Changing Guided Wave Radar Probe Length

How often do you Google for a solution to a problem you face and land in an online community? It happens to me all the time. This was a lot of the reason for the creation of our Emerson Exchange 365 community several years ago—to provide a place where peer-to-peer problem solving could occur around all the Emerson Process Management technologies and solutions.

I mention this because I wanted to share a question and answer in the Measure & Analyze section’s Level group. The question involved guided wave radar (GWR) level measurement technology [hyperlink added]:

Hello Everybody!

I have a Rosemount 5300 Series Coaxial probe guided wave radar (HTHP type). I wanted to install it in place of a displacer in a displacer chamber. The original length of the probe assembly (Flange bottom to probe tip) was 762 mm. I reduced it to 693 mm to match my chamber’s height. The procedure for cutting the coaxial type probe is provided in Rosemount reference manual.

Before cutting away the extra length the transmitter was indicating the actual physical length of probe dipped in the liquid level, quite precisely. After cutting the probe to desired length I configured the new “probe length” and “Tank height” parameters in the transmitter. However, there is a constant error of about 10 to 15 mm in the indicated level.

Also there is a dead zone created of about 20 mm at the tip of the probe. In the dead zone the level is shown 0 mm regardless of the actual level.

Anybody having experience with this type of GWR please guide me about what other configurations I have to change to remove these errors?

Emerson’s Ingemar Serneby responded:

Emerson's Ingemar-Serneby

Ingemar Serneby
Senior Application Specialist

First of all, the HTHP seal contain dual ceramic seals and even though the ceramic is embedded in shock absorbents, it is essential that precautions are taken in order to protect the ceramics from damages while cutting the probe. Given that the seal still is intact, I recommend you to check the following: Check in the database that the probe type is correct. Note that the offset differ between the probe types and if you for instance erratically select “coaxial probe with a standard seal” while you have a “coaxial probe with HTHP-seal”, the measurement will be a couple of centimeters off.

If you however already have checked that the correct probe and seal are selected, it would be good to know whether the deviation varies with the distance or is consistent across the measuring range. If the latter, you can enter the offset value as “Distance Offset (G)” in the database. If this still doesn’t help, I recommend you to take plots and database backup from the instrument and send it to your local Emerson service contact in order to investigate whether the seal is intact, the database is correctly setup, the probe is in good order or not, etc. Please do not hesitate to contact us if you need further instructions – we are here to help you!

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