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Temperature is by far
the most measured variable in a typical industrial plant. The
importance of temperature for most processes, as well its
affordability, explain this.
There are two very distinct segments when it comes to
temperature measurements: Monitoring and Control & Safety.
As will be presented, technology is having a significant impact
on both segments.
Monitoring
A significant portion
(60-70%) of the temperature points falls in the monitoring
segment. This means that the measurement is used as a supporting
reading to the more critical control variable.
Because of its supporting or secondary role, accuracy is
not the main concern for these points; reliability,
repeatability and affordability are.
In the past monitoring
points were wired directly to the control room without the
benefit of a transmitter. This
practice is quickly disappearing because of the inherent
technical problems of “wire-direct”,
the higher wiring costs (particularly in the case of
thermocouples), and the affordability of temperature
transmitters.
New technologies are
having a significant impact on temperature transmitters. For
Monitoring applications the use of open digital protocols (e.g.
Foundation Fieldbus) allow significant wire cost savings, as
multiple transmitter readings can be channeled through the same
pair of wires. In addition, multiple sensor input transmitters
extend this advantage even further. Temperature transmitters
with eight inputs, communicating via a single pair of wires are
a reality today. These multi-sensor transmitters can at the same
time be configured to share the same pair of wires, reducing the
wiring costs even further.
Control & Safety
The rest of the
temperature points (30-40%) fall in the control & safety
segment. For control loops accuracy, stability, reliability, and
repeatability are the key drivers.
A process managed tightly has enormous financial
benefits. Affordability
(cost) plays a secondary role, as a top performing transmitter
will more than pay for itself. In the case of safety loops, the
transmitter is left with the responsibility of triggering the
installation in case of an emergency. A poorly performing
transmitter could trigger the plant unnecessarily, at a large
expense. Even if multiple transmitters are used for the same
measurement point to provide redundancy, these loops deserve the
best transmitters available on the market. . For safety loops
reliability, and repeatability are the key drivers
New technology is having
an impact on the Control & Safety side as well. Digital
protocols have opened up the full transmitter intelligence to
the users. This means that the advanced transmitters can
diagnose not only themselves, but also the process, through
advanced signal processing algorithms. Transmitters currently on
the market can perform a temperature measurement on a dual
element sensor. In the case of the breakdown of one of the two elements, the
transmitter will transition to the surviving element, without
any disruption. The condition is then reported to let the
personnel schedule the replacement at a convenient time. The
transmitter can also tell when its sensors have fallen outside
of the acceptable accuracy range and that they need replacement.
Transmitters available today and using the Foundation Fieldbus
protocol can act as the primary or secondary controllers as
well.
In summary, Temperature
is a very common measurement variable. It is important to
consider the use the measurement will have and base the
equipment selection accordingly. Technology is having a
significant impact in the way we measure Temperature today, both
in Monitoring as well in Control & Safety loops.
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