Flow measurements thermocouples

Heat transfer models of the autoclave process are the most accurate and well understood of all the process models. Much of this understanding is because the models are so easily verified through thermocouple measurements. Thermocouples are the most common part-sensing technique used in production. The challenging aspects are the incorporation of the affects of resin flow, resin kinetics, and autoclave position on heat transfer properties. The importance of incorporating resin kinetic models is to properly predict conditions that may lead to exotherms, especially for thick laminates [17]. 

A good experimental method has yet to be devised to measure the temperature of highly viscous fluids flowing at high flow rates. Thermocouple measurements (16-18) have not been successful because they disrupt the flow field and become heated by the viscous fluid flowing past their surface. 

A variety of optical techniques have been used to measure gas temperatures in combustion applications, particularly in flames. There are potentially some important advantages of optical techniques compared to contact techniques such as suction pyrometers (see Figure 5.7). Optical measurement techniques do not disturb the flow, where thermocouples may have a significant impact on the fluid dynamics. Optical techniques can potentially measure higher temperatures as there are not the materials issues compared to thermocouples. For some optical techniques, temperature profiles can be measured at one point in time without the need to make multiple individual measurements over some length of time. Optical techniques often have a much faster response time compared to contact methods. This is particularly important in turbulent and transient flows. 

Other Instruments, In addition to the foregoing reactor instruments, all of which measure radiation, there are various temperature and water flow measurements to be mentioned. Each of. the 37 sampling tubes is actually part of a triple-function assembly sampling tube, pitot tube for flow measurement, and thermocouple for water temperature measurement. Knowledge of distribution of water flow velocity and temperature is of obvious value. There... 

Confirm accuracy of all flow measurement, level indicators, thermocouples, pressure gauges, weighing devices, etc. 

Flow measurement using thermoelectric devices and sensors implies the use of heat transfer and temperature measurements in microchatmels to determine the near-wall velocity. With appropriate calibration procedures, the mean fluid flow velocity or mass flow rate can be determined by measurements of the local wall temperature. Thermoelectric temperatrue probes and sensors, also known as thermocouples, rely on the Seebeck effect, where a temperatrue difference between two different metal contacts induces a voltage drop which can be electrically mea-strred. An electrical resistance heater introduces a heat flux into the fluid flow. The temperature is measured directly either at the heater, in its vicinity, or at the wall downstream of the heater. Often, the upstream mean temperature of the fluid flow is also measured to provide a comparison. Thermoelectric flow rate measurement is a very common measuring technique and for laminar flow one of the most accurate, reliable, and cost effective. 

The more generalized parameters of the thermal conductance, G, and the temperature conductivity, C, are a function of the sensor geometry and the heat and temperature diffusiv-ity. The device parameters and characteristics are determined by the properties such as the thermal conductivity or the Seebeck coefficient of the thermoelectric materials as well as the geometric setup and fabrication method. The main sensor signal output is determined by the number of thermocouples n, the Seebeck coefficient, and the measured or applied temperature differences. With a higher temperature difference, the signal increases with the heat loss. Hence, the sensitivity of the sensor depends mainly on the properties of the thermocouple, as well as the thermal conductivity of the fluid and the substrate. For gas flow measurement and high temperature differences, the emissivities... 

Sample connections, together with pressure, temperature and flow measurement points, are located at the inlet and outlet of the reformer and membrane modules to measure the performance of the RMM. A multipoint thermocouple is installed inside the first reformer tube in order to monitor the axial temperature profile along the heated catalyst length while two glass peepholes allow the reformer tube metal temperature to be measured by an infrared pyrometer. The control room is located in a safety area with a bird s eye view of the plant area. 

The experimental apparatus employed to study the catalytic performance of the samples consists of a flow measuring and control system, the reactor and an on line analytical system. The reactor is a 30 cm long pyrex tube with an expanded 2 cm long section in the middle (8 mm l.D.) in which the catalyst sample is placed. The catalyst powder is held in dace by means of quartz-wool pieces. The furnace temperature is controlled by means of a temperature contrcdler using a K-type thermocouple placed between the reactor and the walls of the furnace. The temperature inside the catalyst bed is measured by means of a K-type thermocouple (0.5 mm O.D.) placed in a 1/16 O.D. ss well which runs through the center of the cell. 

Danley 2003,2004). In addition to the sample and reference sensors, an additional center thermocouple, denoted To (Tzero), is utilized for the heat flow measurements. Similar to the 910,2910, and 2920 modules, there are two raised platforms for the sample and the reference on a constantan disk, which acts as a heat leak. The sample and reference disk thermocouples are attached to the underside of each platform. Two AT measurements are made. The first is taken between the chromel wires that are attached to the chromel disk area detectors. In addition, ATo is measured between chromel wires attached to the sample chromel disk and the To sensor. A chromel wire is looped between the sample chromel disk and the Tq sensor, which measures the sample temperature at the raised pod. The Tq sensor temperature is measured at the junction of the constantan and chromel wires attached at the center of the heat leak base. 

For the Mettler Toledo thermopiles, the thermocouples form a star-shaped symmetric pattern around a measuring point. Because of the symmetric arrangement of the thermocouples, the thermopiles will cause imbalances to cancel that would otherwise result in distortions in heat flow. Each thermocouple in a thermopile forms a junction under the pan and away from the pan so that each has an associated AT signal at both measuring points. Each thermopile signal is the summation of the AT signals from each thermocouple junction. A final AT signal is obtained from the difference between the sample and reference thermopiles. The heat flow rate (Q) can now be expressed as... 

Gavignet E, Lanzetta F and Nika P (2003), Thermocouple flow sensor with a-c heating for simultaneous temperature and gas flow measurements, ITBM-RBM, 17, 98-100. 

The vertical thermocouple rake used in the flow tests was supported by the transfer tube in the center of the vessel. This rake was composed of 19 gold-cobalt versus copper thermocouples connected differentially to a reference at the hydrogen vapor bulb. The relative position of each thermocouple was known, and this provided the basis for actual volumetric flow measurement. 

The instrumentation of the facility consists of 260 thermocouples, coolant flow measurements, and a LDA facility to measure the local 2-D velocities inside the pool. The heat fluxes can be obtained by two independent means first, by measuring the coolant flow together with coolant inlet and outlet temperatures of each cooling unit and second, at selected units by measuring the temperature gradient in the brass wall. The fluid temperatures inside the pool are measured with 38 stationary thermocouples. 

Flow measurement can also be deducted by correlating thermal noise in the flow channel. The basis for this technique is the measurement of transport time of the noise pattern (caused by local turbulence and bubble collapse) between two thermocouples located a few pipe diameters apart in the fluid. In this case accuracies in the range of 3 to 5% are possible. The use of ultrasonic transducers allows another possible correlation, and in this case no penetration in the pipe is necessary, and, over a limited range of flow, we can have accuracies in the order of 5%. Another possible correlation is the measurement of activity between two distinct points in the coolant system piping. This technique is actually under study, and can be used to attend the requirement of functional diversity for flow measurement. 

The temperature of the melt downstream from the breaker plate may exceed the front barrel temperature, because of the mechanical work transmitted to the resin by the screw it varies with screw speed and flow rate. The melt temperature is measured by a thermocouple inserted into the melt downstream from the breaker plate. A hooded exhaust placed over the extmder die and feed hopper removes decomposition products when the extmdate is heated. 

Instrument Society of America 400 Stanwix Street Pittsburgh, Pa. 15222 Standards l ibrary for Measurement and Control, 12th ed., 1994. Instmmentation standards and recommended practices abstracted from those of 19 societies, the U.S. Government, the Canadian Standards Association, and the British Standards Institute. Covers control instmments, including rotameters, aimunciators, transducers, thermocouples, flow meters, and pneumatic systems (see... 

Not all elements of the industrial thermocouple need to be wine. For example, if a copper pipe contains a flowing fluid whose temperature is to be measured, a constantan wine attached to the pipe will form a T, or copper—constantan, thermocouple. Such arrangements ate difficult to caUbrate and requite full understanding of the possible inherent problems. For example, is the copper pipe fully annealed Homogeneous Pure, or an alloy Many ingenious solutions to specific measurement problems ate given in Reference 6. 

Thermocouples Temperature measurements using thermocouples are based on the discovery by Seebeck in 1821 that an electric current flows in a continuous circuit of two different metalhc wires if the two junctions are at different temperatures. The thermocouple may be represented diagrammaticaUy as shown in Fig. 8-60. A and B are the two metals, and T and To are the temperatures of the junctions. Let T and To be the reference junction (cold junction) and the measuring junc tion, respectively. If the thermoelectric current i flows in the direc tion indicated in Fig. 8-60, metal A is customarily referred to as thermoelectricaUy positive to metal B. Metal pairs used for thermocouples include platinum-rhodium (the most popular and accurate), cmromel-alumel, copper-constantan, and iron-constantan. The thermal emf is a measure of the difference in temperature between To and T. In control systems the reference junction is usually located at... 

Static temperature is the temperature of the flowing fluid. Like static pressure, it arises because of the random motion of the fluid molecules. Static temperature is in most practical instaUations impossible to measure since it can be measured only by a thermometer or thermocouple at rest relative to the flowing fluid that is moving with the fluid. Static temperature will increase in a diffuser and decrease in a nozzle. 

Since this temperature requires the thermometer or thermocouple to be at rest relative to the flowing fluid, it is impractical to measure. It can be, however, calculated from the measurement of total temperature and total and static pressure. 

The heated-thermocouple anemometer measures gas velocity from the cooling effect of the gas stream flowing across the hot junctions of a thermopile supplied with constant electrical power input. Alternate junctions are maintained at ambient temperature, thus compensatiug for the effect of ambient temperature. For details see Bunker, Proc. Instrum. Soc. Am., 9, pap. 54-43-2 (1954). 

Analysts should review the technical basis for uncertainties in the measurements. They should develop judgments for the uncertainties based on the plant experience and statistical interpretation of plant measurements. The most difficult aspect of establishing the measurement errors is estabhshing that the measurements are representative of what they purport to oe. Internal reactor CSTR conditions are rarely the same as the effluent flow. Thermocouples in catalyst beds may be representative of near-waU instead of bulk conditions. Heat leakage around thermowells results in lower than actual temperature measurements. 

At each of the measurement locations, pressure probes may be attached to a harness, and these probes will direct the air flow to external pressure transducers for measurement while serving as a sheath for the appropriate thermocouple at that location (each thermocouple will be seated inside a pressure probe). 

This instrument was developed from the hot-wire ammeter, some examples of which can still be found. In the modem equivalent, the current to be measured (or a known proportion of it) flows through a small element that heats a thermocouple, so producing a rms voltage at its terminals, which is a function of the current. This voltage then supplies a current to a permanent magnet, moving-coil movement. 

A bare thermocouple is used to measure the temperature of a gas flowing through a hot pipe. The heat transfer coefficient bet ween the gas and the thermocouple is proportional to the 0.8 power of the gas velocity and the heat transfer by radiation from the walls to the thermocouple is proportional to the temperature difference. 

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. 

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