Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
2000-05-12
2003-01-21
Lee, Benjamin C. (Department: 2632)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S629000, C340S501000, C340S507000, C431S025000, C431S075000, C250S389000
Reexamination Certificate
active
06509838
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to temperature probes, or sensor tips, of the type used for the control and safety monitoring of gaseous fuel burners as used in various heating, cooling and cooking appliances. In particular, the present invention relates to flame ionization sensor probes used in gas combustion control/safety environments where contamination coating of the in-flame sensor probe shortens the useful life of the sensor.
2. Discussion of the Related Art
Flame ionization sensing provides known methods and apparatus for monitoring the presence of a flame for a gaseous fuel burner.
It is known that hydrocarbon gas flames conduct electricity because charged species (ions) are formed by the chemical reaction of the fuel and air. When an electrical potential is established across the flame, the ions form a conductive path, and a current flows. Using known components, the current flows through a circuit including a flame ionization sensor, a flame and a ground surface (flameholder or ground rod).
FIG. 1
illustrates a flame ionization sensor system
10
with a typical sensor/burner circuit loop as may be used in accordance with the present invention. Flame ionization sensor
11
having a probe
12
, will be mounted near the burner
13
. The output
15
of sensor
11
will be fed into a computer-controller
17
. The sensor loop can provide information regarding the status of a flame
18
in the burner
13
. If there is no flame, then the sensor
11
will not generate a signal which will cause the controller
17
to instruct the system to shut off fuel flow.
In utilizing a flame sensor as previously described, a voltage, such as a 120 AC voltage
21
, will be applied across the sensor loop, with the flame holder, or burner
13
, serving as the ground electrode
20
. Flame contact between the sensor probe
12
and the burner
13
will close the circuit. The alternating current (AC) output of the sensor/ground circuit, can be rectified, if the ground electrode (flameholder or burner
13
) is substantially larger in size than the positive electrode, since, due to the difference in electrode size, more current flows in one direction than in the other.
Flame ionization sensor probes
12
are thus electrodes, made out of a conductive material which is capable of withstanding high temperatures and steep temperature gradients. Hydrocarbon flames conduct electricity because of the charged species (ions) which are formed in the flame. Placing a voltage across the probe and the flameholder causes a current to flow when the flame closes the circuit.
Unfortunately it has been found that contaminants in the air stream of the fuel/air mixture can result in the build up of an insulating contamination layer on the probe, rendering it much less effective. At a certain level of coating, which prevents sufficient electron flow to the probe surface, the sensor is rendered useless, creating a premature or false system failure. Often these airborne contaminants are organosilicones found in personal and home care products which are oxidized by the flame
18
to silicon oxides (SiOx) which in turn build up through impact on the probe
12
providing the insulative contaminant coating.
It is thus desirable to find ways to increase the useful life of flame ionization sensor probes in spite of this insulative build up resulting from normal use of the flame ionization sensor system.
SUMMARY OF THE INVENTION
The voltage potential between the flame sensor and the burner (ground) is the driving force for the flame ionization signal, therefore an increased voltage should yield a higher signal in spite of the probe being covered with insulative contaminants. However, merely applying a higher sensor voltage will likely yield mixed results for increased sensor life, at least in part because the higher voltage may increase contaminant build up on the sensor probe. It was therefore determined that what was needed was a means of regulating the applied voltage to the minimum value needed for a desired flame signal, and incrementally increasing the applied voltage only as required, while limiting the circuit current overdrive ratio (ratio of maximum sensor signal to the minimum threshold detection level) and allowing for a low signal threshold equal to the baseline configuration of standard commercial sensor apparatus, despite the increasing voltage.
The present invention therefore provides a sensor circuit which maintains a reasonably constant current, e.g. 5 microamps, over the operating life of the furnace by incrementally increasing the sensor voltage as the contamination buildup increases. This circuit allows the flame sensor circuit to draw on a higher voltage as needed, without incurring circuit overdrive issues, thereby eliminating the need for raising the signal threshold. Laboratory trials have shown an improved time to failure of the sensor circuit of four to seven times the life of known, or baseline, sensor circuit models.
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Fowler J. Thomas
Goppel Kristin Powers
King Darrell J.
Payne Peter P.
Schmidt Stephan E.
Fejer Mark E.
Lee Benjamin C.
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