Thermocouples transfer standard

Transfer or working-standard thermocouples (including connecting wires—see Fig. 16.17) are individually calibrated by comparison calibration against a defining standard thermometer (such as an SPRT) or another transfer standard thermometer (usually a thermocouple). 

The type-S thermocouple, though no longer used as a defining standard for ITS-90, is still a reasonably accurate transfer standard thermometer. The precision of a type-S thermocouple at temperatures between 600 to 1000°C is about 0.02°C, and its accuracy is about 0.2 to 0.3°C. At lower temperatures (between about 0 and 200°C), a base-metal-type thermocouple (e.g., type T) is capable of a precision of about 0.01°C and an accuracy of 0.1°C. 

Although, as shown in this monograph, mechanical agitation is provided in a number of different ways, the most common method is by a stirrer in a standard vessel. In many mechanically agitated reactors, the vessel contains internals such as baffles (particularly for low-viscosity fluids), feed and drain pipes, heat transfer coils, and probes (e.g., thermometers or thermocouples, pressure transducers, level indicators). The degree of mixing and power requirement depend on the nature of the internals present in the vessel. 

The experiments were carried out in a Netzsch STA 409 C (Simultaneous Thermal Analysis - STA) in the TGA/DSC configuration. The STA has a vertical san le carrier with a reference and a sample crucible, and in order to account for buoyancy effects, a correction curve with empty crucibles was first conducted and then subtracted from the actual experiments. Platinum/Rhodium crucibles were used in order to get the best possible heat transfer. The thermocouple for each crucible was positioned Just below and in contact with the crucible. The ten rerature obtained from the measurement is the temperature in the reference side. This temperature is converted to the temperature in the sample side by using the DSC-signal in pV and a temperature-voltage table for the thermocouple. The product gases were swept away by lOO Nml/inin nitrogen which exited the top of the STA, The STA was calibrated for temperature and sensitivity (DSC) with metal standards at each heating rate. 

The test element was connected in series with an NBS calibrated 0.1-ohm standard resistor, and with a stabilized battery. The resistances were compared, by standard techniques with a measuring current of approximately 0.1 amp, at various temperatures as measured with the bulk liquid thermocouple in the same bath. At these low currents (heat transfer measurement currents ranged from about 5 to 50 amp) the bath temperature could be maintained constant at any desired value to within 0.05 for an indefinite period of time, and the temperature difference between the buUc liquid and the test element was insignificant. Thus the resistance of the test element was determined in terms of the emf of the thermocouple used to measure the bulk liquid temperature over the range from the triple point of N2 (63.16T<) to 150°K. 

Standard temperature measurement in heat transfer experiments is still done using thermocouples. Thermocouple wires have diameters down to 12.7 (im. For shielded thermocouples, the smallest diameters available are in the region of 100 pm. The drawbacks are conduction losses through the thermocouple wire and flow disturbance. These errors are obviously more pronounced in microfluidic flows. 

To measure the uniform temperature distribution of the empty shelf is not sufficient to ensure a uniform heat transfer. There are several ways to check the heat transfer under load. If the shelves are of equal design, one shelf, for example, may be loaded automatically with vials as used in the pilot plant. The vials are filled with water to 15 mm height 25 vials are filled the same way but through the stoppers a thermocouple or another temperature sensor is fixed approximately in the middle of the water layer. These 25 vials are exchanged with vials on the shelf by hand in such a way that 15 vials are placed on the border of the shelf as widely distributed as mechanically possible. The other 10 vials are placed in different positions (again as widely distributed as mechanically possible) in between the center part of the vials. The positions of the vials are marked. The temperature in the product of the vials is recorded as a function of time. The temperature of the brine inlet and outlet is also recorded approximately every 30 s. The freezing behavior in the product of the vials will not be identical (see plot 2 in Figure 9), but if the temperature difference between the brine inlet and outlet are <0.2°C, the temperatures in the product should be very similar with a standard deviation of < 2°C. If the temperature plots... 

Figure 5.6 Thermal conductivity apparatus and coaxial-cylinder cell, developed by Yata et al (1979a). (a) 1 high pressure vessel 2 fluid separator 3 heater 4 heat insulator 5 support table for bath 6 heat transfer fluid (water or glycerin) 7 screw propeller 8 standard resistance thermometer 9 thermocouples and heaters, (b) 1 inner cylinder 2 outer cylinder 3 upper guard cylinder 4 lower guard cylinder 5 inner heater 6 thermocouples 7 upper alumina insulator 8 lower alumina insulator 9 mica spacer 10 alumina piece 11 brass screw 12 alumina pin 13 brass screw 14 compensative heater 15 top closure of high pressure vessel.

For steady-state conditions, the standard thermometer and thermocouple-transmitter outputs are identical. Assuming that the transmitter/thermocouple can be modeled by a first-order transfer function, find K and t. 

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