2 wire RTD w/ 4 wire transmitter

Hi sparktest,

Regards

JP

Thanks, but im looking at the same thing and it doesnt work that way. I tried to connect a three wire and still nothing. Although I pulled a four wire RTD from another process and presto everything worked fine. I suppose I should just get a four wire RTD and be done with it unless there is something obvious Im missing.

Join terminals 1 and 2 with a wire link. Join wires 3 and 4 with a wire link. Connect the 2-wire RTD across terminals 2 and 3. What happens?

"and it doesnt work that way."

Please tell us exactly, what didn't work.

What are the symptoms of it not working?

Sorry for the lack of details. With the 2 wire RTD wired im getting 21.8 mA back to the DCS (delta V) and no output change based on the simulated temp..this is the same reading with no RTD landed. Same scenario with the 3 wire but with the 4 wire it reads an accurate temp. on a meter and and back to the DCS. Im wondering if this 148 transmitter isnt working properly...

Sparktest,

I have never worked with a rosemount transmitter 148, but I can recommend that you check the configuration of this equipment(148). I do not know if this instrument automatically detects the type of connection made with the RTD wires, ie if it is 2 wire 3 wire or 4 wire RTDs.

Acording your first post, you are checking a transmitter 148 selected originally for use a 4 wire RTD sensor, Now, you are putting two and three wires RTD, please check the configuration in the transmitter, maybe that is the problem.

Regards

JP

I doubt the transmitter autosenses the RTD configuration. The transmitter is likely configured as a 4 wire RTD transmitter, and will only produce a valid reading when a 4 wire RTD (or 4 wire resistance) is connected to it. Otherwise it goes to fault condition.

Page 1257 (paper) or page 1277 (pdf) of Mr. Kuphaldt's Lessons in Industrial Instrumentation book
illustrates the operating basics of a 4 wire RTD analog input.

If the any of the 4 sensor wires are missing, the voltage cannot be sensed, the transmitter detects an open circuit and responds by driving the output to a fault condition, typically up-scale burnout; in this case, 21.8mA.

The transmitter's configuration needs to be changed to 2 wire RTD, or you can add two additional copper wires as shown in the wiring illustration to create a 4 wire RTD.

Hi,

These type of transmitters do not auto detect the type of temperature elements connected to them.

Sounds like your temperature transmitter doesn't know what type of temperature element you are using.

Use a communicator to make sure that the transmitter is set up for 2 wire PT 100(sounds like it's set up for 4 wire)

After you set it up for the correct temperature element connect the 2 wire RTD and see if you get a good transmitter output.

If you still don't get a good output then either ohm out the 2 wire RTD to make sure it's good or use the 4 wire RTD as a 2 wire RTD by connecting the two common legs together.

If the transmitter still doesn't work then invest on 4wire RTD just joking!

Hope this helps

Thanks

Hi Sparktest,

The subject of how to connect RTDs to RTD transmitters of differing wire count can be confusing. Here is a screenshot of the connections you'll need:

If you need more commentary on this, read section 21.3.5 "Proper RTD sensor Connections" and you'll be sure to find the information you're looking for. The "incorrect" wiring scheme shown will function, but it won't be as accurate as the "correct" scheme.

- Tony

Thank you for sharing that information and the link to your book, what a great piece of work!