Expert’s Answer: How to read 3 wired PT100 MCS-XV
Nice question. Before I dive straight into the connection of the MCS-XV it is important to understand the principle behind the connection. Especially since this can be pertinent information regardless of which device are you are connecting to.
The PT100 is basically a resistance that changes its value according to the temperature. By definition, a PT100 has a 100 Ohms resistance at 0 degrees Celsius. Yeah, as you probably guessed, that’s where the name come from. You can depend on the Pt100 for its accuracy and wide temperature range.
When using a 2-wire PT100 it is important to be mindful of the cable length. If they are too long they will have their own resistance. It can turn into a chain reaction. If your Pt100 is influenced your temperature reading will be influenced as well.
If you use a 3-wire connection you can avoid this problem entirely.
An additional wire actually has the ability compensate the error we experienced in the 2-wire configuration.
The 3-wire connection tries to compensate this error caused by the lead wires by adding a new lead resistance. When we calculate their final resistance their influence is canceled out. However, in order for this method to work the other 2 cables must have the same length, therefore the same resistance. You can see the calculation for the 3-wire connection below.
Ok, too much theoretical information, right? But this is important to understand in order to properly set up the 3 wires so you don’t get an incorrect reading out of your PT100.
That being said, let’s see how can you can connect the PT100 to the MCS-XV. In both our calibrator and the PT100 there are 4 inputs for a Resistance Temperature Detector (RTD). This means that you can connect 2, 3 and 4 wires to thePT100s. So here we will only consider the 3 wire connection.
Since we have 4 inputs, we will have to connect 2 of them in series. Here let’s consider that our PT100 has 1 white cable and 2 red ones. Usually, there is 1 white cable and 2 cables of a different color. Fear not! If you have 2 black cables instead of 2 red cables you can still follow these directions.
Connect the red cable to the RTD1 and 2 in series and the red ones in the RTD 3 and 4 respectively. Remember that the cables must have the same length in order to have the same lead resistance to get a proper reading.
Well, that’s basically it. The connection is pretty straight forward. However, it is important to know the basics so you can identify a possible wrong reading and correct it.