lamaPLC: PTA8C04 4-channel PT100 Modbus Modul

RTDs (Resistance Temperature Detectors) are straightforward devices: simply a small strip of platinum that measures precisely 100 Ω or 1000 Ω at 0°C. Bonded to the PT100/PT1000 are two, three, or four wires.

Thus, the 4-wire RTD has two wires attached to each side of the sensor. Each wire has about 1Ω of resistance. When connected to the amplifier, the innovative amp measures the voltage across the RTD and across the wire pairs.

For example, the approximate resistances of a 4-wire PT100 RTD at 0 °C are as follows. (For a PT1000, the middle resistance would be about 1002Ω rather than 102Ω).

When the amp measures this sensor, it first assesses the resistance between one set of red and blue wires. It then measures the resistance between the red wires and the blue wires. To get the resistance of a single wire, divide each resistance value by two. The final calculation is 102 - 1 - 1 100Ω.	These are very similar to the 4-wire type, but only have one 'pair' of connected wires. This is because the wires for the RTD are all the same gauge and length; therefore, instead of two pairs, the amplifier reads one pair and uses that resistance for both wires.	It is as simple as it gets, with only one wire on each side. You might need to calibrate the sensor by placing it in an ice bath to measure the resistance at 0°C (around 102 Ω), then subtract 100 Ω to find the total resistance of the connection wires!

Connect the two ends of the PT100/PT1000 resistor to the RTD+ and RTD- terminals on the sensor module. For example, a resistance of 102 Ohms can be measured. In a 3-wire or 4-wire setup, the wire connections go to the F+ and F- terminals. These connections might differ from the resistance values of the respective sides by only a few Ohms, meaning the resistance between F+ and RTD+ or F- and RTD- may vary slightly, just a few Ohms.

Basic resistance values in Ohm PT100 sensors according to DIN/IEC 751