Page 218 - Instrumentation Reference Book 3E

Transmitter: a field-mounted device used to -

202 Microprocessor-based and intelligent transmitters
Secondary sensor: The sensor mounted adja- Differential
cent to the primary sensor to measure the phys- Pressure
characteristic of the primary sensor (e.g., the effect I I Temperature
ical parameter which adversely affects the basic
of temperature on a pressure sensor).
Pressure
sense a physical parameter (e.g., temperature) at
the point where it is mounted and to provide a Flow
signal in a 4-20mA circuit which is a function n (other than AP)
(usually proportional) of the parameter. I
Microprocessor-based transmitter: a transmitter Level
in which a microprocessor system is used to correct
non-linearity errors of the primary sensor through Density
interpolation of calibration data held in memory, or
to compensate for the effect of secondary influences E3 Analysis
on the primary sensor by incorporating a secondary
sensor adjacent to the primary sensor and interpol- Figure 13.5 Relative numbers of process transmitters.
ating stored calibration data for both the primary
and secondary sensors. the characteristic curves for various thermocou-
Intelligent transmitter: a transmitter in which ples, platinum resistance, and other temperature
the functions of a microprocessor system are detectors that have to be embedded in memory,
shared between: (i) deriving the primary measure- for interpolation by the microprocessor through-
ment signal; (ii) storing information regarding the out the selected range of the transmitter. For
transmitter itself, its application data, and its loca- pressure and differential pressure transmitters, it
tion; and (iii) managing a communication system is the calibration data for both the primary and
which enables two-way communication between secondary sensors that have to be embedded in
the transmitter and either an interface unit con- memory for interpolation by the microprocessor
nected at any access point in the measurement throughout the selected range of the transmitter.
loop or at the control room. Differential pressure transmitters are particularly
Smart transmitter: an intelligent transmitter important because about half of the level mea-
with simultaneous digital communication super- surements and about two-thirds of flow measure-
imposed over the analog signal, i.e., smart is a ments are based on this measurement, although
special case of intelligent. this latter proportion is declining.
Fieldbus network: a single communication The second important development arises from
medium such as a twisted pair of copper conduc- the potential benefits to be derived from being
tors, coaxial cable, etc., which carries information able to communicate with a remote instrument
such as address, control data, and process para- over the same network that carries the measure-
meter data between a number of transmitters, ment signal. Hitherto, in the process industries,
actuators, controllers, etc. instruments have been regarded as devices which
are located in remote positions and connected by
a twisted pair of wires to the point where the
13.3 Background information measurement signal is required. The technology
which enables two types of information to be
The range of microprocessor-based measuring communicated over the same pair of wires, with-
systems now in use covers a wide variety of appli- out mutual interference, has long been available
cations, but those which are smart or intelligent but only recently has it been applied and
have been developed specifically for use in the exploited for process measurements.
process industries and, in this chapter, consider- Having developed transmitters in which micro-
ation is restricted to this particular range. The processors are used to enhance the performance by
types and relative numbers of process measure- linearizing the basic sensor characteristic or com-
ments varies from one industry to another, but pensating for secondary influences, it is a logical
Figure 13.5 (based on transmitter sales data) progression to add further memory so that other
shows that the most widely used measurements data, specific to the transmitter itself, such as its
are temperature, pressure and differential pres- type, serial number, etc., are stored at the trans-
sure, flow, level, density, and analysis. mitter. The microprocessor can be used to control
The principal development of smart transmit- a communication facility, so that information
ters has been concentrated on temperature, pres- stored at the transmitter can be interrogated via
sure, and differential pressure transmitters. For the measurement circuit in which it is installed, and
temperature transmitters, it is the data regarding other functions, such as setting or resetting the