Guided wave radar suits measurements in confined spaces

In cardboard manufacturing, a beast roll is used to deposit an even coating of diluted paper stock on a 'wire' (a fine mesh conveyor). The level in the vessel feeding the roll must be constantly monitored and controlled to obtain a consistent layer.

Originally the measurement was made with a 'bubbler' system that needed ongoing maintenance because of regular blockages in the tube. Furthermore, a hydrostatic transmitter installed as a replacement fluctuated due to the turbulent pressure effects in the liquid caused by the rotation of the roller. This measurement principle is also dependent on the density of the paper stock.

Because of the proximity of the 'wire' and the confined space between the vessel and the roll (approximately 5omm), a guided microwave radar measurement was used instead. The rod used as the 6mm diameter waveguide can also be bent to suit the vessel's shape.

Guided microwave radar technology is unaffected by build-up and, because it actually measures the liquid surface level, changes in density of the stock and the pressure effects caused by the movement of the breast roll through the liquid, have no effect.

Guided wave radar works by transmitting microwave radar pulses, which are guided along the surface of a rod or a cable suspended from the top to the bottom of a vessel. These doughnut-shaped pulses travel in a field of around 400mm diameter, unless the instrument is mounted inside a tube or standpipe.

When the pulses reach the material surface, they are reflected back up the guide to the sensor. The elapsed time period between transmission and reception of the signal (at the speed of light) is measured and calculated as a distance, which can be converted to a reading of the level or volume.

Benefits:

• Insensitive to build-up and the pressure effects caused by turbulence from the roller
• Unaffected by density changes
• The rod can be bent and cut to suit the vessel
• Minimal maintenance