Measuring and testing – Gas analysis – By thermal property
Reexamination Certificate
2001-04-19
2003-11-11
Kwok, Helen (Department: 2856)
Measuring and testing
Gas analysis
By thermal property
C073S025050, C073S031050, C422S090000, C422S098000, C422S109000, C324S443000
Reexamination Certificate
active
06644098
ABSTRACT:
BACKGROUND OF THE PRESENT INVENTION
1. Field of the Present Invention
The present invention relates generally to the field of gas sensors, and, in particular, to the art of detecting halogenated refrigerants by applying control theory to an improved “heated electrode” technology to control the operation of the detector using an advanced sensing device and one or more control loops.
2. Background Art
Gas detectors for sensing the presence of halogenated gases and other gases are well known.
FIG. 1
shows prior art gas detector type suitable for this purpose, commonly referred to as a “heated electrode” sensor. This sensor utilizes a cathode wire and an anode wire made of platinum, palladium or an alloy thereof. Typically, the cathode is repeatedly coated with a ceramic material such as a mixture of an alkali metal silicate and oxides of aluminum or silicon, with a drying period between each coat, and then inserted into an anode coil formed by several turns of the anode wire. The anode/cathode assembly is then coated further with the same mixture, except for the ends of the anode and the exposed end of the cathode, and dried. After the final drying, the anode/cathode assembly is fired in a kiln and then installed in a housing, with the exposed ends of the anode and cathode connected to anode contacts and a cathode contact, respectively. The final assembly is then energized and biased over many hours by applying a electrical current through the anode coil and a voltage across the anode coil to the cathode wire.
The ceramic forms an electrically resistive layer between the electrodes. When heated by an electrical current passing through a first of the electrodes, an outer layer depleted of ions develops along the electrodes. When this layer is exposed to reactive gases like halogen, ions flow across the depletion zone and the conductivity of the device is increased. Thus, the presence of halogenated gases may be determined by monitoring the current generated through the second electrode, referred to as the bias current, for a sudden increase in magnitude created by introducing the device to such gases. These sensors are commonly utilized by technicians to determine whether a refrigerant leak exists and to pinpoint its source.
Advantageously, heated electrode sensors have low electrical power requirements and good sensitivity, and such sensors exhibit excellent selectivity in that they tend to ignore most chemical vapors which may be present in a typical test environment, as well as water vapor. Unfortunately, prior art heated electrode sensors also suffer a number of drawbacks. First, and most significantly, the bias current is dependent not only upon the presence or absence of halogenated molecules at the electrodes, but by the temperature of the device as well. Thus, sudden changes in temperature are frequently misinterpreted as an indication of the presence of halogenated molecules because their respective effects are the same: each causes an increase in the bias current of the sensor.
U.S. Pat. No. 4,305,724 to Micko (the “'724 patent”) discloses a combustible gas detection system including a sensor temperature control system. The detection system includes a sensor element having active and reference sensors for detecting combustible gases, a controlled current source for providing electrical power to the sensor element, a voltage-to-duty cycle converter for providing a square wave control signal of variable duty cycle and a bypass switch for bypassing the active sensor element in response to the control signal. By increasing or decreasing the duty cycle, the amount of electrical energy flowing to the active element is likewise affected and the temperature of the active sensor may correspondingly be either upwardly or downwardly biased. When the presence of combustible gas begins to cause the temperature of the active sensor to increase, the increase is detected by the temperature control system and the duty cycle is adjusted to counteract the increase and maintain the temperature constant.
Unfortunately, the detection system of the '724 patent suffers from some drawbacks. First, the detection system of the '724 patent requires the use of a reference sensor. Perhaps more importantly, the temperature control system is used only to equalize the temperature of one sensor with respect to the other sensor. In particular, it includes no means for measuring the absolute temperature of either sensor, or for independently setting the absolute temperature of either sensor to a particular chosen value. This is sufficient in the active sensor type of the '724 patent because the presence of the gas sought may generally be indicated merely by the heat given off by the oxidation process, as indicated by the temperature of the active sensor compared to that of the reference sensor. This characteristic makes the active sensor of the '724 patent impervious to fluctuations in absolute temperature due to ambient conditions. However, in heated electrode refrigerant detector systems, the presence of the gas sought is indicated generally by an increase in bias current, which is also affected by the ambient temperature of the sensor. As a result, a heated electrode refrigerant sensor using the temperature control system of the '724 patent would still be affected by ambient conditions because it is incapable of controlling the absolute temperature of the sensor. In addition, the absolute temperature of the sensor cannot be controlled to prevent damage during warm-up of the system and the like. Thus, a need exists for a temperature control system suitable for use with a heated electrode refrigerant detection system which does not make use of a reference sensor and which may be utilized to control the absolute temperature of the heated electrode.
U.S. Pat. No. 3,912,967 to Longenecker (the “'967 patent”) discloses a circuit for providing regulation of the absolute temperature of a heater-anode of a refrigerant gas sensor. A power supply outputs two different DC voltage levels, one of which is connected through a transistor switch to the heater-anode coil of a heated electrode gas sensing element. The circuit monitors the approximate absolute temperature of the heater-anode based on its effective resistance. When the absolute temperature of the heater-anode drops enough below a desired value, a temperature regulation circuit closes the switch, and a greater amount of current is supplied to the heater-anode. When the temperature of the heater-anode reaches the desired value again, the temperature regulation circuit opens the switch and a lesser amount of current is supplied to the heater anode. Thus, as the temperature of the sensing element fluctuates, greater or lesser heating may be applied to the heater-anode by the temperature regulation circuit. Unfortunately, although this circuit provides some control over the absolute temperature of a heated electrode refrigerant sensor, the regulation is relatively crude, effectively permitting control only by turning an auxiliary heat source on and off. At best, the temperature of the sensor is thus roughly held in a general range, with the upper approximate limit being the desired temperature and the lower approximate limit being the temperature at which the transistor of the switch is cool enough to allow the auxiliary power supply to be coupled in. At worst, however, such a crude controller may allow the temperature of the sensor to oscillate wildly and even dangerously under certain conditions. Further, the circuit allows no adjustment to be made to the triggering temperatures at which the auxiliary source is turned on or off. Thus, a need exists for a more sophisticated temperature control system suitable for use with a heated electrode refrigerant detection system which allows the temperature of the sensor to be rigidly maintained at a particular absolute value, rather than within a wide range of temperatures, and wherein that value is adjustable.
Another disadvantage of prior art heated electrode sensors is that their
Cardinale Dennis
Zubik Robert
Advanced Test Products, Inc.
Baker & Hostetler LLP
Kwok Helen
Wiggins David J.
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