What is the difference between thermowell and thermocouple

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We offer a local website for country. Change now Stay here. For country , we recommend our corporate website. Websites Locations. Why WIKA? Benefits Job Opportunities. Display Dial Thermometers Digital Indicators. Switch Float Switches Optoelectronic Switches. Some of the most common are: Threaded Weld-In Socket Weld O-Ring Flange Threaded connections are made of materials that can be welded or brazed and provide additional strength.

In processes where contaminants from threads must be avoided such as in the food and pharmaceutical industries, weld-in connections are commonly used. O-ring connections use an O-ring to seal inside a sleeve welded to a tank. In processes where several types of measurement devices are used, choosing a standard bore size allows greater flexibility. It enables a single thermowell to be used for a thermocouple, RTD, bimetal thermometer or test thermometer. These standard bore sizes accommodate the most commonly used temperature sensing devices and include the following: 0.

Selecting the right material is crucial to the longevity of a thermowell. The type of chemical, temperature and flow rate the thermowell will be exposed to should be considered when specifying the material. The corrosive effects of chemicals are increased at higher concentrations and temperatures.

In addition, particles suspended in fluid can cause erosion. The most commonly used material for thermowells is stainless steel. A stainless steel thermowell is cost effective and highly resistant to heat and corrosion. The addition of molybdenum improves resistance to corrosion.

Junction - The point at which the two dissimilar metals of a thermocouple are joined. Lag - 1 The delay of time between the output of the sensor and the instrument's response to the signal. This extension is used to get through insulation or get away from high temperatures. These are tolerances within which various calibrations of thermocouples must conform.

Linearity - The deviation of a sensor's signal from a straight line. Magnesium Oxide MgO - An insulation of high purity ceramic that offers dielectric strength and high resistivity. Measuring Junction - The thermocouple junction at which the two dissimilar metals are joined to measure temperature. Melting Point - The temperature at which a substance changes from a solid to a liquid. Mineral-insulated Thermocouple - Thermocouple style which has an outer jacket of metal tubing with tightly compacted magnesium oxide insulation separating the internal pair of thermocouple wires.

Negative Temperature Coefficient - The negative slope of a line that indicates the change in resistance with the increase in temperature. It is used as an outer jacket with some sheath material. Nicrosil - The nickel chrome alloy used for the positive leg of a type N thermocouple. Nisil - The nickel silicone alloy used for the negative leg of a type N thermocouple. Noise - Electrical interference. Passivation - The changing of a chemically active surface of a metal to a much less active state.

Our use is restricted to the use of Nitric Acid and Water to treat Series stainless steel at room temperature. This procedure should not be used with other metals. Platinel - High temperature platinum thermocouple alloy whose emf output is similar to a Type K thermocouple.

Platinel is a trademark of Englehard Industries. Platinum - The pure form noble metal which is used for the negative leg of Type R and Type S thermocouple. Polarity - A characteristic of electricity in which there are two oppositely charged poles, one negative and one positive. Positive Temperature Coefficient - The positive slope of a line that indicates the change in resistance which the increase in temperature. Power Supply - A separate unit that supplies power to a circuit or system.

Probe - A general term used to describe thermocouples and RTD 's. Protection Tube - A cylindrical tube made of metal or ceramic that is used to strengthen and protect the temperature probes which are inserted inside. Range - The area between two established temperature points.

Expressed by stating the lower variable first and the higher variable second. Reference Junction - The cold junction of a thermocouple which is either at a constant or at a known temperature.

Repeatability - The ability of a temperature sensor to duplicate the same output under repeated identical conditions. Resistance - The opposition to the flow of electric current measured in ohms. Resistance Temperature Detector RTD - A sensor which measures temperature by the principle that the resistance of a metal changes with temperature. The flange bolts to a mating flange mounted on the process. Such a technique is more appropriate for large pipe diameters and for high pressure applications.

Flanges are usually of 2 to 3 inches in diameter. The diagram below shows a flanged parallel well:. Welded connections can be used when the process is not corrosive and routine removal is not required. High integrity is achieved and this technique is suitable for high temperature and high pressure applications such as steam lines. Removal of a welded-in well usually involves considerable effort and time.

The diagram below shows a weld-in Thermowell:. Lagging Extensions Lagging extensions are provided on thermowells or even directly on temperature probe assemblies for use on lagged processes. A lagging extension distances the terminal head of the sensor from the immersion part of the assembly to allow for the depth of lagging thermal insulation.

This technique is useful in allowing the head, perhaps with an integral transmitter, to reside in a cooler ambient temperature region rather than adjacent to the much hotter process. Lagging extensions take various forms depending on overall probe or well construction, fitting method and type of termination.

Installing temperature sensor assemblies into thermowells or directly into the process requires the use of some kind of brass or stainless steel fitting. Fittings include various threaded unions, bayonet caps and adapters and flanges.