about signal transmission from the field instruments

i am a fresh engineer currently working in a power plant. i came to know that output signal from thermocouples and rtd are taken special care, since its output should be sent as a raw signal. why can't we sent them as analog signals as like others.

kindly guide me this doubt.
thanks and regards

Dear Friend

Thermocouple makes mV signal and it is possible to transmit it as an analog signal but we should use thermocouple cable for any length is necessary.

In far distance applications it is not economic to use thermocouple cable, while it would be affected by environment temperature through the cable. Using conventional cable also need to contact it in somewhere to thermocouples and it makes another thermocouple in the contact point and makes another fault in measurement.

But RTD's are different. RTD is a resistor which varies by temperature.So it effects on current value and we can measure this variation by measuring the voltage on cable in destination point.

best regards,
Soheili

Hi,

Thermocouples and RTDs are already analog signals in nature but considered as special type and this is because the native characteristics for these signals.

what i mean is that as mentioned in the previous reply that the feedback is in mv which require special handling and also don't forget that For thermocouples it's required to get Cold junction compensation and getting temperature value via suitable TC Curve which differs from type to type.

Also, RTD require special circuit. mainly excitation wire, signal and ground.

Conventional analog signals use Voltage loop or current loop but some signals require more than that which can be called special type signals.

i hope this answers your question
Thanks, Max.

Hi,

I agree with other replies. These signals must be treated in a different way from other field instruments because they are "raw sensors". I mean, they have not electronic to fit and linearize the raw signal into a typical V or mA signal. That´s why special PLC module are usually used to acquire these signals.

In the other hand, special signals converters are available to linearize thermocouple and RTD signals. These converters are normally installed into the "head" of the sensor, and after them copper cable can be used and a normal V or mA input in PLC. You have to know the type of the sensor (K,J,S...etc) and the scale of the signal to program the converter (0-1200 DEGC to 4-20 mA, 0-250 DEGC to 0-5 V....etc)

In a power plant you have many thermocouples in close proximity to the control system so it may be more cost effective to run multi-pair thermocouple cables. Also you don't have the hassle of all those transmitters to calibrate, only one or two multiplexed thermocouple cards.

Most plants only have a few scattered over a wider area so it's more cost effective to change the signals into 4-20 mA for transmission. The newer plants are on some sort of fieldbus so you might see a local module converting thermocouple to fieldbus.

I haven't seen any plant of recent construction where the thermocouples are bought back. Having said that we still do it occasionally on some of our experimental skids where there can be 20-30 thermocouples just a few meters from the PLC.