E 220-02 Standard Test Method for Calibration of Thermocouples By Comparison Techniques.E 207-00.Method of Thermal EMF Test of Single Thermo element Materials by Comparison with a Secondary Standard of Similar EMF-Temperature Properties.The exotic class includes several tungsten alloy thermocouples usually designated as Type W (something).ġ.8 to 7.9 oF or 0.4% of reading above 32 oF, whichever is greaterĠ.9 to 3.6 oF or 0.4% of reading above 32 oF, whichever is greaterĢ.5 oF or 0.25% of reading, whichever is greater The upper crust are types B, S, and R, platinum all to varying percentages. The home bodies are the Types E, J, K, N and T. the exotic class (standards and developmental devices).the rarified class (refractory metals) and,.the upper crust class (called rare metal or precious metal),.The home body class (called base metal),.There are four "classes" of thermocouples: The ASTM Standard E230 provides all the specifications for most of the common industrial grades, including letter designation, color codes (USA only), suggested use limits and the complete voltage versus temperature tables for cold junctions maintained at 32 oF and 0 oC.
Some of the thermocouple types have standardized with calibration tables, color codes and assigned letter-designations. Each calibration has a different temperature range and environment, although the maximum temperature varies with the diameter of the wire used in the thermocouple. The four most common calibrations are J, K, T and E. Thermocouples are available in different combinations of metals or calibrations. If the thermocouple is removed and placed in a calibration bath the output integrated over the length is not reproduced exactly since the temperature difference from one end of the wire to the other is the sum of all voltage differences along the wire from end to end. The calibration of a thermocouple should be carried out by comparing it to a nearby thermocouple.The relationship between the process temperature and the thermocouple signal (millivolt) is not linear.The materials of which thermocouple wires are made are not inert and the thermoelectric voltage developed along the length of the thermocouple wire may be influenced by corrosion etc. Thermocouples operation are relatively complex with potential sources of error.To avoid error the cold junction temperature is in general compensated in the electronic instruments by measuring the temperature at the terminal block using with a semiconductor, thermistor, or RTD. Temperature measurement with a thermocouple requires two temperatures be measured, the junction at the work end (the hot junction) and the junction where wires meet the instrumentation copper wires (cold junction).The thermocouple junction may be grounded and brought into direct contact with the material being measured.Capable of being used to directly measure temperatures up to 2600 oC.Comparing thermocouples to other types of sensors should be made in terms of the tolerance given in ASTM E 230. Thermocouples can be made from a variety of metals and cover a temperature range 200 oC to 2600 oC. The thermoelectric voltage resulting from the temperature difference from one end of the wire to the other is actually the sum of all the voltage differences along the wire from end to end. This voltage is measured and interpreted by a thermocouple thermometer. If the circuit were made with wire of the same material there was no current flow.Ī thermocouple consists of two dissimilar metals, joined together at one end, and produce a small unique voltage at a given temperature. Regardless of temperature, if both ends were at the same temperature there was no voltage difference. He noted that a voltage difference appeared when the wire was heated at one end. It was discovered by Thomas Seebeck's in 1822. One of the most common industrial thermometer is the thermocouple.
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