Three wire thermocouple

Figure 6.21. Three-wire thermocouple assembly for high-pressure DTA apparatus 74).

The tube wall temperature was measured by a single three-wire thermocouple welded to the outside of the copper tube. A numerical transient analysis of the tube for a sine-squared power input showed that for a lOOOF temperature rise of the inner surface, the outer tube wall temperature would be within 20F of this value. It was concluded the temperature drop through the wall cotild be neglected. 

Experimentally, average burning rates have been measured using thin wire thermocouples imbedded in the sample at the mid-point and at the three-quarter... 

There are two different types of thermocouple gauges One has three wires and the other has four. Both have a dc meter (or voltmeter) that reads the voltage from the thermocouple. The three-wire unit uses ac to heat the filament wire, whereas the four-wire unit may use ac or dc. Although there are essentially no differences in performance between the two, they will likely require different controllers (or different settings) for use. 

Many researchers have used fine wire thermocouples (T/Cs), with and without coatings. Costa et al. [77] used a bare 300 pm diameter wire uncoated Pt/Ptl3%Rh thermocouple to measure gas temperatures inside an industrial glass-melting furnace. Mital et al. [78] used three different diameter (76,125, and 200 pm) fine wire Type R thermocouples to measure the gas temperature inside porous ceramic radiant burners by extrapolating the measurements to zero diameter. 

In most cases, three-wire RTDs are used in the industry, so these will be considered here. Mainly, bridge circuits using a modified Wheatstone bridge are used. In these circuits, measured output voltage is a function of the RTD resistance. Requirements for RTDs are similar to thermocouples. Use of a double-channel AC barrier with parameters discussed later will be a cost-effective solution. 

Two types of continuous flow solid oxide cell reactors are typically used in electrochemical promotion experiments. The single chamber reactor depicted in Fig. B.l is made of a quartz tube closed at one end. The open end of the tube is mounted on a stainless steel cap, which has provisions for the introduction of reactants and removal of products as well as for the insertion of a thermocouple and connecting wires to the electrodes of the cell. A solid electrolyte disk, with three porous electrodes deposited on it, is appropriately clamped inside the reactor. Au wires are normally used to connect the catalyst-working electrode as well as the two Au auxiliary electrodes with the external circuit. These wires are mechanically pressed onto the corresponding electrodes, using an appropriate ceramic holder. A thermocouple, inserted in a closed-end quartz tube is used to measure the temperature of the solid electrolyte pellet. 

Figure 6.9 shows an ordinary thermowell-and-thermocouple assembly. The thermocouple junction consists of two wires of different metals. When this junction of the wires is heated, a small electric current, proportional to the junction temperature, is produced. Different metal wires make up the three most common junctions J, H, and K. It is not uncommon for a thermocouple, regardless of the type of junction, to generate too low a temperature signal. 

Two combinatorial studies also utilized thermocouple arrays to measure temperature distribution. Yan et al.133 used an electrically segmented plate with three thermocouples to investigate the effects of a dynamic load on current and temperature distribution. The results are described further in Section 3.2. Maranzana et al.134 developed a transparent cell with 20 thermocouples and 20 electrically isolated segments of gold wire to monitor performance... 

The heat transfer through the same composition mastic by flame temperature of 2000 F. directed against it is illustrated by three test panels of steel 3/16 inch thick and 12 inches square. One panel was soated with 9/16 inch of mastic, and the second bad steel wire lath welded on a five points and then covered with 1/2 inch of mastic the third panel was left bare. The source of fire for the tests was a large gas burner with forced air feed. Temperatures of the heat source and heat transfer on the uninsulated side were made with the thermocouple. The results of the fire test were as... 

If you look at a thermocouple calibration table, you will see that it has a reference junction at 0°C. This reference point is used because of an interesting complication that arises when a thermocouple is hooked up to a voltmeter. To explain this phenomenon, first look at one specific type of thermocouple, a type T (cop-per-constantan design). Also, assume that the wires in the voltmeter are all copper. Once the thermocouple is hooked to the voltmeter, we end up having a total of three junctions (see Fig. 2.31) ... 

Equation 16.18 is particularly useful in deriving the net EMF of a complex thermoelectric circuit. In many reference works, three laws—the law of homogeneous materials, the law of intermediate materials, and the law of intermediate temperature—are used to show how the EMF at a measuring device is affected by various lead wires from a thermocouple junction [33], These laws can be derived from Eq. 16.17 but are generally more difficult to apply in a complex circuit. 

Problem Three thermocouples are used to sense temperature in a reactor. The three signals are wired into a safety PLC, and a trip will occur if only one of the sensors indicates a trip. The probability of failure in the safe mode (causing a spurious trip) for a one-year mission time is 0.005. What is the probability of a spurious (false) trip ... 

For measuring the conductivity and thermoelectric power, the molybdenum cell incorporates a 6-bore alumina tube leading from the cell into the cold closure structure via the capillary. Four bores contain molybdenum wires that serve as electrodes for a conventional four-terminal conductivity measurement with an external potentiometer. Two more wires consisting of platinum run from the center of the cell to the cold end of the alumina tube. These wires together with the sample form a mercury-platinum thermocouple the absolute thermoelectric power of mercury can be derived from the measured e.m.f. In the configuration illustrated in Fig. 7.2, the conductivity and thermoelectric power samples are located in the center of the density sample, and therefore the three quantities are measured on adjacent portions of the same bulk mercury sample. 

Figure 4 shows their location. Ten thermocouples for measuring the gas temperature were located along the centerline of the tank at four elevations. Wall temperatures were measured with nine couples at three elevations. No. 36 wire was used in the beads, and calculations indicate a response time of less than a few seconds. Multi-channel strip recorders provided a permanent record of the temperature during all runs. 

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