Maximum wire length

[Greg Dake]

Generally speaking, what is the maximum wire length going from an RTD input on a PLC to the RTD? 100Ohm Platinum, 3 wire are typically what we use. Obviously it depends on the care which is taken for running the wire, I'm just trying to get a general answer.

TIA,

Greg

 

[Unregistered]

stick a 4-20ma loop converter in the head , and then you don't have so much of a problem - easier to protect against interference etc

 

[Greg Dake]

Right, but that wasn't my question. Thanks for the help anyway.

 

[elevmike]

Not sure if this is what your looking for.. but AD says 1000 meters for two wire RS 485, and 50ft for RS 232.

 

[Unregistered]

Rs 232? on an RTD ?

 

[Tom Jenkins]

I've never seen a specific limit, and I'm not sure there is one. There are a few practical considerations though:

1) On a two wire RTD the lead resistance will look like a higher temperature. You should select lead length and guage so that the false temperature is insignificant for your process.

2) On a three wire or four wire RTD the lead resistance is compensated. However, you are still subject to EMI and RFI induced voltages, and these will cause errors. Good shielding and routing helps, but all other things being equal the longer the lead the greater the exposure to EMI/RFI.

 

[rsdoran]

I posted some information on this recently but haven't located that thread. The gist of it was that RTD wire for long lengths may need to be larger but reduces precision. It is also recommended to make the runs using a solid section of wire..ie no splices etc, if you do need to splice then use connectors/terminals for that purpose should be used.

I dont have all my links on this new laptop but will see if I can find more detailed information on this subject.

 

[Greg Dake]

Thanks guys.....the Pyromation book says 0.099 degree error per 10 feet of wire for a 3 wire RTD. I'm going about 300' so I'd be off by possibly 3 degrees, which can be calibrated out at the chart recorder.


Thanks

 

[ayman metwally]

A related question:

What I think is that I can't just let the RTD wires just as tall as I want, There must be a "red line" for that.. why?
Because if the 2 wires used for excitation (i.e. injecting constant current in the RTD) are too long (i.e. higher resistance), this will be like a heavy "burden" to this excitation current ( acts like an open circuit).. Am I right?

 

[CaseyK]

You're on the right track Greg.

The shorter the better. LArger wire size to keep resistance down.

Avoid interference cuasing items.

I've had some 300'out in the pits below tire curing presses. They did pick up some interference, and calibration was a big problem, specially due to the ambient heat and humidity.

Overall, it can be done, but gennerally not recommended.

regards.....casey

 

[rsdoran]

The shorter the better. LArger wire size to keep resistance down

I am not sure that is the best way to describe it. RTD is resistance detection so a larger wire may "decrease" the resistance of the system..it depends. The wire and its resistance/length would need to be part of the calculation(s).

The nominal resistance for RTD is 100ohms@0°C. Devices may vary from standard to standard worldwide which means an RTD from one standard my not work with a device from another standard. This can be a "gotcha".

Here are some references for RTD's.
http://www.omega.com/temperature/Z/pdf/z054-056.pdf

Omega has alot on temperature products:
http://www.omega.com/temperature/