Provided are a liquid testing apparatus, a holding tank, a device, and a testing method. A first acoustic wave testing component (21) of the testing apparatus is used, at a preset distance, to transmit an acoustic wave and receive a test wave corresponding to the acoustic wave in a liquid (11); a temperature measurement component (22) is used to measure the temperature of the liquid (11); a power supply component is electrically connected to the first acoustic wave testing component (21), the temperature measurement component (22) and a controller; and the controller is electrically connected to the first acoustic wave testing component (21) and the temperature measurement component (22), so as to calculate an acoustic wave propagation velocity in the liquid (11) according to the preset distance, the time at which the first acoustic wave testing component (21) transmits a first acoustic wave, and the testing time at which the first acoustic wave testing component receives a test wave corresponding to the first acoustic wave, and to then determine a liquid type or a liquid concentration according to the acoustic wave propagation velocity in the liquid (11) and the temperature of the liquid (11) measured by the temperature measurement component (22). The liquid testing apparatus automatically tests a liquid in real time, and determines a liquid type or a liquid concentration in real time, such that the labor costs used are reduced.
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