the following detailed description and accompanying drawings are provided for purposes of illustrating and describing presently preferred embodiments of the invention and are not intended to limit the scope of the invention in any way . it will be recognized that further embodiments of the invention may be used . referring now to the drawings wherein fig1 is a diagram of a spa system showing the spa heater 10 with typical equipment and plumbing installed . the system includes a vessel for holding water 1 and a control system 2 with one or more microprocessors 58 to activate and manage various spa components and adjust and maintain various parameters of the spa . connected to the vessel for holding water 1 through a series of plumbing lines 4 are one or more pumps 3 for pumping water , a skimmer 5 for cleaning the surface of the spa , a filter 6 for removing particulate impurities in the water , an air blower 7 for delivering therapeutic bubbles to the spa through one or more air pipes 8 , and a spa heater apparatus 10 for maintaining the temperature set by the user . a light 9 is provided for internal illumination of the water . service voltage power is supplied to the spa control system 2 by electrical service wiring 11 , which can be 120v or 240v single phase 60 cycle , 220v single phase 50 cycle , or any other generally accepted power service suitable for commercial or residential service . an earth ground 12 is connected to the control system 2 and therethrough to all metal parts and all electrical components that carry service voltage power and all metal parts . the spa control system 2 with one or more microprocessors 58 is electrically connected through cables 13 and / or cables in conduit to one or more control panels 14 . all components powered by the control system are connected by cables 13 and / or cables in conduit suitable for carrying appropriate levels of voltage and current to properly operate the spa . water is drawn to the plumbing system generally through the skimmer 5 or suction fittings 16 , and discharged back into the spa through therapy jets 17 . temperature sensing devices 50 and 52 such as thermistors are typically located throughout the system to provide temperature data to the spa control system 2 . fig2 shows a plan view of an embodiment of the water heater 10 having a pipe 70 with a pipe inlet 72 and a pipe outlet 74 for heating water flowing therethrough . the inlet and outlet pipes can be flanged or additional end flange couplings 32 made of pvc , plastic or equivalent polymer material can be attached to the ends to facilitate connecting the pipe with the plumbing system of a spa . the pipe is preferably made of stainless steel , but it is understood that the pipe material can made of copper , copper - nickel allow , aluminum , aluminum alloys , magnesium , magnesium alloys , titanium , titanium alloys , steel , corrosion resistant varieties of steel , brass , ceramic , glass , or any other suitable material which is resistant to known changes in water chemistry of spas , hot tubs , pools , hydrotherapy pools , bath tubs , and similar bodies of water used indoors , outdoors , or both indoors and outdoors . the inner diameter of the pipe is preferably 1¾ inches or 2¼ inches , which corresponds to current pipe sizes typically used in spa plumbing , however , it is understood that the invention will work with virtually any diameter pipe . a binding material 36 is formed on the outer surface of the pipe to bind a dielectric layer 34 to the outside of the pipe 70 . the preferred embodiment uses preheated stainless steel as the material for the pipe 70 . when the stainless steel is preheated , a chromium oxide coating is formed on the outer surface 78 of the pipe , which acts as the binding material 36 to allow the dielectric layer 34 to be attached thereto . if the pipe 70 is made of a non - conductive material such as pvc , the need for a binding material 36 and dielectric insulating layer 34 can be eliminated and the resistors 38 or resistive layer as well as the conductive strips or conductive layer 40 can be attached directly onto the pipe 70 . an alternative means for providing the thermal resistance to a pipe made of non - conductive material is to disperse electrically conductive particles in the binding material 36 . a plurality of resistors 38 are attached to the dielectric layer 34 and connected by a conductive layer 40 . the conductive layer 40 is preferably a series of conductive strips interconnected to electrically connect the plurality of resistors 38 . a plurality of terminals 54 are connected to the conductive layer 40 for connecting wires from an electronic controller 56 , which has at least one microprocessor 58 adapted to process signals from a plurality of devices providing water parameter information such as temperature , ph , and the presence or absence of water within the heater 10 . the electronic controller 56 is also connected to a power supply 60 for energizing the system . the electronic controller 56 is arranged to control the operation of the water heater by regulating the temperature and controllably energizing the water heater 10 . as further shown in fig2 , temperature sensors 50 and 52 are located on the surface of the pipe 70 , to provide temperature data to the electronic controller 56 and to a separate high limit switch 62 ( more readily seen in fig5 ). the terminals 54 for coupling cables 13 from the various controls and sensors to the conductive layer 40 can be multi - strand percussion welds or other methods of attachment well - known in the art , for example a stud welded onto the conductive layer . by maintaining all electrical elements of the heater on the outer surface 78 of the heater 10 , virtually all of the typical failures associated with traditional spa heaters are eliminated . the result is a smooth seamless inner heating surface without the need to pass electrical leads into the inner wet region of the heater , thereby eliminating the need for bulkhead fittings and reducing the risk of leaks . additionally , there are no heating elements exposed to high temperature chemically treated water , which eliminates the risk of corrosion . fig3 shows a top plan view of the heater 10 showing the pipe 70 cut lengthwise and unrolled to show the layout of the resistors 38 , the dielectric layer 34 , and the conductive layer 40 . the dimensions and layout of the dielectric layer 34 , resistors 38 , conductive layer 40 , and the terminals 54 are configured to provide variable operating resistance values . the preferred resistance pattern or layout provides two separate operating resistance values of 1 . 5 kilowatts and 4 . 0 kilowatts ( kw ) and a combined operating resistance value of 5 . 5 kilowatts when both the 1 . 5 kw and 4 . 0 kw resistance patterns are both energized . the dimensions and layout of the resistance pattern can vary depending on the particular application and can be determined in accordance with well - known methods . the pattern of resistors 38 and conductive layer or conductive strips 40 are preferably screen - printed onto the binding material 36 , however , the same pattern or layout can be configured onto the binding material 36 and pipe 70 by various other methods such as depositing an electrically conductive composition onto the binding material , bonding , or electrostatic spraying with the use of a stencil . additionally , when the pipe 70 is made of a non - conductive material , the resistance layer can comprise electrically conductive particles dispersed in the binding material 36 applied directly onto the outer surface 78 of the pipe 70 . fig4 is a section view along lines a — a of fig3 showing the cross - section of the heater 10 . the bottom layer is the pipe 70 , which has the binding material 36 to enable the dielectric layer 34 to adhere to the pipe 70 . the pattern of resistors 38 is screen - printed onto the dielectric layer 34 and the conductive layer 40 electrically connects the resistors 38 to the power supply 60 and controller 56 through the terminals 54 to form an electrical circuit for energizing the heater 10 . in the embodiment shown in fig4 , there is shown an insulating overcoat 66 , preferably of a glass insulating material covering the dielectric layer 34 , the resistors 38 , and the conductive layer 40 to provide thermal insulation and to provide scratch protection for the various layers . fig5 is a block diagram showing the interconnectivity of the water heater 10 to various control mechanisms and the power supply 60 . electrical service wiring 11 is connected to the electronic controller 56 , which is connected to a high limit switch 62 . the high limit switch 62 is in series with the electronic controller 56 and is connected to the temperature sensors 50 and 52 on the pipe 70 to cause power to be disconnected from the water heater when the temperature exceeds a predetermined temperature . the high limit switch 62 preferably automatically reconnects the power once the water temperature has dropped below a predetermined temperature , however , a manual reset can also readily be used to reconnect the power to the heater . the high limit switch 62 can employ either electric circuitry or mechanical means for disconnecting and reconnecting the power supply . the electronic controller 56 is connected to the temperature sensors 50 and 52 for receiving temperature data from the heater 10 . the temperature sensors 50 and 52 are preferably thermistors , however , it is understood that traditional temperature sensors such as a bulb and capillary device can effectively be used . the electronic controller 56 is also connected to a control panel 64 for receiving user preferences . in a preferred embodiment the electronic controller 56 has a microprocessor 58 , which is adapted to process signals from a plurality of devices providing water parameter information , including temperature signals from the temperature sensors 50 and 52 . in one embodiment a separate water presence sensor 84 is located in the water flow path near the heater 10 for indicating the presence or absence of water within the heater . the water presence sensor 84 can be a pressure switch 86 ( shown in fig8 ) or other device to sense the presence of water in the heater 10 , such as a flow meter or vacuum switch . in a preferred embodiment the electronic controller 56 in conjunction with the temperature sensors 50 and 52 can detect the presence or absence of water in the heater by operating the water heater for a given time interval and determining whether water is present as a result of the difference in the before and after temperature values . the electronic controller 56 will turn off the water heater in the absence of water within the heater 10 , and turn the water heater on upon subsequent receipt of water within the heater . additionally , the electronic controller 56 is configured to deactivate operation of the heater 10 if the water temperature rate of rise at the first or second temperature sensor location exceeds a specified value . a control panel 64 is connected to the electronic controller 56 for inputting user preferences . the electronic controller regulates power supplied to the heater based on user inputs from the control panel 64 and temperature data from the temperature sensing devices 50 and 52 coupled to the heater 10 . fig6 is a block diagram showing the interconnectivity of the water heater 10 to the power supply 60 and to traditional control mechanisms that do not employ a microprocessor . electrical service wiring 11 is connected to the power controlling device 68 , which is connected in series to a high limit switch 62 . the high limit switch 62 is connected to at least one temperature sensor 50 to cause power to be disconnected from the water heater when the temperature exceeds a predetermined temperature . a grounding connection 82 is also connected to the heater 10 to ground the device . when only one temperature sensor is employed the preferred location of the temperature sensor is at near the outlet 74 of the water heater 10 . the high limit switch 62 preferably automatically reconnects the power to water heater once the temperature has dropped below a predetermined temperature . a manual reset can also be used to reconnect the power to the heater . the high limit switch can employ either electric circuitry or mechanical means . the power controlling device 68 is also connected to the temperature sensor 50 , to the power supply 60 , to a water presence sensor 84 , which is located on or near the heater 10 , and to a control panel 64 for inputting user preferences . the power controlling device 68 receives temperature data from the temperature sensor 50 for regulating power to the heater 10 . the power controlling device 68 receives water presence data from the water presence sensor 84 and shuts off power to the water heater 10 in the absence of water within the pipe and turns power on to the water heater 10 when the water presence sensor 84 detects water present within the pipe . the power controlling device can employ electrical circuits , mechanical controlling means , or solid state technology controlling means . fig7 shows a perspective view of an alternate embodiment of the water heater 10 for use in spas , hot tubs , pools , hydrotherapy pools , bath tubs , and similar bodies of water that can be used indoors , outdoor or both . the water heater 10 has a heating chamber 20 connected in a water flow path to heat the water flowing through the chamber . the heating chamber 20 has an inlet pipe 28 and an outlet pipe 30 for connecting the heater to a spa &# 39 ; s plumbing lines . the embodiment shown has two circular thick film on substrate heaters with heating surfaces 22 to form two of the walls of the heating chamber . the heating surfaces have an inner wet surface 24 to contact the water to be heated , and an outer dry surface 26 for maintaining all of the electrical connections . the configuration of the heating chamber provides seamless inner heating surfaces with maximum heater water interaction to efficiently heat the water to desired temperatures . the heating surface 22 has a substrate 18 , which is preferably stainless steel that has been preheated to form a chromium oxide binder 36 on the outer surface for coupling a dielectric layer 34 thereon . resistors 38 are attached to the dielectric layer 34 and are connected by a conductive layer 40 , which is connected by terminals 54 to the electronic controller 56 and power supply 60 to controllably energize the water heater 10 . temperature sensors 50 and 52 are located on the heater 10 for sensing temperature and providing temperature data to the electronic controller 56 . fig8 is a perspective view of yet another alternate embodiment of the water heater 10 , having a heating chamber 20 connected in a water flow path to heat the water flowing through the chamber . the heating chamber 20 has an inlet pipe 28 and an outlet pipe 30 for connecting the heater to a spa &# 39 ; s plumbing lines and the electronic controls shown in fig5 or fig6 . the embodiment shown has four rectangular thick film on substrate heaters with heating surfaces 22 to form four of the walls of the heating chamber 20 . a separate water presence sensor 84 is shown as a pressure switch 86 located in the water flow path near the outlet pipe 30 and is connected to the electronic controller 56 for indicating the presence or absence of water in the heating chamber . the inlet pipe 28 and outlet pipe 30 are sized to fit preexisting spa plumbing lines . the advantage of the embodiment shown in fig8 is that the layout of the resistive heating components can be configured to maximize heater surface to water interaction and produce less external heat thereby requiring less external insulation on the heater . additional temperature sensing devices can be used at the heater and / or in the spa plumbing to sense water temperature at various locations throughout the spa system . if the temperature sensor 40 is located within the water flow path it is generally potted in a potting compound such as epoxy or the like and in stainless steel housings . the stainless steel housings are mounted into the side of the heater pipe with an insulating collar , which provides a water pressure seal and an insulative barrier from the heater pipe . while the description above refers to particular embodiments of the present invention , it will be understood that many modifications may be made without departing from the spirit thereof . the accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention . the presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive ; the scope of the invention being indicated by the appended claims , rather than the foregoing description , and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein .
an improved water heater for use in spas , hot tubs , pools , hydrotherapy pools , bath tubs , and similar bodies of water used indoors , outdoors , or both indoors and outdoors are used for both therapeutic and recreational purposes . the water heater uses heating element technology know as thick film on substrate comprising resistive elements bonded to the outer dry surface of a pipe to heat the pipe which in turn heats the water flowing therethrough . the heater is highly efficient due to the direct contact of the wet heating surface with the water and provides a smooth seamless inner heating surface by eliminating the need to pass electrical leads into the wet region of the heater . this virtually eliminates the risk of leaks in the water heater due to bulkhead fittings . the invention further eliminates the need for a heating element to be contained in the inner wet region of a spa heater , thereby reducing the risk of corrosion . the water heater is used in combination with an electronic controller having a microprocessor to control and regulate the operation of the water heater . the water heater can be used with electrical , electro - mechanical , and mechanical control systems for spas and can be retrofitted into existing spa applications .