 58 via another of the parallel input/output ports 66 of microcomputer 58. 
     The microcomputer 58 also provides a signal to a serial-to-parallel display driver 94 via one of the parallel input/output ports 66 of microcomputer 58. The serial-to-parallel display driver 94 drives a 4-digit display 96. The display 96 is used to help the diver position the transmit transducer 50 and the receive transducer 52 for optimum results. The serial to parallel display driver 94, the 4-digit display 96, the preamp 82, the separation distance transducer 54, the ultrasonic transmitter 72, and the pressure transducer 70 are mounted in a water-tight housing 109. 
     Power is input to the system via a 120 Volt AC line connected to a switch 110. The switch 110 also receives a signal from a vent door 112. If the vent door 112 is closed, the switch 110 is open and prevents the operation of direct transmission system 40. The output of the switch 110 is input to a vent fan 114 and to an AC to DC power supply 116. 
     The AC to DC power supply 116 provides DC power to a battery charger 118 that provides signals to a charger on indicator 120, a fully charged indicator 122 and to a relay 124. The relay 124 is connected to a battery latch out circuit 126 to provide signals thereto. A switch 128 is connected to the battery latch out circuit 126 for controlling operation thereof. 
     The relay 124 is also connected to a battery pack 130. The battery pack 130 is connected to a charger circuit 132, which is in turn connected to a pump battery 134. The pump battery 134 is connected to a relay 100, which also receives an input signal from a pump switch 138 and from the microcomputer 58 via one of the parallel to serial input/output ports 66 of microcomputer 58. 
     The AC to DC power supply 116 also provides DC power to a voltage regulator 140. The output of the voltage regulator 140 is input to the power supply 64 of the microcomputer 58 and to a power on indicator 142. 
     The direct transmission system 40 further includes a diver earphone 146 that is driven by an audio amplifier 148. The audio amplifier 148 receives inputs from a microphone 150 and a volume adjust circuit 152. The diver earphone 146 is used to communicate with the diver. 
     The RS-232 serial input/output port of the microcomputer 58 preferably is connected to a serial printer 144. 
     Referring now to FIG. 4, an indirect transmission system 42 for testing concrete by means of indirect transmission of ultrasonic waves through the concrete is similar to direct transmission system 40 of FIG. 3 for direct transmission of ultrasonic waves for the concrete structure under test. The indirect transmission system 42 of FIG. 4 does not include a separation distance transducer. In the indirect transmission system 42 the distance between the transmit transducer 50 and the receive transducer 52 is preferably fixed at about 12 inches. The indirect transmission system 42 includes a suction pump 160 that is used to actuate suction cups (not shown) that are used to attach the transmit transducer 50 and the receive transducer 52 to a planar surface, such as the surface 34 shown in Fig. 2. 
     Ordinarily a minimum of two people are required for proper operation of the ultrasonic test system according to the present invention. A diver positions the transmit transducer 50 and the receive transducer 52 underwater on the structure to be tested. A technician stationed topside operates equipment for receiving and storing the data. 
     The ship board or top side portion of the direct transmission system 40 comprises a receptacle 162, FIG. 5F, which connects the direct transmission system 40 to an external one hundred twenty volt alternating current power source. Receptacle 162 is connected to a power line filter 164 for filtering the 120 VAC which is then provided to a twenty four volt switching power supply 166 (power supply 116 in FIGS. 3 and 4) through a switch S1. Closure of switch S1 by an operator also connects a battery power supply 168 (battery pack 130 in FIGS. 3 and 4) to the direct transmission system 40. Thus, 18 to 24 VDC which is required to power direct transmission system 40 is provided by either the twenty four volt switching power supply 166 or battery power supply 168 which includes three six volt lead acid rechargeable batteries connected in series. Battery power supply 168 is, in turn, a self contained power source which provides for approximately eight hours of continuous power to direct transmission system 40. 
     It should be noted that battery power supply 168 is used as a source of power when the ultrasonic test system of the present invention is used underwater to protect the diver using the system. 
     At this time it should be noted that there is a vent fan 170 (vent fan 114 in FIGS. 3 and 4) electrically connected via a switch 172 (switch 110 in FIGS. 3 and 4) to receptacle 162. Vent fan 170 is used to cool a battery charger 174. Switch 172 is activated by a vent door, not shown, which is a component of the housing for the ship board or top side portion of direct transmission system 40. When the vent door is open switch 172 is in a closed position, as shown in FIG. 5F, and vent fan 170 and power supply 166 are operational. 
     When direct transmission system 40 is utilizing battery 168 as the only source of power for the electrical components of the direct transmission system 40, closure of a momentary contact switch S2 by the operator energizes a coil 176 within relay 178 (relay 124 in FIGS. 3 and 4) which closes contacts 180, 182, 184 and 186 as shown in FIG. 5F and 5G. It should be noted that prior to closing momentary contact switch S2, the operator must first close switch S1 to allow for the energization of coil 176 of relay 178. 
     Energizing coil 176 of relay 178 connects battery 168 directly to voltage regulators 188, 190, 192, 194 and 196 as well as a DC to DC converter 198. Voltage regulator 196 is connected directly to a DC to DC voltage converter 197. When coil 176 is energized voltage regulators 188, 190, 192, 194 and 196 respectively provide at their outputs+12 VDC, +15 VDC,+12 VDC,+5 VDC and+SVDC which is then supplied to the electrical components of the ship board and underwater electrical components of the direct transmission system 40. Voltage converter 197, in turn, converts the+8VDC at its input to about-6.8 VDC which is provided at its output. 
     At this time it should noted that the energization of coil 176 of relay 178 allows battery 168 to energize DC to DC converter 198. DC to DC converter 198, which functions as a charger circuit, provides an output of approximately +15VDC at about 200 mA to a pump battery 200. Pump battery 200 (pump battery 134 in FIGS. 3 and 4) is then utilized in the indirect transmission system 42, FIG. 4, to energize suction pump 160, FIG. 4, that is used to actuate suction cups (not shown) that are used to attach the transmit transducer 50 and the receive transducer 52 to a planar surface, such as the surface 34 shown in FIG. 2. It should further be noted that closure of $3 and closure of contact 184 within relay 178 energizes a coil 202 within a relay 206 (relay 100 in FIGS. 3 and 4) thereby closing a contact 204 within relay 206. This energizes suction pump motor 160, FIG. 4. 
     In a similar manner, when power supply 166 is the source of power for the electrical components of direct transmission system 40 closure of switch $1 supplies power supply 166 with 120 VAC from receptacle 162. Power supply 166 then provides approximately 24 VDC and about a 1.2 amp maximum current via diode 204 to voltage regulators 188, 190, 192, 194 and 196 as well as a DC to DC converter 198. 
     When the current drain on battery 168 results in a voltage of approximately 15 volts at the positive terminal of battery 168 a sensing circuit 211 (battery latch out circuit 126 in FIGS. 3 and 4) comprising a twelve volt zener diode 210, transistor 212, and a twelve Kohm resistor 218 activates a coil 220 of relay 178 opening contacts 180, 182, 184 and 186 of relay 178 to a reset position which disconnects battery 168 from the electrical components of the direct transmission system 40. This sensing circuit 211 in combination with relay 178 protects battery 168 against over-discharge by isolating battery 168 from the electrical components of the direct transmission system 40. It should be noted that diodes 214 and 216 are protective diodes preventing back EMF when coils 220 and 176 are deenergized. 
     When battery 168 is fully charged to approximately 22 VDC, zener diode 210 maintains the base voltage of transistor 212 at a voltage level which is sufficient to restrict current flow from the emitter to the collector of transistor 212 and thereby maintain coil 220 in a nonenergized state. As battery 168 discharges the base voltage for transistor 212 decreases, the current flow through coil 220 increases and the base current for transistor 212 increases. When battery 168 discharges to approximately 15 VDC, the emitter to base voltage of transistor 212 will be at 0.6VDC turning on transistor 212 allowing sufficient current flow through coil 220 to energize coil 220. The energizing of coil 220 opens contacts 180, 182, 184 and 186 of relay 178 thereby disconnecting battery 168 from the electrical components of direct transmission system 40. 
     It should be noted that power supply module 64 of microcomputer 58 supplies a+5VDC to contact 182 of relay 178. When coil 176 of relay 178 is energized contact 182 is in the position illustrated in FIG. 5G such that the+5VDC provided by microcomputer 58 is supplied to NAND gates 227 and 229, flip-flops 222 and 224, a D flip-flop 240, decoder 69, a liquid crystal display 246, and a serial to parallel data converter 248. 
     Referring to FIGS. 5F and 6, direct transmission system 40 also includes a battery charger 174 (battery charger 118 in FIGS. 3 and 4) which is used to recharge battery 168 when power supply 166 is being utilized by direct transmission system 40. A charge cycle begins with charger 174 in a bulk charge state (state 1) and charger 174 functioning as a constant current source that provides a constant current of I MAX  to battery 168 which is defined by the following equation ##EQU2## where R S  is 0.33 ohms. Either of two conditions, in turn, initiates the bulk-charge state at the beginning of the cycle. The first condition occurs when an operator connects receptacle 162 to 120 VAC and the second condition occurs while battery charger 174 is in the float state, that is battery charger 174 senses a low voltage on battery 168. This constant current bulk charge state returns seventy to ninety percent of the charge capacity of battery 168. 
     Battery charger 174 senses the battery voltage for battery 168. When the voltage reaches a transition threshold voltage V 12 , charger 174 begins an overcharge cycle (state 2). V 12  is defined by the following equation 
     
         V.sub.12 =0.95 V.sub.OC                                    (2) 
    
     where V OC  is 22.2 VDC and is defined as the overcharge voltage. The overcharge voltage V OC  may be calculated by using the following equation ##EQU3## where R X  is defined by the following equation ##EQU4## and R A  =R A1  +R A2 , R B  is 4.7 Kohms and R C  is 49.9 Kohms. R A  is, in turn, determined by adjusting ten kohm variable resistor R A1  such that the overcharge voltage is set at 22.2 VDC. The overcharge voltage is, in turn, measured at the positive terminal of battery 168. 
     During the overcharge state, charger 174 regulates battery 168 at V OC  until the charge rate drops to a specified transition current I OCT  which is defined by the following equation ##EQU5## where R S  is 0.33 ohm. 
     When the current provided by battery charger 174 tapers to I OCT  with the battery&#39;s 168 voltage at V OC  the capacity of battery 168 is nearly one hundred percent. At this point, battery charger 168 functions as a voltage regulator with a precise output defined by the following equation ##EQU6## where R A  =R A1  +R A2 , R S  is 0.33 ohm and V REF  is a reference voltage for battery charger 168 which is temperature dependent and is typically between 2.425 VDC and 2.18 VDC for a temperature range of zero to seventy degrees celsius. 
     The output voltage of battery charger 168 is now in the third state V F , that is the float state. If the charge current increases sufficiently to reach I MAX  while in float state V F  then V 31  is reached which leads to the state 1 bulk charge condition after I MAX  is reached. 
     V 31  is, in turn defined by the following equation 
     
         V.sub.31 =0.9V.sub.F                                       (7) 
    
     At this time it should be noted that a complete written description of the operation of the battery charger circuit used in the present invention is provided in an article entitled &#34;IC Provides Optimal Lead-Acid--Battery Charger Cycles&#34; by Richard Valley published in Engineering Design News, Oct. 31, 1985, pages 161-178, which is incorporated by reference. It should also be noted that the constant current bulk charge first returns seventy to ninety percent of battery capacity and the remaining charge capacity is returned during the elevated constant voltage overcharge. In addition, it should be noted that external resistors R A1 , R A2 , R B , R C  and R S  determine the programming of all battery charger&#39;s 174 voltage and current levels which are set forth in equations one through seven. 
     The float charge state is sensed by a comparator 175 which activates a light emitting diode 177 to indicate battery 168 is fully charged. The battery charger used in the preferred embodiment of the present invention is a Unitrode Model UC3906 battery charger integrated circuit. 
     It should also be noted that a light emitting diode 179 is connected to the output of voltage regulator 188 to indicate that either switching power supply 166 or battery 168 is operational and thereby supplying current to the electrical components of the direct transmission system 40. It should also be noted that contact 184 of relay 178 must be closed before light emitting diode 179 is activated. 
     Direct transmission system 40 also includes a light emitting diode 181 which indicates the battery charger 174 is on and which requires the closure of contact 184 of relay 178 to be operational. 
     Referring to FIGS. 5A and 5B, ultrasonic transmitter 72 sends high voltage pulses to transmit transducer 50. The high voltage pulse may be about 1000 volts. The high voltage pulse from the ultrasonic transmitter 72 causes the transmit transducer 50 to produce an ultrasonic pulse. The high voltage pulse from the transmitter 72 also produces an appreciable amount of electromagnetic interference. A concrete piling (not shown) or like structure will be positioned between the transmit transducer 50 and the receive transducer 52. Ultrasonic pulses pass through the concrete structure and impinge on the receive transducer 52. 
     The output of the receive transducer 52 is then input to preamp 82, which provides an amplified signal to an ultrasonic amplifier 221, which is shown in FIG. 5D. The output of the ultrasonic amplifier 221 is then input to blanking circuit 74 shown in FIG. 5C. The blanking circuit 74 includes a monostable multivibrator 222, a monostable multivibrator 224, a NAND gate 223 and a NAND gate 225, a NAND gate 227 and a NAND gate 229. 
     At this time it should be noted that flip-flops 222 and 224 are National Semiconductor Model MM74C221 Dual Monostable multivibrators and NAND gates 223, 225, 227 and 229 are RCA Model CD4093B Qaud 2-Input NAND Schmitt Triggers. The flip-flops 222 and 224 are connected to function as monostable multivibrators. The monostable multivibrator 222 is arranged so that when the input B1 receives a pulse, output Q1 of multivibrator 222 outputs a 15 microsecond positive going pulse. The NAND gates 223, 225, 227 and 229 are connected to function as Schmitt trigger circuits. 
     Simultaneously with outputting each high voltage pulse through the concrete structure under test, ultrasonic transmitter 72 outputs a logic pulse T&#39; at its T output that is input to pin 8 of NAND gate 227. Pin 9 of the NAND gate 227 is connected to the CLR inputs of multivibrators 222 and 224 which, in turn, receive+5VDC from the power supply module 64 of microcomputer 58 via contact 182 of relay 178. The logic pulse T&#39; when input to NAND gate 227 causes the output of NAND gate 227 to provide a negative going pulse, that is a logic zero pulse. This logic zero pulse is inverted by NAND gate 229 and is then supplied to the T input of V-meter electronics module 76, FIG. 5D, and the B1 input of multivibrator 222. 
     It should be noted that the logic pulse T&#39; provided by transmitter 72 is a voltage spike and that NAND gates 227 and 229 are used to convert the logic pulse T&#39; to a square wave pulse. 
     The logic zero to one transition of the pulse occurring at the output of NAND gate 229 triggers multivibrator 222. The Q1 output of multivibrator 222 is connected to the A2 input of multivibrator 224, which results in multivibrator 222 providing a 15 microsecond positive going pulse to the A2 input of multivibrator 224. 
     The logic one to zero transition of the 15 microsecond positive going pulse triggers multivibrator 224 which provides a one millisecond positive going pulse to pin 1 of NAND gate 223. Pin 2 of NAND gate 223 is connected to the output of ultrasonic amplifier 201 to receive the signal output of the receiver 52 after amplification by preamp 82 and amplifier 201. The one millisecond pulse input to NAND gate 223 allows the output of the ultrasonic amplifier to pass to NAND gate 225. 
     The one millisecond pulse input to pin 1 of NAND gate 223 acts as a window. If the output of ultrasonic amplifier 221 reaches NAND gate 223 during the one millisecond window, then the output of the ultrasonic amplifier 221 will pass to NAND gate 225. NAND gate 225 inverts the pulse provided by NAND gate 223 and then inputs it to the ultrasonic V-meter electronics module 76. James Instruments, Inc. Model C-4901 may be used as the ultrasonic V-meter electronics module 76. The ultrasonic V-meter electronics module 76 has a T input that is connected to the output of the NAND gate 229. When the square wave pulse from NAND gate 229 is supplied to the ultrasonic V-meter electronics module 76, counting circuitry within module 76 (not shown) is activated. 
     The leading edge of the pulse from NAND gate 225 when supplied to ultrasonic V-meter electronics module 76 turns off the counting circuitry within module 76, holding the count between the pulse to the T input of module 76 and the pulse received from NAND gate 225. The blanking circuit 74 is desirable because it is undesirable for the ultrasonic V-meter electronics module 76 to respond immediately after the pulse from NAND gate 229 is received. A finite time is required for the ultrasonic signal to travel through the concrete and reach the receiver transducer 52. Additional time is required for the electrical signal from the receiver transducer 52 to pass through the circuitry of direct transmission system 40 to the ultrasonic V-meter electronics module 76. Without the blanking circuit 74 there is a problem with cross talk between the 1000 volt pulse output from the ultrasonic transmitter 72 and the receive line that connects the preamplifier 82 and the ultrasonic amplifier 221. 
     The V-meter electronics module contains circuitry including the counting circuitry for computing the transit time of the ultrasonic wave through the concrete structure under test. The transit time for the ultrasonic wave through the concrete structure under test is displayed by four digit display 80, illustrated in FIGS. 3 and 4. The transit time is also provided from BCD output 86 through parallel to serial converter 88 to microcomputer 58 through its PB3 input. It should be noted that the parallel to serial converter 88 comprises a pair of parallel to serial converters 250 and 252. 
     The separation distance transducer 54 output signal is input to an amplifier 243 and then input to a data acquisition module 65 of the microcomputer 58. The microcomputer 58 then calculates the velocity of the ultrasonic waves in the concrete by dividing the distance the waves travel in the concrete structure under test by the time interval required to traverse the distance. 
     Referring to FIGS. 3, 5G and 5H, keypad 67 is used by the operator to input certain digital data/information into microcomputer 58 such as the number of measurements to be made on a concrete structure under test. When the operator desires to enter data into microcomputer 58 a sixteen key decoder 69 which is electrically coupled to keypad 67 provides a logic zero to one transition at its DA output (data available). This logic zero to one transition clocks the logic one at the D input of Flip-Flop 240 to the Q output thereof which causes the not Q output of Flip-Flop 240 to transition from the logic one to the logic zero state thereby providing an interrupt to the/RSTA input of microcomputer 58. This interrupt provided by Flip-Flop 240 indicates to microcomputer 58 that decoder 69 is ready to provide data to microcomputer 58. 
     At this time it should be noted that the decoder used in the preferred embodiment of the present invention is a CMOS key encoder, Model MM74C992 16 Key Encoder manufactured by National Semiconductor. 
     The functions of the keys on keypad 69 are briefly described as follows: L selects the alpha character located in the left hand corner of each alphanumeric key; M selects the alpha character located in the middle of each alphanumeric key and R selects the alpha character located in the right hand corner of each alphanumeric key. DELETE allows deletion of the last entry without affecting other data entered; SELECT allows a cursor to be moved around liquid crystal display 246 to select a desired function and DONE allows data to be entered into the static RAM module 71 of microcomputer 58 or executes the selected function. The alphanumeric keys &#34;0-9, A-Z, +, -, #, SP&#34; allow either numbers or letters to be entered microcomputer 58 via decoder 69. Numbers are selected by pressing the desired key, while letters are entered by pressing a position key (either L, M or R) and the key with the desired letter on it. A blank space is generated by selecting the SP key. 
     The liquid crystal display 246 used in the preferred embodiment of the present invention is a 4 line by 40 character LCD display Model LM44A4C40CBW LCD manufactured by Densitron. Data provided by microcomputer 58 for display by liquid crystal display 246 is provided to a serial to parallel data converter 248 which converts the data from an eight bit serial word to an eight bit parallel word and then supplies each eight bit parallel word to liquid crystal display 246 for display thereby. 
     Direct transmission system 40 also has a diver communications circuit which includes a microphone jack 249, FIG. 5C, adapted to receive microphone 150, FIGS. 3 and 4, diver earphone volume control 152 connected to jack 249, audio amplifier 148 connected to jack 249 and diver earphone 146 connected to the output of the amplifier 148. The diver communications circuit allows for ship or shore to diver communication with the diver using direct transmission system 40 to test an underwater concrete structure for deterioration. 
     A program listing is attached to this disclosure as an Appendix A. The program listing is an example of software that may be used with the apparatus shown in FIGS. 3, 4 and 5A-5H. 
     The structures and methods disclosed herein illustrate the principles of the present invention. The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects as exemplary and illustrative rather than restrictive. Therefore, the appended claims rather than the foregoing description define the scope of the invention. All modifications to the embodiments described herein that come within the meaning and range of equivalence of the claims are embraced within the scope of the invention. ##SPC1##