Measurement Principle and Basic Flow Selection
Temperature
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 temperature reading. 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 thermocouple can 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 is also 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 to the 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.
