**3.2 Standard thermocouple types**

472 Solar Radiation

Iron, Chromel) and the other as the negative (Constantan, Alumel). Basic system suitable for the application of thermoelectricity in power generation is that of several thermocouples connected in series to form a thermopile (a device with increased output relative to a single thermocouple). The junctions forming one end of the thermocouple are at the same low

The thermopile is connected to a device in which the temperature TL is fixed when connected to a heat sink. The temperature TH is determined by the output of the heat source and the thermal output of the thermopile. The load is run by the charges generated. With a thermopile, the multiplication of thermocouple involves a corresponding increase of resistance, hence it follows that one thermocouple can be better than another at one temperature but less effective at a second temperature. In order to take maximum advantage of the different materials, the thermocouples are cascaded, producing power in stages and

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

Suitability for conditions of use, expected service life and compactable installation

 Adequate sensitivity S over a wide range of temperature, stability against physical and chemical changes under various conditions of use and over extended periods of times,

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

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

temperature TL and the other junctions at the hot temperature TH.

other selection factors among others to be addressed include:

Availability, moderate costs, abrasion and vibration resistance.

increase power output.

requirements

Fig. 8. Thermocouple materials

and shielded by the sheath.

**3.1 The choice of thermocouple** 

ASTM and ANST standards explicitly stated that the letter designations identifying only the reference tables might be applied to any thermocouple with a temperature-emf relationship that complies with the table within the specified tolerances, regardless of the chemical composition of the thermocouple (Quinn, 1983). Any randomly chosen pair of dissimilar wires will produce some kind of thermal emf when subjected to a temperature difference from end to end, however, the emf so produced may be unpredictable and of little use. Hence certain thermoelement combinations have been commercially developed over the years that have proved to be useful, reproducible, and readily available.

Eight of the most widely used of these combinations have been assigned letter-designations for ease of reference, Base metal thermocouples types designated as E, J, K, and T. The rear (Noble) metal thermocouple types are S, R, and B types.
