Measuring and testing – Gas analysis – Moisture content or vapor pressure
Reexamination Certificate
1999-09-20
2001-08-21
Williams, Hezron (Department: 2856)
Measuring and testing
Gas analysis
Moisture content or vapor pressure
C073S335020, C073S025040
Reexamination Certificate
active
06276202
ABSTRACT:
The invention relates to a device for detecting snow and ice including a humidity sensor, with which a heating current circuit is associated, and an evaluation circuit controlled in dependence on a humidity sensor—measured variable which produces a switching and/or control signal at predetermined humidity values. The invention further relates to a method of operating the snow and ice detecting device.
DE 4032734 C1 discloses a snow and ice detecting device for controlling a heating system. This device prevents the formation of snow and ice on limited roadway regions, road ramps or roofs. Humidity is determined by means of at least two exposed electrodes by measuring a electrical resistance between the electrode arrangement and humidity is indicated when the resistance falls below a predetermined value. Since water is non-conductive in the solid aggregate state (in the form of ice or snow), a heating element had to be provided in or on the humidity electrodes in this known humidity sensor arrangement in order that an adjacent layer of ice and snow is melted and converted into electrically conductive water. In addition to the humidity sensor, a temperature sensor, which detects the ambient or surface temperature, is necessary. If the humidity sensor produces a humidity signal after the temperature value has fallen below a “critical” value (near to 0° C.), the switching or control signal is produced and the associated heating system switched on since the formation of snow or ice can be inferred. This known device is characterised by a high precision and has proved satisfactory in practice. It was, however, previously not readily possible to protect the metallic humidity electrodes, which are constantly exposed to environmental influences, from corrosion, contamination and/or short circuits from the exterior. The operational reliability of the known ice and snow detector therefore depends on periodic maintenance and cleaning work. Such work can only be performed, under certain circumstances, by experienced personnel and is expensive, particularly if it is relatively difficult to gain access to the humidity sensor, for instance in a roof gutter. It is also not easy to provide a sealed encapsulation of the electrical components of the known humidity sensor since the metallic electrodes must be exposed.
It is the object of the invention to provide a snow and ice detecting device in which reliable functioning is ensured with significantly reduced maintenance expense and low manufacturing expense.
Starting from a snow and ice detecting device of the type referred to above, this object is solved in accordance with the invention if an ambient temperature sensor is arranged in a control circuit which activates the humidity sensor when a temperature within a predetermined temperature range is detected; if the humidity sensor has a PTC heating element whose operating state with a resultant electrical current consumption is used as an indicator of the humidity; and if the two sensors are mutually spaced within an elongate sensor cartridge such that the temperature sensor is thermally decoupled from the PTC heating element.
The invention starts from the usage of a humidity sensor with maintenance-intensive, exposed humidity electrodes. In order to measure humidity, the invention uses the temperature-dependent current consumption of a PTC (positive temperature coefficient) resistance or heating element. This PTC heating element may be manufactured very economically in the form of a pill which is inserted into a good thermally conductive capsule, for instance a sleeve of metal or glass. The current consumption of a PTC heating element or resistance is dependent not only on its environmental temperature but also on its energy dissipation to the environment. It is known that the heat transfer between solid bodies and gaseous media is significantly worse than that between solid bodies and liquid media. The heat abstraction from the PTC heating element by air (when the sensor is dry) is significantly worse than by a damp or even liquid ambient atmosphere. The diagram of Figure A was plotted in an ambient temperature window between +5° C. and −20° C. with the humidity sensor including a PTC heating element used in accordance with the invention. At an operating voltage of 12V, the following currents were recorded.
Sensor dry, wind calm
100 . . . 120 mA
Sensor dry, high wind
140 . . . 170 mA
/
Sensor damp
190 . . . 200 mA
Sensor very wet
200 . . . 250 mA.
These relatively unmistakable differences in the current consumption of the PTC sensor influenced by the ambient conditions may be detected reliably and without difficulty. The evaluation can then distinguish unambiguously between a dry sensor and a moist sensor. The system operates practically without maintenance as a result of the complete encapsulation of the new PTC sensor and the low current consumption values.
A considerable advantage of the invention also resides in the particular construction of the overall sensor combination in an elongate sensor cartridge. The sensor cartridge has small dimensions overall and may be installed without difficulty at a suitable position in the zone in which the formation of snow and ice is to be monitored. The humidity sensor is always disposed in the so-called distal region of the elongate sensor cartridge whilst the ambient temperature sensor is spaced from the humidity sensor and is thermally decoupled. The heating action of the PTC heating element does not influence the ambient temperature detection of the temperature sensor; the two sensors may nevertheless be combined into a compact unit in the sensor cartridge.
Precisely because of the compact construction of the sensor cartridge and its maintenance-free operation, expanded fields of application are produced for the new ice detecting device. These include roof gutters which are accessible with difficulty, roofs, antennae arrangements, which are to be maintained free of ice and snow, particularly parabolic mirrors, external doors and roadway coverings, road ramps, approaches to houses and the like.
In a preferred embodiment of the invention the sensor cartridge is constructed in the form of a plastic tube, arranged on one end of which there is a cable entry and in whose opposite end region the PTC heating element is arranged in a thermally conductive metal or glass sleeve. At its distal end the plastic tube has an opening at which the good thermally conductive sleeve or shell of the PTC heating element is exposed to the environment and offers the possibility of dissipating heat. The ambient temperature sensor is arranged close to the cable entry, that is to say remote from the PTC heating element, in the plastic tube.
A heating device for thawing purposes is generally controlled with the switching and/or control signal from the snow and ice detecting device in accordance with the invention. It can be installed, for instance, in a roof and ensure that the roof is reliably kept free of relatively large amounts of snow. The advantage of such a roof heater, in addition to avoiding dangerous falls of ice and snow from the roof, is the possibility of constantly deloading the roof construction and thus of a lighter and cheaper roof construction.
The control circuit, which acts in dependence on the ambient temperature, closes the measuring current circuit of the humidity sensor in a preferred embodiment of the invention. The humidity sensor is thus only powered when the ambient temperature is in a critical range close to 0° C., that is to say when there is some danger of snow and ice forming. After switching on the current to the PTC heating element, the latter initially has a very high current consumption substantially independent of humidity. In an advantageous embodiment of the invention, during this first phase a lead time suppresser is operative which is activated by the control circuit and blocks the production of the switching and/or control signal for a predetermined period of time until the PTC heating element has reached a steady operating
Blakely & Sokoloff, Taylor & Zafman
Tekmar GmbH
Wiggins David J.
Williams Hezron
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