PURPOSE: To enable exact measurement of liquid level and density by automatically controlling the stiring air sent to the tank. based on the signal of the tank temperature and liquid level and correcting the effect of bubble on the density.

CONSTITUTION: Air is blown in the liquid in a tank 1 from a header 2 to stir and generate bubbles. Through bubbler pipes 4A to measure the liquid level and bubbler pipes 4B to measure the density, a constant air flow is given. And the back pressure is measured as differential pressure with a differential pressure transmitter for liquid level 5 and a differential pressure transmitter for density 6. The density signal is stabilized in a delay circuit 8, corrected in a density correction circuit 11 as a true density value and output on a density indicator 12. The liquid level signal is corrected with density in a liquid level correction circuit 9 as a true liquid level value and output on a liquid level indicator 10. The air flow control is done by outputting a control signal to an air flow control valve 14 after calculating the temperature signal detected with a temperature detector 7 and the true signal of the liquid level in an air flow control circuit 13.