I have been researching guided wave radar level instrumentation and I've run into an issue where vendors limit the solid rod and coaxial probes to 4 to 6 meters while wire rope probes can be used in lengths up to 75 meters. Based on my understanding of the technology involved (high-frequency material surface conduction) I would think that a coaxial probe would conduct the signal as well as or better than a wire rope. I have asked vendor representatives and the answers I received have been unsatisfying at best or technically inaccurate at worst. Can anyone give me a good, technical reason for this limitation?
Hello Paul Butchart ,
The limited length of the rod and coaxial GWM (Guided Wave Measurement) probes versus that of the rope type probe is a mechanical limitation not an electrical limitation. The rod and coaxial GWM probes are only supported at one end of the probe (the proximal end - electrical interface end), whereas the rope probe is mechanically supported at both ends (proximal and distal ends). Take a look at the figures on p. 7 of the document at the following URL: http://www.manoraz.com/_Uploads/dbsAttachedFiles/n2gb_ngm.pdf . In the case of the rope probe, there is a collet at the distal (far) end of the rope that allows it to be attached to the bottom of the tank. This two-point mechanical support prevents any lateral dispalcement of the probe due to the contents of the tank. Any significant lateral displacement can affect the accuracy of the GWM. This is especially true in the case of the non-coaxial probes due to their wider range of influence, but it is also a factor in the coaxial probe due to bending and buckling.
Regards,
Thomas Cuff
Paul Butchart Microwave technology underpins guided wave radar. Microwaves are only impacted by materials that reflect energy, therefore temperature changes, dust, pressure, and viscosity have no effect on accuracy. A low-energy microwave pulse is sent down a probe by the gadget.
Fill the level transmitter chamber all the way to the top with water. Check the transmitter's LCD for the level measurement (or in the HART communicator). Set this condition to 100% using the HART communicator. Using a multimeter, measure the transmitter's mA output.
Thomas Cuff
For my application, essentially a high temperature boiler, I have been looking specifically at a coaxial probe. One of the benefits of this type of probe, unless my understanding is wrong, is that the outer "sleeve" is electrically grounded. This allows the use of a fixture at the bottom of the probe to prevent movement. It also allows the use of mounting fixtures at points along the length to prevent flexing.
My original supposition was that the limitation was imposed by the length of a shipping truck. Obviously, a wire rope can be coiled into a relatively small package but a solid rod or coaxial would not. However, segmented probes are available but they still seem to have the same limitation on length. Hence, my original question.
I'm beginning to think that I need to go into business manufacturing GWR probes for applications that don't fit into the "normal" use cases.
Thank you for your thoughts,
Paul Butchart
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