Float Type Level Indicator

Is the vacuum strong enough to boil the liquid?

What is the liquid, the temperature and pressure?

A vacuum has no effect on the laws which govern bouyancy.
So I'd say, any level indicator that will work in the tank when it's not pumped down.
Del

Further - is the float itself rated for full vacuum?

A level control that works very well in refrigeration surge drums is the Sporlan Level Master LMC series expansion valves. I don't know if the vacuum will have some effect on the probe, since it uses a heater to evaporate a fluid in the sensing bulb. The heat conductance changes when the liquid level covers the bulb as opposed to when it does not. There is a change in volume with the change in state from liquid to vapor, and this applies pressure on a diaphragm. In turn, this opens or closed the expansion needle-seat valve. For a price, I suppose, the fluid in the sensing bulb and capillary tube could be selected to suit your process. It would have to be a compound suitable for the temperature range your liquid level will be.