What is the advantage of using duplex RTDs?
I hope that the lead wire resistance compensation will be taken care of in the Duplex RTDs (4 wire & 3 wire).
There are several reasons why you might use dual RTDs.
1) If you need to monitor the temperature with two different instruments, e.g. control and record, you can't do that with one unless you first convert it to 4-20 mA.
2) As a backup, although personally if one failed I would be reluctant to use the other.
3) For reliability, some transmitters can use the second RTD as a redundant spare, alarming if either drifts or fails (3144P for example).
4) As part of a Safety Instrumented System (SIS) for critical safety related loops. I don't think you would be allowed to use a dual with 2 transmitters but you might take advantage of point 3) with two pairs of RTDs and two transmitters.
The dual RTDs I have used were only 3 wire which is OK, once you convert a 4 wire to mA you negate the advantage of better accuracy anyway. The only time I have seen a 4 wire RTD actually wired with 4 wires is in the case of an integral transmitter, otherwise one of the wires has been left unconnected in the head with a triad cable to the transmitter/control system. Perhaps someone else can make the case for 4 wires.
First of all, lead wire resistance has nothing to do with simplex/duplex RTDs.
In any RTD with 3 wires, 3rd wire acts as the compensation wire.
In simplex type RTDs, there will be only one element with 3 wires. In duplex type RTD, there will be 2 temperature sensing elements of 3 wires each.
Now coming to the advantages/applications:
1. In some of the hazardous areas/high temperature zones, if you install simplex RTDs, in case of a fault, you may not be able to remove the RTD from the line and replace the same with a new one. In such cases, people prefer to go with duplex RTDs. So, if one element fails, you need not physically remove the RTD from the line but you can use the other element for temperature sensing.
2. In some of the cases (typically in glass heating furnaces) the temperature sensing elements come embedded with the body. In these cases, you have to replace the entire body if the temperature sensing element fails. In such cases, they use either one duplex thermocouple or 2 independent thermocouple sensors for temperature sensing.
3. Some applications require local as well the remote indication for temperature. In these cases, people prefer duplex RTDs. One element is used for local indication while the other is used with 4-20mA transmitter for remote monitoring & control.
4. In some cases, especially in case of ATEX guidelines, when you are designing critical systems it is mandatory that each measurement should have more than one sensor in a line, more than one physical property to be sensed.
For example, I had an application where I had to sense the flow of water in a return pipeline to ensure that there is a continuous flow. According to the ATEX guidelines, I had to measure more than one parameter between temperature, flow, pressure. So I had installed 2 temperature sensors, 2 flow transmitters and one pressure sensor. I could have avoided pressure sensor but still went for it.
Hope it is clear.
I agree with Sastry on most points but don't agree that the un-used one can be a spare. If the on-line sensor fails there's a strong possibility that the spare has failed also, I doubt they ever fail for electrical reasons, however if you use both at once to check each other the chances are you will get prior warning. I know the Rosemount 3144P transmitter uses both sensors to give several options and displays a warning message if one fails or starts to drift, I'm sure the other manufacturers have a similar product.
If the sensor is installed in a well you should be able to replace it.
There is an interesting product from Moore Industries called Worm. You can use it to replace the sensor, without replacing the RTD body from the line. very interesting product.
We offer an Insertion RTD that can be installed (hot tapped) or removed from the line under pressure. Very handy if the line needs to be cleaned with a "pig" or for seasonal shutdowns.
The only advatage is you have a spare RTD should one RTD fail. Hence the term "Duplex" meaning two.
However the term "duplex" should not be used always. Duplex RTDs are usually two RTDs located in one sensor probe. Not all equipment have space for this type of arrangement, such as 3Ph motor windings, where the RTD is imbedded into the winding prior to varnishing. In this case the motor maker may provide two single winding RTDs per phase (one as spare) but they are not considered as "duplex" RTDs.