Approvals like the German Federal Water Act (WHG) obtained from TUV, or the Flemish Regulation on Environment (VLAREM), certify Overfill Proof Protection, defined as the tested, reliable operation when the transmitter is used as overfill alarm. However, it is assumed in the analysis that the installation is designed in such a way that the vessel or side mounted cage will not physically overfill.
In other words, transmitters can obtain Overfill proof approval, without necessarily having the capability to measure level to the top of the probe. The only requirements are that the transmitters be installed properly and used within their defined measuring range.
In reality, there are practical applications where a Guided Wave Radar (GWR) probe can be completely flooded with level all the way up to the process connection (face of the flange). Due to the physics of the technology, when this occurs, there can be adverse interaction between the desired level reflection and residual reflections at the top of the probe.
This affected area at the top of a GWR probe is dependent not only on the probe itself, but also on the application and installation. Typical GWR probes have a transition zone (or possibly even a dead zone) at the top of the probe where interacting signals can either affect the linearity of the measurement or, more dramatically, result in a complete loss of signal.
The Risks of “Inferred” Level Measurement
While some manufacturers of GWR transmitters may use special algorithms to “infer” level measurement when this undesirable signal interaction occurs and the actual level signal is lost, more advanced GWR transmitters offer a unique solution by using a concept called Overfill Safe Operation. An overfill safe probe is defined by the fact that it has predictable and uniform characteristic impedance all the way down the entire length of the waveguide (probe).
With a probe physically designed to be overfill safe, signal loss will not occur when level reaches the top of the probe. Inferring level measurement instead of actually measuring true product level always comes with some assumptions, which can create an unacceptable risk in your industrial process control environments. That’s why a new concept called Overfill Safe Operation, which uses overfill safe probes to provide true product level measurement across the entire length of the waveguide, is gaining in popularity.
Caged Coaxial Probes
Today, caged probes that combine the performance/sensitivity advantages of a coaxial probe and the viscosity immunity of a single rod probe are available. These solutions consist of a caged GWR single rod probe that uses an existing or new cage, bridle, or stillwell as the second conductor to re-create the same signal propagation of a coaxial GWR probe.
Caged GWR probes are designed for 2” (DN50), 3” (DN80) or 4” (DN100) diameter chambers and use a specially designed impedance matching section that results in the same overall characteristic impedance of a coaxial style GWR probe. Caged GWR probes offer the same sensitivity and performance as coaxial GWR probes, but the single conductor design allows it to be used in applications with viscosities up to 10,000 cP.
These caged coaxial probes allow the GWR to measure accurate levels up to the process flange without any non-measureable zone at the top of the GWR probe, and their signals look very much like those of a coaxial probe.
Overfill safe operation provides a safer solution for today’s industrial process control environments. Instead of using algorithms to infer level readings in uncertain areas of a GWR probe, overfill safe GWR probes ensure that actual, true product level can always be measured.