Q: Please clarify how to install three magnetostrictive level gauges (reflex/transparent type) in a vessel to measure the same level. What should the nozzle projection be? Is it possible to install three magnetostrictive level gauge transmitters in a two-of-three configuration? Are three gauges applicable or not necessary? If it's not necessary to use three magnetostrictive, level-indicating transmitters (LITs), how should one install two level gauges (reflex/transparent type) in a vessel to measure the same level? Please provide any sample sketch or isometric drawing for a level gauge that I might use for reference and review.
A: In my experience, we now use a median selector in all cases, and do not use averaging when using three measurements. The only time we're using averaging for level measurement is on large-diameter tanks, when there would be measurement differences due to waves.
A: The options to increase level measurement reliability are these: You can use two sensors and average the readings, but in this case, if one fails or needs recalibration, you don't know which it is. If you use three LITs and a "median selector" strategy to control on the basis of the reading of the gauge in the middle (2 in Figure 1), you will get good control, but you will not know which of the remaining two needs recalibration. Therefore, on the most critical applications, such as nuclear reactor cooling water level measurement, I use the two-out-of-three voting system in the following way: If (A + B) < 2 in., I just add up the three readings and average them. If, on the other hand, (A + B) > 3 in., I compare the distances A and B in Figure 1, select the two that are closer to each other (1 and 2), and average them for control. At the same time, an alarm is actuated which requests immediate maintenance and recalibration for sensor 3.
As to the proper installation of magnetostrictive LIT (Figure 2, p. 86), it's the same as that for large-chamber reflex gauges. You can buy them as a unit, or insert their cable version (up to 20-m long and good for high temperature and 2,000 psig) into existing gauges.
Q: I am a student of instrumentation engineering. I have a diploma of associate engineer (DAE), and I'm starting more education. I want instrument books and other references that would be helpful for me.
Can you advise me on what instrumentation books to choose? I need books that relate to level, flow, temperature, pressure, pneumatics, hydraulics, electromechanical, viscosity, gravity, etc.
A: Understanding that, as a student, you're on a limited budget, I recommend joining the International Society of Automation (ISA) as a student member. (They have subsidies, I believe.) Also, I'd investigate signing on to the LinkedIn bulletin boards relating to our field, which are accessible through the ISA and others. Perhaps, at this point, some of the topics discussed are a little obscure or advanced, but getting exposed to the vernacular of the trade and, more importantly, to actual field applications helps with the learning experience.
A: The best start on instrumentation reading is Omega's instrumentation handbooks. They're available as printed books, and most are also available under "Technical Reference" on Omega's web pages. [Editor's note: They're also available as PDFs.]