Thermocouple & Rtd

It simply means that two of the same device type occupy the same housing. The most common use is to have one connected to a temperature controller, or indicator and the other connected to an overtemp safety shutdown device. But they can also be used for comparison to tell when one is beginning to go bad, or to have a spare in place for the purpose of a quick changeout on the fly.

is there any specific conditions in which this duplex type to be choosed?

Typically a duplex temperature element is utilized where the temperature measurement is to be used by two different systems; for instance one TE will be wired to the plant DCS (via temperature transmitter or directly to temperature input modules) for indication and/or control and the other TE is wired to the plant SIS (via transmitter) for ESD and safety interlock functions.

Also I have seen duplex TEs utilized for redundancy i.e. to satisfy application SIL requirements. Each TE is wired out to separate transmitters each reporting independently to the monitoring and control system. A side note however; I'm not convinced that this is acceptable, to truly satisfy a SIL 2 requirement I believe that two completely separate installations are required.

Duplex TEs are also not much more expensive than simplex TEs so for new projects they are selected because there is a readily available spare in the event of failure of one TE.

Right now I and another engineer are drawing up designs for a control panel for a pharmaceutical tank that has a duplex RTD in a well on the tank bottom. As Ace Boeringa has noted, one often goes to a local indicator and one goes remote. In this case, one is connected to a local recorder mounted in a panel on the portable tank, and one goes to a transmitter which utimately connects to the plant DCS via an ethernet jack in the panel wall.

This is not real common, and is usually a customer driven option. In 22 years I have seen this applied to pharmaceutical equipment we build maybe a dozen times. It is more common to have a single RTD and use that signal for both the local readout and the plant system.

Google duplex thermocouple

http://www.google.co.uk/search?q=duplex+thermocouple&sourceid=ie7&rls=com.microsoft:en-gb:IE-SearchBox&ie=&oe=&rlz=1I7DKUK_en-GB&redir_esc=&ei=lP3yS-m-Lpm80gSJq-mhDQ

thank u very much for ur valuable comments

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