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measure the level of fluids and solids. The most basic method is the manual process of viewing the medium through a sight glass. Other traditional techniques include electro- mechanical devices such as floats and displacer switches, capacitance point level switches, ultrasonic technology, differential pressure sensors, laser-based devices and load cells. In pulp mills, however, these


methods are increasingly being replaced by modern electronic technologies such as vibrating fork level switches, guided wave radar (GWR) transmitters, non-contacting radar transmitters and acoustic scanners. These advanced devices provide enhanced functionality and increased diagnostics capability, leading to improved measurement accuracy and repeatability, reduced maintenance requirements and lifecycle costs, and greater reliability.


Non-contacting radar Non-contacting radar technology provides a top-down, direct measurement of the distance to the surface, which is highly accurate and reliable and can be used with liquids, sludges, slurries and some solids. With built-in diagnostics, a lack of moving parts, and straightforward installation and commissioning, non-contact radar transmitters provide both ease-of-use and low maintenance requirements. To perform continuous level measurements, non-contacting radar transmitters use one of two main modulation techniques – either pulse or frequency modulated continuous wave (FMCW). In pulse systems, thousands of short radar pulses


May/June 2019


the surface, thereby enabling the level to be measured. FMCW radar transmitters


The Södra Cell mill in Sweden was facing two demanding level measurement challenges, which it solved by installing advanced non- contacting radar transmitters from Emerson


are emitted from an antenna positioned at the tank top directly towards the material below. These pulses are reflected back to the transmitter from the material’s surface. The transmitter measures the time delay between the transmitted and received echo signal, and an onboard microprocessor calculates the distance to the material’s surface, and consequently the level measurement. Transmitters using FMCW


technology transmit a radar signal with increasing frequency over time to create a signal sweep. The signal echo reflected from the surface is picked up by the antenna. Because the frequency of the


transmitted signal constantly varies, the echo frequency always differs slightly from the transmitted signal at any given moment. The difference between these frequencies is directly proportional to the distance from the transmitter to


have more than 30 times higher sensitivity than pulsed devices, which maximises their signal strength and enables them to deliver greater measurement accuracy and reliability. However, FMCW technology uses more electrical energy than pulse. Although level measurement


technology selection is very much application- dependent, non-contacting radar transmitters provide a broad range of important benefits that enable them to meet many of the toughest challenges presented during pulp production.


Turbulence and foaming Heavy turbulence or a large amount of foam can affect the signal strength of non- contacting radar transmitters. But recent advances include dedicated software algorithms that counteract the effects of turbulence and surface foaming. Alternatively, devices can operate in bypass chambers or stilling wells to negate these problems.


Corrosive media and condensation Devices must be designed to cope with aggressive media and condensation. The latest non-contacting radar devices overcome this with all-PTFE process seal antenna, engineered to maximise resistance to corrosive chemicals.


Coating, viscosity and dirty antennas Non-contacting radar transmitters are unaffected by coating and viscosity but a device’s antenna can become


Pulp Paper & Logistics


dirty and this can affect its performance. The latest radar transmitters can detect a dirty antenna using signal quality metric diagnostics, enabling preventative maintenance to be scheduled.


Density, temperature and pressure Liquid density can change as part of a reaction, along with


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