Abstract: The liquid testing apparatus comprises a housing incorporating a device for measuring the refractive index and a device for measuring the sonic velocity of the liquid under test. The housing comprises a chamber into which a prism protrudes and approximately collimated light is transmitted through the prism. By measuring the amount of totally internally refracted light, the refractive index of the liquid in the chamber is determined. A further chamber is provided in the housing and a sonic pulse transducer is disposed at one end of the further chamber which also receives the reflected pulse from the other end of the further chamber. A temperature sensor is also provided. The apparatus provides one or more signals to display the quality of the liquid and/or to provide a control of the liquid flow or of a device using the liquid. The apparatus may form part of a container drain plug and incorporate a drain valve.