Measuring temperature with RTDs over a long distance

[nate]

I have created an application using the FP-RTD-122 to take RTD mearurements from multiple locations in a plant. The measurments are correct when the RTD leads are connected directly to the module (about 6ft). However when we wire the RTDs in the plant to the module, the measuremtns are incorrect. For instance the temerature is 79C at one location, but shows as between -193C and 19C in Measurment and Automation Explorer. The distance is about 150 ft. I have found a reference in the internet that suggests using a temperature transmitter that converts the RTD signal to a 4 to 20 mA signal. Does anyone have any experience with measuring RTD over long distances? Do I really need a temperature transitter? If so, do I need to use the FP-AI-111 module instead?

[Aaron_G]

Nate,

RTDs allow temperature measurement by reading the resistance of a sensor. This is normally done by exciting the sensor with a known current and measuring the resulting voltage. There are three general types of RTDs; two-wire, three-wire, and four-wire. Two wire RTDs are less accurate because the wire resistance is indistinguishable from the sensor resistance. Three wire RTDs use two wires on one leg (one to carry the known current and one to measure the voltage) allowing one portion of the wire resistance to be nullified. 4 Wire RTDs use two wires on each leg (one for the excitation current) to nullify the wire resistance. As a result, 4 Wire RTDs are the best ones to use for high precision or long distance measurements. To measure 4 Wire RTDs with FieldPoint, you will need to use the [c]FP-RTD-124 module instead of the [c]FP-RTD-122. With that said, I do not know how well 4 wire RTDs will do at 150 feet. 4-20 mA transmitters will easily handle 150 feet so they will probably be your best option. To measure the 4-20 mA signal, you can use the [c]FP-AI-100, [c]FP-AI-110, [c]FP-AI-111, [c]FP-AIO-600, or [c]FP-AIO-610 modules.

Regards,
Aaron

[GregLPease]

Once the twisted pair from the 4 to 20 mA RTD temperature transmitter connects with the FP-AI-100 will we have to do our own linear proportional calculation to convert this amperage to a temperature? Is there a way for the FP-RTD-122 to grab values from the FP-AI-100 and do the calculations itself? Is the FP-RTD-122 even needed anymore if a 4 to 20 mA RTD transmitter and the FP-AI-100 are used?

[Marni]

Hello Greg,

If you are using the FP-AI-100 instead of the FP-RTD-122 you will have to convert the voltage to temperature in software. The FP-RTD-122 is not programmed to take the values from the FP-AI-100, but if you have a FP-RTD-122, I would recommend using it. As long as you have a FP-AI-100, you don't "need" the FP-RTD-122, but it is easier to use and saves programming time. Please let me know if you have any questions. Have a great day!

Marni S.
National Instruments