Abstract: An apparatus and method of the gas-bubbling type for determining the level of a body of liquid sewage standing in a tank which is vented to a partial vacuum; that is a tank whose internal gas pressure above the liquid is less than ambient gas pressure outside the tank. Measurements are obtained by drawing a small flow of ambient gas thru a gas jet into a bubbler tube set with its opening in the lower drain line of the tank. This allows the gas to bubble up to the surface through the liquid sewage. Gas pressure in the bubbler tube is communicated to one side of a differential pressure transducer. The other side of the transducer is connected to the partial vacuum above the liquid sewage in the tank. The differential pressure transducer, sensing the difference between these two pressures transmits an electronic, linear output, measurement signal. The accuracy of this measurement system is not affected by varying ambient or internal gas pressures as long as gas bubbles are being emitted from the bubbler tube.