EL ORDEN JURÍDICO EN UTOPÍA Y LA IDEA DE JUSTICIA

Mineral insulated thermocouple cables have an outer sheath made of metal and for any one design, 2...6 internal wires made of a thermal material. The insulation consists of highly compressed metal oxide powder, preferably Magnesium oxide MgO, or Alumi-num oxide AI₂O₃.

They are used where particularly high mechanical, electrical and chemical stability is required. Because they are readily bendable, these cables are preferred where prob-lematic space requirements exist and a flexible installation is desired, e.g. in machine building, laboratories and experimental test facilities.

The minimum bending radius is approx. 3 x outside diameter of the cable. As a result of the development of economical manufacturing processes, sheathed cables are find-ing more and more applicability as an essential part for the production of standard ther-mocouples, especially in the industrial measurement and control sector as well as for automotive sensors.

Due to the metallic outer sheath, these thermocouples are essentially unaffected by field induced electromagnetic interference (EMI), provided that they are grounded cor-rectly.

Insulation and Insulation Resistance

The achievable insulation resistance is a function of the purity of ceramic insulation material. Aside from the standard material MgO with a purity of > 97 %, also MgO with a purity of 99.4 % and Al₂O₃ can be used. Since these oxides are highly hygroscopic, care must be exercised when handling the cable. After removing the sealing or cutting the cable, it has to be dried properly. Afterward the open ends have to be immediately sealed against moisture entry. Storing for any length of time with open ends must be avoided.

Since the insulation material of the mineral insulated thermocouple cables and sheathed thermocouples has a low rest conductivity, the insulation resistance de-creases as the length of the cable or thermocouple inde-creases. Therefore a length related resistance with the units Ω x m or MΩ x m is specified.

For lengths less than 1 m the insulation resistance is specified independent of the length. Based on EN 61515 the insulation resistance must be tested with a voltage of 75 ±25 V DC for outside diameters ≤1.5 mm and with 500 ±50 V DC for outside diam-eters >1.5 mm.

Tbl. 3-3: Minimum insulation resistance of mineral insulated thermocouple cables according to EN 61515

Tbl. 3-4: Minimum insulation resistance of sheathed thermocouples with insulated measurement spot locations according to EN 61515

Insertion depth

temperature 1 (3) 20 ±15 (68 ±27) 1000

Increased temperature

temperature ≥ 1 (3) Total length

20 ±15

(68 ±27) 1000 –

Ambient

temperature < 1 (3) Total length

20 ±15

It should be noted when using sheathed thermocouples that the insulation resistance of the insulating ceramic decreases appreciably with increasing temperatures. When longer lengths of the sheath material are exposed to high temperatures, measurement errors could result due to shunt currents or cross talk between adjacent measurement installations along the length of the cable.

Sheath Materials

Basically, mineral insulated thermocouple cables could be made of materials suffi-ciently ductile, preferred however, are those made entirely of austenitic stainless steel.

Nickel alloys are also useful for special applications. Though not all sheath material/thermocouple combinations are possible, e. g., for high heat resistant sheath materials the required intermediate annealing temperatures required for processing may, in part, be appreciably above the allowable temperature limits for the thermo-couple materials. The most common sheath materials are:

1.4541 (corresponds to AISI 321)

Max. operating temperature: 800 °C (1472 °F).

Application areas: Nuclear plants and reactor construction, chemical system engineer-ing, heat treating furnaces, heat exchangers, paper and textile industries, petrochemi-cal and petroleum industries, lubricant and soap industries.

Material properties: Good intercrystalline corrosion resistance, also after welding.

Good resistance against crude oil products, steam and combustion gases. Good oxi-dation resistance. Good welding properties for all standard welding processes, no sub-sequent heat treatment required after welding, good ductility.

1.4571 (corresponds to AISI 316 TI)

Max. operating temperature: 800 °C (1472 °F)

Application areas: Nuclear plants and reactor construction, chemical system engineer-ing, furnace manufacture, chemical and pharmaceutical industries.

Material properties: Increased corrosion resistance to specific acids due to the addition of Molybdenum. Resistant against pitting, salt water and aggressive industrial influenc-es. Good welding properties for all standard welding processes, no subsequent heat treatment required after welding, good ductility.

1.4749 (corresponds to AISI 446)

Max. operating temperature: 1150 °C (2102 °F)

Application areas: Petrochemical industries, metallurgy, energy technologies and for recuperators, heat treatment ovens, systems for controlling fluidized bed coatings, waste incineration plants.

Material properties: Extremely good resistance against reducing atmospheres contain-ing Sulphur. Very good resistance against oxidation and air. Good resistance against corrosion by combustion products, Copper, Lead- and Tin melting. Good welding prop-erties for applications using arc or WlG welding. Preheating to 200...400 °C (392...

752 °F) is recommended. Subsequent heat treatment is not required.

1.4841 (corresponds to AISI 314)

Max. operating temperature: 1150 °C (2102 °F)

Application areas: Steam boilers and blast furnaces, cement and tile ovens, glass man-ufacture, petroleum and petrochemical industries, furnace manman-ufacture, power plants.

Material properties: Exceptional corrosion resistance, even at high temperatures. Suit-able for Carbon and Sulphur containing atmospheres. Air oxidation resistance to 1000 °C (1832 °F) (batch operation) or 1150 °C (2102 °F) (continuous operation). Very good for higher alternating temperature changes. Long term continuous operation is not recommended for temperature ranges from 425...850 °C (797...1562 °F). Good welding properties for applications using arc welding. Subsequent heat treatment is not required. Good ductility in the as received condition. After longer use some slight brit-tleness can be expected.

1.4845 (corresponds to AISI 310 S)

Max. operating temperature: 1100 °C (2012 °F)

Application areas: Steam boilers and blast furnaces, cement and tile ovens, glass man-ufacture, petroleum and petrochemical industries, furnace manman-ufacture, power plants.

Material properties: Good resistance against oxidation and sulfidization. Due to the high Chromium content resistant to oxidizing aqueous solutions as well as good resis-tance against Chlorine induced stress crack corrosion. Good resisresis-tance in Cyanide melters and neutral fused salt at high temperatures. Not sensitive to “Green rot“. Read-ily weldable. It is recommended that heat be added during welding. When intercrystal-line corrosion may occur, solution heat treat after welding.

1.4876 (corresponds to Incolloy 800®)

Max. operating temperature: 1100 °C (2012 °F) in air

Application areas: power plants, petroleum and petrochemical industries, furnace man-ufacture.

Material properties: Due to the admix of Titanium and Aluminum the material has especially good heat resistance. Suitable for applications, where highest loading is required. Resistant to scale. Exceptionally stable where carburization and nitration can be expected. Good welding properties for applications using arc or TlG welding. Sub-sequent heat treatment is not required.

2.4816 (corresponds to Inconel 600^®) Max. operating temperature: 1100 °C (2012 °F)

Application areas: Pressurized water reactors, nuclear power plants, furnace manufac-ture, plastic industry, heat tempering, paper and food industries, steam boilers, airplane engines.

Material properties: Good general corrosion resistance, resistant to stress crack corro-sion. Exceptional oxidation resistance. Not recommended for CO₂ and Sulphur con-taining gases above 550 °C (1022 °F) and Sodium above 750 °C (1382 °F). Stable in air to 1100 °C (2012 °F). Good welding properties for all welding techniques. The ma-terial should be annealed before welding. Subsequent heat treatment is not required.

Exceptional ductility even after long term use.

Platinum 10% Rhodium

Max. operating temperature: 1300 °C (2372 °F)

Application areas: Glass, electrochemical and catalytic technology, chemical industry, laboratory applications, melting, annealing and firing ovens.

Material properties: High temperature resistance to 1300 °C (2372 °F) under oxidizing conditions. In the absence of Oxygen, Sulphur, Silicon, high heat resistance to 1200 °C (2192 °F). Especially resistant to halogens, acetic acid, NaOCI solutions etc. Embrittle-ment due to absorption of Silicon from sheath ceramics. Sulphur eutectic formation possible above 1000 °C (1832 °F). Phosphorous sensitivity.