Thermal measuring and testing – Transformation point determination
Reexamination Certificate
1999-05-07
2001-12-11
Gutierrez, Diego (Department: 2859)
Thermal measuring and testing
Transformation point determination
C374S025000
Reexamination Certificate
active
06328467
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention concerns the field of environmental sensors, and more particularly a sensor for detecting the buildup of frost or ice.
2. Description of the Related Art
Moisture condenses onto a surface whose temperature is cooled below the air's dew point. If the surface temperature is freezing then a frost or ice layer forms and gradually thickens with time. This deposition of frost can have detrimental effects on the operation of machinery which is operated outdoors. For example, frost on refrigeration coils is known to degrade the efficiency of refrigeration machines and can compromise reliable operation of the equipment.
In addition to the efficiency concerns raised by ice buildup, the icing of airplane wings is a major safety concern for the aviation industry because of the sudden loss of lift caused by wing icing can lead to catastrophic accidents. Similarly, frost deposits on roadways can be a serious safety hazard. For example ice formation on roadways, bridges and overpasses is the cause of many automobile accidents. The risk of such automobile and aircraft accidents may be minimized if it were possible to provide drivers and pilots with some warning of the hazardous condition which exists in each case.
HVAC manufacturers limit the ill effect of frosting by using an inexpensive and reliable time-and-temperature defrost control. However, such controls suffer from a drawback in that they require an adaptive logic because the sensors do not directly measure the frost buildup. For example, HVAC manufacturers use detectors that measure the increase in air pressure drop across the refrigeration coil or in another scheme they measure the temperature gradient from the refrigeration coil to the surrounding ambient air temperature. However, these measures are known to suffer from the false indications of frost buildup in inclement weather.
Other embodiments of sensors proposed for refrigeration are based on ultrasonic, emissivity and photo cell technologies.
In the transportation and aviation industries, several techniques are available for detecting an icing condition of a road or wing section. As an example, such sensors have been known to employ a network of thin, flexible microstrip antennas, distributed on aircraft wing, to measure the unique electrical properties of compounds that accumulate on the wing surface. Temperature and acoustic data is gleaned and processing of data shows the presence or absence of ice. Another known ice sensing technique uses a temperature sensor and a parallel arrangement of electrodes whose coefficient of coupling is indicative of the formation of ice. Still another technique applies a controlled heat to a sensing element which when dry displays a linear time-temperature curve. Ice or frost modifies the sensing element's time-temperature curve and the shift in the curve is used to indicate an icy road condition.
As noted above, existing frost sensors used in refrigeration do not directly detect the buildup of frost. They sense the pressure drop through the coil or the temperature gradient from the outdoor-ambient air to the refrigerant. Optical, ultrasonic and emissivity technologies use the effect of sound or electromagnetic radiation in their respective detection schemes. In both transportation and aviation, the detection schemes measure the temperature and either measure the electrical, acoustic or heat transfer characteristics from some sensing element.
SUMMARY OF THE INVENTION
The proposed sensor uses thermocouples or other heat sensor whose time-temperature history reveal the formation of frost or ice because of a change in the thermal properties of its surroundings. No form of electromagnetic radiation, nor any optical system is needed for detection.
According to one aspect of the invention, the method involves a series of steps including: applying a predetermined quantity of heat to a temperature sensor positioned at the interface to cause a thermal perturbation; measuring an output signal of the sensor subsequent to the applying step; and comparing a measured value of the output signal to a reference value to determine the presence of the frozen deposition.
The measuring step can be accomplished by recording two or more measured output values defining a transient response curve, and comparing the transient response curve to a reference transient response curve. The transient curve advantageously includes at least that portion of the transient curve defining a thermal decay response for the sensor.
According to another aspect of the invention, the predetermined quantity of heat applied to the sensor can be defined as the maximum quantity of heat required under the coldest anticipated operating conditions to raise the sensor temperature above a freezing temperature, except when the frozen deposition is present at the interface. In this embodiment, the predetermined quantity of heat applied to the sensor is determined based on a variety of criteria which can include a minimum anticipated operating temperature of the surroundings, a maximum anticipated wind velocity and the thermophysical properties of the substrate. The scientific basis for this embodiment is that the predetermined quantity of applied heat is not sufficient to affect a phase change, i.e., change ice to water. Therefore, the if ice is present temperature of the heated sensor will not rise above freezing temperature of water.
The temperature sensor may be any of a variety of commercially available temperature sensor units which have an electronically readable output. The heat applied to the sensor can be provided by an integrated heating device formed as part of the sensor or a separate heating element provided for this purpose. According to a preferred embodiment, the temperature sensor is a thermocouple device so that by passing a current through the thermocouple, it can be heated without the need for a separate heater unit.
A system for carrying out the foregoing method can be formed with a temperature sensor positioned at the interface to cause a thermal perturbation; a heater element is provided for applying a predetermined quantity of heat to the temperature sensor. However the heater element may be combined with the sensor for instance when a thermocouple is used for this purpose. Electronic circuitry is provided for measuring an output signal of the sensor and for comparing one or more measured value of the output signal after application of the predetermined quantity of heat to one or more corresponding reference values. Where the measured values match the reference values obtained for the presence of the frozen deposition, a suitable display or audible warning device may be provided to notify a user. Alternatively, a signal indicating the presence of ice can be used to control a related piece of equipment to respond to the presence of such ice.
The electronic circuitry or software for measuring can include suitable equipment for recording a set of measured output values defining a transient response curve. Additional circuitry or software can also be included to compare the transient response curve to a reference transient response curve. According to one aspect, the transient response curve represents a thermal decay time.
As with the method previously described, the predetermined quantity of heat applied to the sensor can be defined as the maximum quantity of heat required under the coldest anticipated operating conditions to raise the sensor temperature above a freezing temperature, except when the frozen deposition is present at the interface. In that case the predetermined quantity of heat applied to the sensor is determined based on criteria which can include factors such as the minimum anticipated operating temperature of the surroundings, the maximum anticipated wind velocity and the thermophysical properties of the substrate on which the sensor is mounted. The scientific basis for this embodiment is that the predetermined quantity of applied heat is not sufficient to affect a phase change,
Akerman Senterfitt & Eidson, P.A.
De Jesus Lydia M
Gutierrez Diego
University of Tennessee Research Corp.
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