| |
| 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.
www.spitzerandboyes.com
|
|
