Ultrawave Qi Series Operator's Instruction Manual page 20

Screen 6: Fluid level sensor
The Qi-Series ultrasonic bath is fitted with a fluid level sensor to ensure it is
not under-filled prior to or during the cycle. This ensures that the optimum
cleaning process can occur and potential damage to the tank is minimised.
The level sensor is a resistive device, so will not detect fluids with very low
conductivity such as pure deionised (DI) water.
The level sensor can be disabled for specialist cleaning applications where
deionised (DI) water is used.
Use the ON and OFF keys to set the fluid level function. When the desired
option is selected, use the MENU key to move to the next screen. The fluid
level setting will return to ON each time the unit is switched on.
When the off button is pressed and the  key is selected to continue, a
message will appear to check that the operator has filled the tank up to the
level indicated.
When the level sensor is disabled the liquid set temperature is
automatically reduced to 5°C.
If 'NO' is selected an instruction will appear requesting the tank to be filled
before use.
Qi Operating Manual 20 C6027205