Temperature Sensors for Use in Aerospace and Automotive Industries - Part 1

Hello to all our English speakers. For this entry I would like to share the contribution prepared by Laura Oviedo from GTS Mexico. If you have interesting information related to heat treatment, do not hesitate to contact me at victor.ttmexico@gmail.com. We will be happy to review and publish every value added article with the TTM community.

1.-General Sensor Information

a) Thermocouple Definition

A thermocouple is a sensing device that consists of two conductors of dissimilar electro thermal characteristics. The conductors are joined together at one end (hot junction), where the junction is placed in contact with the item whose temperature is to be measured. At the other end (cold junction), a voltmeter is used to measure the electrical voltage that is produced by the difference in temperature between the two junctions as shown in figure 1

Figure 1 Example of thermocouple hot junction and cold junction. 

As shown in figure 2, there are different types of sensors for thermocouples and their classification is as follows. 

• Base Metal Thermocouple: Thermocouple whose thermo elements are composed primarily of base metals and their alloys. Examples include E, J, K, N and T.
• Noble Metal Thermocouple: Thermocouple whose thermo elements are composed primarily of noble metals (example: platinum/platinum-rhodium) and their alloys. Examples include types R, S and B.
• Expendable Thermocouple: Thermocouples made of fabric or plastic covered wire. The wire is provided in coils or on spools. Insulation usually consists of glass braid or ceramic fiber cloth on each conductor plus glass braid overall. 
• Nonexpendable Thermocouple: Thermocouples that are not covered with fabric or plastic insulations. One type consists of ceramic insulators over bare thermocouple wire, sometimes inserted in a tube for stability and protection. A second type consists of a combination of thermocouple wires, mineral insulation, and a protecting metal sheath compacted into a small diameter. The thermocouple thus constructed is protected, flexible and, within the temperature limits of the sheath material, may be used many times without insulation breakdown.

Figure 2 Types of Thermocouple Sensors 

b) The Basic Temperature Sensor

Every type of thermocouple corresponds to different thermo-element compositions depending on its base/noble metal composition as follows, and affects directly to their temperature ranges in which they can work effectively as shown in Table 1.

Para leer la nota completa da clic aqui....