17 - Level measurement

Abstract

This chapter discusses a wide variety of instruments that are available for measuring the level of liquids. Some of these can also be used to measure the levels of solids that are in the form of powders or small particles. Dipsticks offer a simple means of measuring level approximately. The ordinary dipstick is the cheapest device available. Capacitive devices are widely used for measuring the level of both liquids and solids in powdered or granular form. Ultrasonic level measurement is one of the non-contact techniques available. In alternative versions, the ultrasonic source is placed at the bottom of the vessel containing the liquid, and the time of flight between emission, reflection off the liquid surface, and detection back at the bottom of the vessel is measured. Level-measuring instruments using microwave radar are an alternative technique for non-contact measurement. Liquid level measurement is therefore obtained in terms of the output frequency of the oscillator when the tube is resonating. Consequently, elements in the liquid have a different temperature, and therefore a different resistance to those in air. One laser-based method is the reflective level sensor. An alternative, laser-based technique operates on the same general principles as the radar method described above, but uses laser-generated pulses of infrared light directed at the liquid surface. Most types of level gauge are now available in an intelligent form. 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.