| 摘要 |
A sensor 10 including a temperature dependent resistor 12 and a heating resistor 16 in a casing 20 are placed into an environment to detect a change in the fluid condition of the environment, for example, the flushing of a toilet or urinal, the spillage of a liquid, or the lowering of a liquid level. The temperature dependent resistor is heated to a temperature which lies outside the range of temperatures of the fluid, and in the passive phase, the fluid in the vicinity of the sensor is brought to a constant passive temperature by heat transfer between the sensor element and the surrounding fluid so that the sensor resistance, to which a voltage is applied, provides an approximately constant passive output voltage. In an active phase, the fluid condition of the environment changes which changes the heat transfer between the sensor and the fluid so that sensor produces an active output voltage. If the difference between the active and passive output voltages exceeds a threshold, an indicator signal is generated.
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