Last month we determined that 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. Often the excess capillary tubing for the closer nozzle or tap is coiled for convenience.
In one such flowmeter application, the capillary tubes were of equal length — following the “rule of thumb” to use capillary tubes of equal length. However, the excess capillary tubing to the closer tap (approximately 4 meters) was coiled inside the base of the transmitter enclosure. This would not be a problem if the temperature of the capillary tubing inside the base and exposed to the closer tap were the same as the temperature of the exposed capillary tubing to the farther tap.
In this installation, the instrument enclosure was cooled, so the temperature of the capillary tubing was different. In particular, the temperature of the capillary tubing to the closer tap was colder than that to the farther tap. Compounding the problem, the exposed capillary tubing to the farther tap was exposed to sunlight during part of the day. These conditions affect the measurement in the same direction, so the measurement error would be greater than either individual error.
One of the capillary tubes was coiled while the other was not. Although it might seem that this would affect the measurement, the measurement should not be affected because the pressure increases/decreases in the coiled impulse tube effectively cancel so the static pressure at the transmitter is dependent upon the elevations of the tap and the transmitter.
In many applications the transmitter enclosure is heated and can create similar problems. This often occurs in colder climates where the capillary tubing to the closer tap will be warmer than the capillary tubing to the farther tap that is exposed to the cold climate. These conditions affect the measurement in the same direction, so the measurement error would be greater than either individual error — albeit in the opposite direction as compared to an installation where the instrument enclosure is cooled.
Remember that following a “rule of thumb” may be a good thing, but doing so without a complete understanding can create problems.
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.
