On the use and maintenance of radio frequency admittance level gauges

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This post was last edited by zhenlizhijian on 2012-12-31 at 19:49. Recently, I was repairing an RF admittance level gauge. The process was as follows: The RF admittance level gauge used in a certain tank was of the rod-type, with a rod length of 1.7 meters. The setpoint inside the transmitter was 140 at zero level and 300 at full level. During my shift, the control room informed me that the inaccurate reading of the level gauge was causing overflow, as the liquid level appeared to be around 27% according to the gauge. I rushed to the site immediately. Given the inherent characteristics of RF admittance level gauges and my own experience, such transmitters are not prone to failure. After a quick inspection of the transmitter, I thought that the rod or some other factor might be the cause. However, due to the vacuum-pumping process in use, it was not possible to disassemble the gauge at that time. After completing the necessary process steps, the flange was removed after a few hours, and the rod was taken out; it was found that there was a large amount of crystallization at the flange opening of the mounting base, which essentially blocked it. The rod itself was not damaged – it had some scratches, but the PTFE layer remained intact. Initially, I thought that the crystallization might be the cause of the problem. After cleaning it, the PF value dropped, but when I contacted the control room to lower the liquid level, the PF value did not change. It was really frustrating. Trying to replace the module had no effect either. In the end, I had no choice but to replace the entire level gauge. After checking the inventory, I couldn’t find a gauge with exactly the same model and length, so I used a thinner rod with a length of 1.5 meters. After installing it, the measured liquid level was around 90, with a PF value of around 38. The zero point value used was an approximate value of 15; the exact zero point requires an empty tank, which was not possible under the current process conditions. After replacing the level gauge, there were changes in the liquid level as material was added or removed, showing a certain trend. However, due to the extremely cold weather, waiting and working on-site took around 6–8 hours. It’s also not possible to continuously measure and verify the liquid level based on its display value, as the vacuum-pumping process cannot be carried out. I would like to ask my colleagues: Can the PF value differ so much when the liquid level remains almost unchanged? The guide rods are different, so there are definitely differences, but will they be this big? “The level gauge that was replaced – on the second day of work, I used it on another tank and it worked fine; I’m really speechless.” Also, today I had off from work, and a colleague told me that this level gauge was replaced after the tank reached a level of 1.5 meters, with both the inlet and outlet closed. The level indicated by the gauge keeps rising slowly; we won’t know the exact situation until we go to work tomorrow. Please help analyze this together, guys