Thermocouple extension wires

Temperature Measurement shift. Measurement not representative of process. Indicator reading varies second to second. Ambient temperature change. Fast changing process temperature. Electrical power wires near thermocouple extension wires. Increase immersion length. Insulate surface. Use quick response or low thermal time constant device. Use shielded, twisted pair thermocouple extension wire, and/or install in conduit. 

A 1500-W tubular heater is wrapped around the reactor. The autoclave is placed inside a copper pipe, with Fiberfrax bulk fiber insulation packed between the copper pipe and the reactor. Four 1000-W quarter-circle ceramic heaters are placed around the copper pipe. The space between the copper pipe and the ceramic heaters is filled with aluminum balls for efficient and uniform heat transfer from the heaters to the copper pipe. The bottom of the copper vessel is closed with a copper plate. Beneath the copper plate, there are two heaters (a small one of 500 W and a larger one of 1000 W), which are tightly clamped to the bottom copper plate. The top of the copper pipe is similarly covered with another copper plate, and two slots are provided for the feed and product gas lines, coolant inlet and outlet lines, thermocouple extension wires, and the lead wires of the tubular heater. Two ring heaters (660 and 1000 W) are placed on the top copper plate. One thermocouple and a tape heater are placed on the shaft extension to minimize the heat loss through the shaft. No insulation is placed around the ceramic heaters to achieve better control of... 

In industrial applications it is not uncommon that the thermocouple must be coupled to the readout instrument or controller by a long length of wire, perhaps hundreds of feet. It is obvious from the differential nature of the thermocouple that, to avoid unwanted junctions, extension wire be of the same type, eg, for a J thermocouple the extension must be type J. Where the thermocouple is of a noble or exotic material, the cost of identical lead wire may be prohibitive manufacturers of extension wire may suggest compromises which are less cosdy. Junctions between the thermocouple leads and the extension wire should be made in an isothermal environment. The wire and junctions must have the same electrical integrity as the thermocouple junction. Because the emf is low, enclosure in a shield or grounded conduit should be considered. 

In order to protect the thermocouple against chemical or mechanical damage, it is normally enclosed in a sheath of mineral packing or within a thermowell (Fig. 6.24). Any material which contains the junction should be a good conductor of heat on the one hand, but an electrical insulator on the other. A potentiometric converter is frequently employed to convert the thermocouple signal to the standard 4-20 mA current range prior to further processing and control room presentation. The extension wires which connect the thermocouple element to the control room should have similar thermoelectric properties to those of the thermocouple junction wires. 

Extension wires having same thermoelectric properties as thermocouple elements... 

When using a measuring junction at a remote site relative to the reference junction, it is important to use the proper extension wire between the junctions. Usually, a wire with the same composition as the thermocouple wire itself, but not made to such a high specification, is used. 

Thermocouple Components and Fabrication. A thermocouple measurement assembly includes a sensing element assembly, extension wires (when used), reference junction, connecting wires, an EMF-measuring device (possibly with signal-processing equipment), and other hardware needed for applications in adverse environments such as protection tubes, connectors, adapters, and so on. Each of the above components will be discussed in the following paragraphs. 

When using an RTD or a TC the time constant considerations are similar, but the actual response times of the devices will vary. The FTS and RTD will have response times of nearly the same magnitude, whereas the TC is somewhat faster. For a thermocouple, tb varies with the device s constmction and length of the extension wires. Hence, a TC made of small wire with short extension wires will give a fast response. A typical TC response is around 0.5 s. 

The installation of extensive thermocouple equipment requires the services of qualified instrument technicians, and special attention should be given to extension wires, reference junctions, switches, and terminal assemblies. 

For the connection of a thermocouple to the measurement instrument, the best way is to use leads made of the same materials (extension leads) these avoid lead junction errors. For economic reasons, however, cheaper alloys having similar e.m.f. output, at least over a limited temperature range close to room temperature, are often used. These compensating leads are often supplied by the same producers of the thermocouple wires. 

Platinum metal and its alloys have numerous applications. As a precious metal it is used extensively in jewelry. Other important applications include construction of laboratory crucibles and high temperature electric furnaces in instruments as thermocouple elements as wire for electrical contacts as electrodes in dentistry in cigarette lighters and for coating missile and jet engine parts. 

Vallan et al. (2005b) proposed and patented a weighing device that works inside the vacuum chamber it is composed of a motorized balance which is able to rise and weigh several vials (up to 15 small vials in the first prototype, but extensible even to a much larger number and adaptable to diEFerent sizes of vials with a proper loading cell), and a miniaturized radio-controlled thermometer, connected to the balance tray, that can measure the temperature of these vials without altering the mass measurement because of the force transmitted by the thermocouple wires, as is sketched in Fig. 4.5 (Vallan, 2007). 

When compared with thermocouples, RTDs have higher accuracy, better linearity, long-term stability, do not require cold-junction compensation or extension lead wires and are less susceptible to noise. However, they have a lower maximum temperature limit and are slower in response time in applications without a thermal well (a protective well filled with conductive material in which the sensor is placed). 

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