AU608563B2 - Integrating temperature-averaging sensor - Google Patents

Description

AIJ-0-1021/8 WORLD INTELLECTUAL PROPERTY ORGANIZAT!ON INTERNATIONAL APPLICATION PUBLISHED UN 8 E aN 5 OCraA I N TREATY (PCT) (51) tolernational Patent Classification 4 (11) International Publii!-ton Number: WO 88/ 05160 1/00(43) International Publication Date: 14 July 1988 (14.07.88) ~t (21) Internation 1l Application Number: PCT/AU87,100445 (74) Agent: GRIFFITH HASSEL FRAZER; G.P.O Box (22) Internationul Filing Dai.-; 24 December 1987 (24.12.87) 14 yny NW20 Aj (81) Designated States: AT (European patent), AU, BE (Eu- (31) Priority Application Numbers: PH 9671 ropean patent), CH (European patent), DE (Eu~o- P1 2550 pean patent), DK, Fl, FR (European patent), GB (European patent), IT (European patent), JP, KR, LU (32) Priority Dates: 24 December 1986 (24.12,86) (European 'patent), NL (European patent) NO, SE 18 June 1987 (18.06.87) (European patent), US, (33) Priority Country: AU Pulse (71) Applicants (for all designated States except Wt:nentonlsac eot RHEEM AUSTRALIA LIMITED (AU/AU]; Brodie Stueet, Rydalmere, NSW 2116 THE UNIVERS- ITY OF SYDNEY [AU/AU]; Parramatta Road, Syd- &OL I SEP 1988 ney, NSW 200f4 (AU).

(72) Inventors; and

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hiaventors/Applic,-nts (for US only) COLLINS, Richard,AUrAv~

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E, 84 C~idgegong Road, Riverstone, NSW 27JLT8 2765 PAILTUORPE, Bernard, A. (AU/AU]; 97 27JL18 Holden Street, Ashield, NSW 2131 BOURKE, Brendan, V, (AU/AU]; 107 Archer Street, Chatswood, PATENT' OFFICE NSW 2067 (AUJ).

(54) Title:. INTEGRATING TEMPERATUlRE-AVERAGING SENSOR (57) Abstract A temperature sensor comprising a tempera- 4 ture to electrical quantity transducer or array of In, terconnected transducers in thermal contact with a defined space such as a tank in which a non-isoth- I ermal temperature profile exists, Sensor may be arranged wth respect to the shape of the space and Its proflo enabling single output of sensor to rep- resent inelegral summation of temperature within i// the spoce, linearly interpretable for a given space as representing average temperature or thermal en-.I ergy conitent of the space. f Thi dcment ct~ntains the Scct ionl 49 and is z*orrect for p i i rinti- 1 This invention relates to a device capable of sensing the average temperature of thermally stratified water contained within the metal storage tank of a hot water heater. The thermal stratification is a result of temperature induced density bands of the water in the tank.

In such a hot water heater, bulk values such as the thermal energy content or the average or mean temperature or heat content, are not linearly related to a single point measurement of temperature at any one location either within the tank of the heater or at the boundary of the tank of the heater. It is often of much greater use to be able to measure or estimate these bulk values rather than one or more localised point values.

The invention is capable of useful application in the field of electrically heated off-peak heat energy storage appliances wherein means to measure the average temperature as provided by this invention can be used in conjunction 20 with signal processing means to control the undesirable effects of excessive or sudden loading in a community electricity s'upply network resulting at the beginning of "t 300O S a 7769A/as energy storage appliances in the community is met by substantially simultaneous switching of supply to at least significant portions of the t tal load. It can be used to significant advantage also to maximise the amount of lowest-generation-cost electricity supplied to the community.

In a significant example of the use of tIe invention involving off-peak hot water systems, it is explained that at the completion of a heating cycle, all the water in a hot water storage tank is at approximately the same temperature. This occurs because the off-peak heating element and the single thermostat controlling it are located near the bottom of the tank and natural convection ensures that ery small temperature gradients remain when the thermostat switches off the element to terminate the heating cycle. Subsequently, as some hot water is drawn from the tank, cold water enters at the bottom and the body of cold water and the body of hot water in the tank remain essentially separate, that is, temperature stratified, subject to no intervening heat input. This stratificmation occurs because the density of hot water is significantly less than that of cold water and the tank is not normally mixed by the simultaneous withdrawal and acompanying mareplenishment of water. As the iterface between th two bodies of 1 ater approaches the top of the tank due to draw off of hot water from dnear the top, the useful thermal, energy content in the tank nears 2 exhaustion. The temperature of the water leaing the tank therefore remains approximnately constant over substantially all of the draw off, ,,,finally dropping rapidly to close to the inet temperature.

heat When the i vention is applied to off-peak storage hot water 30 systems, supplied with power on a daily basis during a defined off-peak period, the energy sensor of the invention can be used to provide a ee: delay in switching on of the heating element in the tank, in *conjunction with suitable signal processing circuitry outside the scope of this specification. Normally, this delay would be applied when a 35 shorter time than the full off-peak period would suffice to fully replenish the design thermal energy content of the tank.

i !0 -3- Therefore the invention can be of important benef it to a supplier of electricity to the community, who could by its use be able to progressively build up to peak load rather than other more difficult procedures and expensive investmen~cs to enable adequate balance between generation capacity and load on a day-to-day basis.

one method of estimation of the average temperature in a non-isothermal hot water storage tank would be to distribute over the height of the tank a number of separate desr~rete temperature measuring devices such as thernao coupl1es or thermistors at a number of points either inside the tank or, conveniently, in thermal contact with the external wall of the tank. Each temperature dependent output could then be connected separately to a measuring, readout, control device c- other information processing system. Thi,- method is discussed in Australian patent application no. 33728/84.

The reading of each thermocouple or other discrete sensor a a: could be averaged to give an indication of total thermal @020 energy content stored. This method, however, would involve many wires emerging from the tank into the information processing system. in fact such a multiple parallel connected discrete sensor method would provide more information than is actually required, It is sufficient to know the tot ?tl energy in the storage tank, which, as will btz shown, for vessels of substantially uniform cross-section~al area is linearly related to the integral of the temperature profile from top to bottom of the tank. Therefore a sensor which integrates this temnperature would provide adequate :3Q information for the required purposes. The present ivention provides a very much less expensive and more servikc able alternative to that proposed in patent *Goo 0000 application no. 3372e/84 to enable effective achievement of an at least equivalent end result.

The invention has useful, application in solar hot water systems havir~g a storage tank heated by solar energy backed 6691S/as 4 up by electrical heating. In such systems the sensor can be coupled with a readout of average temperature of the tank, located at a conveniently accessible point remote from the tank. This can be useful in indicating to a user of such a system whether it is necessary to take steps to initiate electrical bost heating of the tank at a particular time.

Boost heating controlled in this manner is aimed at improving the solar contribution of the system.

The invention is readily applicable to tall tanks such as the cylindrical type very commonly used in domestic off-peak mains pressure hot water storage systems, such tanks having a capacity of 160 litres or more. Such tanks normally display distinct thermal stratification of the water in the tank and indeed, owe their effectiveness to that tendency in the context of off-peak once-per-day heat up.

The invention operates on the principal that the 20 electrical resistance of a conductor comprising for example a length of wire is dependent upon the volume resistivity of the material of the wire, its length and cross-sectional area and its effective temperature. When the wire passes through regions of variable and differing temperature and each increment of length of the wire is at least substantially isothermal with its immediate surroundings, the resistance measureable over the full length of the wire can be calibrated to be indicative of the integral summation of temperature of each increment of length of the wire. If 30 the temperature of each increment of the wire is representative of a larger element of some hot medium substantially isothermal within that element then the resistance of the wire can be tranduced to be also indicative of the integral summation of temperature of the hot object and hence is indicative also of its thermal energy content related to a reference temperature.

6691S/as 6691S/as c~,f _i IL-YI-~- In applying this to determining the average temperature of a non-isothermal storage tank for hot water it is knowi that in such a tank the wall of the tank at any given height will tend to equilibrate at a temperature quite close to that of the contents at the same height, especially if the tank is externally heat insulated. It is also known that the temperature profile of the water occupying the tank space is related to vertical distance from the bottom (or top) end. A resistance in the form of an elongate wire can be made to serve the purpose of indicating the energy content of the tank if for example arranged in thfrmal contact with the external wall of the tank and extending from between high and low points on the wall of the tank.

When using an elongate resistance wire it is known that within the temperature range involved that the resistance of the wire is substantially linearly variable with temperature. Electrically conductive materials display different temperature coefficients of resistance, these :,420 being tabulated in standard electrical engineering and other handbooks. It has been found that for many metals the change in resistance is sufficiently distinct to be readily measurable for the purpose of providing a useful measure of

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the average temperature of the tank.

Accordingly, the present invention provides the combination of an integrating temperature sensor and a hot water heater. The sensor is mounted in thermal contact with but is electrically insulated from the exterior of the metal 30 storage tank of the hot water heater. Conventional thermal *Goes: insulating material surrounds the storage tank. The sensor comprises an elongate member that extends along substantially the full depth of the storage tank. The terminal ends of the sensor are located adjacent one another and are connectable to associated electrical circuitry.

Along its length the sensor exhibits temperature dependent resistance and therefore an electrical measurement may be 6691S/as 6derived of the average temperature of the thermally stratified water contained within the storage tank.

In a preferred form the aforementioned integrating temperature sensor is mounted linearly along the depth of the metal storage tank. This form is of low cost and is easily installed.

In a further preferred form the integrating temperature sensor is mounted as a continous length containing a plurality of substantially parallel legs along the depth of the metal storage tank. This enables improved accuracy when compared with a linear arrangement.

15 In a further preferred form the integrating temperature sensor is mounted non-linearly along the depth of the metal storage tank. This enables the making of allowance for the variation in cross section of the tank with respect to the 1* direction of stratification of the temperature within the 20 tank.

Notwithstanding any other forms that fall within its :i scope, one preferred form of the invention and variations thereof will now be described, by way of examples only, with reference to the accompanying drawings, in which: Figure 1 is a digrammatic part ealvation-part cross section of a hot water storage tank to which one form of temperature sensor according to the invention is shown 30 schematically, affixed tc the tank.

Figure 2 is a graph showing resistance of a temperature sensor according to the invention when applied to a 250 litre storage tank, as a function of energy draw-off from the tank, together WIth a schematic illustration of the temperature a sor and its mode of attachment to the tank.

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7- Figure 3 is a diagrammatic elevation view to a reduced scale of a hot water storage tank to which the integrating temperature sensor in one further form according to the invention, is applied.

Figures 4(a) to 4(c) are a related family of diagrammatic perspective views to a reduced scale of sensors according to the invention applied to a hot water storage tank in a disposition as commonly mounted on roofs of buildings in the case of Fig and in the case of Figs 4(b) and showing two further alternative embodiments of sensors according to the invention applicable for installation in conjunction with a horizontally installed tank. Figure 4(d) illustrates division of the tank of Fig 4(a) into notional segmental zones or strata appropriate to the temperature profile. Figs 4(b) and 4(c) are a flat development of sensors applicable to attachment to the cylindrical wall of a horizontally oriented tank.

e Referring to Fig 1, an insulated hot water corage tank is shown comprising a steel cylindrical tank 11, a heat insulation layer 12 partly cut away to show a resistive transducer element in the form of a length of wire 13 attached to the wall of the tank and extending from a top extreme point 14 to a bottom extreme point 15. The wire is electrically insulated from the tank by a thin dielectric layer 16, which is selected to provide a negligible thermal separation of wire 13 from the adjacent tank wall, especially in view of the protection from convective heat 30 loss from the wire by virtue of the surrounding heat insulation layer 12.

The ends 14 and 15 of the wire are connected by leads 17 and 18 for connection to measuring and control circuitry -not shown- suitable for detecting the relatively small change in resistance of the wire indicating the average temperature of the thermally stratified water enclosed by t the tank.

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9 is/as 8 By means of circuitry outside the scope of this specification the value of the resistance of wire 13 can be measured and adapted to provide a delayed switching of at least one heating element (not shown) of the tank at the commencement of an off-peak time period, thus delaying, in conjunction with suitable timing circuitry, the switch-on time for individual hot water systems among a large population of off-peak appliances connected to the electricity supply network, by an amount in approximate proportion to the heat input required for the particular hot water tank to raise its thermal energy content to the desired extent.

Alternatively, the resistance can be adapted to provide a readout of the average temperature of the tank at a meter remote from the tank. This provision can be applied to enable a solar hot water system to be operated at an improved solar contribution level.

""20 Alternative embodiments include different arrangements for the sensor where the sensitivity is increased by attaching a resistor of increased length to the wall in an up and down or spiral path. A resistance path having the

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start and finish at adjacent points is preferred for 21 convenient connection to associated circuitry.

With reference to Figure 2, in order to test the validity of the use of the invention, experiments were performed on two 250 litre hot water storage tanks of a form as depicted in the illustration. In a first tank a copper o tube was inserted through a top central hole in the tank wall to pass along the cylindrical axis to a point immediately adjacent the bottom of the tank. A second copper tube was arranged parallel to the first but fixed in good thermal contact to the outside of the vertical metal wall of the tank. In both tubes thermocouples wero arranged to be moveable up and down the tubes to selected heights 6691S/as d" I 9 above the lowest point of the tank. It was thus possible to measure the temperature at all points along both the axis and the outside vertical wall of the tank. The temperatures in both these sets of locations were measured for the tank when full of hot water and also when draw-off of known amounts of hot water had been made together with the simultaneos make-up of cold water accompanying draw-off.

Under a variety of conditions it was confirmed that the temperature as measured on the outside of the tank wall was within 1 to 2 degrees Celsius of the temperature at the same height on the axis of the tank.

Figure 2 includes a graph of heat energy removed from the tank versus the resistance of a copper wire which was taped in good thermal contact to the vertical wall of the second 250 litre tank in a second experiment. The copper wire used was 30 B and S gauge and it was run from the top o of the vertical cylindrical wall of the tank to a point within about 20mm of the bottom of the same wall, oo 20 transversing this distance 4 times, The resistance of the wire was in the order of 2 ohms. An approximately linear variation of resistance with respect to the thermal energy content of the tank was confirmed by the experiments. The very minor departure from linearity at the two extremes, that is when the tank is nearly full and when the tank is nearly depleted of hot Water, is due to the fact that the top and bottom of the tank are not of identical shape to the majority ot the tank that lies between these extremes.

I 30 Referring to Figure 3, a hot water storage tank 30 of capacity typically 160 litres to 350 litres is shown, omitting the features irrelevant to the present invention.

The tank is of the mains pressurised type. Two forms of integrating temperature sensors 31 and 32 according to the invention are shown attached to the cylindrical wall 33 of the tank as will be discussed below. The tank 30 is tall in Il 0 relation to its diameter and the convexity of the top end 34 6691S/aS 10 or bottom end 35 has negligible influence on the total tank capacity as compared to a tank having substantially flat ends.

The sensor 31 in the form of a long wire is affixed to the vertical wall, the wire being arranged in two vertical legs so that the two ends of the wire 36 and 37 terminate close together near the bottom of the tank in order to be convenient for attachment to si~tnal processing means (not shown) The sensors 3, or 32 in use carry electrical current typically of the order of I, milliamp at a potential of typicatlly 10 volts.

The sensor are electrically insulated from the metallic wall of the tank by a thin dielectric layer or coating (not shown). The layer is as thin as possible -to assist good, thermal contact between the sensor wire and tank wall and to minimise the response time of the sensory should the temperature profile change quickly. This enables each 20 length increment of wire to be substantially isothermal with the increment of tank~ wall with which it is in contact.

Thus and increment of wire length can be considered substantially isothermal with a horizontal slice of the tank~ interior and the water contained therein.

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0s*0 9006 S0 90 S 0S. 0 30 0 SS 0 00 *0 The sensor 32 Is a longer wire than. the sensor 31 and is applied to the tank wall in a part-SPiral path with its upper extremity and lower extremity at the same height as the respectively corresponding extremity off sensor 31o. The purposo of this method of fixing of the longer sensor wire 32 is for making a single length of sensor wire suitable for application to different length tanks. Thus a shorter tank such as one of 16~0L capacity would use a sensor affi<ed as is sensor 32, namely in a part spiral path, whereas a tcIller tank such as one of 350L Capacity Would have. an identical, length sensor fixed straight and vertical. After fiing to Sthe tank, the sensor as used is encapsulated in heat insulation (not shown).

66918/as II ii ;iji i~r I 11 Figure 4(a) shows a long storage tank mounted horizontally, as is commonplace for roof mounted solar hot water systems. The wall of the tank has applied one version of the integrating temperature sensor of the invention. In figs. 4(b) and 4(c) are shown several alternative forms of sensor as will be further discussed below.

An important principle illustrated in Figs. 4(b) and 4(c) is that of using a length of resistance wire (in contact with the tank wall) to be representative of the volume of a segment or zone of enclosed tank space. The segments are in general bounded by the tank wall and two horizontal planes passing through the (cylindrical) tank and in which an increment of resistance wire makes thermal contact with the increment of the tank wall defining the segment. Convenient forms of segmental division are shown in Fig in which the horizontal tank is shown in endwise cross-section. Above the horizontal axis x-x is shown a division into labelled zones or strata 1 to 6, formed notionally by the subtended angle as illustrated.

a* The small volume of tank not included above the intersection f* Qf the 55 degree line has been ignored as it only represents approximately 4.5% of the total tank volume. Below x-x is

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shown an alternative division into zones labelled 1A to 6A where the horizontal thicknesses of each zone are equal again ignoring the insignificant lower extreme. The 0 41' division into horizontal zones is appropriate for hot water tanks since the thermal stratification which is observable in such tanks is also into horizontal layers.

With reference to Fig 4(a) elongate resistance wire 52 is affixed to the tank in a serpentine path where each 0090 length of straight resistance wire in the serpentine conceptualiy represents a reference level in a segmental zone of the tank* For a level of accuracy sufficient to the purpose, it is not of significance how the exact position of 6691S/as 12 the horizontal lengths of serpentine specifically relate to the arbitrarily selected division into segments. It is possible that the spacing of each horizontal leg of the serpentine when projected onto a vertical plane will be chosen to be uniform from leg to leg, but this choice is arbitrary.

In Figure sensor wire 53 is affixed to the tank such that it traces two parallel pairs of smooth curved paths from a point high on the tank wall and from a point low on the tank wall to a point at mid-height. The dimensions shown on Fig 4(b) relate to one example only in which the sensor as a whole wraps around a portion of a cylinder of 475 mm diameter and the opposite extremes of the sensor wire transducer subtend an included angle at the centre of the cylinder of 1i0 degrees (see Fig, a In this arrangement the purpose of the path shape is analogous to the above described serpentine shape. This is :20 that the horizontally aligned segments having small volume S near the top and bottom of the tank have a short length of the Wire affixed to the wall of the segment and the large volume segments near the centre of the tank havo length of wire attached to the wall of the resp6 iVo segment. Thus the sensor wire has a steeper lope the upper and lower extremities of the tank and a lessor slope near the centre.

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O00 AS may be appreciated from Figs 4(a) to 4(d) in a horizontally installed cylindrical tank an appropriate path OO of resistor wire can perform the function of integratin the m ~product of temperature and enclosed voluio. Tho sensor wire a Doe, can be arranged so as to provide an indication of the .0 thermal energy content possessed by the water occupying the internal space of the tank assuming a valuc Oan be anoribed to the Specif ic heat of the water occupying tho ipaee, In San analogous manner a combination of resistane devizes may 6691s/as N- 06 9 IS/ aa 4i 13 be arranged with respect to any tank whose cross-sectional area varies in the same direction in which a temperature profile change exists in the internal space contained by the tspk.

An alternative method available for use in producing a sensor for such applications includes forming conductive coatings on a dielectric supporting film. For example, with reference to Fig. the coating can be formed having variation of width along its length so that the resistance varies in the required relation to the part of the non-uniform cross sectional area tank to which it is applied.

Thus the various forms of sensor illustrated in Figs 4(a) to 4(c) provide an indication of the integral summation of the temperature and associated volume of the segmental zone of the tank space, or of any medium occupying the o 0 segmental zone of the tank space with which the relevant portion or length of the respective sensor is in thermal 20 contact, *040 In the foregoing descriptions it has been implied that a sensor which integrates the temperature profile gives a tseans to linearly relate the output to the thermal energy content Of the enclosure. This assumes that the specific heat of the medium occupying the enclosure and its mass are known if in fact it is thermal energy per se (referred to a datum temperature) which is required. Otherwise it is a mean temperature from which a thermal energy content is 30 linearly related which is available by the use of the sensor type! of the invention. The utility of the invention is unaffected by the actual definition of the quantity actually measured whether it be thermal energy, mean temperature, I average temnperature, heat content or some other related expression. For the purpose of characterising the invention the term thermal energy is used.

L, J I~ JJLIIJA__:l:g I -14 if In support of t.Ie foregoing implication, using the tall vertically oriented cylindrical tank of Figure 1, as an example, it can be shown mathematically that a sensor which integrates the temperature profile gives an output which is spu-tantially linearly related to energy content of the tank, provided that the sensor spans a major part of the full height of the tank. Applying calculus to a considered horizontal increment, dy, of the height of the tank in the cylindrical region:- 0 00 60 *0 ;Coo *0 o S 0

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15 1 Thermal energy (referred to a datum temp, o Area x density x specific x dy x of heat tank

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4 Cp where T(y) is the temperature of the layer of water at a height of y units above the bottonm of the tank.

Total thermal energy in the tank

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A c 7P *r )J le 0oLC L since A is constant for most of the tank, the top and bottom ends having negligible volume compared to the remainder in a tall tank @0&eo .u S

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0 S Similarly, considering the resistivity of the sensor resistance wire as a function of height of the tank:- Resistivity SResistiviy 1+ alpha Resistivity I alpha

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of

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S S S S as a function of temperature at height y at datum temp where alpha is the temperature of resistivity and T(y) is the of the resistance at height y coeffi cient temperature A 4

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16 1 Resistance dR resistivity x dy/a of element '..here a is the between y and cross-section of the y+dy resistance wire Aa 1o -n <fo -ry Thus it may be seen that both total thermal energy and resistance are linearly related to the integral of over the height of the tank, the other factors in the analysis being substantially constants.

Of resistor materials in the form of wire, film or tape, many are considered suitable for the illustrated and cited applications. These include those of iron, aluminium, nickel or platinum. Preferred materials have a reasonably high resistivity and an adequate temperature Coefficient of resistivity. For example, copper has a 0960.: temperature coefficient of resistivity of .004 per degree C giving a total variation of resistance of about 20% between the applicable extremes of temperature in water heating applications, say 20 to degreest. A generally suitable material for a' elongate resistor has been found to be annealed mild steel wire in the diameter range 0.3 to 0.9 nm, the thinner end of the range being especially preferred.

It has been found possible to measure the energy content in a hot water storage tank to an accuracy of about 5 to 10%. Long term stability, annealing effects on the resistor material and variation in wire dimension uue to thermal expansion are not likely to detract from 35 the accuracy of the sensor to any significant extent, L 1, ii As

Claims (5)

1. The combination of an integrating temperature sensor and a hot water heater, the sensor being mounted in thermal contact with but electrically insulated from the exterior of a metal storage tank of the heater and being covered by thermal insulation that extends about the storage tank, the sensor comprising an elongate member that extends along substantially the full depth of the storage tank and having terminal ends which are located adjacent one another andv are connectable to associated electrical circuitry, the sensor exhibiting temperature dependent resistance along its length %hereby an electrical measurement can be derived of the average temperature of thermally stratified water within the storage tank.

2. The combination of an integrating temperature sensor and a hot water heater as claimed in claim 1 where the integrating temperature sensor is mounted linearly along the depth of the metal storage tank.

3. The combination of an integrating temperature .0.0:20 sensor and a hot water heater as claimed in clain 1 where the integrating temperature sensor is mounted as a OO" continuous length containing a plurality of substantially parallel legs along the depth of the metal storage tank.

4. The combination of an integrating temperature sensor and a hot water heater as claimed in claim 1 where the integrating temperature sensor is mounted non-linearly along the depth of the mei:,al storage tank.

5. The combination of an integrating temperature sensor and a hot water heater substantially as herein herei desoribed with reference to any one of Figures 1, 3, 4(a), 4(b) or 4(c). *Goo DATED 17th day of December, 1990 0 RHEEM AUSTRALIA LIMITED AND THE UNIVERSITY OF SYDNEY by their Patent Attorneys $LU4 GRIFFITH HACK CO 6691S/as L I