Thermocouples come in many styles from direct immersion, to thermowell, to hand-held, to surface mount and many others. Each has a specific way of implementing the sensor, but here are a few things to keep in mind:
Thermocouples have positive and negative wires, therefore it is important to maintain the polarity during installation. For thermocouples with ANSI/ASTM Color-Codes, the negative wire is always red. In IEC color-coded thermocouples the negative wire is always white.
When using extension wire or connectors, the same thermocouple type must be used in order to not introduce an error. Thermocouple connectors have the polarity listed on the connector shell and have unique pin sizes to insure that the connectors are properly combined.
If any of the connections are reversed it will result in an error in the measurement.
When connecting thermocouples to other devices care needs to be taken to ensure that the proper polarity is maintained. Thermocouples manufactured to the ANSI/ASTM color-codes always have the negative wire as the red wire. This is opposite the normal electrical standard of positive wire is red. Also, when extension wire is needed, thermocouple wire must be used to maintain accuracy.
The crossing of the polarity, and the use of non-thermocouple wire as extension wire, are two of the most common causes of errors in installing thermocouples.
If more than one measurement is needed, then dual or multiple thermocouples should be used. In dual thermocouples, separate measuring circuits are contained in the sensor so that each thermocouple signal is independent of the other. This insures that one measurement signal does not have an adverse effect on the other.
Thermocouple signals are low-voltage signals that are easily affected by electro-magnetic noise. High voltage devices and electro-magnetic emitters such as motors and radios can get into the thermocouple signal and overwhelm it. In cases with long cable runs are needed, or where electro-magnetic interference is expected, shielded cable can be used to help protect the thermocouple signal from these sources.
A better option for many is the use of temperature transmitters or signal conditioners. These devices can be located close to the sensor and will not only convert the low-voltage signal to a 4-20 milliamp or other more robust signal, but will also provide the reference junction.
The question of how long a thermocouple cable can be is more a function of the capability of the measurement instrument.
When investigating reading errors here are some places to start: