Two separate classes of level sensors can be distinguished according to whether they
make contact or not with the material whose level is being measured. Contact devices
are less reliable for a number of reasons, and therefore non-contact devices such as
radar, laser, radiation or ultrasonic devices are preferred when there is a particular
need for high reliability. According to the application, sensors that are relatively unaffected
by changes in the temperature, composition, moisture content or density of the
measured material may be preferred. In these respects, radar (microwave) and radiation
sensors have the best immunity to such changes. Further guidance can be found
in Liptak, (1995).