# What's the meaning by "RTD's max. cable resistance "

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#### netc_kefi

I want to choose a RTD input module which can receive 1km distance signal away from RTD. then I found the Schneider company's BMXART0814. In their product datasheet has a term called "max cable resistance, related to 3 wire RTD, this resistance value is 20 ohm. Does it mean I can choose the cable under 20ohm resistance?? If so, I can calculate the length of the cable whether could reach 1km.

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#### David

> max cable resistance, related to 3 wire RTD is 20 ohms. Does it mean I can choose the cable under 20ohm resistance?? If so, I can calculate the length of the cable whether could reach 1km. <

Yes, but . . .

I would assume that the 20 ohms is the total wire resistance for the two legs, out and back, and does not include the 3rd compensating leg.

So, yeah, calculate the wire size needed to stay lower than 20 ohms out and back (2km of wire).

Your 1km distance application is what RTD transmitters were designed for. You don't have a 4-20mA input to use with a 2 wire, loop powered RTD transmitter?

And by the time you run 1km of copper wire, doesn't wireless I/O look less expensive?

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#### Temperature

20 ohms is a ridiculously low impedance rating for an RTD transmitter. They are normally close to 10,000 ohms. It appears you want high accuracy (You choose and RTD over a thermocouple) yet you want to go cheap.

I would suggest a 4 wire transmitter with a matching RTD. You can get very accurate, reliable and versatile temperature from about any source for less than \$100.

Speaking of temperature, the new ASME standard for thermowells is now available through their website. ASME PTC 19.3 TW 2010. There is lots of free auxiliary software available on the web. Google SwiftyCalc for one example.

Good luck with your temperature measurement situation.

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#### Eric Ratliff

A direct answer about 'calculate the length of the cable whether could reach 1km' would be to use 14 gage copper wire. This would have a resistance of 8 Ohms on each leg. So, the two legs have a resistance of 16 Ohms. See http://www.powerstream.com/Wire_Size.htm , my favorite reference on wire gauges. This is pretty heavy wire, like used in a 15 amp house wiring circuit.

An example of a 'transmitter' that will change the RTD signal to 4 to 20 mA is http://www.icpdas-usa.com/products.php?PID=3072 . That one requires DC power of 10 to 30V. With a transmitter, you can use a much lighter gauge wire, such as 24 or 26 gauge.