1.Type DesignationThe thermocouple type designation shall meet the actual operating temperature requirements of the temperature measurement site (see "Recommended Operating Temperature Ranges for Thermocouples").Compensation cables of the same type designation must be used to connect to the display/control instrument of the same type designation.
2.Measuring Junction TypeAssembled thermocouples are generally insulated type, and can also be grounded junction type (i.e., low-inertia thermocouples).Thermocouple elements can also be used directly without a protective tube.Mineral-insulated (MI) thermocouples include insulated and grounded types; the insulated type is generally used.Grounded junction thermocouples have a faster thermal response time but are susceptible to electromagnetic interference, requiring reliable earthing of the outer sheath.
3.Protective Tube Material
General applications: Stainless steel 1Cr18Ni9Ti.
Corrosive atmospheres: 1Cr18Ni12MoTi or SUS316L.
Strong oxidizing media such as sulfuric acid, nitric acid, hydrochloric acid: Tantalum tube or Hastelloy alloy.
Above 900°C: Heat-resistant steel Cr25Ti or SUS310S.
Above 1000°C: High-temperature alloy or high-alumina corundum ceramic tube.
Above 1300°C: Corundum ceramic tube.
Vacuum, inert or reducing atmospheres: Aluminum, tantalum or tungsten tubes.High-alumina and corundum protective tubes are not suitable for use in atmospheres containing moisture or reducing gases.
4.Diameter and Length of Protective TubeThe larger the diameter of the protective tube, the longer the thermal response time and the better the mechanical properties.For temperature measurement below 600°C without special mechanical requirements, mineral-insulated thermocouples with smaller diameters are generally selected.Thermocouples measure the temperature at the welding point of the hot junction.The insertion length shall be determined according to process requirements.The total length of the thermocouple is generally insertion length + 150 or 100, or + 250.To ensure accurate temperature measurement, the insertion length of assembled thermocouples into the measured medium is generally required to be more than 100 mm.
5.Mounting and Fixing Device
Non-sealed applications: No fixing device, movable flange, movable ferrule thread, or movable ferrule flange.
Sealed applications: Threaded or flange mounting.
Common specifications:
Assembled thermocouples: Commonly used thread M27×2; flanges φ95 (for φ16 protective tube) or φ105 (for φ20 protective tube).
MI thermocouples φ2/3/4: Thread M12×1.5, ferrule flange φ55.
MI thermocouples φ5/6/8: Thread M16×1.5, ferrule flange φ65.
Connecting tube of MI thermocouples: Commonly used thread M20×1.5.
6.Mounting and Wiring DeviceThe mounting and wiring device shall be selected for convenience and reliability.
Mineral-insulated thermocouples without junction box: Ceramic terminal board, connectors, protective cap with compensation cable, protective cap with metal hose and compensation cable.
Assembled thermocouples without junction box: Terminal board, simple wiring posts.Thermocouples with junction boxes are mainly used in dustproof, waterproof and explosion-proof environments,
including:waterproof junction box, anti-spray junction box, flameproof junction box, junction box with local display, and multi-point junction box.