2 Sensors
2.4 Heights and Positions

Measuring Heights and Positions

This is a typical measuring automation task specific particularly by the type of concrete measurement, e.g. liquid surface, bulk materials. Height measurement is usually a primary indication of quantity calculation. It is possible to calculate quantity using the data recorded while measuring height, while it of course depends on the shape of the container, for example the reservoirs in which a specific surface level is measured. If the measurements are carried out in containers of an unalterable cross-section and height, the evaluation is very easy.

Ultrasound Height Measuring

Altimeters (mostly in the form of level gauges) based on the principle of ultrasound use two methods. In one case, the transit time of the ultrasonic wave from the transmitter via the reflection of the surface back to the receiver is measured and distance is calculated from the measured time and the known speed of ultrasound in a known environment. This method is used for continuous measuring of surface level position. The second method evaluates the attenuation of ultrasonic waves in dependence on the environment’s composition, which the ultrasound passes through. They are apt for continuous contactless measuring of surface level positions in open and closed tanks with liquid or bulk materials. They can be used with polluted slush and pasty matters and, in certain circumstances, to detect the level of foam. The benefits are the absence of moving parts, noncontact continuous measurement, the possibility installation on the outside of the tank without breaking its seal, the compact design of sensors, resolution up to 1 mm, and high accuracy of measurement. A disadvantage can be in the possibility signal affection by the presence of heavy fume and dust and the disruptive action of the turbulent surface level and the presence of foam.

Radar and Laser Measurement of Height and Distance

Radar level sensors work analogically with ultrasonic level gauges, however, when using electromagnetic waves, which spread through the medium at the speed of light. Microwave radiation is defined as the frequency of waves larger than about 2 GHz. Radar level gauges use two methods of measurement, the time (pulse) method and the frequency method (with chirp continuous signal).

Radar level gauges operate without moving mechanical parts, and show great accuracy (± 1 mm) and reliability even at very difficult operating conditions (high temperature, pressure, aggressive environment). The laser sensor for distance measuring transmits million short pulses of laser light per second and records the time it takes them to reach their goal and back to the sensor. Generally, the large range of the sensor allows it to measure the distance of small elements or objects even when the sensor is installed within a long distance from the dangerous area of the manufacturing process. The clear and well visible light emitted by the laser beam expedites the sensor’s activation.