There are also a great number of both simple and sophisticated level measurement devices...
Ultrasonic level measurement depends on a transducer sending an ultrasonic pulse to the liquid surface which is reflected back to the transducer. Electronics convert the ultrasonic lag time into a distance corresponding to depth... Although ultrasonic level indicators require little maintenance and are unaffected by the nature of the liquid (acidity/basicity, dielectric constant, or specific gravity), they cannot be used in circumstances of excessive foaming or turbulence or in high-temperature situations where stratified vapor layers are present.
On the other hand, radar detectors can be used in vessels exhibiting a wide variety of conditions: Radar energy passes through air or vapor space with imperceptible changes. The greatest limitation to the use of radar detectors is price, because of the complexity of the microwave and timing circuitry. In an advanced measurement unit, the radar level gauge sends out a continually swept microwave signal with varying frequency. The transmitted signal is compared with the signal sent back in a microwave mixer device, the difference in frequency being directly proportional to the distance. It will be obvious that high-quality signal processing is necessary since the liquid surface in a working reactor is never calm and unwanted echoes within the reactor need to be separated out.I've never had the opportunity to use either ultrasonic or radar measurement of levels in the reactor; the reactors that I have worked with have had pre-measured landmarks that are quite accurate in diagnosing the volume of solvent in the reactor. That said, I can imagine instances (crystallization, perhaps) in which knowing the exact amount of solvent is necessary. (Also, if there's a good bit of solvent vapor in the reactor, it can be difficult to see!)
(Incidentally, a quick Google search indicates the use of radar to track volumes of cooling water in the nuclear power industry. Yeah, that makes sense.)