Expert answer:

Nice question. Before I get to the answer, I want to introduce the device and talk a bit about the difference between a 2 and 3 wire RTDs (Resistance Temperature Detectors). In this way, we can help other people that might have similar questions.

VIVACE VTT10-F Device Overview

The VIVACE VTT10-F is a temperature transmitter that you can install directly onto the sensor. There are two options for installation. You can install it through a bracket on a 2 inch pipe or fix it directly on a wall or panel. You can use the VTT10-F in conjunction with different types of temperature sensors, such as RTDs, thermocouples, Ohm and mV sensors. It has a measurement range that goes from -40 to 85 degrees Celsius.

This temperature transmitter also receives and retransmits 4-20 mA signals. Therefore you can use it as a 4-20 mA isolated signal repeater.

RTDs and Connections

Now let’s talk about the different types of RTDs and their connections. RTDs are widely used in the industry because of their accuracy and wide temperature ranges. Usually, they can be used in process temperatures of up to 500 degrees Celsius. There are basically 3 different configurations for this device. You options here are 2, 3 or 4 wire. Let’s understand the difference between them.

2-wire connection

The 2-wire connection is the simplest but least accurate of all the connections. This happens because the lead resistance of the cables influence the reading if they are too long (more than 100 meters). Therefore if you have a big distance between your transmitter and your controller this is not the best connection to choose.

3-wire connection

When we add an additional wire to the equation the error caused by the length of the cables in the 2-wire connection is compensated. However, in order for this configuration to work, the length of the other two cables should be the same. In this way, their resistances are also the same. You can have a look at the diagram below.

3 Wire RTD
Courtesy of Wikipedia

This configuration is more accurate and allows up to 600 meters of cable.

4-wire connection

If you need high accuracy in your readings, then you should use the 4-wire connection. While using this configuration, you can eliminate the voltage drop in the measuring leads. Voltage drop is responsible for incorrect readings. In this configuration, you will also need a constant current generator of 1 mA. The generator creates a reversal current which eliminates any residual thermoelectric voltages that might come up when you have different wire types or screwed connections.

VTT10-F Connection

Now that we understand the difference between the RTDs, let’s get to the connection in the VTT10-F. To get access to the terminal block you can just remove the device’s rear cover. In this block you will find 4 sensor terminals. For a 3-wire RTD you should follow the diagram below.

VIVACE VTT10-F 3 Wire RTD Diagram
Courtesy of VIVACE Instruments
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