Abstract:
A dry fire protection system for a spa and the spa&#39;s associated equipment. A heating element heats the spa&#39;s water. A resistive water level sensor senses that the level of water around the heating element is higher than a predetermined height or lower than a predetermined height, and a heating element deactivation device electrically deactivates the heating element when the water level around the heating element falls below a predetermined level. In a preferred embodiment, the heating element deactivation device is an electric circuit comprising a comparator circuit and a control circuit.

Description:
BACKGROUND OF THE INVENTION 
     A spa (also commonly known as a “hot tub” when located outdoors) is a therapeutic bath in which all or part of the body is exposed to forceful whirling currents of hot water. When located indoors and equipped with fill and drain features like a bathtub, the spa is typically referred to as a “whirlpool bath”. Typically, the spa&#39;s hot water is generated when water contacts a heating element in a water circulating heating pipe system. A major problem associated with the spa&#39;s water circulating heating pipe system is the risk of damage to the heater and adjacent parts of the spa when the heater becomes too hot. 
     FIG. 1 is a drawing showing the main elements of a prior art hot tub spa system  1 . Spa controller  7  is programmed to control the spa&#39;s water pumps  1 A and  1 B and air blower  4 . In normal operation, water is pumped by water pump  1 A through heater  3  where it is heated by heating element  5 . The heated water then leaves heater  3  and enters spa tub  2  through jets  11 . Water leaves spa tub  2  through drains  13  and the cycle is repeated. 
     Some conditions may cause little or no flow of water through the pipe containing heating element  5  during the heating process. These problems can cause what is known in the spa industry as a “dry fire”. Dry fires occur when there is no water in heater  3  or when the flow of water is too weak to remove enough heat from the heating element  5 . Common causes of low water flow are a dirty filter or a clogged pipe. For example, referring to FIG. 1, if a bathing suit became lodged in pipe  17 B clogging the pipe, flow of water through heater  3  would be impeded and a dry fire could occur. 
     KNOWN SAFETY DEVICES 
     FIG. 1 shows a prior art arrangement to prevent overheating conditions. A circuit incorporating temperature sensor  50  serves to protect spa  1  from overheating. Temperature sensor  50  is mounted to the outside of heater  3 . Temperature sensor  50  is electrically connected to comparator circuit  51 A and control circuit  52 A, which is electrically connected to high limit relay  53 A. 
     As shown in FIG. 1, power plug  54  connects heating element  5  to a suitable power source, such as a standard household electric circuit. Water inside heater  3  is heated by heating element  5 . Due to thermal conductivity the outside of heater  3  becomes hotter as water inside heater  3  is heated by heating element  5  so that the outside surface of heater  3  is approximately equal to the temperature of the water inside heater  3 . This outside surface temperature is monitored by temperature sensor  50 . Temperature sensor  50  sends an electric signal to comparator circuit  51 A corresponding to the temperature it senses. When an upper end limit temperature limit is reached, such as about 120 degrees Fahrenheit, positive voltage is removed from the high temperature limit relay  53 A, and power to heating element  5  is interrupted. 
     A detailed view of comparator circuit  51 A and control circuit  52 A is shown in FIG.  4 . Temperature sensor  50  provides a signal representing the temperature at the surface of heater  3  to one input terminal of comparator  60 . The other input terminal of comparator  60  receives a reference signal adjusted to correspond with a selected high temperature limit for the surface of heater  3 . As long as the actual temperature of the surface of heater  3  is less than the high temperature limit, comparator  60  produces a positive or higher output signal that is inverted by inverter  62  to a low or negative signal. The inverter output is coupled in parallel to the base of NPN transistor switch  64 , and through a normally open high limit reset switch  66  to the base of a PNP transistor switch  68 . The low signal input to NPN transistor switch  64  is insufficient to place that switch in an “on” state, such that electrical power is not coupled to a first coil  70  of a twin-coil latching relay  74 . As a result, the switch arm  76  of the latching relay  74  couples a positive voltage to control circuit  52 A output line  78  which maintains high limit relay  53 A in a closed position (FIG.  1 ). 
     As shown in FIG. 4, in the event the switch arm  76  of the latching relay  74  is not already in a position coupling the positive voltage to the output line  78 , momentary depression of the high limit reset switch  66  couples the low signal to the base of PNP transistor switch  68 , resulting in energization of a second coil  72  to draw the switch arm  76  to the normal power-on position. 
     If the water temperature increases to a level exceeding the preset upper limit, then the output of the comparator  60  is a negative signal which, after inversion by the inverter  62 , becomes a high signal connected to the base of NPN transistor switch  64 . This high signal switches NPN transistor switch  64  to an “on” state, and thus energizes the first coil  70  of latching relay  74  for purposes of moving the relay switch arm  76  to a power-off position. Thus, the positive voltage is removed from the high temperature limit relay  53 A, and power to heating element  5  is interrupted. Subsequent depression of the high limit reset switch  66  for resumed system operation is effective to return switch arm  76  to the power-on position only if the temperature at the surface of heater  3  has fallen to a level below the upper limit setting. 
     In addition to the circuit incorporating temperature sensor  50 , it is an Underwriters Laboratory (UL) requirement that there be a separate sensor located inside heater  3  in order to prevent dry fire conditions. There are currently two major types of sensors that are mounted inside of heater  3 : water pressure sensors and water flow sensors. 
     Water Pressure Sensor 
     FIG. 1 shows water pressure sensor  15  mounted outside heater  3 . As shown in FIG. 1, water pressure sensor  15  is located in a circuit separate from temperature sensor  50 . It is electrically connected to spa controller  7 , which is electrically connected to regulation relay  111 . 
     Tub Temperature Sensor 
     Spa controller  7  also receives an input from tub temperature sensor  112 . A user of spa  1  can set the desired temperature of the water inside tub  2  to a predetermined level from keypad  200 . When the temperature of the water inside tub  2  reaches the predetermined level, spa controller  7  is programmed to remove the voltage to regulation relay  111 , and power to heating element  5  will be interrupted. 
     Operation of Water Pressure Sensor 
     In normal operation, when water pressure sensor  15  reaches a specific level, the electromechanical switch of the sensor changes its state. This new switch state indicates that the water pressure inside heater  3  is large enough to permit the heating process without the risk of dry fire. Likewise, in a fashion similar to that described for temperature sensor  50 , when a lower end limit pressure limit is reached, such as about 1.5-2.0 psi, positive voltage is removed from regulation relay  111 , and power to heating element  5  is interrupted. 
     However, there are major problems associated with water pressure sensors. For example, due to rust corrosion, these devices frequently experience obstruction of their switch mechanism either in the closed or open state. Another problem is related to the poor accuracy and the time drift of the pressure sensor adjustment mechanism. Also, water pressure sensors may have leaking diaphragms, which can lead to sensor failure. The above problems inevitably add to the overall expense of the system because they may require relatively frequent replacement and/or calibration of water pressure sensor switch. 
     Water Flow Sensor 
     Another known solution to the dry fire problem is the installation of a water flow sensor  16  into the heating pipe, as shown in FIG.  2 . However, like the water pressure sensor, water flow sensor  16  is prone to mechanical failure in either the open or close state. Moreover, water flow sensor switches are expensive (approximately $12 per switch) and relatively difficult to mount. 
     Microprocessor Utilization 
     It is known in the prior art that it is possible to substitute a microprocessor in place of the comparator circuit and control circuit, as shown in FIG.  3 . Microprocessor  56 A is programmed to serve the same function as comparator circuit  51 A and control circuit  52 A (FIG.  1 ). When an upper end limit temperature limit is reached, such as about 120 degrees Fahrenheit, microprocessor  56 A is programmed to cause positive voltage to be removed from high temperature limit relay  53 A, and power to heating element  5  is interrupted. 
     Resistive Water Level Sensor 
     Resistive water level sensors (also known as resistive fluid level sensors) are known. A resistive water level sensor functions by utilizing a probe to sense the presence or absence of water in a water container. FIGS. 8A and 8B illustrate the operation of a resistive water level sensor. FIG. 8B shows water  204  in container  203 . Electrically conductive probe  201  is held in place inside container  203  by insulating sleeve  200 . A conductive wire extends from the top of probe  201  to electronic circuit  206 . Conductor  202  is mounted to the side of container  203  and is grounded. As shown in FIG. 8B, the water level is below probe  201 . Therefore the resistance between probe  201  and conductor  202  is substantially infinite. Hence, no current would flow through the electronic circuit. In FIG. 8A, the water level has increased so that it is above the tip of probe  201 . The resistance through water  204  is relatively low and a current carrying path is established between probe  201  and conductor  202 , completing the electronic circuit. 
     A popular application of resistive water level sensors is their utilization to sense to presence or absence of boiler water in heating plant boilers. Advantages of resistive water level sensors are that they have a relatively simple design, requiring low maintenance and are relatively inexpensive. 
     What is needed is a better device for preventing dry fire conditions in a hot tub spa. 
     SUMMARY OF THE INVENTION 
     The present invention provides a dry fire protection system for a spa and the spa&#39;s associated equipment. A heating element heats the spa&#39;s water. A resistive water level sensor senses that the level of water around the heating element is higher than a predetermined height or lower than a predetermined height, and a heating element deactivation device electrically deactivates the heating element when the water level around the heating element falls below a predetermined level. In a preferred embodiment, the heating element deactivation device is an electric circuit comprising a comparator circuit and a control circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a prior art hot tub spa utilizing a water pressure sensor. 
     FIG. 2 shows a prior art heater utilizing a water flow sensor. 
     FIG. 3 shows a prior art utilization of a microprocessor. 
     FIG. 4 shows a prior art circuit comprising a comparator circuit and a control circuit. 
     FIG. 5 shows a hot tub spa utilizing a preferred embodiment of the present invention. 
     FIG. 6 shows another preferred embodiment of the present invention. 
     FIG. 7 shows another preferred embodiment of the present invention. 
     FIGS. 8A and 8B show the operation of a resistive water level sensor. 
     FIG. 9 shows another preferred embodiment of the present invention. 
     FIGS. 10-12 show preferred embodiments of the present invention. 
     FIG. 13 shows another preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A detailed description preferred embodiments of the present invention can be seen by reference to FIGS. 5-13. 
     Protection Against a Dry Fire Condition 
     The present invention provides protection against a dry fire condition. A dry fire can occur if heating element  5  is on and there is no water or very little water inside heater  5  to remove heat from heating element  5 . A cause of a low or no water condition inside heater  3  could be blockage in pipe  17 B or in drains  13  or a closed slice valve  70 . Also, evaporation of water from spa tub  2  could cause a low water condition inside heater  3 , leading to a dry fire. If there is no water or only a small amount of water inside heater  3  so that the level of the water does not reach the tip of probe  250 , the resistance between between probe  250  and conductor  251  will be substantially infinite. Then, positive voltage will be removed from regulation relay  53 B, and power to heating element  5  will be interrupted. 
     Preferred Embodiment 
     In a preferred embodiment, resistive water level sensor probe  250  is a stainless steel pin, as shown in FIG.  5 . Probe  250  is mounted inside insulating enclosure  252 . Insulating enclosure  252  serves as a holder to maintain the probe in place inside heater  3 . Conductor  251  is mounted to the inside of heater  3 . The resistance measurement between probe  250  and conductor  251  is used to determine if the level of water is adequate around heating element  5 . 
     Probe  250  is part of an electrical circuit that includes comparator circuit  51 B, control circuit  52 B, and regulation relay  53 B. When the resistance between probe  250  and conductor  251  is greater than a predetermined limit level, control circuit  52 B causes positive voltage to be removed from regulation relay  53 B, and power to heating element  5  will be interrupted. In a preferred embodiment, the predetermined limit level is approximately 3.75 MΩ. For example, if the water level inside heater  3  is such that it does not reach the tip of probe  250 , then there will be substantially infinite resistance between the tip of probe  250  and conductor  251 . This resistance would be greater than the predetermined limit level and power to heating element  5  would therefore be interrupted. 
     Whirlpool Bath Application 
     Although the above preferred embodiment discussed utilizing the present invention with spas that do not incorporate separate fill and drain devices, those of ordinary skill in the art will recognize that it is possible to utilize the present invention with spas that have separate fill and drain devices, commonly known as whirlpool baths. 
     A whirlpool bath is usually found indoors. Like a common bathtub, a whirlpool bath is usually filled just prior to use and drained soon after use. As shown in FIG. 7, tub  2 A is filled with water prior to use via nozzle  100  and drained after use via tub drain  102 . Once tub  2 A is filled, whirlpool bath  104  operates in a fashion similar to that described for spa  1 . Spa controller  7  is programmed to control the whirlpool bath&#39;s water pumps  1 A and  1 B and air blower  4 . In normal operation, water is pumped by water pump  1 A through heater  3  where it is heated by heating element  5 . The heated water then leaves heater  3  and enters spa tub  2  through jets  11 . Water leaves spa tub  2  through drains  13  and the cycle is repeated. 
     When the resistance between probe  250  and conductor  251  is greater than a predetermined limit level, control circuit  52 B causes positive voltage to be removed from regulation relay  53 B, and power to heating element  5  will be interrupted. For example, if the water level inside heater  3  is such that it does not reach the tip of probe  250 , then there will be substantially infinite resistance between the tip of probe  250  and conductor  251 . This resistance would be greater than the predetermined limit level and power to heating element  5  would therefore be interrupted. 
     FIG. 13 shows another preferred embodiment of the present invention in which signals from both microprocessor  200  and probe  250  are used to control regulation relay  53 B 
     Heater Pipe Embodiments 
     FIG. 10 shows a preferred embodiment of heater  3  in which heater pipe  600  is metal. Probe  250  is mounted to heater pipe  600  by insulating enclosure  252 . Ideally, when the water level inside heater  3  reaches the tip of probe  250 , current will flow from probe  250  to the side of metal heater pipe  600  and then leave through conductor  251 . When the water level is below the tip of probe  250 , no significant current should flow. However, it is possible due to condensation on the surface of insulating enclosure  252  inside heater  3 , for current to flow from probe  250  across insulating enclosure  252  to the side of metal heater  600  prior to the water level reaching the tip of probe  250 , thereby causing a false reading. Utilizing the embodiments shown in FIG. 11 or  12  can eliminate this risk. FIG. 11 shows probe  250  mounted inside plastic heater pipe  601 . In this embodiment by making the heater pipe out of non-conducting plastic, the path to ground is drastically increased. Hence, the risk of a false read due to condensation is lessened. FIG. 12 shows metal pipe  600  with plastic fitting  602  attached to its end. In this embodiment, the amount of metal around probe  250  has also been decreased, decreasing the risk of a false read due to condensation. 
     Microprocessor Embodiments 
     FIG. 6 shows probe  250  as part of an electric circuit that includes microprocessor  80  in place of comparator circuit  51 B and control circuit  52 B. In this preferred embodiment, microprocessor  80  also receives input from tub temperature sensor  112 . Microprocessor  80  controls regulation relay  53 B. FIG. 9 shows another preferred embodiment that includes circuit  510  and microprocessor  80 B. In this preferred embodiment, voltage from DC voltage source  508  feeds op-amp  506 . Filter  500  is inserted in the circuit to protect the circuit against noise and ESD. Current limiting resistor, Rlimiter  504 , has a much lower value than Rweak  502  and is placed between earth ground  514  and digital ground  512 . If there is no water in heater  5 , the resistance between probe  250  and conductor  251  is substantially infinite. So, there is no current through Rweak  502  and the voltage drop across Rweak  502  is approximately 0V. Consequently, the input voltage at op-amp  506  is approximately 5 Volt and the op-amp output voltage is also approximately 5 Volt. When there is water in heater  3  between probe  250  and conductor  251  a current path is set up that flows through filter  500  through the water in heater  3 , through Rlimiter  504 , to digital ground  512 . This current path creates a voltage drop between the Rweak  502  terminal. As a result, the input signal to op-amp  506  and the output signal from op-amp  506  are both decreased to a voltage level between 0 to 2.5 Volt. Microprocessor  80 B is programmed to make a determination based on the signal coming from op-amp  506  whether or not there is sufficient water inside heater  3 . If the level of water is insufficient inside heater  3 , then positive voltage will be removed from regulation relay  53 B, and power to heating element  5  will be interrupted. 
     Although the above-preferred embodiments have been described with specificity, persons skilled in this art will recognize that many changes to the specific embodiments disclosed above could be made without departing from the spirit of the invention. Therefore, the attached claims and their legal equivalents should determine the scope of the invention.