Ultrawave Qi Series Operator's Instruction Manual page 21

WARNING
Setting the liquid level sensor to OFF means that the Qi-Series ultrasonic
bath will operate as if the bath were full of liquid.
This means that without water, the heaters will continue to operate and the
surface inside the bath will become extremely hot. It is therefore the
operator's responsibility to ensure that the tank is filled with sufficient water
when the liquid level sensor is disabled. If the Qi-Series ultrasonic bath
operates with insufficient liquid, the heaters will be disabled as a safety
feature and the following screens will be displayed:
To continue using the Qi-Series ultrasonic bath, refill with liquid up to the
level sensor, switch the unit off and on again.
If the bath is run dry this will invalidate the warranty on your Qi-Series
ultrasonic bath.
Qi Operating Manual 21 C6027205