The HAKKO 191-211 and 191-212 Sensor Wires are K-Type thermocouple sensors that are designed for use with the HAKKO 191 and FG-100 Tip Thermometers and the HAKKO 192 and FG-101 Soldering Station Testers.
As with all thermocouples, polarity of the sensor is important. The red and blue indicators on the sensor wire are there so you can identify the polarity of the wire and install it on your HAKKO unit properly.
The HAKKO units suspend the sensor wire between three posts in a Y-configuration. Two of the posts are marked with red and blue dots next to them, and the third post that is top-center in the Y-configuration, is the one that is spring loaded to provide proper tension of the wire.
When you install the sensor wire, it is important that you match the color indicator of the post with the color indicator of the sensor, otherwise the polarity will be reversed and the unit will not function properly.