Resistive water sensor for hot tub spa heating element

Electric heating – Heating devices – With protective means for heater

Reexamination Certificate

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Details

Other Related Categories

C219S497000, C004S541100, C392S441000, C340S618000

Type

Reexamination Certificate

Status

active

Patent number

06476363

Description

ABSTRACT:

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'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'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'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 relativ

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