The purpose of standardization is to make the components of the loop compatible, and thereby make products of different manufacturers interchangeable. One of the tasks of engineering societies, such as ISA, is to promote standardization. This is not easy (particularly not in our digital age), because standardization conflicts with the business interests of the manufacturers whose goal often is to create "captive markets" and to eliminate competition. Standardization eliminates such practices.
A: RTD sensors are typically terminated in field transmitters sending a 4-20 mA signal to a panel-mounted controller or to a DCS. Wiring between the transmitter and the RTD itself will usually be a four-wire copper cable of which only three wires are used. The RTD sensor exists as one resistor of a four-resistor bridge circuit. Two of the wires connect the RTD into the bridge. The third wire balances the bridge for the length of the copper extension wire that is used. The transmitter circuit balances the bridge adjusting one of the resistors, such that no current flows through the RTD. A fourth wire may also be used in the bridge circuit, but does not increase accuracy. Please refer to many textbooks on measurement technology listed in the ISA book catalog for more information. Béla's comments are accurate for thermocouple measurements. The various types of thermocouples are necessary to cover the range of temperatures that may be needed.
A: The benefits of compensation are concerned with linearizing the relationship between the physical process condition sensed and the output signal of a transmitter.
Linearization is useful because a PID loop has stability problems if the loop gain changes over the range (span) of the transmission signal. The PID gain term must be reduced to be stable with the highest transmitter gain. The quality of control is reduced as the non-linear transmitter goes away from the region of highest gain. Compensation also reduces errors due to offsets or zero shifts as ambient or process pressure or temperature changes.
A thermocouple has cold junction compensation (CJC) because the potential of each junction of different metals adds to (or subtracts from) the voltage of the sensing junction. The cold junction is installed to subtract from the sensing junction and is held at some reference temperature, so that only the sensing junction voltage appears at the amplifier in the transmitter. CJC may also be done by a clever arrangement of electronic parts.
The number of wires used to carry the resistance of the RTD to its transmitter depends on the resistance range of the RTD and the resistance of the wires. If the resistance of the RTD is 1,000 or more times the loop resistance of the wires, then two wires are enough. Three wires are used with 100-ohm RTD sensors. Four wires are used in a Kelvin connection for low-resistance sensors, such as the 10-ohm sensor in a motor winding.
Generally, four-wire transmitters require more power than 60 milliwatts for their operation, and a second pair of wires is used to deliver this external power to the device.
The transmission signal does not matter as long as both ends of the transmission medium know how to encode and decode the signal. The first instruments were pneumatic, partly because electricity wasn't all that reliable and partly because pneumatic power for actuators made for simpler systems. As time went on, manufacturers in the United States converged on using 3 PSI to 15 PSI ranges for the pneumatic transmission signals. When 3 to 15 PSI became standard, a manufacturer could produce only one loop component , such as transmitters or valves or controllers, and their devices could be connected with other vendors' devices to form control systems. Each vendor became free to do what it did best.
As science and technology kept advancing the bleeding edge further into society, pneumatic transmission gave way to electrical with 4-20 mA DC (after a battle over 10-50 mA DC). Again, electrical signaling was simple, but there was no power for actuators, so we still need air compressors and dryers. Microprocessors began appearing in devices for calculated compensation, leading to devices that could do control in the field, which requires digital signal transmission.
Digital transmission should be as transparent to the user as a digital telecom network, but we aren't there yet.