| | Thermocouples with magnesium oxide insulation are recommended where the thermocouple is immersed in liquids, high moisture, corrosive gases, or high pressures. The thermocouple can be formed to reach otherwise inaccessible areas. The magnesium oxide has a high dielectric strength, responds quickly to temperature changes, and is very durable. T/C MgO insulated thermocouple wire is manufactured from premium quality wire encased in pure magnesium oxide, and processed into a chemically clean outer metal sheath. The wires are individually selected and matched, and are of uniform cross section with smooth surfaces. Finished stock is warranted to meet ANSI standard limits of error set forth in MC96.1. The unique preparation of MgO insulated thermocouple wire produces a uniform thickness of insulation with high density. The result is a product that is mechanically strong and resistant to penetration of corrosive gases and moisture. The diameters of 0.040” and 1/16” are useful for applications requiring fast response. Junction Construction: Ungrounded （insulated）: Thermocouple insulated from sheath with MgO. Stray EMF is prevented from affecting the reading. Response from rapid or frequent temperature cycling is slower than for grounded style. | | Exposed: Thermocouple junction is not protected by welded cap. Used for quick response, but is susceptible to corrosive failure. Time Constants: The time constant is the amount of time required for a thermocouple to indicated 63.2% of step change in temperature of a surrounding media. Some of the factors influencing the measured time constant are sheath wall thickness, degree of insulation compaction, and distance of junction from the welded cap on an ungrounded thermocouple. In addition, the velocity of a gas past the thermocouple probe greatly influences the time constant measurement. In general, time constants for measurement of gas can be estimated to be ten times as long as those for measurement of liquid. The time constant also varies inversely proportional to the square root of the velocity of the media. In general, time constants for measurement of gas can be estimated to be ten times as long as those for measurement of liquid. The time constant also varies inversely proportional to the square root of the velocity of the media. Approximate time constants for different sheath diameters in water are shown below for a step change from 0 to 100°C: |
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