Final answer:
The alarm bell on a pressure demand apparatus usually operates at around 2000-3000 psi.
Step-by-step explanation:
A pressure demand apparatus's alarm bell usually sounds at a certain pressure level. It is challenging to give a precise response without knowing more detailed information about the apparatus in question, though. Firefighters use self-contained breathing apparatuses (SCBAs), which are pressure demand apparatuses with an alarm bell that sounds when the pressure falls below a predetermined threshold, signaling a low air supply. Although this threshold pressure varies depending on the apparatus, it is typically set between 2000 and 3000 psi (pounds per square inch).