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Anonymous Poster #1

RTD

06/12/2016 9:12 AM

We have obsolete surge arrestor used for winding temp. Now,I need to connect 4 wires RTD to a new surge arrestor. Is it possible to use SD32x,SD32 or SDRTD

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#1

Re: RTD

06/12/2016 4:03 PM

Have a look online at the Weidmuller datasheets, 3 wire RTD yes, 4 wire RTD not sure.

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Anonymous Poster #1
#5
In reply to #1

Re: RTD

06/14/2016 4:44 AM

Thanks jack. I will use SDRTD. I will modify my 4 wire to 3 wire by connecting 2 wire together to make it 3 wires. Hopefully it will work.

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#2

Re: RTD

06/12/2016 7:25 PM

Are you sure that the analog input is truly 4 wire with all the attendant increase in accuracy that 4 wire implies? Does the AI state a different accuracy spec for 4 wire vs 3 wire?

Or is it just a 3 wire analog input with a screw terminal for the 4th wire?

The vast majority of field instruments have good 3 wire lead wire compensation.

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#3

Re: RTD

06/13/2016 10:39 AM

There should not be a surge arrestor connected to an RTD. You may want a surge arrestor on the power line for the electronics that reads the RTD but not on the RTD itself.

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#4
In reply to #3

Re: RTD

06/13/2016 3:39 PM

I have never personally come across an application with one but Weidmuller do make one so there must be some sort of RTD application where it would be useful.

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#6

Re: RTD

06/14/2016 6:43 PM

>I will modify my 4 wire to 3 wire by connecting 2 wire together to make it 3 wires. Hopefully it will work.

It will 'work', meaning it will give you a temperature reading, but it will, in all liklihood, have an error due to incorrect lead wire resistance compensation. Connecting two lead wires together does not make a "3 wire RTD".

A 3 wire RTD has three equal gauge/diameter core copper wire leads. When you double up the wires on one side of the sensor element, the total lead wire resistance on that side is halved. That affects the lead wire compensation circuit in the analog input with a resulting offset error.

The conventional way to deal with a 4 wire RTD and a 3 wire input is to cut off the 4th wire and use the 3 remaining wires to connect to the 3 wire input.

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