In my last two installments of “Applications Corner,” we determined the capillary tubing for differential-pressure measurements should generally be the same length — even though the added physical length may seem wasteful, expensive and cumbersome. In particular, we discussed a flow measurement system where the capillary tubes were designed and installed such that their temperatures were often different. Level measurement using a transmitter with diaphragm seals and capillary tubing can exhibit different problems.
In particular, differential-pressure level transmitters with diaphragm seals are often calibrated in the instrument shop — especially prior to initial installation. To simulate the actual installation in the instrument shop, the diaphragms should be mounted at different elevations that correspond to the actual installation to calibrate the transmitter. This procedure becomes impractical in many applications due to tank heights that limit this simulation to but a few meters.
In place of this simulation, differential-pressure level transmitters are often calibrated with their diaphragm seals mounted at the same elevation. The calibration values will be different than those in the actual vessel due to the different elevations of the diaphragm seals in the shop as compared with the field. Sometimes transmitters are calibrated with their diaphragm seals just “laying around”.
Aside from these potential shop calibration issues, installing the diaphragm seals on the actual vessel may result in minor calibration shifts due to the physical nature of positioning the diaphragm seal and tightening its bolts. As a minimum, a zero calibration should be performed after the transmitter and its diaphragm seals are permanently installed.
Performing a shop calibration has its virtues because it can find transmitter and diaphragm seal problems before installation in a field environment. However, appropriate valves and calibration ports should be installed to allow field calibrations that will improve measurement accuracy. One such device is a “filler flange” that is a thin wafer spool piece that contains drain and bleed ports that can be used for calibration. Better yet, it would be more convenient to purchase the diaphragm seals with these features already included.
David W. Spitzer is a regular contributor to Flow Control. He has more than 30 years of experience in specifying, building, installing, startup and troubleshooting process control instrumentation. He has developed and taught seminars for over 20 years and is a member of ISA and belongs to the ASME MFC and ISO TC30 committees. Mr. Spitzer has written a number of books concerning the application and use of fluid handling technology, including the popular "Consumer Guide" series, which compares flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing and distribution consulting for manufacturing and automation companies. He can be reached at 845 623-1830.
