Abstract:
A temperature measuring and monitoring arrangement, in particular for leak detection on pipelines. A number of temperature sensors ( 30 ) are arranged in a distributed array along the item to be monitored and are connected to one or more computers ( 15, 19 ) by way of communication units ( 10 ) and at least one serial connecting line ( 11 ). The temperature sensors ( 30 ) have a measuring member and a reference member which are included as a frequency-determining member in a measuring oscillator and a reference oscillator respectively. The temperature can be corrected and in particular drifts can be substantially eliminated by suitable mathematical treatment of the frequency values. Anomalies in respect of temperature distribution can be interpreted as a leak in a pipeline.

Description:
FIELD OF THE INVENTION 
     The invention concerns a temperature measuring and monitoring system for leak detection on pipelines. 
     BACKGROUND OF THE INVENTION 
     There are many different methods for locating leaks in pipelines, including those methods which are based on the variation in temperature when leaks occur. In German patent application DE 30 49 544 A1 sensors in respect of temperature, moisture and radioactivity are arranged between the pipeline wall and the insulation and are connected to an analysis station by way of optical waveguides or fiber optics. Many sensors and accordingly many optical waveguides are needed in order to analyze propagation of the leakage. The handling of such optical waveguides in the ground gives rise to problems. 
     German patent application DE 195 09 129 A1 describes a further known method and apparatus for checking and monitoring the condition of pipes, containers, vessels, pipelines or the like, wherein local fluctuations in temperature are detected. This involves establishing the ambient temperature distribution over portions of the item to be monitored, and using an elongate temperature sensor for distributed temperature measurement. When a local anomaly is detected in the temperature distribution, it is concluded that there is a leak. For establishing the ambient temperature distribution, use is made of an optical-fiber sensor cable which is operated with laser light, and evaluation in respect of transit time and intensity of the backscattered laser light is implemented. That mode of temperature detection is subject to error sources which vary with time so that reliable evaluation over long periods of time becomes difficult. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a novel temperature measuring and monitoring system for leak detection in pipelines. It is a further object of the invention to keep drifts in temperature measurement at a low level or even to eliminate such drifts. 
     With the present invention, a number of temperature sensors are provided which each have a measuring member and a reference member. The measuring member is included as a frequency-determining member in a measuring oscillator which is of the RC- or LC-type and the reference member is included as a frequency-determining member in a similar RC- or LC- reference oscillator. Such oscillators have active components; which are disposed on a common carrier and are made up of similar materials or comprise materials which age in a similar manner. Associated with each sensor is a communication unit which has transmitting and receiving units in order to respond upon receipt of an associated address so as to interrogate the temperature sensor. In that situation the oscillator frequency of the reference oscillator and of the measuring oscillator is sent to an evaluation circuit which evaluates the measuring frequency to determine the temperature at the measuring member. A mathematical function including the measuring frequency and the reference frequency is used to correct the respective provisionally acquired temperature value and to substantially eliminate drifts in respect of temperature measurement. 
     In the case of extended items as pipelines and pipeline systems are, groups of temperature sensors and associated communication units are formed and are connected by way of a serial connecting line having two or three wires. Associated with each group is a group computer as an evaluation circuit for each serial connecting line and the group computer operates each temperature sensor with its address allocated thereto and interrogates it in order to compare temperature values obtained in that way to predetermined temperature values. In that fashion, anomalies in temperature distribution at the item to be monitored are detected. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further details of the invention are described with reference to the drawing in which: 
     FIG. 1 is a diagrammatic view of a temperature measuring and monitoring system of a pipeline network, 
     FIG. 2 is a diagrammatic view of a pipeline portion with communication unit and temperature sensor, 
     FIG. 3 is a view in section through the pipeline portion shown in FIG. 2, 
     FIG. 4 is a view in section through buried pipelines, 
     FIG. 5 is a diagrammatic view of a communication unit, and connected to a block circuit diagram of the temperature sensor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, shown therein is a pipeline system with two groups of pipelines  1  and  2  which include individual pipeline sections  4 ,  5  and  6  which can be communicated with each other and which are to be monitored jointly in respect of leakages. For that purpose, disposed along each pipeline  4 ,  5  and  6  are temperature sensors and associated communication units  10  which are connected by way of a serial connecting line  11  and  12  respectively to an associated route or group computer  15  and  16  respectively. The group computers  15  and  16  communicate by way of suitable data lines, for example telephone lines  17  and  18 , with a central computer  19 . The serial connecting lines  11  and  12  comprise two or three wires and are laid along the respective pipelines  4 ,  5  and  6  and taken to the respective route or group computers  15  and  16 . 
     FIG. 2 shows a portion of a pipeline  4  which includes an inner pipe  7 , an outer pipe  8  and an insulating sheathing  9  disposed therebetween. A communication unit  10  is carried an the outer pipe  8  or is disposed in the proximity thereof. Theserial connecting line  11  leads to further communciation units on the pipe, as indicated in FIG.  1 . Usually, leakages occur in the proximity of the connecting locations between individual pipes, and for that reason the communication units  10  are disposed in the proximity of such locations which may comprise a polyethylene coupling sleeve or socket (FIG.  3 ). 
     FIG. 4 shows the configuration of two parallel pipelines  5 ,  6  in the ground  20 . Normally, a trench  21  is dug, in which the lines  5 ,  6  are laid and enclosed with sand or the like filling material  22 . The communication unit  10  is disposed at a protected location on the pipeline, for example at the upper apex point of a coupling sleeve or socket. 
     FIG. 5 shows one temperature sensor  30  and the communication unit  10  associated thereto, i. e. arranged adjacent to one another, as indicated in FIG. 2, in the same casing. The communication unit  10  includes an addressing logic  40  which is connected by way of the serial connecting line  11  (or  12 ) to an associated computer  15  (or  16 ). Addressing logics are known per se and do not need to be described in greater detail herein. The function thereof is to evaluate addressing data arriving an the line  11  (or  12 ), that is to say to respond or not to respond, and, in the event of responding, to produce switching operations in the temperature sensor  30 , that is to say to connect same to the power supply and to switch the frequency signals F t , F r  on to the line  11  (or  12 ). 
     The temperature sensor  30  comprises a chip  31  and two resistors R 1  and R 2  in series with capacitors C 1  and C 2 . A supply voltage Ub of for example 15 V is fed on a line  32  and a line  33  serves for the current return. Within the chip  31 , the line  32  has a branching  34 . Extending between the lines  32 ,  34  and  33  respectively are two voltage dividers formed by resistors  35   a ,  36   a ,  37   a  and  35   b ,  36   b ,  37   b  respectively so that ⅔ Ub and ⅓ Ub can be tapped off at those voltage dividers. The tapped-off voltages are fed to a series of comparators  41 ,  42 ,  43 ,  44  which each have two inputs, wherein the first inputs are connected to the above-mentioned tappings of the voltage dividers while the second inputs are connected by way of lines  38  and  39  respectively to the capacitors C 1  and C 2  respectively. The comparators  41 ,  42  are connected at their outputs to the inputs Q and {overscore (Q)} of a bistable multivibrator  45  and the comparators  43  and  44  are connected at their outputs to Q and {overscore (Q)} of a bistable multivibrator  46 . The multivibrators  45  and  46  each have a respective output line  47  and  48  which can be connected by way of the addressing logic  40  to the serial connecting line  11 . The multivibrator  45  controls an electronic switch  49  and the multivibrator  46  controls an electronic switch  50 , which switches are each in the form of single-pole change-over switches. The connecting lines of the electronic switch  49  are identified by  32   a ,  33   a  and  51  while those of the electronic switch  50  are identified by  33   b ,  34   b  and  52 . The capacitor C 1  is either connected by way of the resistor R 1  and the switch  49  to supply voltage Ub and is charged up, or the capacitor is discharged in the switch position illustrated. The same applies in regard to the capacitor C 2 , the resistor R 2  and the switch  50 . When the capacitor Cl charges and reaches ⅔ Ub voltage, the comparator  41  switches the multivibrator  45  into the ‘low’ state, whereupon the electronic switch  49  assumes the illustrated position in which the capacitor C 1  discharges by way of the resistor R 1 . When ⅓ Ub voltage is reached the comparator  42  responds and switches the multivibrator to ‘high’, whereby in turn the electronic switch  49  is switched over and the capacitor C 1  is; charged by way of the resistor R 1  and the lines  51 ,  32   a ,  32 . When the voltage at the capacitor C 1  again reaches ⅔ Ub, that is detected at the comparator  41  and the multivibrator  45  again goes into the ‘low’ state. That procedure is continuously repeated and produces a frequency signal F t  on the line  47 , which depends on the instantaneous resistance value of the resistor R 1 . The lower the resistor R 1  is, the correspondingly higher is the frequency F t . 
     The capacitor C 2 , the resistor R 2 , the switch  50 , the comparators  43 ,  44  and the bistable multivibrator  46  co-operate in the same manner as described in respect of the components C 1 , R 1 ,  49 ,  41 ,  42 ,  45  so that a frequency signal Fr is also produced at the output line  48 , that frequency signal being characteristic in respect of the resistance value of the resistor R 2 . The resistor R 2  is produced from a metal-film resistor with the lowest possible temperature coefficient so that the frequency F r  which is tapped off at the line  48  remains fairly constant and can be used as a reference frequency. The resistor R 1  is a temperature-dependent resistor and the signal F t  is a temperature-dependent frequency. There are resistors with a positive temperature coefficient, for example a platinum resistor or a silicon PTC resistor, and there are resistors with a negative temperature coefficient, for example with an NTC resistor. Both kinds of resistors are suitable. The temperature sensor  30  is distinguished in that, as a result of the supply voltage division by means of the voltage dividers  35   a ,  36   a ,  37   a  and  35   b ,  36   b  and  37   b  respectively, fluctuations in the supply voltage Ub do not have any falsifying influence on temperature detection as such error sources cancel each other out or can be compensated from the measuring frequency and the reference frequency. While an RC-member has been used in the oscillator circuit as the frequency-determining member, it is also possible to use an LC-member in the oscillator as the frequency-determining member. In addition it is also possible to use a quartz crystal with a special ground plane, in the gase of which the TC-reversal point is far outside the temperature range to be measured. The ground quartz crystal is used in place of the RC- or LC-member in the respective oscillator. In both of the oscillators, the temperature/frequency characteristic is selected in different linear regions. It should be noted that, the resonance frequency of the quartz crystal depends only oil its mechanical dimensions- Those dimensions alter with temperature and thus the resonance frequency also alters therewith.