A: One-mm accuracy in custody transfer cases is not quite warranted. Vapor pressure and temperature controls in Horton spheres and similar storage tanks for ammonia will require wave dampening controls and closed loop refrigeration systems.
Manufacturers claim that kind of accuracy using servo systems. The radar transmitters have come a long way in the past three years, including open-wave and guided-wave transmitters. The problem is that tank geometry, suction piping and spurious signal/noise ratios and interference (metal bottoms) affect accuracy at low liquid levels.
Other problems include premature instrument failures due to ambient ammonia presence, which can't be helped because of leaky PSVs nearby, and the temperatures inside the instrument housing, which leave many of these installations as installed monuments for engineers.
For proper operation the dielectric constant should exceed 2, which is usually no problem, as anhydrous ammonia usually contains a few ppm of water. For non-contact, sealed-diaphragm type guided wave sensors, a stand pipe type stilling well is recommended, and it's also advisable to mount the transmitter electronics remotely to avoid lighting strikes. The accuracy is around 3 mm over 30 meters. Major manufacturers like SAAB, Ohmart-Vega, Enraf and others will provide more application help, though they may not give details on the shortcomings of their product line for competitive commercial reasons in ammonia applications.
Q: I have a degree in instrumentation technology and also have acquired some software skills such as Java programming. I'm looking at additional courses and came across the Certified Automation Engineer (CAE) program. Would such a course be helpful in finding a job later?
Satya Dev Koneru
AISA's Certified Automation Professional (CAP) is an excellent program. The person passes an exam and becomes certified. He or she then must continue practice of that profession and take a number of courses to remain certified. ISA offers a course to help pass the CAP exam.
The certification, while admirable, carries no legal status. The CAP program is gradually becoming recognized as a status achievement by our profession, but far from universal. Some of the best experts instrumentation and controls are not CAPs, nor do they feel a need for certification.
Richard H. Caro, CAP