EP0087308A2 - Systèmes de surveillance de température - Google Patents

Systèmes de surveillance de température Download PDF

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Publication number
EP0087308A2
EP0087308A2 EP19830300895 EP83300895A EP0087308A2 EP 0087308 A2 EP0087308 A2 EP 0087308A2 EP 19830300895 EP19830300895 EP 19830300895 EP 83300895 A EP83300895 A EP 83300895A EP 0087308 A2 EP0087308 A2 EP 0087308A2
Authority
EP
European Patent Office
Prior art keywords
temperature
resistance
signal
cable
value
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP19830300895
Other languages
German (de)
English (en)
Inventor
Roy Frederick Nailor
Paul Owen Sanders
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Dreamland Electrical Appliances PLC
Original Assignee
Dreamland Electrical Appliances PLC
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Dreamland Electrical Appliances PLC filed Critical Dreamland Electrical Appliances PLC
Publication of EP0087308A2 publication Critical patent/EP0087308A2/fr
Withdrawn legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G08—SIGNALLING
    • G08B—SIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
    • G08B17/00—Fire alarms; Alarms responsive to explosion
    • G08B17/06—Electric actuation of the alarm, e.g. using a thermally-operated switch

Definitions

  • This invention relates to temperature monitoring systems and is particularly concerned with the way in which cables of such systems are energised.
  • a known type of temperature monitoring system particularly (but not exclusively) used for detecting fires, includes a cable comprising two or more conductors separated by elongate temperature-sensitive means whose impedance varies with temperature and detection means operative to provide an output signal if such impedance achieves a predetermined or threshold value indicative of the cable being at a predetermined temperature.
  • the elongate temperature sensitive means may for example comprise polyvinyl chloride (PVC) which has a negative temperature coefficient such that at room temperature it acts as an insulator and such that its impedance drops with increasing temperature in a known manner.
  • PVC polyvinyl chloride
  • the impedance (or a component thereof) of the temperature sensitive means is measured by sending a current through the temperature sensitive means and generating a signal related to the impedance (or a component thereof) of the temperature sensitive means.
  • d.c. current it is particularly convenient to employ d.c. current for this purpose, since one can make use of a simple circuit network in the form of a resistive divider to develop a voltage proportional to the resistance of the temperature-sensitive means.
  • the resistance of the temperature-sensitive means in particular when such means is PVC, increases with time when a d.c. voltage is applied across it, i.e. when a d.c. current flows through it.
  • This phenomenon which is believed at least partly - due to a polarisation effect, is very pronounced in the case of PVC.
  • the initial resistance of the PVC may for example increase by a factor of about 10 once the system has been in use for some while.
  • a temperature monitoring system comprising:
  • the periodic reversal of the direction of current flow through the temperature-sensitive means at least alleviates the problem of ageing and may in some instances be able to reduce it to substantially the same extent as if the energisation were a.c.
  • the use of what is essentially switched d.c. at a frequency low enough to avoid transient phenomena due to cable reactance (e.g. capacitance) means that one can avoid the use of difficult, cumbersome and not altogether accurate low frequency a.c. measurement techniques.
  • the above-mentioned reference value may be a fixed value determined for a particular ' cable and a particular operating temperature. However, it is within the scope of the invention for the reference value to be variable in a sense to compensate at least partly for changes in the ambient temperature of the cable.
  • the temperature monitoring system shown in Figure 1 includes a cable 10 comprising two wires 12 that are twisted together and, preferably, enclosed within a sheath (not shown). (The cable 10 could however be of coaxial construction). The cable 10 is disposed in proximity to an object or an area whose temperature is to be monitored, e.g. above a conveyor belt or on a ceiling, in particular to detect a fire.
  • the wires 12 comprise respective conductors 14 and 16 having sheaths 18.
  • the sheaths 18 are of a material of which the resistance has a temperature coefficient, for example a negative temperature coefficient.
  • the material can be silicone rubber or a form of rubber known in the art as "EP rubber".
  • the material is polyvinyl chloride (PVC), which may or may not be doped with a material that enhances its conductivity.
  • PVC polyvinyl chloride
  • the resistance of PVC whether doped or undoped, drops from a very high value at room temperature, in a substantially logarithmic value, as its temperature increases.
  • the material of the sheaths 18 presents a distributed shunt resistance between the conductors 14 and 16.
  • the resistance is shown as comprising a plurality of discrete resistors connected in parallel, the total resistance thereof as viewed at an end of the cable being R .
  • the sheaths 18 naturally also constitute a distributed capacitance between the conductors 14 and 16. Again, for convenience of representation, such capacitance is shown in Figure 1 as comprising a plurality of discrete capacitors connected in parallel, the total capacitance thereof as viewed at an end of the cable being C .
  • a resistor R is connected in series with the parallel combination of the resistance R and the capacitance C between a +V rail 20 and an OV rail.
  • a switch arrangement 22 which is preferably an electronic switch arrangement and, for simplicity of comprehension, is represented as a pair of changeover switches.
  • the switch arrangement 22 is reversed the direction of flow of d.c. current from the source through the material of the sheaths 18 (i.e. through the resistance R s and the capacitance C ) is reversed.
  • the switch arrangement 22 is s operative periodically to reverse the way in which the temperature sensitive material of the sheaths 18 is connected into the resistive divider circuit network constituted by such material and the resistor R.
  • the speed of operation of the switch arrangement 22 is controlled by control logic 24 which is driven by a clock 26.
  • the periodic reversal of the direction of current flow through the material of the sheaths 18 reduces the net d.c. current flow through them and therefore reduces the above-described ageing effect.
  • the reduction of the effect can be maximised if the net d.c. current is reduced to zero, e.g. if the applied d.c. voltage and its duration are identical in the two positions of the switch arrangement 22.
  • the voltages are kept identical by using the common voltage source +V and ensuring that the switches making up the arrangement 22 have negligible "on" resistance.
  • the durations are kept identical by suitable design of the control logic 24.
  • the control logic 24 may comprise an edge-triggered counter/divider driven by the clock 26 to produce an output switching waveform whose two halves are of exactly equal time span.
  • the switching frequency provided by the control logic 24 is chosen to be sufficiently low that the capacitance C charges in good time for the signal on the line 28 to be considered to have changed to a substantially steady value before the next current reversal.
  • a sample and hold unit 30 the operation of which is synchronised by the control logic 24, is provided in the line 28 to sample the waveform during each 'plateau' portion, i.e. after the end of each spike, whereby the output voltage of the sample and hold unit is periodically updated so that its magnitude represents the latest sampled value of the signal representing the resistance R s.
  • This signal is applied to one input of a comparator 32.
  • a reference value or signal appropriate to a particular application is applied to another input of the comparator 32.
  • the magnitude of the reference signal is such as to be equal to that provided by the sample and hold unit 30 when the cable 10 has been heated to a temperature which is sufficiently high that an alarm is required.
  • the comparator 32 is responsive to the signal from the sample and hold unit 30 achieving this reference value to provide an alarm output signal on a terminal 34.
  • the reference value applied to the comparator 32 may be a value which is fixed for a particular application. Preferably, however, the reference value is varied to at least a certain extent to compensate for changes in the ambient temperature of the cable.
  • a modified system, in which the reference value is in fact varied to compensate for ambient temperature changes of the cable thereby enabling an increase in the maximum length of cable that can be used for a particular differential between the maximum ambient temperature and required operating temperature, will now be described with reference to Figure 3.
  • the system of Figure 3 includes a modified cable 10'.
  • the cable 10' is similar to the cable 10 of Figure 1, except that it comprises two further wires 12 having conductors 14', 16' and sheaths 18 the same as for the two wires 12 of the system of Figure 1. All four wires 12 are twisted together and are preferably enclosed in a sheath (not shown).
  • the wires 12 having the conductors 14, 14' are joined at one end of the cable 10' (the left-hand end as shown in Figure 3) - or comprise a single wire folded back on itself - thereby to form a single looped conductor 36 whose ends are accessible at the other end of the cable 10'.
  • the wires 12 havingthe conductors 16 16! are joined - or integral - at the left-hand end of the cable 10' as shown in Figure 3 to form another looped conductor 38.
  • the looped conductors 36, 38 are useful for a number of reasons, especially in that they enable the continuity of the wiring to be checked by checking that the loops are not broken. However, at least one of the loops is used for a further purpose, the nature of which is explained below.
  • the wires 12 are shown in Figure 4 as being so arranged that the conductors forming each of the looped conductors 36, 38 are diagonally opposite each other. They could instead be arranged with such conductors adjacent each other.
  • a current source 40 is connected via a gate 42 controlled by the clock 26 to the looped conductor 38 such that, when the gate 42 is opened, a predetermined d.c, current is sent through the looped conductor 38.
  • the current source 40 could be a constant current source, but preferably comprises simply a resistor where resistance is high with respect to that of the looped conductor 38 whereby the predetermined d.c. current is substantially unaffected by changes in the resistance of the conductor 38).
  • the looped conductor 38 is of a material whose resistance changes in known manner with temperature. The material may for example be copper, whose resistance changes by approximately 0.4 per cent per deg C and in fact increases by a factor of only about two between normal ambient temperature and its melting temperature.
  • the resistance R can be considered as being constituted by a multiplicity of incremental resistance elements connected in parallel, the resistance R can drop to a low value either by general heating of the whole cable or by more intense localised heating of a part of the cable.
  • the resistance/temperature characteristic of a typical sheath material 18, that is to say the value of the resistance R is represented in s Figure 5.
  • the relationship between R and temperature is approximately linear, there being typically a 17 deg C change in temperature per decade of resistance or in other words a doubling or halving of resistance for a change in temperature typically of about 5 deg C.
  • the magnitude of the signal on the line 28 representing the parameter R will vary in similar manner, though not in exactly the same manner because R forms a potential divider chain with the resistor R.
  • the signal on the line 44 which represents the series resistance of the looped conductor 38, is processed by a converter or amplifier 46 which has a logarithmic characteristic or transfer function such as to produce from the signal on the line 44 a modified (reference) signal which is applied to the comparator 32 and which varies with temperature in similar manner to that on the line 28.
  • the reference signal from the converter 46 provides a degree of ambient temperature compensation for the signal on the line 28 in that both signals vary in a similar manner with changes in the ambient temperature of the cable 10', but does not preclude the generation of an output alarm signal on the terminal 34 in the event of a fire situation in that the reference signal, unlike the signal on the line 28, does not respond substantially to localised heating, for the reasons explained above.
  • the gate 42 is operated by the control logic 24 such that the current from the source 40 is sent through the looped conductor 38 in the form of a pulse for a short interval at the end of each 'plateau' of the signal on the line 28, i.e. just before the next spike.
  • the converter 46 incorporates a sample and hold unit (not shown) to sample each pulse on the line 44 whereby the reference value supplied by the converter 46 represents the latest updated value of the series resistance of the looped conductor 38.
  • the converter 46 can be arranged so that it does not continue to alter the reference value if the signal on the line 44 goes beyond a value equivalent to a limit temperature value of, say, 32 deg C greater than a predetermined temperature value equal, for example, to a normal ambient temperature value. Looking at the matter another way, the converter 46 causes the system to cease to preserve the fixed differential between the ambient temperature and alarm temperature - i.e. to stop the alarm temperature tracking the ambient temperature - once the ambient temperature exceeds said limit temperature value.
  • the converter 46 may in fact be designed so that it does not provide a continuously varying output but instead provides an output which can adopt only one of (say) eight discrete values each corresponding to the signal on the line 44 indicating a respective step in temperature of (say) 4 deg C above said predetermined temperature value (e.g. normal ambient temperature value).
  • a predetermined temperature value e.g. normal ambient temperature value
  • the invention can of course be embodied in various different ways than those described above by way of example.
  • other forms of cable than the cables 10 and 10' could be used.
  • the cable 10' happens to have two looped conductors 36, 38, it should readily be apparent that for present purposes only one looped conductor is needed.
  • the looped conductor 38 does not have to be common with one of those conductors between which the resistance R is measured.
  • the s looped conductor could in fact be wholly separate and need not even be sheathed by temperature-sensitive material. It is in fact possible for the conductors associated with the resistance R s and those forming the loop to be separate cables laid together or at least reasonably close to one another to form a common cable arrangement.
  • the parameter of the loop 38 that is measured may not be its resistance. It is contemplated that a cable might be provided in which the inductance of the loop 38 might vary with temperature, in which case the inductance could be measured and used to provide the ambient temperature compensation reference signal.
  • a device having a different characteristic for positive and negative polarities e.g. a diode
  • This can enable different properties of the cable arrangement to be measured by .reversing the polarity of energisation.
  • the shunt-impedance detection means in effect monitors the shunt impedance (or a component thereof) between the two conductors when the polarity is such that the diode does not conduct.
  • the diode With the opposite polarity, the diode is forward-biased whereby the diode effectively short-circuits the remote ends of the conductors together and enables their continuity to be checked by measuring the resistance presented to the detection means. If the conductors monitored by the shunt-impedance detection means and the series-circuit impedance monitoring means are common, e.g. if there are only two conductors, the continuity check can be carried out by the monitoring means monitoring the impedance (e.g. resistance) of the two conductors. That is to say, the impedance of the temperature-sensitive means is monitored when the energisation is of one polarity (e.g.

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  • Business, Economics & Management (AREA)
  • Emergency Management (AREA)
  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Measurement Of Resistance Or Impedance (AREA)
EP19830300895 1982-02-23 1983-02-21 Systèmes de surveillance de température Withdrawn EP0087308A2 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
GB8205338 1982-02-23
GB8205338 1982-02-23

Publications (1)

Publication Number Publication Date
EP0087308A2 true EP0087308A2 (fr) 1983-08-31

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EP19830300895 Withdrawn EP0087308A2 (fr) 1982-02-23 1983-02-21 Systèmes de surveillance de température

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Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2149167A (en) * 1983-11-04 1985-06-05 Graviner Ltd Electrical circuit arrangements in fire alarms
GB2157871A (en) * 1984-04-17 1985-10-30 American District Telegraph Co Apparatus for providing an environmental alarm indication
FR2598239A1 (fr) * 1986-05-01 1987-11-06 Gen Electric Dispositif de detection de chaleur et/ou de fumee
RU2651490C1 (ru) * 2017-07-14 2018-04-19 Общество с ограниченной ответственностью "Упаковочные решения" Комплексная упаковка для куриного яйца и способ формирования штабеля на ее основе
CN113108914A (zh) * 2021-03-04 2021-07-13 国网浙江省电力有限公司嘉兴供电公司 一种输变电线路智能温度动态监控系统

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2149167A (en) * 1983-11-04 1985-06-05 Graviner Ltd Electrical circuit arrangements in fire alarms
US4628301A (en) * 1983-11-04 1986-12-09 Graviner Limited Electrical circuit arrangements
GB2157871A (en) * 1984-04-17 1985-10-30 American District Telegraph Co Apparatus for providing an environmental alarm indication
FR2598239A1 (fr) * 1986-05-01 1987-11-06 Gen Electric Dispositif de detection de chaleur et/ou de fumee
RU2651490C1 (ru) * 2017-07-14 2018-04-19 Общество с ограниченной ответственностью "Упаковочные решения" Комплексная упаковка для куриного яйца и способ формирования штабеля на ее основе
CN113108914A (zh) * 2021-03-04 2021-07-13 国网浙江省电力有限公司嘉兴供电公司 一种输变电线路智能温度动态监控系统

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Inventor name: SANDERS, PAUL OWEN

Inventor name: NAILOR, ROY FREDERICK