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How do we select between RTD and thermocouple? On what basis do we select them?

RTD and thermocouple serve the same purpose which is to measure and give the temperaturereading. However the selection of them depends on the accuracy and range of temperature we

measure. RTD can give more accurate reading compare to thermocouple. However in term of

range, thermocouple can give wider range compare to RTD. In other simple words, RTD can

only measure up from -270 degree C to 650 degree C (920 degree C range) while thermocouplecan measure from -185 degree C to 1620 degree C (1805 degree C range).

The other factor is the price. RTD is more expensive compare to thermocouple, but this factor isalso linked with accuracy. If you have something that requires more accuracy, then you might

put the price aside.

How is the connection of RTD or thermocouple from site to DCS?

Well, it depends on types and also plant. Mostly older plant use direct connection from site to the

control room. For example the thermocouple from site is connected directly using thermocouple

wire to MTL barrier (specifically the converter). Newer plant use thermocouple or RTD

connected to the temperature transmitter and the transmitter transmit normal 4-20mA/fieldbus tothe DCS.

The reason was that the transmitter was quite expensive compare to thermocouple wire for a

decade ago. Nowadays, as technology evolves and transmitter has become cheaper, economically

purchasing a transmitter is cheaper compare to using thermocouple way from site to the control

room.

Normally orifice plate has turndown-ratio of 3:1. How to increase the performance of the

reading without changing the orifice plate?Use 2 transmitters instead of 1. Select the range of the transmitter differently so that one of them

cover the total measurement reading, while the other only cover the normal reading.

What's the main difference between shutdown PLC and DCS?

Main difference between PLC and DCS is their function. Distributed Control System (DCS) uses separate

hardware to control the unit. It distributes the load to all the controller. PLC is processing all the logic

inside its primary controller. If one DCS controller fails, it will only caused shutdown to only one unit of

the process. If the PLC controller fails, it will shutdown all the process due to the reason that it only has

one controller to process the sequence/logic (we put aside the redundancy here).

In term of hardware, both of them use different standard. DCS is following the general industrial

standard which is not strict as compare to PLC which is using military standard. Specifically the respond

time for DCS can be longer compare to safety PLC which requires 300ms.

In term of application, DCS generally being used in process control while PLC is being used mostly in

batch process. This is due to the reason that DCS program runs in serial mode while PLC program runs in

ladder.

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The early DCS looked dramatically different from PLC. Initially operator use analog instrumentation for

control while for safety they used relay system.

http://www.red-bag.com/jcms/specifications/172-bn-sp-k01-engineering-and-design-specification-for-

instrumentation.html#6