Introduction to Thermocouples and Thermocouple Assemblies What is a thermocouple? relatively rugged, they are very often thermocouple junction is detached from A thermocouple is a sensor for used in industry. The following criteria the probe wall. Response time is measuring temperature. It consists of are used in selecting a thermocouple: slower than the grounded style, but the two dissimilar metals, joined together at • Temperature range ungrounded offers electrical isolation of one end, which produce a small unique • Chemical resistance of the 1 GΩ at 500 Vdc for diameters voltage at a given temperature. This thermocouple or sheath material ≥ 0.15 mm and 500 MΩ at 50 Vdc for voltage is measured and interpreted by • Abrasion and vibration resistance < 0.15 mm diameters. The a thermocouple thermometer. • Installation requirements (may thermocouple in the exposed junction need to be compatible with existing style protrudes out of the tip of the What are the different equipment; existing holes may sheath and is exposed to the thermocouple types? determine probe diameter). surrounding environment. This type Thermocouples are available in different offers the best response time, but is combinations of metals or ‘calibrations.’ How do I know which junction type limited in use to dry, noncorrosive and The four most common calibrations are to choose? (also see diagrams) nonpressurized applications. J, K, T and E. There are high temperature Sheathed thermocouple probes are calibrations R, S, C and GB. Each available with one of three junction What is ‘response time’? calibration has a different temperature types: grounded, ungrounded or A time constant has been defined range and environment, although the exposed. At the tip of a grounded as ‘the time required by a sensor to maximum temperature varies with the junction probe, the thermocouple wires reach 63.2% of a step change in dia. of the wire used in the thermocouple. are physically attached to the inside of temperature under a specified set of the probe wall. This results in good conditions. Five time constants are How do I choose a thermocouple type? heat transfer from the outside, through required for the sensor to approach Because thermocouples measure in the probe wall to the thermocouple 100% of the step change value.’ wide temperature ranges and can be junction. In an ungrounded probe, the Exposed junction thermocouples are the fastest responding. Also, the Operating Atmosphere – Typical Sheath Materials smaller the probe sheath diameter, Application Atmosphere the faster the response, but the Maximum maximum temperature may be lower. Material Temperature Oxidizing Hydrogen Vacuum Inert Be aware, however, that sometimes 304, 310, 316, 900°C Very Very Very the probe sheath cannot withstand and 321SS (1650°F) GoodGood Good Good the full temperature range of the 1150°C Very Very Very thermocouple type. Inconel 600 (2100°F) GoodGood Good Good Grounded Super 1335°C Very Very Junction, ® Excellent Good OMEGACLAD XL (2440°F) Good Good OMEGACLAD® Probes Platinum- 1650°C Very Poor Poor Rhodium Alloy (3000°F) Good Poor A grounded junction is recommended for the measurement of static or flowing 2200°C Not Fair Good Fair corrosive gas and liquid temperatures Molybdenum (4000°F) Rec. and for high pressure applications. The junction of a grounded thermocouple is 2300°C Not NotGood Not Tantalum (4200°F) Rec. Rec. Rec. welded to the protective sheath, giving faster response than the ungrounded OMEGACLAD® Bend radius should be not less than junction type. twice the diameter of the sheath. Ungrounded Specifications Delivery: Off-the-Shelf, other sheaths Junction, Diameters: Standard diameters: available; call for price and delivery. OMEGACLAD® 0.25 mm (0.010"), 0.5 mm (0.020"), Probes 0.75 mm (0.032"), 1 mm (0.040"), Dual Elements: Thermocouples with a 1 1 3 An ungrounded junction is 1.5 mm ( ⁄16"), 3 mm ( ⁄8"), 4.5 mm ( ⁄16"), sheath diameter of 1 mm (0.040") thru 1 1 recommended for measurements and 6 mm ( ⁄4") with two wires 8 mm 6 mm ( ⁄4") are available in dual element. Accuracy: The wires used in in corrosive environments where it is (0.313") and 9.5 mm (0.375"). desirable to have the thermocouple ® OMEGA® thermocouples are selected Length: Standard OMEGA electronically isolated from and thermocouples have 300 mm and matched to meet ANSI Limits of Error. Special limits of error shielded by the sheath. The welded (12 inch) immersion lengths. wire thermocouple is physically Other lengths available. thermocouples can be made from 0.25 mm (.010") O.D. to 9.5 mm (.375") insulated from the thermocouple Sheaths: 304 stainless steel and sheath by MgO powder (soft). Inconel are standard. Other sheath OMEGACLAD®. Thermocouple wire. materials available; call for price and ANSI Polarity: In the thermocouple Exposed availability. industry, standard practice is to color Junction, Insulation: High Purity Magnesium the negative lead red. Other standards OMEGACLAD® Oxide is standard. Minimum insulation that OMEGA® uses are: the negative Probes resistance wire to wire or wire to sheath lead of bare wire thermocouple is An exposed junction is recommended 1 is 1 GΩ at 500 Vdc in diameters above approximately 6 mm ( ⁄4") shorter than for the measurement of static or flowing 1 1.5 mm ( ⁄16") the positive lead, and the large pin on non-corrosive gas temperatures where Calibration: Iron-Constantan (J), a thermocouple connector is always the fast response time is required. The CHROMEGA®-ALOMEGA® (K), negative conductor. junction extends beyond the protective Copper-Constantan (T), and Extension Wire: Thermocouple alloy metallic sheath to give accurate fast CHROMEGA®-Constantan (E) wire must always be used to connect response. The sheath insulation is are standard calibrations. a thermocouple sensor to the sealed where the junction extends to Bending: Easily bent and formed. instrumentation to assure accurate prevent penetration of moisture or gas measurements. which could cause errors. A-13