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MEASURMENT AND INSTRUMENTATION

TABLE 3

82

High-Temperature Sheath Materials

Sheath Material

Maximum Operating Temperature

Workability

Working Environment

Molybdenum∗

2205◦C (4000◦F )

Tantalum†

2482◦C (4500◦F )

Malleable

Inert, vacuum

1677◦C (3050◦F )

Malleable

Oxidizing, inert, vacuum

1149◦C (2100◦F )

Malleable

Oxidizing, inert, vacuum

Platinumrhodium alloy

Inconel 600

Brittle

Inert, vacuum, reducing

Approximate Melting Point 2610◦C (4730◦F )

3000◦C (5425◦F )

1875◦C (3400◦F )

1410◦C (2570◦F )

Remarks Relatively good hot strength; sensitive to◦ oxidation above 500 C (930◦F ); resists many liquid metals and most molten glasses Resists most acids and weak alkalies; very sensitive to oxidation above 300◦C ◦ (570 F ) No attack by SO2 at 1093◦C (2000◦F ); silica is detrimental; halogens attack at high temperatures Excellent resistance to oxidation at high temperature; do not use in presence of sulfur above 538◦C (1000◦F ); hydrogen tends to embrittle

∗ Refractory metals are extremely sensitive to any trace of oxygen above approximately 260◦C (500◦ F ). They must be used in vacuum or in very pure inert gases such as helium and argon. † Suitable for exposure to certain reducing atmospheres as well as inert gases and vacuum.

Thermocouple Wire Insulators The dissimilar wires of a thermocouple must be insulated. Traditionally, ceramics have been used (Fig. 8). Special Thermocouples In addition to the traditional industrial thermocouples just described, there also are surface probes and cement-on styles. Thermocouples can be drawn in metal-sheathed form to as small as 0.25-mm (0.01-inch) OD. In wire form, 0.013-mm (0.0005-inch) thermocouples can be made. Types K and E surface probes are commercially available. Type E usually is preferred because of its high accuracy in most low-temperature applications. Type K is used where high temperatures must be measured. Types J and T are not commonly used as probes. Cement-on style thermocouples have grown in popularity in recent years. Special fast-responding techniques include thin-foil couples with plastic laminates for cementing directly on equipment. The full sensor is embedded between two thin glass-reinforced, high-temperature polymer laminates that both support and electrically insulate the foil section as well as provide a flat surface for cementing. The polymer-glass laminate, in general, determines the maximum temperature of the construction, which is 260◦C (500◦F ) in continuous service (Fig. 9). Thermocouple Circuit Flexibility Normally one envisions the use of thermocouples one at a time for single temperature measurements. As shown in Fig. 10, thermocouples may be used in parallel, in series, and in switching and differential circuits. FOR MORE DETAILS VISIT US ON WWW.IMTSINSTITUTE.COM OR CALL ON +9199994621

Mechanical BE (Measurement and Instrumentation)  

Mechanical BE (Measurement and Instrumentation)

Mechanical BE (Measurement and Instrumentation)  

Mechanical BE (Measurement and Instrumentation)

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