Issues with the output of PT100 temperature transmitters
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My temperature variation module (24V, 4-20mA, dedicated for PT100) and the PT100 are placed separately. There is a 40-meter-long shielded twisted pair cable in my PLC cabinet, which is grounded on the side of the cabinet. I’m a bit confused: when this temperature sensing module is placed in the cabinet, the PT100 sensor on site (3-wire type, with two wires shorted together) is connected directly to the twisted pair cable. Using a multimeter to measure the resistance of the twisted pair cable on the cabinet side shows values that correspond accurately to the actual temperature. However, once the temperature sensing module is installed, the milliampere value sent by the module to the PLC – which only accepts milliampere signals – becomes significantly too high. On the other hand, when the temperature sensing module is installed on-site, the milliampere value returning from the twisted pair cable on the PLC side becomes normal (24V can be measured at the PLC terminals). How can this phenomenon be explained?
1. First, lay a wire: the temperature sensor module is installed on the side of the cabinet, while the PT100 sensor is installed at the site. 2. This is because there is only a two-core shielded wire available
It won’t work properly if you don’t follow the specifications; for a three-wire thermal resistance PT100, if a multi-core cable is not available, it’s better to opt for an integrated temperature sensor
First, let’s clarify one thing: the temperature sensor module (24V, 4-20mA, designed for PT100) receives resistance signals and outputs a 4-20mA signal to the PLC card, right? Therefore, the resistance from the PT100 sensor at the installation site should be transmitted to the temperature sensor module using a three-wire cable; it is not acceptable to short the BC terminals at the PT100 side and use only two wires to transmit the signal to the control room. This approach does not comply with the standard requirements, so it is prohibited – and that’s normal.
First, let’s clarify one thing: the temperature sensor module (24V, 4-20mA, designed for PT100) receives resistance signals and outputs a 4-20mA signal to the PLC card, right? Therefore, the resistance from the PT100 sensor at the installation site should be transmitted to the temperature sensor module using a three-wire cable; it is not acceptable to short the BC terminals at the PT100 side and use only two wires to transmit the signal to the control room. This approach does not comply with the standard requirements, so it is prohibited – and that’s normal.
That’s what I did, but the error was more than double. And by installing the temperature sensing module on-site as well, that is, connecting the PT100 directly to the temperature sensing module, with the latter connected to the shielded twisted pair cable that has been laid previously, and the twisted pair cable leading into the PLC card, there is basically no error in this case; I just don’t understand why
But even if it’s not allowed, it won’t exceed twice that amount
I know it doesn’t meet the standards either, but the conditions are limited
When you connect the temperature sensor to the field, the connection between the temperature sensor and the thermal resistor is a three-wire system; the signal transmission between the temperature sensor and the PLC takes place via current, which naturally results in lower errors.
All thermistor-to-transmitter interfaces are 3-wire, with a 4–20 MA output, and the connections to the system consist of one positive and one negative wire. Our company specializes in the production of instruments and meters
The original poster’s description is unclear; it seems they don’t understand the concept of thermal resistors. It’s estimated that the resistance of the wire was taken into account during the measurement; the resistance for 40 meters of wire back and forth is several ohms. Approximately 3 degrees per ohm, so you can estimate it by yourself. ;P