Temperature control furnace calibration

In principle, calibration of the control of the analyser requires the same approach as temperature calibration the information from temperature control should be compared with that of various standards. However, to save repeating the whole process again software often uses an automatic procedure to match the information already obtained with the control routines. Some parameters for this, e.g. the temperature range, may need to be chosen. 

---

Electrolytic Procedure. The cell is placed inside a vertical-column, Hevi-Duty, multiple furnace and is surrounded by an Inconel heat shield (Fig. 13). The temperature-control thermocouple (Pt vs. 90% Pt — 10% Rh) is located between the crucible and the heat shield and is protected by an outer ceramic tube. A calibration run is initially made without a charge in the crucible. A second thermocouple is inserted through the heat shield and centered on the cover of the electrolytic cell. This thermocouple is read directly by means of a potentiometer the temperature of this thermocouple corresponds closely to that of the melt. The temperature controller is adjusted until the second thermocouple indicates the desired temperature for the electrolysis. ... 

As mentioned before, this system operates at low temperatures. A furnace around the reactor is used to heat the system to the desired temperature. The temperature is measured by an Omega K-type thermocouple that is attached to the outside of the reactor near the catalyst bed. The temperature measured on the outside has been calibrated against the internal temperature of the reactor, and has been discussed elsewhere (Liu, et al. 1996 Marafee, et al. 1997). However, when the operating temperature is below 373 K it is necessary to use cooling air to control the temperature since the plasma itself does heat the... 

Experimentally, TMA consists of an analytical train that allows precise measurement of position and can be calibrated against known standards. A temperature control system of a furnace, heat sink, and temperature-measuring device (most commonly a thermocouple) surrounds the samples. Fixtures to hold the sample during the run are normally made out of quartz because of its low CTE, although ceramics and invar steels may also be used. Fixtures are commercially available for expansion, three-point bending or flexure, parallel plate, and penetration tests (Fig. 4). 

Constant-temperature baths or furnaces are needed to maintain the uniform temperature environment for comparison calibration. Stirred-liquid baths and temperature controllers have good characteristics as constant-temperature media due to their ability to maintain temperature uniformity [100]. The liquids used include refrigerants, water, oils, molten tin, and molten salt. Water can be used for temperatures between 0 and 100°C and oils above 300°C. Refrigeration units are available for commercial constant-temperature baths with alcohol as the working fluid and temperatures as low as -80°C. At NIST, special cryostats [8, 25] with liquid nitrogen or liquid helium as the coolant are used for comparison calibration at low tem-... 

Temperature control can be no better than the sensors upon which it relies. Although operators and engineers are inclined to trust the measurement of temperature to those who specialize in that field, the operating engineers must be aware that they cannot expect greater accuracy from a control than is put into it by the sensors. (This applies to pressure and other sensors as well.). While T-sensors are usually very good at replicating, they need to be calibrated. And it is the duty of everyone involved around a furnace to be alert to conditions that may cause sensors to deteriorate. 

Temperature and Pressure Measurements. Temperatures were measured with Type K inconel-sheathed thermocouples calibrated against the melting point of NaCl (801 C) in evacuated SiO tubes placed within the pressure vessels with the same configuration as the Au capsules. Temperature gradients were minimized to less than 1 C by careful positioning of the vessel within the furnace and were measured within the vessel at pressure of 2000 bars. A variety of temperature controllers were used. The narrow range of temperature... 

The furnace and thermostatic mortar. For heating the tube packing, a small electric furnace N has been found to be more satisfactory than a row of gas burners. The type used consists of a silica tube (I s cm. in diameter and 25 cm. long) wound with nichrome wire and contained in an asbestos cylinder, the annular space being lagged the ends of the asbestos cylinder being closed by asbestos semi-circles built round the porcelain furnace tube. The furnace is controlled by a Simmerstat that has been calibrated at 680 against a bimetal pyrometer, and the furnace temperature is checked by this method from time to time. The furnace is equipped with a small steel bar attached to the asbestos and is thus mounted on an ordinary laboratory stand the Simmerstat may then be placed immediately underneath it on the baseplate of this stand, or alternatively the furnace may be built on to the top of the Simmerstat box. 

The x-ray studies were made with a high-temperature x-ray diffraction camera of Hume-Rothery design. This camera had a 9-cm. diameter and employed the Straumanis film setting. A fine Chromel-Alumel thermocouple within the furnace cavity and adjacent to the sample served to measure and control temperature. The sample temperature was calibrated against the thermocouple e.m.f. through a series of lattice-constant measurements on pure silver. 

The catalytic activity of the nickel molybdates [11] was tested in a 1/4 inch quartz flowthrough tubular reactor operated at atmospheric pressure. The reactor was contained within an electrically heated tube furnace. The temperature of the reactor was controlled according to the temperature of the gases at the base of the catalyst bed. The compiosition and flow rate of the gas feed mixture was measured using MKS mass flow controllers calibrated for each specific gas. Certified gas mixtures with Grade 5 helium (99.999%) as the balance gas were used throughout. 

Clearly, it would be desirable if the area under the peak was a measure of the enthalpy associated with the transition. However, in the case of DTA, the heat path to the sample thermocouple includes the sample itself. The thermal properties of each sample will be different and uncontrolled. In order for the DTA signal to be a measure of heat flow, the thermal resistances between the furnace and both thermocouples must be carefully controlled and predictable so that it can be calibrated and then can remain the same in subsequent experiments. This is impossible in the case of DTA, so it cannot be a quantitative calorimetric technique. Note that the return to baseline of the peak takes a certain amount of time, and during this time the temperature increases thus the peak appears to have a certain width. In reality this width is a function of the calorimeter and not of the sample (the melting of a pure material occurs at a single temperature, not over a temperature interval). This distortion of peak shape is usually not a problem when interpreting DTA and DSC curves but should be borne in mind when studying sharp transitions. 

Two types of systems are commonly used power compensation and heat flux DSCs. In the power compensation apparatus temperatures of the sample and the reference are controlled independently by using separate but identical furnaces. The power input to the two furnaces is adjusted to equalize the temperatures. The energy required for the temperature equalization is a measure of the enthalpy or heat capacity in the sample relative to the reference. In heat flux DSC, the sample and the reference are interconnected by a metal disk that acts as a low-resistance heat-flow path. The entire assembly is placed inside a furnace. The changes in the enthalpy or heat capacity of the sample cause a difference in its temperature compared to the reference. The resulting heat flow is small because of the thermal contact between the sample and the reference. Calibration experiments are conducted to correlate enthalpy changes with the temperature differences. In both cases, the enthalpy changes are expressed in the units of energy per unit mass. 

In electrothermal atomization methods, the microcomputer also controls the furnace temperature, a key factor for this technique. Guevremont and Whitman used a microcomputer based on a Z80 microprocessor for the automatic control and data acquisition from a graphite furnace [13] they could heat the furnace from 0 to 2500 C In about 3 s, with an approximate gain In precision of 50% under the control of the microprocessor. The system Is shown schematically In Fig. 10.9. The automatic functions afforded are data acquisition, measurement of the furnace temperature and calibration, temperature programming, control of the gas flow, control of the sampler and delivery of results. 

Some electric tube furnaces incorporate a p3n ometer, which is not required in any of the experiments described in this manual, though Experiments 13 and 14 can be run by using an electric tube furnace at controlled temperature instead of an oil bath if so desired. A pyrometer may easily be improvised with iron and constantan wires twisted together and spot-welded to provide the junctions. The hot junction is placed against the outside of the combustion tube in the center of the heated portion and bound in position by asbestos tape, which also serves as insulation. The cold junction is kept in ice and water, and the electromotive force is measured with a millivoltmeter. With the cold junction at 0°C and the hot junction at 200°C, the electromotive force of the iron-constantan couple is given in tables as 10.77 millivolts. For accurate work the couple used should be calibrated and to assure uniform temperature distribution and electrical shielding, the combustion-tube and thermocouple wires should be encased in a tubular metal shield that fits inside the furnace and is groimded. 

---