ELECTRONIC CIRCUITRY FOR DETECTING LEAKAGE OF CHEMICAL SUBSTANCE

Abstract: Electry circuitry is described which employs time domain reflectometry techniques and impedance mismatches in a detection cable to record faults occurring in the cable. Problems relating to the apparent rise of impedance over distance are overcome by use of a single pulse and use of circuitry that permits setting of a voltage threshold that allows high sensitivity to be achieved at the far end of the cable.

Description: TITLE OF THE INVENTION Electronic Circuitry For Detecting Leakage of Chemical Substances FIELD OF THE INVENTION This invention relates to electrical leak detection circuitry and especially to circuitry designed to detect leaks of liquid chemicals, such as hydrocarbons, from piping or storage tanks. BACKGROUND OF THE INVENTION Leakage of liquid hydrocarbons can be detected electronically by placing an electric detection cable along hydrocarbon storage tanks or along piping for carrying the hydrocarbon. Such cables are well know, as seen, for example, in USP 4,877,923 (Sahakian). In such cables, the leaking liquid penetrates insulation between conductor wires causing the electrical characteristics of the cable to change. Usually, time domain reflectometry (TDR) procedures are used to detect these changes. In TDR procedures, an electrical impulse is sent down the conductor wires and the impedance is monitored. Any change in the impedance caused by, for example, a break in the cable or a change in the dielectric of the insulation is recorded as a reflected wave of the pulse. A difficulty arises with TDR in that TDR equipment inherently records a rise in impedance as the length of the cable increases. Thus, when sensitivity limits are set against a voltage zero standard, the apparent rise in impedance means that near the far end of the cable, the sensitivity limit will be much wider than the original limit. This means that leaks near the far end of the cable could go undetected if the magnitude of the reflected wave does not encompass or exceed the wider threshold limit. It is a purpose of this invention to overcome this difficulty. SUMMARY OF THE INVENTION The apparent rise of impedance over distance in TDR circuitry is eliminated in the invention herein by: 1) reducing the TDR to a single launch pulse and monitoring the effects of the single pulse so that no pulse overlap occurs; 2) by providing a time delay component that resets the trip latches when they are tripped by the strength of the launch pulse so that only reflections are monitored after the latches are reset; 3) by providing a digital-to-analog coverter programmed to set the upper sensitivity limit against voltage zero and programmed to adjust the lower threshold sensitivity to the most negative voltage dip created by imperfections in the cable. The rise in impedance is eliminated at the far end, enabling good sensitivity to be achieved. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram of the circuitry of the system of the invention. Figure 2 depicts a voltage zero line and upper and lower voltage threshold levels. DESCRIPTION OF THE INVENTION In this invention, time domain reflectometry (TDR) techniques are used to monitor leak detections in cables, such as coaxial cable. TDR is used to sense changes in the impedance of a cable. "Impedance", as used herein, is the electrical resistance to a signal being transmitted down the cable. Maximum signal transfer is attained when impedances of the wires are matched because no reflections occur. A terminated cable is used since a matched resistive load placed across the open end of the cable makes the cable appear to be infinitely long. A properly terminated cable will have no reflections of the pulse. However, once the cable is broken or the impedance changed due to a leak that changes the dielectric of the insulation, a "cable fault" has been created at that point. This creates an impedance mismatch. It is this mismatch that causes a reflected wave return of the original pulse, and this phenomena is known as "reflectance". Not only can an impedance mismatch occur from a break or a change of the insulation dielectric, but it can also occur from faults inherent in the cable, such as a connector in the line or a kink in the line. It is desirable to eliminate or minimize such inherent imperfections in order to set the sensitivity limit as close to an actual break or leak as possible. This becomes particularly important as the cable length increases since attenuation or power strength loss increases as the cable length increases. Thus, the smaller impedance change recorded near the far end of the detection cable can be recorded by the circuitry of this invention. The "reflectance" can be of two types. It can be what is called a "positive reflectance" in which the amplitude of the reflected pulse is above the system ground and is due to a low to high impedance transition (which means it is caused by an open or connector mismatch). The reflectance can also be a "negative reflectance" in which the amplitude of the reflected pulse is below the system ground and is due to a high to low impedance transition (which means it is due to a short or leak). As shown in Figure 2, the black horizontal line 30 represents a voltage zero line of an oscilloscope. The dotted lines 31 and 32 denote the upper and lower voltage threshold respectively and taken together show typical sensitivity limits. A positive reflection is denoted by Peak "A" and a negative reflection is denoted by Peak "B". Referring now to Figure 1, the leak detection circuitry comprises a pulse generator 10, time delay gate 11, two RS latches 12A and 12B (one for a positive reflection and one for a negative reflection), a high speed window comparator 13 and a digital-to-analog connector 14 (DAC). The pulse generator, of course, generates the single pulse and sends it down the cable, which, in Figure 1, is denoted by 15 (the terminated leak detection cable) with a 50 ohm resistor on the end. Use of a single pulse is important because it precludes pulse overlapping which results in impedance rise at the furthest end of the cable. The time delay gate 11 is used to reset the trip latches when they are tripped by the intensity of the launch pulse so that only reflections are monitored. The status outputs of the RS latches 12A and 12B record the effects that the launch pulse has on the cable due to impedance mismatches and their resulting reflections. The high speed signal comparator 13 is an electronic device that compares the pulse pattern with the sensitivity limits. If the dip in the pulse reflectance exceeds the sensitivity limit, the latch is tripped and an appropriate warning is indicated. The digital-to-analog converter is used to set the upper sensitivity limit and is used to electronically adjust the lower threshold at a variable level that, for a particular cable, will be just off the level of the most negative voltage dip caused by cable imperfections. It is convenient to employ a microprocessor chip to continually process the data and automatically monitor and control the various functions. The microprocessor initiates the sequence of events and monitors the results. It also control the operation of the digital-to-analog converter which in turn establishes and records the upper and lower sensitivity thresholds. The sequence of events to test a terminated cable for faults or leaks is as follows: A single pulse is launched at one end of the cable. Since the connector and latches must be located near the launch point in order to detect reflections along substantially all the cable line, the latches are tripped immediately upon launching of the pulse. In order to reset the latches so that they can then sense reflections of a predetermined magnitude, a portion of the launch pulse is tapped off, delayed a specific amount of time (long enough for the launch pulse to be on its way down the cable), and applied to the reset inputs of the latches. This results in self-resetting circuitry that resets the latches. There are two RS latches, one to capture positive reflections, the other to capture negative reflections. If no reflections occur on a properly terminated cable, the latches will not be set and the cable is considered undisturbed. If any of the latches are set, then a fault has occurred and been detected in the cable. The type of fault can then be determined by distinguishing which latch has been set. The sensitivity of the latches to reflections is established by adjusting the comparator's voltage "window". The microcontroller and the DAC are used for this to obtain an auto-referencing circuit. Referencing is initiated by pressing a momentary switch. Since the upper (positive) voltage of the comparator window only detects a cable open, it is set to a stationary value. However, the lower (negative) voltage of the comparator window determines how sensitive the comparator is to sensing leaks (or shorts) and will vary from cable to cable. Upon referencing, the DAC initially sets up the comparator with a wide window and a pulse is sent down the cable. If the latches remain cleared (no faults), then the lower (negative) window is closed one DAC step and again a pulse is sent and the latches read. This process is repeated until at some point the latch representing a leak becomes set. This occurs at the greatest reflection point found on the cable. This could be from a connector or just the steady state noise level of the cable. The DAC then opens the window slightly and the circuit is ready to detect leaks.

Claims: WE CLAIM: 1. Process for monitoring impedance over the length of a coaxial, resistor terminated cable, which comprises: a) causing a single electrical launch pulse to be sent down the cable; b) tapping off a portion of the launch pulse, then delaying the portion and then the sending the portion to a pair of trip latches thereby causing the trip latches to be reset; c) causing a digital-to-analog converter to record both an upper and lower voltage threshold sensitivity levels to establish a sensitivity range limit for a high speed signal comparator. 2. An electrical system for detecting leakage of chemical substances which comprises: a single electrical pulse generator electrically connected to a terminated leak detection cable that uses impedance changes between two conductors to record faults; a tap means comprising a conductor leading to an electronic detection assembly; said components of the electronic detection assembly comprising: a time delay gate, a latch to record a positive reflection, a latch to record a negative reflection, a high speed signal comparator, a digital-to-analog converter, said components of the electronic detection assembly being constructed and arranged to record faults in the leak detection cable by recording wave reflections created by a mismatch of the impedances of the cable conductors.

Inventors: BERKMAN SAMUEL, REDDY WILLIAM JOHN, BERKMAN, SAMUEL, REDDY, WILLIAM, JOHN
Applicants: GORE & ASS, W.L. GORE & ASSOCIATES, INC.
Application year: 1991
Main classification: G01R31
IPC codes: G01M3, G01R27, G01R31