03.01.2015 Views

handbook of modern sensors

handbook of modern sensors

handbook of modern sensors

SHOW MORE
SHOW LESS

You also want an ePaper? Increase the reach of your titles

YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.

16.2 Thermoelectric Contact Sensors 487<br />

Fig. 16.18. Some thermocouple assemblies.<br />

to health hazard). In addition, thermocouples must be protected from atmospheres<br />

that are not compatible with the alloys. Protecting tubes serve the double purpose<br />

<strong>of</strong> guarding the thermocouple against mechanical damage and interposing a shield<br />

between the wires and the environment. The protecting tubes can be made <strong>of</strong> carbon<br />

steels (up to 540 ◦ C in oxidizing atmospheres), stainless steel (up to 870 ◦ C), ferric<br />

stainless steel (AISI 400 series), and high-nickel alloys (Nichrome, 6 Inconel, 7 etc.)<br />

(up to 1150 ◦ C in oxidizing atmospheres).<br />

Practically all base–metal thermocouple wires are annealed or given a “stabilizing<br />

heat treatment” by the manufacturer. Such treatment generally is considered<br />

sufficient, and seldom is it found advisable to further anneal the wire before testing<br />

or using. Although a new platinum and platinum–rhodium thermocouple wire as sold<br />

by some manufacturers is annealed already, it has become a regular practice in many<br />

laboratories to anneal all Type R, S, and B thermocouples, whether new or previously<br />

used, before attempting an accurate calibration. This is accomplished usually<br />

by heating the thermocouple electrically in air. The entire thermocouple is supported<br />

between two binding posts, which should be close together, so that the tension in the<br />

wires and stretching while hot are kept at a minimum. The temperature <strong>of</strong> the wire is<br />

conveniently determined with an optical pyrometer. Most <strong>of</strong> the mechanical strains<br />

are relieved during the first few minutes <strong>of</strong> heating at 1400–1500 ◦ C.<br />

Thin-film thermocouples are formed by bonding junctions <strong>of</strong> foil metals. They are<br />

available in a free-filament style with a removable carrier and in a matrix style with<br />

a sensor embedded in a thin laminated material. The foil having a thickness in the<br />

order <strong>of</strong> 5 µm (0.0002 in.) gives an extremely low mass and thermal capacity. Thin<br />

6 Trademark <strong>of</strong> the Driver-Harris Company.<br />

7 Trademark <strong>of</strong> the International Nickel Company.

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!