IL43373A - Heating device - Google Patents

Abstract

1446412 Fluid heater L M ELECTRONICS (HOLDINGS) Ltd and D F LENTON 4 Oct 1973 [5 Oct 1972] 45988/72 Heading H5H [Also in Division G3] An apparatus for heating fluids (e.g. blood transfusion liquid or anaesthetizing gases) flowing in a flexible tube, to a desired temperature, comprises a heating block 20 (Figs. 6, 10) having a channel 22 to receive the tube (e.g. nylon), electric heating elements 39, 40 formed of wire 43 wound around insulating strips 41, 42, e.g. mica, and embedded in parallel slots. The channel is U-shaped (or alternatively may be straight as in Figs. 1-5, not shown) with circular cross-section and one limb has an extended straight portion 35 whereby a single straight run of tube may be accommodated. The block is of die-cast aluminium/zinc alloy. A control circuit maintains the block at a fore-set temperature whereby fluid passing through will attain a required temperature, e.g. normal body temperature. Control circuit. Fig. 12.-Heater elements 65 (i.e. 39, 40 of Fig. 10) are connected in series with a transistor 67 and a thermal cut-out 66 across 12 Volt supply lines 60, 61. The block temperature is sensed by a thermistor 68 inserted in a hole 45 (Fig. 6) and connected in a bridge circuit including a desired temperature setting resistor 73 and resistors 71, 70, 74, 73, 75. The bridge output is a function of temperature error and is applied to an amplifier 69 and thence to an inverter 78 via a transistor 77. The inverter output controls a transistor 81 which in turn controls the transistor 67 and hence the heater current. In the event of excess temperature being sensed the cut-out 66 opens and a normally conducting transistor 90 will turn-off so that a green light-emitting diode 83 will be extinguished and a bleeper alarm 91 sounded. The alarm is energized under the control of an inverter 94 and NOR gate 96. Any fall in supply voltage is sensed by a differential amplifier 102 having one input connected to a divider 105, 106 across the supply lines 60, 61. If the potential at line 60 falls below a predetermined value a transistor 111 is rendered conducting, the potential in a line 112 will fall and as a result the green light diode 83 will cycle ON and OFF and the bleeper 91 will emit an intermittent signal. The electric supply may be derived from the mains or from a battery. [GB1446412A]

Description

HEATING APPARATUS FOR- A FLUID FLOWING THROUGH A FLEXIBLE TUBE This invention relates to a device for heating a fluid flowing throug a .flexible -cube to a required temperature within a predetermined temperature range, and in. particular for heating a sterile- liquid, such as blood, plasma or saline solution to body temperature in a transfusion operation. The invention is also . applicable to the heating of anaesthetic gases as they are.being fed to a patient.

.When supplying a liquid by drip into the vein of a patient, careful control of the rate of flow and the temperature of the fluid' entering the patient's circulation, is essential. A liquid ¾entering the cir-dilation intraveneously at the wrong temperature can cause shock to the patient. , · The acceptable temperature range of the liquid as 'it is delivered to the patient is appreciably smaller in infants and a' patient already suffering from hyperthermia or shock, if further damage to the patient is to be prevented and the greatest benefit ' from 'bhe liquid attained. More particularly, liquids given rapidly at too low a temperature can have very serious effects on such patients and in extreme cases can cause death., ¾ In conventional transfusion apparatus the sterile liquid is in a suitable container, such as a bag of sjmthetic plastics material, which is suspended above the part of the patient's body to which the liquid is supplied and a flexible tube for example of clear nylon extends downwardly rom the container to the ■· ' ' Appln.No.43373 2 body) any curves in the tube must be within well knownf limits of radius of curvature* The liquid, for example blood plasma or saline solution, being supplied must be brought to temperature near to body temperature, close to the point of introduction to the patient's body and in conventional apparatus the liquid is heated as it flows through the tube to the patient by elementary means such as hot water bottles placed against the tube* During the. process of introducing the requisite amount of liquid to the patient's body, the hot water bottles often need to be changed and replenished to maintain the flowing liquid within the required temperature range* Such methods require operation of the transfusion apparatus by experienced personnel* for example a qualified burse i attendance, which is costly and prevents such personnel from attending to other patients under their charge* When working out of doors under more primitive conditions at an accident site or under war conditions it is much more dif icult to keep transfusion liquids warm as they flow* "7 There is known U.S.Patent Specification No* 3370153 which teaches a method of heating fluid flowing along a tube by preforming the tube to a special shape to suit the shape of heater body, and then pushing the preformed tube into a slot in the body, and filling the space between the preformed tube and the body with a liquid to achieve some sort of heat conducting relationship between the liquid in the tube and the material of the body* There is further known Canadian Patent specification No* 794470 in which a tube is clamped to one side o a casing containing an electrical heater, and spaces are filled d m r : Attempts have been made to provide suitable potable equipment or heating the flexible iubes in transfusion apparats but they are of complicated construction and expensiv and include means for controlling the flow of the liquid to the patient* A main object of the present invention is to provide an inexpensive apparatus for heating a fluid flowing through a flexible tube and in particular for heating 'a liquid to body temperature in a transfusion operation which is not subject to the aforesaid disadvantages, in which the tube does not have to be specially formed as taught in U.S. specification No. 3370153, and in which the tube has,more than half its. surface area in intimate contact with the heater body in distinction f^om Canadian specification 794470. y. t . · , \ A further object of the invention is to provide such aii apparatus which is adapted to sense a cessation in the flow of liquid to a patient in a transfusion operation and emit a visual and audible warning signal thereby providin s a means of detectin malfunction o the transfusion apparatus or heart failure or coma in the patient* The present invention consists in an apparatus for heating a fluid flowing through flexible tube to a required temperature comprising a heat sink in the form of a block (4) of thermally conductive material formed with a channel (11) the side of which is not closed, electrical heatin means (5) in thermal contact with the block; and an electrical supply control circuit ^9) connected to the heating means and operable in response to the block temperature to control the current in the heating means to tend to maintain the block at tem erature such that fluid flowin throu h the tube length retained in the channe is of part-circular cross-section, the channel edges subtending an angle of more than 180° at the central axis of the channel, whereby the channel is capable of receiving a flexible walled circular tube (1) in intimate thermal contact with a substantial part of the side vail area of the channel without having access to either end of the tube and retaining it therein.

In an apparatus accordin to the invention which is particularly suitable for heating a liquid to body temperature in a transfusion operation the blook is elongate and one face of the block is formed with a. channel in the form of a groove shaped to retain a flexible tube for transfusion liquid in close contact with the. walls of the groove, the electrical heating means comprises an electrical heater element extending along and in contact with the block, and the electrical supply . circuit includes means for measuring the temperature of the block and eettable means for controlling a circuit responsive to the sensed temperature which regulates sufficient pov/er .supply to the heater element .to maintain the block at a required body temperature.

In a preferred embodiment of the invention the groove is a circular cross-section to receive a flexible tube of matching circular cross-section, and .the upper edges of the groove are shaped as inwardly, extending lips between which the tube can be pressed, and which retain the tube in the groove· .

The said, one face o the block may be formed •with a straight/ tube-retaining groove extending from end-to-end of the block. Alternatively, or in addition said one face of the block may be formed with a tube-retainin .groove of U-configuration having l mbs' terminating at one end of the block. With a groove of ' U-configuration the closed end of the groove is pre-ferably a curve having a diameter in. the range 35 mm to 40 ram.

Preferably the electrical heating means comprises electrical resistance heaters wire wound on insulating strips inserted i slots in the block.

According, to a further aspect of the invention there . is provided a device according to the invention in combination with a container for transfusion liquid, and a flexible tube leading from the container to a needle, a length of which tube is retained in the groove in the block.. ' ..··. · • The flexible tube is of plastics material for example a clear nylon tube.

In order that the invention inay be more fully ^ understood some embodiments in accordance therewith., in the form of devices suitable for heating a liquid to body temperature in a transfusion operation, will now be described, by way of example, with reference to the accompanying drawings, in which: - Figure 1 is a front elevation partly in section of a heating device with a flexible tube of the kind used in .a transfusion operation '·· retained ther-ein; „·: Figure 2 is a side view of the heater shown in Figure 1; Figure 3 is a section on the line III-I!I of Figure 1; Figure 4 i a ear view of the hea er shown in Figure 1; · Figure 5 is an end view looking in the direction of the arrow 13 in' Figure 4; Figure ' 6 is a view of the underneath of a block of thermally conductive • material of another embodiment of a heating device; . Figure 7 is a top plan view of a cover . fitted on to the other face of the block of the device shown in ■■ . Figure 6; · ■ Figure . 8 is an end view in the direction of arrow A of the device shown in Figure 9. is a side elevation of the device ^ shown in Figures 6 to 8; Figure 10 is a section through the thermally ' conduc ive block on line X~X of .

Figure 6; Figure 11 is a schematic representation of a heating element in the device of • Figures 6 to 10, with the block · cut away; and Figure 12 is a schematic diagram of an electrical heating',' control and warnin 'Circuit of the heating device of Figures 6 to 11.

Referring to Figures 1 to 5 the heating device is shown with a flexible silicone rubber tube 1 retained therein in a manner to be described, the tube being of the kind used in blood transfusion apparatus and connected at its upper end to' the flexible bag or glass container through blood flow control equipment, not. shown, of conventional kind. An injection needle is connected to the lower end of the tube in known manne o . · . .

The heater comprises a housing 2 of heat conducting material which may be die cast from a zincr-aluminium alloy, for example it may be injection moulded from a foam of a zinc-aluminium alloy, resulting in a block of. high tensile strength and light, weight. The housing 2 defines a 1 chamber 3 in which is mounted an electrical. resistance heating element.5 within a support block 4 of a heat conductive material such as aluminium, together with a temperature sensing ' element 17 The support 4 has heat transfer fins 4A and is secured to the inner face of housing 2 by screws 6; apertures closed by fluid tight screws 7 (of which one is 'shown) are provided for access of tools to tighten the screws 6. . · ■ The heating element 5 is controlled by an electronic circuit 9 represented schematically in Figures 1 and 2; the circuit, includes temperature control means 8, an amplifier 9 and a lamp 10 which lights' when the heating element is operating. The circuit includes a power transistor which. controls current flow through the heating- element 5 together with the temperature sensing element 17. · . .

While the heater shown in the ' drawings is mounted within the support block 4, it could equally be secured directly to the chamber and/or enclosed -i-n-- a flexible wrapper or like cover.

The tube-housing block 2 has along its ' outer "face an elongated slot 11 shaped to receive a flexible tube 'by a press or sliding fit and to be retained therein by friction. The slot may have a pressure sensitive •adhesive surface to adhere. to the outer surface of the tube. As shown, the slot is narrower at its mouth at the surface of the block than its inner portion so that it embraces the tube not only to grip it but, also so that It has a large heat conducting surface in contac with it. ■ .' ■ The heating circuit is supplied with electric current through leads 12 from a source of mains boss 13 may be provided with electrical connectors to receive an electric dry battery (not shown). Where a mains supply is used, a transformer . is provided to . transform 'the mains supply to 12 volts at v/hich the heating circuit is constructed to operate. Where a battery is used, this may be of the expendible or rechargeable type and the boss 13 may be provided with an- adaptor v/hich allows, the device to be plugged into a car or ambulance 12 volt D.C.. supply or to a rechargeable or dry battery pack.. If a battery pack is. used, this may include a separate on/off switch.

The casing is formed w th a cavity 16 in v/hich is located a temperature monitoring thermometer 18, v/hich may be an ordinary clinical thermometer. VJhen the device is used in an upright position this cavity is partially filled with liquid 15 to give good heat • transfer to the thermometer, and a rubber buffer 116 is located at the base of the' cavity to prevent breakage of the thermometer bulb.

The thermometer continuously monitors the internal temperature of the device.

Another embodiment of the invention is illustrated in ' Figures 6 to 12 which show a heating device- comprising a rectangular block 20 of thermally conductive material, for example . a zinc-aluminium all.oy, from which it may be formed by die casting. A ^ convenient size for the block 20 is 24.5 cm x 5.1 cm x 3.8 era. One face 21, the outer face, of the block 20 is formed with an elongated channel 22 for receiving a • flexible tube while the opposite face 23 is in thermal contact with electrical heating means and provides a base for the electrical control circuit to be described with reference to Figure 12. The circuit is enclosed by a cover 24 of for example plastics material such as acrylonitrile-butadiene-styrene .

The . channel 22 is in the form of a groove of circular cross-section of 4.5 mm diameter, , and is of U-configuration, both limbs 25, 26 of which terminate at one end. 27 of the block 20, and the. closed curved end: of the channel is a curve having a diameter in the range 35 to 40 mm.- As shown in Figure 7 the upper edges of the groove are shaped as lips 28, 29 inwardly extending to within ipproximately 2 mm of each other. The inside surfaces 30, 31. of the lips 28, 29 follow the circumference of : the circle of cross-section of the channel while the outer surfaces 32, 33 of the lips, 28, 29 are in the plane of the surface of face 21 of the block.20.

A flexible tube of 6 mm diameter, of silicone rubber or.'clear nylon, such as. are used as standard equipment in blood trcmsfusion apparatus is inserted into the channel 22 by press fit through the gap between .the lips' 28, 29 and retained in the channel by those lips.

■'·. -. The- flexible tube is normally of slightly greater .diameter than the channel 22 so that presses tightly against the inner surface of the channel 2.2 to give , good thermal contact "between the wall of the tube and the surface of the channel 22.

As shown in Figure 6 the lips 28, 29 are provided only on the limbs 25, 26 of the channel. The grooves forming one limb 25 is continued in a straight line from tli'e point 34 where the curved end of. the U begins, to form an extension 35 of the limb 25 providing a straight channel extending from end 27-to-end 36 of the block 20.

The provision of this extension 35 is optional, and the block can be formed with a single groove either of straight or U-con iguration. · For a given rate of. liquid flow from the container ' of- transfusion liquid through a flexible 'tube retained in the groove in the heating block, the time of heating of "the liquid in the tube - is greater when the tube is retained in the channel of U-configuration than when the tube is retained in the straight channel. In a .transfusion operation when 'the rate of liquid flow is higher than the normally used rate of 1 litre/4 hour f . ' · . ' is the tube^/retained in the- whole of the channel of U configuration.

As shown in Figure 10 two narrow slots 37, 38 are in formed/the block 20 and o-pen onto the back face 23 of the block. These slots 36, 37 extend in a direction parallel to the limbs 25, 26 of the channel 22 but are inset from these limbs 25, 26 towards the centre line of •'the block 20. . ..

Inserted within the slots 37, 38 are .heating elements 39, 40, (Figures 10 and 11), ·.which are formed by insulating strips 41 f 4.2 of for example mica or plastic^, material, with insulated electrical resistance wire 43, 44 wound around- the strips 41, 2 along the whole length of the strips. In the electrical circuit, Figure 12, the two heatin elements 39, 40 are connected together in series and under the control., of the circuit they supply .sufficient, heat to the block 20 to maintain its temperature at a value in the range 37 to 40°C - 2°C.

A thermistor 36 for sensing the temperature of the block is mounted in a hole 45.formed in the back face 23 of the block. The thermistor 45 is' connected into the circuit controlling the supply of current to the heating elements 39,' 40, Also mounted on the back face 23 of block 20 is a thermally operable emergency cut-out switch 64 which is connected in series 'with the heating elements 38, 39 to' cut off the supply of heating current when the temperature of the block reaches ' 43°C ·- 2 C.

The control circuit for the heating elements 38, 39 contains red and green light emitting diodes which serve as warning lights and are visible in holes 46 and 47 in the cover shaped to accommodate the tips of the diodes .

A series of openings 4-8 are provided oyer an audible warning device, usually a bleeper.

A knob 4-9 which is a hand control for variable 73 of resistance 61/a potentiometer chain in the electrical circuit is positioned on top of the cover 24 opposite the end 27 of the block- 20. ' The dial is marked with four temperature settings 37°C, 38°C,' 39°C and 40°C, and the dial is turned to bring one of these settings opposite the arrow 50 marked- on the cover 2 ' to set into the -L circuit a resistance- value indicative of the temperature at which the block is to be maintained.

At. the bottom end 51 of the cover 2k an opening is provided for the passage of an electrical cable from a power .source such as ' a transformer unit or a car battery.

A friction clamp, not shown is provided at the · .opening for ' tightly gripping the cable to prevent it. from exerting any strain on the electrical contacts with the .circuit. · ; .

• One side edge 52 of the block 20 is formed with an -outwardly projecting elongate projection 53 shaped with a groove 54 - to receive a monitering clinical thermometer IS without restrictor. . - .

The facing side edge 56 of the cover 24 is formed with two .. abutments 57 , 58 which lie over the ends of the thermometer 55.and hold it in the groove 54.

Centrally of the cover there is a hole 59 opening onto the threaded bore of a bush, (not shown) secured to the face 23 of block 20 s which bush receives a bolt of a clamping device (not shown) with universal movement by which the heating device can be" clamped in the most convenient position to the stand from which the container of transfusion liquid is suspended at. the bedside- of a patient. By supporting the whole weight of the heating device in such a clamping device no strain is exerted by -the heating device on the flexible tube for the •transfusion liquid, thus . avoiding the possibility of ..' . constrictions in the tube.

The control circuit for the heater elements and for the warning, lights and the audible warning are illustrated in Figure 12. This circuit also provides a warning of a fall in voltage of the power supply voltage, which is of particular importance when operating from batteries, and provides warning. of .a fault condition which causes the temperature of the block to exceed a specified limit.- A rise in temperature of the block may be. due to -a. slowing down of the flow of fluid through the tube, for · example if the patient lapses into coma or in -the extreme condition if the heart stops and there is ■no longer any feed of transfusion liquid taken from the tube; .. When such a situation developes the cooling effect of the liquid flow through the tube ceases and the block 'temperature rises. . The audible signal generated, by the control circuit may therefore be an indication of a deterioration in the patient's condition. %* The control circuit has a power supply line 60 which is normally maintained at' a potential of 12.6 volts ~ 1 volt-. This power supply can be. take either from a mains operated transformer unit, or from a '12 volt battery supply which is maintained within the range 12.6 volts - 1 volt' .at a peak current of 2.5 amps. The positive and ■negative terminals of the supply are connected 'to the positive line .60 and to a zero volts line 61. An. earth line of the power supply cable is connected directly to the block and would also be connected to the main earth' . points in the power supply unit or when operating from the' battery of a vehicle for example an ambulance ; the earth line .would be connected to the chassis of the vehicle„ A + 5 volt supply for operating certain parts of the circuit is maintained on a further line 62 which is"' connected to the junction of a resistor 63 and a Zener diode 64 across which diode there is maintained a stabilised 5 volt dro „ The tvo wire wound heater elements 39, 40 which are fitted in to slots 37, 38 in the block '20 as already •described, are connected in series and are represented in the control circuit at 6.5. he heater elements 65 are connected in series with a thermally operable emergency cut-out switch 66 which is mounted on the inner face of the block and cuts- out when the, temperature of the block is 43°C - 2°C. , Normal body temperature is The cut-out switch 66 is connected to the 12.6 volt supply line- 60 and the series connection of the switch 66 and the heater elements 65 are connected through the collector-emitter circuit of a transistor 67 to the zero volt line 61. Operation of 'the transistor '67 controls the supply of current through the heater elements 65.

• A thermistor 68 for sensing the temperature of the block is mounted in the hole 45 formed in the back face of the block. This thermistor 68 i connected between the zero volts line 61 and one input, of an operational amplifier 69 which · nput is also connected to the power line 60 through a. resistor chain comprising a fixed resistor 70 and a variable ■ resistor 71. The amplifie 69 has a conventional . fe d back loop including a resistor 72. The variable resistor 71 is adjustable to match the cold setting condition of .the .thermistor 68 to balance the input to the amplifier 69.

The second input to the amplifier 69 is from the V slide.r of a variable' resistor 73 which is the- temperature setting potentiometer .adjustable by the knob 49.. The resistor 73 is connected in a potentiometer chain comprising resistors 7 and 75 connected in series with · the potentiometer 73 between the lines 60 and 61. ' Upon switching on, when the device is cold, a differenc signal appears at the output from the amplifier 69 corresponding to the difference in temperature between that set in on the resistor 73 and the actual value of the temperature of the block indicated by the thermistor 68» This difference signal on line 76 is fed through a coupling circuit to the base of a transistor 77 which feeds an amplified outlet to an inverter 78 which is one of a group of 6 inverters forming a single component block.

When there is a positive signal on the output 76 from the aniplifier 69 the transistor 77 'conducts so that the potential on the output line 79 falls, which fall is inverted by the inverter 78 so that a potential rise is transmitted on the output line 80 from the inverter to the base of a transistor 81 which is thereby switched into conduction causing a red light transmitting diode 82 to light up,- · ··' ■ ' ' .· The base of the transistor 67 is coupled to the emitter of the transistor 81. and the transistor 67 conducts so that current flows through the heater elements 6 « At the same time a green light transmitting diode 83 is lit by current flow through a buffer resistor 84 i series with the diode 83, which resistor is connected to the output of an inverter 85. When that output falls due to a rise in potential of the input to the inverter 85, the green diode 83 is . light. Input to the inverter 85 is derived' from a NOR gate 86 having an input on line 87 connected to the output from a further NOR gate 88 and a second input on line 89 connected to the output from the circuit of a transistor 90 which is connected in series with a load resistor 9.1 to the 5 volt line 62 and which receives an input when the cut-out. switch 66 is closed. The line 89 therefore puts a constant potential onto, one input to the NOR. gate 86 as long as the safety cut-out switch 66 is closed.

In the start-up condition;, and when the device is warming up, which usually takes about 4 minutes . under ambient conditions of 20°C in till air, and with a 12.6 volt supply and no fluid flow, both the red and green lights are lit. When the block reaches the temperature . value set by the knob .49, the difference signal on the output line 76 from the operational amplifier 69 falls.' ; The transistor 77. becomes, non-conducting and the potential on line 79 rises so that the .output of the inverter 78 on line 80 falls. This cuts-off the ■transistor 81 so that the red light is extinguished, and the transistor 67 also operates in cascade to reduce the •current through the heating elements 65. When the temperature of the block has fallen sufficiently to produce a positive-going difference signal on the line 76, heater current will again be switched through the transistors 81 and 67 and the red light', switches on c ain. ' While the device is correctly controlling the. temperature of the block and the transfusion liquid is flowing through the tube at the set temperature , the green light remains on ana the red light is cycling on and off. .

If the block becomes too hot the switch 66 opens and current to the heaters is cut off. ' The potential on the base of transistor 90 falls and the potential o line 89 rises so that the output from the NOR gate 86 falls and current ceases to flow through the green diode 83. which is extinguished. In this- condition an audible alarm in the form of a bleeper 91 sounds. This bleeper , is connected in series with' a resistor 92 to an output line .93 of an inverter 94 whose input on line 95 is derived from a NOR gate 96, which has .two inputs connected in common to a line 97 which is connected to the output from the NOR gate 86.. The junction between the bleeper 91 and the resistor 92 is connected to the junction ■ between two series-connected resistors 98 and 99 which are connected, between the power line 60 and the output . ' line 100 of a . further inverter 101 whose input' is also . derived on line 95 from the output from the NOR gate 96. If the temperature of the block rises to 43°C 'which at the normal flow rate of 1 litre per 4 hours means that the liquid, temperature has risen to 4l°C, the NOR gates . 86. and 96 operate by signals derived from the transistor' circuit 90 to ensure that the bleeper 91 emits a continuous audible note. . . '" · ' ■ ' This note continues until the block has 'cooled sufficiently .and the switch 66 closes to reconnect ■ supplies to the heater elements. If a fault condition " again rices above the critical temperature and this sequence will continue until the povrer is switched off by the attendant. · A cause of overheating as mentioned above may be due to a decrease in the rate at which the patient- is accepting the transfusion liquid and the audible note emitted by the device gives a Yearning to the nursing staff of the possible critical condition, of the patient. Λ further safet requirement is that the level of the supply voltage be maintained in .order that the circuit v/ill operate correctly to maintain the liquid at the required set temperature. Any fall in the supply voltage is sensed' by a second operational amplifier 102 having one input on line 103 connected to the junction of a chain of two resistors 105 and 106 which are connected in series between the lines 60 and 61. The potential on line 103 is an indication of the potential oh the supply line 60 and the values of the resistors 105 and 106 for example respectively 5.1 Kohms and 4.7 ohms- ensures that under normal operating conditions with the .voltage on the line 60 at 12.6.volts the -voltage on line 103 is about 5 volts. The other input to the amplifier 102 on line' 107 is derived from the 5 volt line 62 through a resistor 103·. The amplifier has a conventional feedback loop including a resistor 109.

As long as the potential on the line 60 is up at 12.6 volts there is no difference signal output on the output line 110 from the amplifier 102. This means that the transistor 111- whose base is connected to the line 110 is. non-conducting so that an output _ line 112 from the transistor circuit is up in potential near to the- 3 volt ■value of the line 62. This line 112 is fed to one input of the NOR gate 886 The other input to that gate on line 113 is derived through a CR' -coupling circuit from an output line 114 of a further inverter 113 which has an input on line 116 derived from the output from the sixth inverter 117 whose output on line 118 is coupled by a CR coupling circuit to the line 116» Under normal operation with the correct supply- voltage there is no signal oirput from the gate 88. The output line 87 from that gate is connected to both inputs of a .further NOR gate 119 whose output is connected to th input of the inverter 117» The effects of this logic .circuit is that if the voltage on line 60 falls t an ..output signal is derived from amplifier .102, transistor 111 conducts so that the potential on line 112 falls. This results-- in the green diode 83 cycling on and off while the red diode either cycles on and off or remains .on continuously. This condition also causes the ' bleeper ,91. to emit an intermittent audible signal associated with .the cycling . on and off of the two light emitting diodes which condition persists. as long as the supply voltage is below the minimum necessary for adequate temperature control.

. In use for example in the transfusion of plasma or saline or dextrose solution to a patient, the heating device is clamped to a stand at a point close to the ' intended point of entry of the transfusion needle into the patient.

.The bag for the transfusion, liquid is. suspended . at the height above the patient normally used for clear nylon leads from the bag, into one j^ limb 25 or 26 of. the channel 22 in the device and a short length of tube will project from the end of the 'other limb up to the transfusion needle' which is inserted ■ into the patient. Once this, condition has been established transfusion can proceed.

The device will heat up liquid flowing through the flexible tube retained in the channel to the temperature set on the dial up to a maximum liquid flov/ rate' of Ί litre in k hours. With .higher flov; rates the ' temperature attained by the liquid will be correspondingly . lower than that set "on the dial and therefore the operative will take into account the desired liquid flow rate when setting the temperature on the dial, which in cases of flov; rates higher' than 1 litre in 4 hours 'will be higher than that actually desired. By carrying out a number of runs at differe t temperature settings and' .flov; rates the device can be calibrated and the results' used when the device is used. in actual operations.

. In addition to the use of the device in-heating \ transfusion or other liquids flowing through a flexible .tube it envisaged, that the device could be used fo heating gases such as anaesthetising gases used in · . •surgery and dentistry.

\ Appln.No. 3373/3

Claims (4)

1. , Apparatus fo heating a fluid flowing through a flexible tube to a required temperature comprising heat sink in the form of a blook (4) of thermally conductive material formed with a channel (11) the side of which is not closed, electrical heating means (5) in thermal Con-tact with the block, and an electrical supply control circuit (9) connected to the heating means and operable in response to the block temperature to oontrol the current in the heating means to tend to maintain the block at temperature such that fluid flowing through the tube length retained in the channel is of part-Circular cross section, the channel edges subtending an angle of more than 180° at the central axis of the channel, whereby the channel is capable of receiving a flexible walled circular tube (l) in intimate thermal contact with a substantial, part of the side vail area of the channel without having access to either end of the tube and retaining it therein* ,,

2. * Apparatus according to Claim 1 wherein the channel, is straight and extends from end-to-end of one face of the block*

3. 2. Apparatus according to laim 1 wherein the channe is of U-oonfiguration in one face of the block having limbs terminating, at one end. of the block*

4. * Apparatus according to Claim 3 wherein the configuration of the U of the channel is a curve having diameter in the range 35 mm to 40 mm* 5» Apparatus acoording to any preceding claim wherei the electrical heatin moans comprises electrical: re Appln.No.43373/3 7. Apparatus as claimed in any preceding claims having a uniform uncoiled flexible tube in the channel in intimate contact with a substantial part of the channel side wall area, and the tube extending beyond both ends Of the channel* 8« Apparatus according to Claim 7 in which the tube leads from a container for transfusion liquid, to a needle at respective ends of the tube. 9. Apparatus coording to Claim 7 or Claim 8 wherein the flexible tube is a transparent plastics tube* AGENT FOB APPLICANT