The proper connections for all three sensor types (2-wire, 3-wire, and 4-wire) to the user-adjustable transmitter are shown in the images below:
It is very important to note that the common connections shown for the 3 and 4 wire sensors represent the connection points on this sensor. Not the terminals that were jumpered by the technician at the time of installation, nor the internal jumpers of the transmitter. The whole purpose of having 3-wire and 4-wire RTD circuits is to eliminate errors due to voltage drops along the current carrying wires, and this can only be achieved if the sensing wire(s) extend to the RTD itself. And connect there.
If the transmitter terminal(s) is jumpered to only one current-carrying terminal, the transmitter will sense the voltage dropped by the RTD plus the voltage dropped by the current wire(s), resulting in high temperature indications. Misconceptions about proper RTD connections are unfortunately very common among students and industry professionals. With any luck, the following presentation will help you avoid such mistakes and most importantly help you understand why connections are better. Always keep in mind the purpose of connecting a 3-wire or 4-wire RTD:
Avoid doing this to avoid inaccuracies due to voltage drops along current-carrying wires. The only way to do this is to ensure that the non-carrier wire(s) of the transmitter wire(s) extend from the transmitter terminal(s) all the way to the sensor itself. In this way, the transmitter is able to “look at the voltage of the current carrying wires” as before to see that the voltage has only been reduced by the RTD. The images below show both the correct and incorrect ways to connect a 2-wire RTD to a 3- or 4-wire transmitter:
Jumpers placed in the transmitter terminals reduce the functionality of the 3-wire or 4-wire transmitter and reduce its performance compared to the 2-wire system. A similar problem occurs when one tries to connect a 3-wire RTD to a 3-wire transmitter using a readily available 4-wire cable:
3-wire RTD measurement is based on the assumption that both current-carrying wires have exactly the same electrical resistance. By paralleling two of the four wires in the 4-wire cable, you will create unequal resistance in the current path, thus leading to measurement errors in the transmitter (note).
Note: these errors only occur if parallel wires occur. If your two wires happen to be in parallel, connect the transmitter sense terminal to the RTD (one wire with no current), no error will result. However, many RTD transmitters do not acknowledge which of the terminals they sense (carry no current) versus which they are excited (pass current to the RTD), and so if Easy to guess, there is a possibility of error. Given that the parallel wires connecting the sense terminal of the receiver to the RTD are of no real use, my recommendation is to either use all four wires and configure the transmitter for 4-wire mode, or no wires at all. Fourth, do not use. Better solutions for the 3-wire RTD and 4-wire cable scenario include configuring the transmitter for a 4-wire RTD input and actually using all four terminals (shown on the left), or keeping the transmitter for a 3-wire RTD input configuration and Do not use it. 4th wire in the cable at all (shown on the right)
A photo of a modern temperature transmitter capable of receiving input from 2-wire, 3-wire, or 4-wire RTDs (as well as thermocouples, another type of temperature sensor entirely) describes the connection points and labeling of what the sensor looks like. . To connect to the appropriate terminals:
The rectangle symbol shown on the label represents the RTD resistance element. The symbol with the “+” and “-” signs represents the thermocouple connection and may be ignored for the purposes of this discussion. As shown in the diagram, a two-wire RTD is connected between terminals 2 and 3. Similarly, a three-wire RTD is connected to terminals 1, 2, and 3 (using terminals 1 and 2 as the connection points of the two common RTD wires). Finally, the four-wire RTD is connected to terminals 1, 2, 3, and 4 (terminals 1 and 2 are common, and terminals 3 and 4 are common to the RTD). After connecting the RTD to the appropriate terminals of the temperature transmitter, the transmitter must be electronically configured for that type of RTD. In the case of this particular temperature transmitter, configuration is done using a “smart” communication device using the HART digital protocol to access the transmitter’s microprocessor-based settings.
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