GB2039054A

GB2039054A - Monitoring a medium

Info

Publication number: GB2039054A
Application number: GB7939649A
Authority: GB
Original assignee: BICC PLC
Current assignee: Balfour Beatty PLC
Priority date: 1978-11-15
Filing date: 1979-11-15
Publication date: 1980-07-30
Also published as: GB2039054B

Abstract

A system for monitoring a condition (e.g. temperature or pressure) of a medium to determine its average value includes a pair of elongate conductors of such a construction that, over its length, the pair of conductors has a characteristic which will change gradually over a range of values of the condition being monitored. The system also includes means for injecting an oscillating signal or a train of pulses to the pair of conductors and means for detecting a change in frequency or voltage of the oscillating signal or of the train of pulses arising from a change in the characteristic of the pair of conductors. A device for displaying the average value of the condition may be connected to the detecting means.

Description

SPECIFICATION An improved monitoring system This invention relates to monitoring of a condition of a medium in order to determine the average value of the condition.

It is an object of the invention to provide an improved system for this purpose.

According to the invention the improved monitoring system comprises a pair of elongated electric conductors of such a construction that, over at least a part of the length of the pair, the pair of conductors has a characteristic which will change gradually over a range of values of the condition being monitored; means for injecting an oscillating signal or a train of pulses to the pair of conductors; and means for detecting a change in frequency or voltage of the oscillating signal or a change in frequency of the train or pulses arising from a change in said characteristic of the pair of conductors.

A device for displaying the average value of a condition to be monitored may be connected to the detecting means.

The monitoring system may also include a system for controlling the condition to be monitored and/or an audible and/or visible alarm which will be actuated when the pair of conductors of the system detects any unacceptable change from the norm in the average value of the condition being monitored.

Where a pair of conductors is employed that will determine the average temperature of a medium, the monitoring system is especially suitable for use in detecting elevated temperature conditions. By use of appropriate conductor pairs, other conditions of a medium that may be monitored include pressure, humidity and the presence or absence in the medium of a fluid.

Usually, but not necessarily, the pair of elongate electric conductors of the monitoring system is a coznponent part of an electric cable and, where it quire to monitor the temperature of a medium, preferably the electric cable is a temperature detector cable in which two substantially parallel conductors insulated from each other have ferromagnetic material positioned between them or it is a temperature detector cable which is substantially as described and claimed in the Complete Specification of our Patent No. 1544941.

The monitoring system may be employed using either of two principles of operation. In the first of these two principles of operation, the pair of conductors is used as a resonant or near-resonant line. In the second of these principles of operation, the pair of conductors is used as a condition dependent "lumped" element.

In both principles of operation, since it is only required to determine the average value of a condition, it is possible to employ an oscillating signal or train of pulses of lower frequency than has previously been proposed or used to monitor the condition of a medium with the result that the effects of localised discontinuities or irregularities along the pair of conductors are substantially reduced. Consequently, the limitations otherwise present as to the maximum distance within a medium that can be satisfactorily monitored are substantially reduced. For example, for a cable having a length of 1 km, we prefer to empioy an oscillating signal or train of pulses of a frequency lying in the range 20 to 60 kHz.

The invention will be further illustrated by a description, by way of example, of five methods of monitoring the temperature of a medium using the improved monitoring system in order to determine the average temperature of the medium, with reference to the accompanying drawings.

In each of the methods described the electric cable positioned in the medium to be monitored comprises a central conductor which is insulated from a tubular outer conductor coaxial with and surrounding the central conductor by a layer of insulating material in which a quantity of magnetically soft ferromagnetic material is dispersed. A protective sheath surrounds the outer conductor.

Referring to Figure 1 of the drawings, in the first of the five methods to be described, a voltage controlled oscillator 1 is connected to one end of a temperature detector cable (not shown) and is used to 'track' the fundamental resonant frequency of the cable. The oscillator 1 has a range of variation which encompasses the expected range of variation of the resonant frequency of the cable. The operating frequency of the oscillator 1 or the control voltage are measured and compared with a range of values of frequency between pre-set upper and lower limits, and a system (not shown) for controlling the temperature of the medium is activated if the operating frequency or control voltage goes outside this limited range.

In the second method shown in Figures 2a and 2b, an oscillator 11 with a high output impedance is connected to one end of a temperature detector cable (not shown) and is adjusted to a set frequency slightly different from the fundamental resonant frequency of the cable at its normal operating temperature, for example the point X shov... in Figure 2a. The input impedance of the cable is measured using a voltmeter 12 connected directly across the terminals of the cable. As the temperature of the medium surrounding the cable changes, the input impedance of the cable will vary producing a corresponding variation in the terminal voltage of the cable. As in the first method, a control system (not shown) may be activated when the terminal voltage goes outside a pre-set range.

In the third method shown in Figure 3, a temperature detector cable 23 is used as a delay line whose electrical length varies with temperature to control the repetition frequency of a pulse generator 21. The monitoring system is arranged so that oscillation is maintained either by detecting the arrival of the injected pulse at the far end of the cable 23 or by detecting the echo of that pulse from the far end. The frequency of oscillation is measured by means of a frequency measuring circuit 22 and compared with pre-set upper and lower values and a control system (not shown) activated in the event that the frequency of oscillation reaches one or other of these pre-set values.

In a fourth method, it is possible to measure the "lumped" equivalent capacitance (or inductance) of a temperature detector cable using an autobalancing bridge, provided that the operating frequency is significantly lower than the resonant frequency of the cable. The measured "lumped" capacitance (or inductance) is compared with preset upper and lower limits for the "lumped" capacitance (or inductance) and a control system is activated when one or other of these pre-set limits is reached.

Figure 4 shows the fifth method in which a temperature detector cable 33 is used as a part, or the whole, of one of the frequency determining elements in the tuned circuit of an oscillator (not shown), the resonant frequency of the circuit being significantly lower than the fundamental resonant frequency of the cable itself. In this method, the frequency of oscillation is measured and compared with the pre-set upper and lower limits of frequency to activate a control system.

Claims

1. A system for monitoring a condition of a medium in order to determine the average value of the condition, which monitoring system comprises a pair of elongate electric conductors of such a construction that, over at least a part of the length of the pair, the pairof conductors has a characteristic which will çhange gradually over a range of values of the condition being monitored; means for injecting an oscillating signal or a train of pulses to the pair of conductors; and means for detecting a change in frequency or voltage of the oscillating signal or a change in frequency of the train of pulses arising from a change in said characteristics of the pair of conductors.

2. A monitoring system as claimed in Claim 1, wherein a device for displaying the average value of the condition to be monitored is connected to the detecting means.

3. A monitoring system as claimed in Claim 1 or 2, which monitoring system also includes an audible and/or visible alarm which will be actuated when the pairs of conductors of the system detects any unacceptable change from the norm in the average value of the condition being monitored.

4. A monitoring system as claimed in any one of the preceding Claims, which monitoring system also includes a system for controlling the condition to be monitored.

5. A monitoring system as claimed in any one of the preceding Claims, wherein a pair of elongate electric conductors of the monitoring system is a component part of an electric cable.

6. A monitoring system as claimed in Claim 5 in which the condition of a medium to be monitored is temperature, wherein the electric cable is a temperature detector cable in which two substantially parallel conductors insulated from each other have ferromagnetic material positioned between them.

7. A monitoring system as claimed in Claim 5 in which the condition of a medium to be monitored is temperature, wherein the electric cable is a temperature detector cable substantially as described and claimed in the Complete Specification of Patent No. 1 544941.

8. A monitoring system as claimed in any one of Claims 1 to 5, wherein the pair of elongate electric conductors will determine the average pressure of a medium.

9. A monitoring system as claimed in any one of Claims 1 to 5, wherein the pair of elongate electric conductors will determine the average humidity of a medium.

10. A monitoring system as claimed in any one of Claims 1 to 5, wherein the pair of elongate electric conductors will determine the presence or absence in the medium of a fluid.

11. A method of monitoring a condition of a medium in order to determine the average value of the condition using a monitoring system as claimed in any one of the preceding Claims, wherein the pair of elongate electric conductors is used as a resonant or near-resonant line.

12. A method of monitoring a condition of a medium in order to determine the average value of the condition using a monitoring system as claimed in any one of Claims 1 to 8, wherein the pair of elongate electric conductors is used as a condition-dependent "lumped" element.

13. A method of monitoring the temperature of: a medium substantially as hereinbefore described by way of example and with reference to Figures 1 to 4 of the accompanying drawings.