**3.1 The choice of thermocouple**

A primary consideration in choosing which thermocouple type to use in a given circumstance is the range of temperatures over which the device is to be used. Some of the other selection factors among others to be addressed include:


Thermocouples can either be sheathed or beaded with bare thermoelements (Figure 8).

Fig. 8. Thermocouple materials

Sheathed thermocouple probes are available with one of the three junction types:

**Grounded Junction Type**: This is recommended for gas and liquid temperatures and for high-pressure applications. It has faster response than the ungrounded junction type.

**Ungrounded Junction Type:** This is recommended for measurements in corrosive environments where it is desirable to have the thermocouple electronically isolated from and shielded by the sheath.

**The Exposed Junction Type:** This is recommended for the measurement of static or flowing non-corrosive gas temperature where fast response time is required.

**ANSI Polarity**: In the thermocouple industry, standard practice is to colour the negative lead red. The negative lead of a bare wire thermocouple is approximately 6mm ('4'') shorter than the positive lead and the large pin on a thermocouple connector is always the negative conductor (Omega Eng., 2001) Standard Diameters of thermocouple available are: 0.25mm (0.010''), 0.50mm (0.020''), 0.75mm (0.032''), 1.0mm (0.04''), 1.5mm (1/16''), 3mm (1/8''), 4.5mm (3/16''), 6 mm (1/4''). With two wires 8mm and 9.5mm standard Omega thermocouples have 12-inch (300mm) immersion lengths. Other lengths are available.
