BYPASS CHAMBERS FOR LEVEL GAUGES (METERS)
Measurement of fluid or bulk material levels in pressure containers, tanks or silos is an important part of regulatory systems of industrial applications. For this reason, there are used bypass chambers that are either welded or mounted to the container by dismountable flange connection and this way it makes a bypass.
Weld joint of the bypass chamber to the container has the advantage of its low weight and perfect tightness of the connection. Its disadvantage is that it is not possible to disassembly the joint without destruction. Flange connection of the bypass chamber and the container is advantageous due to eventual dismantling of the device, however, its greater weight and imperfect tightness of the connection is its disadvantage. In both cases thanks to the direct connection of the container and bypass chamber the level in bypass chamber corresponds to the level in the measured container.
Bypass chambers are not separate measuring devices. These are just cases into which the meters, co-called level meters, are inserted. They can be either contactless or contact and may function on various physical principles.
Basic type of contactless level meter is magnetic level indicator. In the bypass chamber there is a cylindrical float with incorporated permanent magnet placed directly at the level position. Magnetic field is spreading through the wall of the chamber to outer inductive strip visually indicating level in the chamber and in the container. These are reliable devices that may be used in aggressive surroundings, under high temperatures and pressures, even in places with risk of explosion.
Reflectometric level meters are typical examples of contact level meters. They function on the principle of microwave radar impulses emitted by transmitter’s body. The transmitter is placed on a bar or on a rope under the level of measured medium in the bypass chamber. The radar signal is transmitted towards the level at speed of approx. 300, 000 km/s and reflects itself back from the level. The reflected signal strength depends on the permittivity of the medium. The higher the permittivity, the higher the reflected signal strength. Time between transmission and reception of reflected signal, so called TDR, Time Domain Reflection, is evaluated. Reflectometric level meters are easily put into operation. They are resistant to level fluctuations or to changes of operating conditions.
The bypass chambers can be made of various materials. Most commonly used are refractory alloyed steel, stainless steel and in case of low operational parameters and non-aggressive surrounding of carbon steel.