Ultrasonic liquid level monitor

An apparatus for measuring the level of a liquid having a surface in contact with a gas in a container, which comprises in combination:

• (a) means in physical contact with the outside of a wall of the container located below the surface of the liquid for generating at least two acoustic resonance responses in the liquid substantially perpendicular to the surface, and for determining the acoustic frequencies of at least two resonant responses;
• and
• (b) means for electrically exciting said means for generating at least two acoustic resonance responses over a chosen range of acoustical frequencies and having a chosen waveform, whereby the level of the liquid is determined from the frequency difference between the frequencies of the resonant responses.

A method for measuring the level of a liquid having a surface in contact with a gas in a container, which comprises the steps of:

• a. generating at least two acoustic resonances in the liquid substantially perpendicular to the surface of the liquid;
• and
• b. determining the frequency of at least two acoustic resonances, whereby the level of the liquid is determined from the frequency difference between the frequencies of the acoustic resonances.

An apparatus for measuring the level of a liquid having a surface in contact with a gas in a container, which comprises in combination:

• (a) means in physical contact with the outside of a wall of the container located below the surface of the liquid for generating acoustic resonance responses in the liquid substantially parallel to the surface, and for detecting resonant responses;
• and
• (b) means for electrically exciting said means for generating acoustic resonance responses, whereby if no resonance responses are detected, the level of the liquid is below said means for detecting resonant responses.

A method for measuring the level of a liquid having a surface in contact with a gas in a container, which comprises the steps of:

• (a) generating at least two acoustic resonances in the liquid substantially parallel to the surface of the liquid;
• and
• (b) detecting the presence of acoustic resonances from the liquid whereby if no acoustic resonances are detected, the level of the liquid is below a location where said step of generating at least two acoustic resonances in the liquid can be performed.