Patent Number: 
Section: claims

1. A liquid level sensing apparatus which measures a liquid level in a liquid holding vessel based on temperature signals from a plurality of probes placed at fixed intervals in a vertical direction of the liquid holding vessel, wherein each of the probes includes a temperature sensor and a heater enclosed in the probe and the heater is placed in a neighborhood of a detecting point of the temperature sensor, the liquid level sensing apparatus comprising:a probe selection unit configured to select a probe whose heater is to be activated from among the plurality of probes;an input unit configured to receive an output of the temperature sensor of the probe selected by the probe selection unit, the output being received as a temperature signal directly in a form of an analog quantity;a signal processing unit configured to output a processing signal of the temperature signal in synchronization with activation of the heater;a calculation unit configured to arithmetically process the temperature signal and the processing signal and output a result;a gas/liquid discrimination unit configured to discriminate whether the detecting point exists in a gas phase or a liquid phase based on the output result of the arithmetic processing; anda display unit configured to indicate a discrimination result produced by the gas/liquid discrimination unit,wherein the calculation unit finds one of a difference and a quotient between the temperature signal and the processing signal and outputs a result, andthe signal processing unit is one of a hold circuit and a first order delay circuit, wherein the hold circuit holds the processing signal at a level of the temperature signal at a start time of the heat supply and the first order delay circuit outputs a first order delay response to the temperature signal. 2. The liquid level sensing apparatus according to claim 1, further comprising a liquid level determination unit configured to determine the liquid level based on the discrimination result produced by the gas/liquid discrimination unit. 3. A liquid level sensing method for measuring a liquid level in a liquid holding vessel based on temperature signals from a plurality of probes placed at fixed intervals in a vertical direction of the liquid holding vessel, wherein each of the probes includes a temperature sensor and a heater enclosed in the probe and the heater is placed in a neighborhood of a detecting point of the temperature sensor, the liquid level sensing method comprising:selecting a probe whose heater is to be activated from among the plurality of probes;receiving an output of the temperature sensor of the selected probe as a temperature signal directly in a form of an analog quantity;outputting a processing signal of the temperature signal in synchronization with activation of the heater;arithmetically processing the temperature signal and the processing signal and outputting a result;discriminating whether the detecting point exists in a gas phase or a liquid phase based on the output result of the arithmetic processing; anddisplaying a discrimination result based on at least one of the selected probes,wherein the arithmetically processing finds one of a difference and a quotient between the temperature signal and the processing signal and outputs a result, andthe outputting a processing signal utilizes one of a hold circuit and a first order delay circuit, wherein the hold circuit holds the processing signal at a level of the temperature signal at a start time of the heat supply and the first order delay circuit outputs a first order delay response to the temperature signal.