[0001] This invention relates to temperature monitoring systems.
[0002] A known type of temperature monitoring system, particularly (but not exclusively)
useful for detecting fires, includes a cable arrangement comprising at least two conductors,
two at least of the conductors being separated by elongate temperature-sensitive means,
and detection means for monitoring the shunt impedance (or a component thereof) between
the conductors separated by the temperature-sensitive means. Such systems may be of
two types, depending upon whether the cable arrangement comprises what is known in
the art as a "digital cable" or what is known in the art as an "analogue cable". In
a digital cable, the temperature-sensitive means comprises an electrically insulative
material that melts (liquifies or softens) in the event of a fire or other overheat
situation to allow contact of the conductors, whereby the above-mentioned impedance
is subjected to a large abrupt change from one value to another in response to the
overheat. In an analogue cable, the temperature-sensitive means has an impedance that
varies in an analogue manner with temperature whereby monitoring of the impedance
can produce a signal that is representative of the temperature of the cable arrangement.
[0003] As is known to those skilled in the art, the ambient temperature of the cable arrangement
generally has a significant effect on the operation of the system and it is thus often
desirable reliably to monitor the ambient temperature, e.g. to provide a continuous
indication of such temperature or to indicate to supervisory or operating personnel
and/or equipment that the temperature has gone beyond a predetermined limit or limits.
Reliable monitoring of the ambient temperature may be difficult, bearing in mind in
particular that it may vary along the length of the cable arrangement.
[0004] According to the present invention there is provided a temperature monitoring system
comprising:
a cable arrangement having at least two conductors, two at least of said conductors
being separated by elongate temperature-sensitive means; and
means for monitoring the shunt impedance (or a component thereof) between the conductors
separated by the temperature-sensitive means;
the system being characterised by means for monitoring the impedance (or a component
thereof) between the ends of a series circuit comprising at least one of the conductors
of the cable arrangement, said impedance (or component) varying in accordance with
the temperature of said at least one conductor and thus being indicative of the ambient
temperature.
[0005] Thus, with a system in accordance with the invention, the ambient temperature can
be monitored by using the cable arrangement itself as a sensor. The ambient temperature
can thus be monitored reliably and accurately, in contrast for example to the case
in which one or more point measurements are relied upon.
[0006] The second-mentioned means for monitoring is.preferably operative to monitor - wholly
or predominantly - the resistance between the ends of the series circuit. An advantage
of this feature is that, since the resistance of the conductor material can be considered
equivalent to the sum of a plurality of incremental resistors arranged in series,
any ambient temperature variations are effectively integrated in real time, the total
resistance being effectively representative of a mean or average ambient temperature
value.
[0007] The second-mentioned monitoring means may be operative to monitor, at one end of
the cable arrangement, the series resistance of a series circuit comprising two conductors
connected together at the other end of the cable arrangement to form a looped conductor.
Alternatively, the second-mentioned monitoring means may be operative to monitor the
resistance of a series circuit comprising one conductor and an earth return.
[0008] In one embodiment of the invention described below, in which a digital cable arrangement
is employed, the first-mentioned monitoring means is operative to indicate a fire
or other overheat situation and the second-mentioned monitoring means is operative
separately to indicate when the ambient temperature exceeds a value - e.g. a permitted
maximum ambient - somewhat less than the value necessary to cause the first-mentioned
monitoring means to indicate a fire or other overheat. In other embodiments described
below, in which an analogue cable arrangement is emplyed, the detection and monitoring
means use output signals that' are employed interactively.
[0009] In known analogue systems, the means for monitoring the shunt impedance (or a component
thereof) is generally operative to provide an output signal if the impedance of the
temperature sensitive means achieves a 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. The impedance between the conductors,
for a given length of cable, will decrease as the temperature increases. Looking at
the matter in another way, the impedance between the conductors, for a given temperature,
will decrease as the length of the cable increases. Thus, the impedance of a short
piece of cable at a given temperature is the same as that of a longer piece of cable
at a lower temperature. This leads to the following problem. Suppose, for the sake
of argument, that the system is designed to provide an output signal (hereinafter
also referred to as 'an alarm') if the impedance is such that any one metre length
of the cable is at 100°C, for example due to the cable being heated by a fire. Obviously,
a rise in the ambient temperature of the whole cable or a part thereof substantially
longer than one metre to a value of less than 100°C will also give rise to an alarm.
As the length of the cable is increased, there will come a point when a rise in the
ambient temperature to a value which might reasonably be expected to occur will cause
a spurious alarm to be produced. Thus, a practical limit is imposed on the maximum
length of cable that can be employed. The limit could in principle be increased by
making the monitoring means respond to a lower threshold shunt impedance, e.g. to
provide an alarm only if one metre of the cable is heated to an operating or trip
temperature of greater than 100°C. However, increasing the limit in this way is not
acceptable since to do so to any substantial extent would mean that the temperature
value to which a short length of the cable has to be heated to cause an alarm would
be unacceptably high. Thus, in summary, there is a maximum limit on the length of
the cable that can be employed in the known system if both spurious alarms due to
ambient temperature rises and an unacceptably high operating or trip temperature for
heating of a short length of the cable are to be avoided. In more concise terms, the
maximum length of cable that can be employed is dictated by the differential between
the required operating or trip temperature and the maximum ambient temperature. Thus,
for example, in a particular application where a particular length of cable must be
used, it might be necessary to split that length into several sub-lengths each provided
with its own detection means and associated circuitry thereby increasing cost and
complexity. Looking at matters in another way, in a conventional system a trade-off
or compromise must be made between the maximum length of cable that can be employed
and the differential between the maximum ambient temperature and the operating or
trip temperature.
[0010] According to an embodiment of the present invention, the elongate temperature-sensitive
means has an impedance that varies with temperature, and the system comprises:
means for generating a first signal representing said shunt impedance (or a component
thereof) between the conductors separated by the temperature-sensitive means and thus
varying with local or general variations in the temperature of the cable arrangement;
means for generating a second signal representing said impedance (or a component thereof)
measured between the ends of said series circuit comprising at least one of the conductors
of the cable arrangement;
means responsive to a predetermined relationship between the first signal and a reference
value to provide an output signal indicating that the temperature of at least part
of the cable arrangement has exceeded. a predetermined value; and
compensation means responsive to the value of the second signal to alter said reference
value in a sense at least partially to compensate for changes in the ambient temperature
of the cable arrangement.
[0011] The at least partial compensation for ambient temperature changes provided by such
a system relaxes the above-described constraint on the maximum cable length in that
the differential between the ambient temperature and the operating or trip temperature
is constrained and is in fact preferably held constant at least over a certain range
of ambient temperatures. This advantage is achieved in essence by monitoring and suitably
compensating for the ambient temperature by monitoring the impedance (or a component
thereof) of the series circuit comprising the at least one conductor. As will be appreciated,
with such an arrangement the ambient temperature is measured reliably and accurately,
in marked contrast to what might be the case if, for example, an attempt was made
to provide compensation for ambient temperature changes by means of one or more point
measurements of ambient temperature.
[0012] The second signal is preferably wholly or predominantly representative of the resistance
of the conductor or conductors concerned. An advantage of this feature - additional
to that mentioned above - is that, since the resistance of the conductor material
can be considered equivalent to the sum of a plurality of incremental resistors arranged
in series, then while the overall resistance varies with a general ambient temperature
change it varies little with a local change in resistance caused by local overheating
(e.g. due to a fire) because a local change in resistance will cause little change
in the overall resistance. In this connection, it will be appreciated that the second
signal thus reacts in a totally different way to local heating than does the first
signal, in that the first signal is representative of the shunt impedance (or a component
thereof) between the conductors separated by the temperature-sensitive means and will
thus vary markedly in response to local overheating since local overheating can be
considered to change markedly the impedance of what can be considered to be one of
a plurality of incremental impedors connected in parallel. That is . to say, the second
signal can be considered, at least approximately, to vary only with general heating
of the cable arrangement due to ambient changes, taking little or no notice of local
changes due to a localised fire or the like, whereby the second signal provided in
such a case is very well suited to compensate for ambient temperature changes.
[0013] The above-mentioned compensation means may include means operative to vary the relationship
between the second signal and the reference value in a sense complementary to that
in which the first signal varies with temperature, assuming that both such signals
do not vary with temperature in the same way. For instance, if the first signal varies
antilogarithmically with temperature, then the compensation means may comprise a converter
that provides a generally logarithmic transfer function between the second signal
and the reference value.
[0014] It is conceivable that, in some instances, for example in the case of a fire building
up slowly and not producing a very large amount of local heating of the cable, the
ambient temperature of the cable arrangement could rise gradually to a dangerous value.
In this case, in the absence of some feature to recognise and respond appropriately
to these circumstances, the temperature monitoring system could conceivably not respond
at all to the fire but instead treat the slowly rising temperature as a slowly rising
permissible ambient temperature and do nothing to provide an output signal. (In this
connection, it should be appreciated that preferred systems described below are in
essence operative to provide the desired ambient compensation by preserving a fixed
differential between the ambient temperature and the temperature at which an alarm
is generated.) In a preferred embodiment of the invention described below, this possibility
is avoided by imposing a limit on the extent to which the reference value can be varied.
That is to say, compensation for changes in ambient temperature is stopped - i.e.
the differential is allowed to decrease - once the ambient temperature exceeds a predetermined
value.
[0015] The invention will now be further described, by way of illustrative and non-limiting
example, with reference to the accompanying drawings, in which like reference numerals
designate like items throughout, and in which;
Figure 1 is a circuit diagram - partially in block diagram form - of a first temperature
monitoring system embodying the invention, the system using an analogue cable;
Figure 2 is a cross-section through wires of a cable forming part of the system of
Figure 1;
Figure 3 is a graph showing the approximate relationship of the resistance (R ) with
temperature (T) of an insulating material of the wires s of Figure 2;
Figure 4 is a circuit diagram - partially in block diagram form - of a modified temperature
monitoring system embodying the present invention, also using an analogue cable; and
Figure 5 is a circuit diagram - partially in block diagram form of another temperature
monitoring system embodying the invention, this system using a digital cable.
[0016] The temperature monitoring system shown in Figure 1 includes an "analogue" temperature
monitoring cable 10 comprising four 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.
[0017] The wires 12 comprise respective conductors 14, 16, 18 and 20 having respective sheaths
22. The sheaths 22 are of a material of which the impedance (or a component thereof)
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".
Preferably, however, the material is polyvinyl chloride (PVC), which may or may not
be doped with a material that enhances its conductivity. As is known to those skilled
in the art, 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.
[0018] The wires 12 having the conductors 14,16 are joined at one end of the cable 10 (the
left hand end as shown in Figure 1) - or comprise a single wire folded back on itself
- thereby to form a single looped conductor 24 whose ends are accessible at the other
end of the cable 10. Similarly, the wireε 12 having the conductors 18,20 are joined
- or integral - at the left hand end of' the cable 10 as shown in Figure 1 to form
another looped conductor 26. The looped conductors 24, 26 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 will be explained below.
[0019] The wires 12 are shown in Figure 2 as being so arranged that the conductors forming
each of the looped conductors 24, 26 are diagonally opposite each other. They could
instead be arranged with such conductors adjacent each other.
[0020] As will be appreciated from an inspection of Figures 1 and 2, the looped conductors
24 and 26 are separated by the temperature-sensitive material constituting the sheaths
22. For convenience of representation, the distributed shunt impedance of the sheaths
22 between the conductors 24 and 26 is shown in Figure 1 as comprising a plurality
of discrete resistors connected in parallel, the total resistance thereof as viewed
at the ends of the cable being R
s. As will be appreciated, Figure 1 should, strictly speaking, show further discrete
resistors connected between the conductors 14 and 18 and the conductors 16 and 20.
However, for convenience, these are omitted.
[0021] A resistor R is connected between a +V rail 30 and the looped conductor 24. An 0V
rail is connected to the conductor 26. Thus, the resistor R and the resistance R are
connected in series across a d.c. voltage source +V whereby the voltage on a line
32 connected to the junction of the resistor R and the resistance R constitutes a
signal representing the value of the shunt resistance R
s. This voltage is applied to one input of a comparator 34. A reference value or signal
is applied on a line 36 to another input of the comparator 34 and the comparator 34
is operative to produce an output signal on an output terminal 38 when the resistance
R drops to a value indicating that the temperature of the cable 10 has exceeded a
predetermined trip or threshold value. Note that, because the resistance R can be
considered as being constituted by a multiplicity of incremental resistance elements
connected in parallel, the resistance R
s could drop to the value causing the generation of an output signal either by general
heating of the whole cable 10 or by more intense localised heating of a part of the
cable 10.
[0022] A current source 40 is connected as shown to the looped conductor 26 and to the +V
rail 30 and OV rail such that a predetermined d.c. current is sent through the looped
conductor 26. (The current source 40 could be a constant current source, but preferably
comprises simply a resistor whose resistance is high with respect to that of the looped
conductor 26 whereby the predetermined d.c.current is substantially unaffected by
changes in the resistance of the conductor 26). The looped conductor 26 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 deg C and in fact increases
by a factor of only about two between normal ambient temperature and its melting point.
Consequently, it will be appreciated that the voltage (with respect to the OV rail)
on a line 42 will be representative of the resistance of the looped conductor 26.
That is to say, such voltage will vary with changes in the ambient temperature of
the cable 10. Note, however, that since the resistance of the loop 26 can be considered
to be the series combination of a multiplicity of incremental resistance elements,
the overall resistance of the looped conductor 26 will be subjected only to a small
change if the cable 10 is subject to intense local heating as a result of a localised
fire.
[0023] The resistance/temperature characteristic of a typical sheath material 22, that is
to say the value of the resistance R , is represented in Figure 3. As will be seen,
below a value typically equal to about 60°C, on a logarithmic scale, the relationship
between R
s 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. It will be appreciated
that the magnitude of the signal on the line 32 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 42, which represents the
series resistance of the looped conductor, is processed by a converter or amplifier
44 which has a logarithmic characteristic or transfer function such as to produce
from the signal on the line 42 a modified (reference) signal on the line 36 which
varies with temperature in similar manner to that on the line 32. Thus, the signal
on the line 36 provides a degree of ambient temperature compensation for the signal
on the line 32, 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
signal on the terminal 38 in the event of a fire situation in that the reference signal
on the line 36, unlike that on the line 32, does not respond substantially to le
galised heating, for the reasons explained above.
[0024] If, in the above-described arrangement, the voltage +V is continually applied to
the resistance R and the constant current is sent continuously through the looped
conductor 26, there is a possibility that the output signals on the lines 32 and 42
might mutually interfere. This could be avoided by energising these different parts
of the cable 10 at different times. If this is done, sample and hold units may be
incorporated in the converter 44 and in the line 32 to sample the lines 42 and 32
at the appropriate times.
[0025] In the above-described arrangement, d.c. energisation of both the resistance R and
the conductor 26 is employed. However, it is within the scope of the invention to
instead use a.c. energisation. Further,.a form of pulse energisation described below
may also be used.
[0026] With the circuit as described above, a fixed differential (e.g. say 5 deg C) is maintained,
as the ambient temperature varies, between the ambient temperature and the temperature
at which an alarm will be generated. There is therefore some risk that a gradual rise
in temperature of the cable 10 to a dangerous value, e.g. in the event of an adjacent
but not contiguous fire, could be ignored if it is sufficiently gradual. This can
be avoided by limiting the extent to which the reference value on the line 36 (representing
ambient temperature) can be altered. In other words, the converter 44 can be arranged
so that it does not continue to alter the reference value if the signal on the line
42 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 44 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.
[0027] The converter 44 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 42 indicating a respective
step in temperature of (say) 4 deg C above said predetermined temperature value (e.g.
normal ambient temperature value). Such a feature can readily be accomplished, essentially
by sensing the magnitude of the input signal on the line 42 and allocating one of
eight particular values to the reference signal on the line 36, the reference values
being related in logarithmic manner to the input threshold values whereby the desired
logarithmic transfer function of the converter 44 is obtained.
[0028] A modified temperature monitoring system embodying the invention will now be described
with reference to Figure 4. The system of Figure 4 is closely based upon that of Figure
1 and will only be described in so far as it differs therefrom. The system of Figure
4 incudes a feature, more fully described in our co-pending UK Patent Application
No. 8205338 and in our co-pending European patent application of even date herewith,
which avoids certain disadvantages that may be incurred if the resistance R is energised
with direct current. In the system of Figure 4, an electronic switch arrangement 50
is provided. The switch arrangement 50, schematically represented for simplicity of
comprehension as a pair of change-over switches, is operative so that the current
sent through the resistance R is periodically reversed at a frequency corresponding
to the frequency of switching thereby to reduce the net d.c.current, preferably to
zero. In theory, operation of the switching arrangement 50 would have no affect on
the signal on the line 32. In practice, however, bearing in mind that the cable 10
has considerable capacitance, there is in fact a spike on the signal on the line 32
each time the current is reversed due to the capacitance of the cable being charged.
The resultant waveform on the line 32 is as shown in Figure 4. To ensure that the
signal reliably represents the value of the resistance R , the switching frequency
of the switching arrangement 50, which is determined by control logic 52 driven by
a clock 54, is chosen to be sufficiently slow that the cable capacitance charges in
good time for the signal on the line 32 to reach a steady state value before the next
current reversal. The necessary frequency will of course vary with different materials
and different applications, though a typical frequency will be about 1/6 or 1/7 Hz,
that is to say the period of the waveform will about 6 or 7 seconds in such a typical
application. A sample and hold unit 56, the operation of which is synchronised by
the control logic 52, is provided in the line 32 to sample the waveform during each
'plateau' portion, i.e. after the end of each spike.
[0029] The control logic 52 also controls and synchronises a gate 58 such that current is
sent through the looped conductor 26 towards the end of each plateau of the waveform
on the line 32, i.e. shortly before the next spike. The converter 44 will include
a sample and hold unit or the like to sample the resultant voltage on the line 42
at the appropriate times, but may otherwise be of the same construction decribed above.
The circuit of Figure 4 in fact operates in much the same manner as that of Figure
1, as regards the feature of temperature compensation, the only substantial difference
being that, as mentioned above, the resistance R
s is periodically energised by d.c. of opposite polarity whereby problems associated
with continuous energisation with the same polarity can be avoided or at least reduced.
Such reduction can be maximised (i.e. the problems can be minimised) if the net d.c.
current through the resistance R is reduced to zero, e.g. if the applied d.c. voltage
and its duration are identical in the two portions of the switch arrangement 50. The
voltages are kept identical by using the common voltage source +V and ensuring that
the switches making up the arrangement 50 have negligible "on" resistance. The durations
are kept identical by suitable design of the control logic 52. To this end, the control
logic 52 may comprise an edge-triggered counter/divider driven by the clock 54 to
produce an output switching waveform whose two halves are of exactly equal time span.
[0030] The invention can of course be embodied in various different ways than those described
above by way of example. As mentioned above, a.c. energisation can in principle be
used instead of d.c. energisation. In this case, instead of measuring R , the parameter
measured might be the impedance of the sheaths 22. Over a large range of frequencies
the impedance would in fact be largely or predominately resistive, in particular if
the PVC is doped with a conductivity-enhancing material. However, the capacitive component
may have some affect at high frequencies, in particular if undoped PVC is used.
[0031] Another temperature monitoring system embodying the invention is shown in Figure
5. The system of Figure 5 resembles that of Figure 1 to some extent and will therefore
be described only in so far as it differs therefrom. The system of Figure 5 includes
a cable 10
* which, for convenience, is illustrated as having a like configuration to the cable
10, though it should be appreciated that - as explained below - other forms of cable
can readily be employed. The cable 10' differs from the cable 10 in that it is a digital
rather than an analogue cable. More specifically, the material of the wire sheaths
22, represented by the resistance R , does not necessarily have a temperature coefficient
of resistance, since in this embodiment the arrangement is such that, in the event
of a fire or the like, the material melts (liquefies or softens) to allow contact
of the at least one of the conductors 14, 16 with at least one of the conductors 18,
20, the resultant short circuit resulting in R
s dropping from a very high to a low value.
[0032] Means is provided to provide an alarm output signal on the output terminal 38 when
the resistance R drops to a low value as a result of a fire or the like. For example,
as shown, the resistance R and the resistor R are connected in series as a potential
divider across the +V d.c. supply and their junction is connected to an input of a
trigger circuit 60, the circuit 60 being operative to provide an alarm output signal
when it detects that the voltage on the line 32 has dropped from a relatively high
level to a relatively low level as a result of the resistance R
s having done likewise.
[0033] In the embodiment of Figure 5, as in the embodiment of Figure 1, the signal on the
line 42 is representative (in an analogue sense) of the resistance of the looped conductor
26 and therefore of the ambient temperature of the cable 10'. Such signal is applied
to an input of a trigger circuit 62 set to switch and provide a warning output signal
on an output terminal 64 if the ambient temperature exceeds a predetermined value,
e.g. maximum permitted value.
[0034] In all of the foregoing embodiments it is contemplated that the parameter of the
loop 26 that is monitored may not be its resistance. It is contemplated that a cable
might be provided in which the inductance of the loop 26 might vary with temperature,
in which case the inductance could be monitored and used to provide an indication
of the ambient temperature.
[0035] As should be evident to those skilled in the art from a perusal of the foregoing,
various different forms of cable than those specifically described above could be
used within the scope of the invention. For example, though the cables 10, 10' happen
to have two looped conductors 24, 26, it should readily be apparent that for present
purposes the looped conductor 24 is not needed. Furthermore, the looped conductor
26 does not, as is the case in the cables 10 and 10', have to be common with one of
those conductors between which the resistance R is monitored. The looped conductor
26 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 least reasonably
close to one another to form a common cable arrangement.
[0036] It is not in fact essential in any of the above embodiments to use a looped conductor.
It is instead feasible to feed the current from the source 40 into a single, non-looped
conductor having a good earth connection at its end remote from the source 40, whereby
the voltage of the conductor with respect to earth could be measured to determine
its resistance. (If the remote earth potential were different to that at the monitoring
system, the difference could be trimmed out). In this case, it is the resistance between
two ends of a non-looped conductor (rather than between the two ends of a looped conductor
formed by a series circuit of two conductors) that is monitored. In general, one can
monitor the impedance (or a component thereof) between the ends of a series circuit
comprising at least one of the conductors. The invention can in fact be carried into
effect with a cable having only two conductors.
[0037] It is within the scope of the invention to connect a device having a different characteristic
for positive and negative polarities (e.g. a diode) between that end of the conductors
separated by the temperature-sensitive means that is remote from the detecting means.
This can enable different properties of the cable arrangement to be measured by reversing
the polarity of energisation. For example, if the device is a diode then 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. 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. during half-cycles
of an alternating supply of one polarity) and the conductor continuity and the series
circuit impedance representing ambient temperature are monitored when the energisation
is of the other polarity (e.g. during half-cycles of the opposite polarity).
1. A temperature monitoring system comprising:
a cable arrangement having at least two conductors, two at least of said conductors
(16, 18) being separated by elongate temperature-sensitive means; and
means for monitoring the shunt impedance (or a component (Rs) thereof) between the
conductors (16, 18) separated by the temperature-sensitive means;
the system being characterised by means for monitoring the impedance (or a component
thereof) between the ends of a series circuit comprising at least one of the conductors
(18, 20) of the cable arrangement, said impedance (or component) varying in accordance
with the temperature of said at least one conductor (18, 20) and thus being indicative
of the ambient temperature.
2. A system according to claim 1, wherein the second-mentioned means for monitoring
is operative to monitor - wholly or predominantly - the resistance between the ends
of the series circuit.
3. A system- according to claim 1 or claim 2, wherein the first-mentioned means for
monitoring (+V, R, 60) is operative to indicate a fire or other overheat situation
and' the second-mentioned means for monitoring (40, 62) is operative separately to indicate
when the ambient temperature exceeds a value less than a value necessary to cause
the first-mentioned means for monitoring (+V, R, 60) to indicate a fire or other overheat.
4. A system according to claim 1, wherein the elongate temperature-sensitive means
has an impedance that varies with temperature, the system comprising:
means (+V, R) for generating a first signal representing said shunt impedance (or
a component thereof) between the conductors (16, 18) separated by the temperature-sensitive
means and thus varying with local or general variations in the temperature of the
cable arrangement;
means (40) for generating a second signal representing said impedance (or a component
thereof) measured between the ends of said series circuit comprising at least one
of the conductors (18, 20) of the cable arrangement;
means (34) responsive to a predetermined relationship between the first signal and
a reference value to provide an output signal indicating that the temperature of at
least part of the cable arrangement has exceeded a predetermined value; and
compensation means (44) responsive to the value of the second signal to alter said
reference value in a sense at least partially to compensate for changes in the ambient
temperature of the cable arrangement.
5. A system according to claim 4, wherein the second signal is wholly or predominantly
representative of the resistance of the series circuit.
6. A system according to claim 4 or claim 5, wherein the compensation means (44) is
operative to vary the relationship between the second signal and the reference value
in a sense complementary to that in which the first signal varies with temperature.
7. A system according to claim 6, wherein the first signal varies substantially antilogarithmically
with the temperature and the compensation means (44) provides a generally logarithmic
transfer function between the second signal and the reference value.
8. A system according to any one of claims 4 to 7, wherein the compensation means
(44) is operative not to alter the reference value if the second signal exceeds a
predetermined limit value.
9. A system according to any one of claims 4 to 8, wherein the compensation means
is operative to alter the reference value between a plurality of predetermined discrete
values in response to variation in the value of the second signal.
10. A system according to any one of claims 4 to 9, wherein
the means for generating the first signal comprises energisation means (+V, R) operative
to send d.c. current through a circuit network including the temperature sensitive
means, thereby to generate the first signal in the form of a d.c. signal representative
of the resistance (Rs) of the temperature-sensitive means, and switch means (50) operative
periodically to reverse the connection of the temperature-sensitive means into the
circuit network and therefore to reverse the direction of current flow through the
temperature-sensitive means, the frequency of operation of the switch means being
sufficiently low that, after a change in the first signal that will take place upon
each said reversal due to reactance of the temperature-sensitive means, the first
signal will revert substantially to a steady value before the next reversal,
and wherein the means (34) providing said output signal is responsive to said predetermined
relationship between said steady value of the first signal and the reference value.
11. A system according to any one of the preceding claims, wherein said series circuit
comprises a pair of conductors (18, 20) that are connected together or integral with
one another at one end of the cable arrangement and are connected to the compensation
means (44) at the other end of the cable arrangement.