3 wire RTDs

[Walkerok]

I have a system using a V570 and the V200-18-E6B. I have a customer that is strongly resisting 3 wire RTDs. (they have a large inventory of 2 wire 100 Ohm RTD sensors). This is a repeating customer so I can not ignor their desires. I understand that it is Unitronics position that a 3 wire RTD should be used and that a 2 wire can be used but the accuracy will suffer. What is the difinition of suffer ? .5 - 1 degrees C, or wildly wrong values.

I have used a huge quntity of 2 wire RTDs over the years, and to be blunt, I had never used a 3 wire RTD until Unitronics demanded it. Overs hundreds of projects I have never had issues, but admittedly I have always used 1000 Ohm platinums with .385 alpha and not 100 Ohm sensors and maybe this makes all of the difference in the world? Unitronics is the first piece of equipment I have ever seen to emphatically insist on 3 wire sensors only.

Looking into the inner wiring of RTDs, the only difference between a 2 wire and 3 wire is a fixed resistance on the 3rd wire to mitigate variations due to wire length and ambient temperatures affecting resitivity of the connection wiring. Can I add a fixed risistive ciruit between T+ and where the 3rd wire would connect to mitigate ill affects? Granted I may need to make a permeneant offset on screen and in the program comparisons if adding the exterior fixed resistance but that would be acceptable if that is what is required. If I keep my wiring distances less than 30 feet can I affectively accept the 2 wire version with just a jumper between T+ and where the 3rd wire would land without causing bad things to ensue?

In this case we are talking about systems that has a high temperature shutdown requirement in the 250 Deg F range and a variation of actual to read values in the +/- 5 Deg F will not negatively affect the operation of the equipment.

Thank you in advance

Keith

[Joe Tauser]

You can put a jumper on the module between one of the sensor wires and the third connection. It will be OK.

Here's the long answer-

Let's assume the RTD sensor leads are 24 AWG, which has a resistance of 0.025 ohms/ft at 20 C. For 30 feet, you'll be inducing a constant error of 0.75 ohms.

A standard DIN 100 ohm RTDs resistance can be calculated by R = 100(1 + (0.385 * T), where T is in degrees C and T > 0. Since your "third wire" is a jumper with a resistance of effectively zero, the module's circuitry will always translate the higher resistance as 0.75 / 0.385 = 1.94 degrees high.

The lead resistance will change slightly as the wire gets hotter, but not by much. You may want to Google "24 AWG wire resistance" to get a feel for it so you're going in with your eyes open and your customer understands what's going on.

Joe T.

[Walkerok]

Hi Joe,

Thank you very much that does help. I am thinking..and yes that is dangerous...

If the third wire is that important for 100Ohm RTDs accuracy then whould I improve my situation alot if I made a three wire termination very close to the two wire RDT body. With one connection to the negative side and two connections to the positive side. That way I would only have a few inches of difference between the original positive 2 wire and the sensing leg of the third wire. Or is this line of thinking not worth my time?

Thanks

Keith

[Joe Tauser]

Yes, that would improve your accuracy. In an actual RTD the third wire is connected at the sensing element, so the closer you get to the RTD the better.

Joe T.

[dahnuguy]

conductor resistance is less of an issue with 1000 ohm RTDs as the iwre resistance is small in relation to the whole resistance.

However, I would still prefer to use a 3 wire RTD over a 2 wire RTD.

In addition, I always used a transmitter on all the RTDs I used, and brought a 4-20mA signal from the transmitter back to the PLC.

If the RTD is less than 10 feet away, (mine never were) it's probably never going to be an issue.

Until the RTD opens and you get maximum temperature and the process freaks out. Not common, but I had to be ready for everything working with chemicals.