Liquid level sensing apparatus and method

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; and

a 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, and

the 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; and

displaying 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, and

the 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.