Primary thermocouples

LThe compensated flow transmitter determines the process flow it converts this quantity to a signal that is proportional to the process flow and sends it to the flow controller. The transmitter could be a pneumatic device using a venturi primary element, with compensation for pressure by a pressure element and compensation for tern perature by a thermocouple. The output would be a pneumatic sic nal that is proportional to weight flow. 

Seebeck used antimony and copper wires and found the current to be affected by the measuring instrument (ammeter). But, he also found that the voltage generated (EMF) was directly proportional to the difference in temperature of the two junctions. Peltier, in 1834, then demonstrated that if a current was induced in the circuit of 7.1.3., it generated heat at the junctions. In other words, the SEEBECK EFFECT was found to be reversible. Further work led to the development of the thermocouple, which today remains the primary method for measurement of temperature. Nowadays, we know that the SEEBECK EFFECT arises because of a difference in the electronic band structure of the two metals at the junction. This is illustrated as follows ... 

Thermocouples were placed inside the fuel bed, to be able to follow the temperature history, which contained a great deal of information about the thermochemical processes occurring in the bed. The sample ports were positioned at 45, 145 and 245 mm above the grate. These ports could also be used for gas sample probes. The primary air could be preheated up to 200°C and the air flow rate could be varied between 150-1500 1/min. 

A chromel-alumel thermocouple is used to monitor the temperature of the ceramic boat and also acts as a sensor for the furnace temperature controller. A constant flow of clean, dry N2 gas is maintained through the quartz tube. The metal vapor is carried out of the furnace with N2 and condensed to form a polydisperse aerosol. The primary aerosol is diluted by mixing with a filtered dry air stream. The diluted aerosol is then routed to the chamber as required and the excess aerosol is vented out through a glass fiber filter. 

Although two reactors are shown in Figure 1, they were not used simultaneously. The reactor shown in the center was the fixed bed reactor which is of primary interest in this contribution. It consisted of a 12.7 mm diameter X 250 mm long steel tube packed with 40/50 mesh catalyst (0.3 mm average particle diameter). The reactor was heated by a nichrome wire coil and was well insulated. The coil spacing was adjusted and was packed in insulation with the intent of making the reactor crudely adiabatic. A variac controlled heater on the reactor inlet and a thermocouple sensor kept the feed to the reactor at the nominal reaction (or feed inlet) temperature of 400°C. The tube of the fixed-bed, reactor was fitted with 12 thermocouples to record the axial temperature profile in the bed (Figure 1). 

The temperature measurements were the most difficult to make. The thermocouple used, mentioned before, is a fragile, brittle wire and was subject to frequent breakage as a result of the turbulent bombardment it received in this hostile atmosphere. Sufficient temperatures were determined to verify the combustion gases temperature already known. Most significant, were the measurements of the temperature gradients at the initiation of the carbon black reaction and the disappearance of gradients as the reaction reached its first primary completion. The points within the reactor where these maximum temperatures and their associated gradients occur... 

Use Alloy with platinum for ammonia fuel-cell catalyst, electric contacts and thermocouples, commercial electrodes and resistance wires, laboratory ware, extrusion dies for glass fibers, jewelry. Primary standards of weight and length. 

The two primary temperature-sensing devices used in the CPI are thermocouples (TCs) and resistance thermometer detectors (RTDs). Thermocouples are less expensive and more rugged than RTDs but are an order of magnitude less precise than RTDs. Typically, RTDs should be used for important temperature control points, such as on reactors and distillation columns. 

Figure 11 shows near-surface, gas phase temperature measurements for steady burning of uncatalyzed, medium energy NC/NG propellant (designated propellant-N following Russian notation) at 5 MPa (50 atm) from thermocouples [23]. At this pressure there is a distinct two-stage gas flame structure but only the primary stage closest to the surface, where temperature increases from 7j = 668 K to around... 

In order to perform NMR studies on compressed supercritical steam, which exhibits a number of unusual properties, de Fries and Jonas developed an NMR probe working in the temperature range 25-700 °C and at pressures up to 200 MPa. The design of this probe, consisting of two high-pressure bombs, is given in Fig. 4.29. Both bombs have a 76 mm outer diameter and a 38 mm inner diameter and are made of IMI-680 titanium alloy (Imperial Metal Industries). The primary bomb at the top contains a furnace, the RF coil and two thermocouples. The sample extends from the ceramic tube in... 

In principle, any device that has one or more physical properties uniquely related to temperature in a reproducible way can be used as a thermometer. Such a device is usually classified as either a primary or secondary thermometer. If the relation between the temperature and the measured physical quantity is described by an exact physical law, the thermometer is referred to as a primary thermometer otherwise, it is called a secondary thermometer. Examples of primary thermometers include special low-pressure gas thermometers that behave according to the ideal gas law and some radiation-sensitive thermometers that are based upon the Planck radiation law. Resistance thermometers, thermocouples, and liquid-in-glass thermometers all belong to the category of secondary thermometers. Ideally, a primary thermometer is capable of measuring the thermodynamic temperature directly, whereas a secondary thermometer requires a calibration prior to use. Furthermore, even with an exact calibration at fixed points, temperatures measured by a secondary thermometer still do not quite match the thermodynamic temperature these readings are calculated from interpolation formulae, so there are differences between these readings and the true thermodynamic temperatures. Of course, the better the thermometer and its calibration, the smaller the deviation would be. 

Morrow (52) has described a DTA-light photometer polarizing system for hot-stage microscopy. Thermocouple wires. 0.003 in. in diameter, were used to detect the 7 — Tr) temperature difference fo r simultaneous DT A measurements. Sample capsules were fabricated by bending small, identical stripes of AI foil over both sample and reference thermojunctions. A small hole in the foils permitted the viewing of the sample by transmitted light microscopy. The primary use of the apparatus was to study the thermal properties of thermally sensitive polymers (84). 

Thermocouples are the primary device used to make temperature measurements in industrial combustion applications. A variety of other techniques have been used to measure both gas and surface temperatures that are briefly discussed later in this chapter. Due to the importance of thermocouples, they are considered in some detail in this section. 

Reactor Temperature. Analysis of the data showed that the primary variable governing the composition of the reactor products was the temperature. The temperature as indicated by the thermocouple measurements increased with the amount of combustion gas fed to the reactor per pound of coal. Although the reactor tube was electrically heated. 

Having estimated the optimum primary drying parameters, a pilot study should be performed. It is not good practice to carry out such a study either in an incompletely filled drier or by filling up shelves partly with empty vials. The reliability of pressure and temperature measurements should be checked at this stage. It has sometimes been found that the (apparent) product temperature exceeds the shelf temperature. Since this is an obvious physical impossibility, such an anomaly is due to faulty calibration of the thermocouple probe. 

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