Send ohms via such long cables...

Hi there,

The RTD Pt100 is used to monitor the temperature of devices, and the output of this RTD (Ohms) is sent to the control room which is far away from these devices with the range (350 to 1250 meters), is it ok to send ohms via such long cables? or is it better to use a Temperature Transmitter to convert this Ohms and send them as 0-10 V or 4-20 mA ? And which is better for this purpose: sending as 0-10 V or 4-20 mA and why ? Thanks.
 
Hi,
in my opinion the first thing to find out is the mm2 of the cable. This will limit the length of the cable. A 4..20mA signal is usually based on 24VDC. So I would prefer the 4...20mA. Also because of the wire break indication.

PS: This is my first post, so please be lenient ;)
 
Hi there,

The RTD Pt100 is used to monitor the temperature of devices, and the output of this RTD (Ohms) is sent to the control room which is far away from these devices with the range (350 to 1250 meters), is it ok to send ohms via such long cables? or is it better to use a Temperature Transmitter to convert this Ohms and send them as 0-10 V or 4-20 mA ? And which is better for this purpose: sending as 0-10 V or 4-20 mA and why ? Thanks.
https://newton.ex.ac.uk/teaching/CDHW/Sensors/an046.pdf
 
An RTD input circuit does not 'send Ohms'. Most RTD analog inputs drive about 1mA of constant current through the RTD resistance and then read the voltage drop. A 150 Ohm resistance generates only 150mV of IR drop, a fairly low level signal to read over hundreds of meters of lead wire which can pick up who-knows-what-noise over that distance.

Most quality RTD inputs will publish a limit on the resistance of the lead wire 'legs'. The example below shows that the configured RTD-Low range (up to 316 Deg C) has a limit of 10 Ohms per leg.





Still, 10 Ohms of copper wire can be a fairly long run, I have a note that says 450m of 18 AWG wire (common in the US) is 10.0 Ohms. You'll have to look up what distance that covers for the lead wire in use.

But whether it's possible for the AI card to drive the source current a given long distance and get a reading doesn't mean that such is a particularly good idea.

Personally, I've had problems with 3 wire Pt100 RTD runs in excess of 150 meters and I tell people that that's my limit on direct RTD connections. Beyond 150 meters, I use a transmitter.
 
There are several possibilities to choose from:
You can connect the Pt100 with 4 wire connection, the extra wires compensate for the resistance of the cabling.
You can also consider a transmitter to 4-20mA, see for example these ones: https://cascade.net/en/transmitter/
Or you can switch over to a thermocouple measurement, you need to connect it with thermocouple compensation wire, but you can make it very long since the output of a thermocouple is low-impedant.
 
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