|David W. Spitzer, P.E.|
Signal transmission technology can be an important aspect of flow measurement system design, and improper technology selection can potentially introduce significant error and process control problems.
Analog signal transmission utilizes a continuous signal, such as four to 20 milliamps or zero to 1,000 Hertz, to represent zero to 100 percent of full scale — although some transmitters will produce a signal slightly in excess of full scale. The maximum signal is limited to the transmitter full scale, so the transmitter full scale is typically selected to ensure the maximum measurement — perhaps under extreme conditions — does not exceed the full scale represented by 20 milliamps or 1,000 Hertz. As a result, the transmitter tends to operate relatively low in its operating range where its accuracy is typically degraded. In addition, totalizing continuous signals, such as four to 20 milliamps, usually requires a signal conversion that further degrades accuracy.
Process measurements in excess of the full scale will measure as if the process measurement were at full scale, so flowrates above full scale flow will totalize less than the actual flow. Further, process control system would operate as if the flow were at full scale even though the actual flow could be two or three times higher. This inaccurate information could cause an unsafe or abnormal process condition. It should be noted that some transmitters are capable of producing signals above full scale (typically approximately 10 percent) to mitigate some of these issues.
On the other hand, digital signal transmission utilizes a digital number to represent the measurement. In doing so, the transmitter can often be calibrated at a lower full scale, which is more representative of its typical operation. Calibrating at a lower full scale can result in a measurement that is more accurate. Occasional measurements above full scale are not limited to a maximum signal, such as 20 milliamps or 1,000 Hertz, so the process measurement will be transmitted with reasonable accuracy. In addition, the process control system (and operator) will have better information with which to make decisions.
Note that while the use of digital communications can reduce the calibration range of the transmitter, a different transmitter with a lower upper range limit should not necessarily be selected. This is because the process measurements are limited to the upper range limit of the transmitter, so the actual measurements should not exceed this limit.
David W. Spitzer, P.E., 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. He can be reached at 845 623-1830.