Page 238 - Instrumentation Reference Book 3E

222 Microprocessor- based and intelligent transmitters

p2

Figure 13.26 Measurement of liquid level in a closed
vessel, using two pressure transmitters.
the data regarding the transmitter model and serial
number, etc. However, it is important to remember
that changes in the temperature of the stored liquid
may lead to changes in density which, in turn, may
result in an unacceptable loss of accuracy. A third
transmitter located at a fixed distance above the
bottom transmitter can be used to obtain the den-
sity and make the necessary correction.

13.9 Other microprocessor-
based and intelligent
measurement systems

An interesting application of microprocessor-
based and intelligent instrumentation occurs in
the metering of natural gas. A typical example is
the Spectra-Tek Flow Computer (see References),
shown in Figure 13.27.
The preferred method for fiscal metering of gas
flow is by the measurement of the differential
pressure across an orifice plate. The traditional Figure 13.27 Spectra-Tek flow computer. Courtesy of
Spectra-Tek Ltd.
approach to achieve high accuracy over a wide
flow range has been to install several differential
pressure transmitters, with overlapping ranges, The Spectra-Tek Sentinel Flow Computer pro-
across a single orifice plate, as shown in Figure vides the facilities to communicate digitally with
13.28. With the improvements which have been the transmitter. It can either use the transmitter’s
made in the basic sensors and the introduction of analog output, switching the range of the trans-
the smart transmitters, it has become possible to mitter as the flow rate varies, or it can use the
use a single transmitter, the output from which is digital value received from the transmitter, in
a conventional 4-20 mA analog signal with the which case no range changing is necessary. The
digital measurement signal superimposed on it. unit also provides similar facilities for tempera-
Taking the Honeywell ST3000 Model STD120, ture and pressure transmitters, both of which
the reference accuracy for the digital mode is could be “intelligent,” and it also accepts the
0.0625 percent of calibrated span or 0.125 percent frequency signal from a gas density transmitter.
of reading for spans between 62mbar and The computer calculates the mass flow rate
1OOOmbar. This means that the transmitter can in accordance with IS0 5167, with temperature
be calibrated for the differential pressure corres- correction for pipe and orifice bores. Flowing
ponding to the maximum flow rate and operated density, standard density, and standard compress-
over a wide range of flows, with optimal accuracy. ibility are calculated by the primary method