Dewars calibration

Phenylacetylene. Support a 5-litre glass Dewar flask in a wooden case. Equip the flask with a lid of clear Perspex, provided with suitable apertures for a mechanical stirrer, introducing solids (e.g., sodium) or hquids, a calibrated dip stick for measuring the volume of liquid in the Dewar vessel, a gas mlet tube and an ammonia inlet arrange for an electric light to shine downwards into the flask. 

Figure 8.1 Scheme of a Dewar vessel isoperibol reaction-solution calorimeter. A ampule containing the sample B ampule breaking system C calorimeter head D temperature sensor E stirrer F electrical resistance G Dewar vessel H plunger of the ampule breaking system I, J inlets K plug connecting the calibration resistance to the calibration circuit. 

Figure 11.1a shows a scheme of a widely used reaction vessel for isoperibol titration calorimetry [211]. It consists of a silvered glass Dewar A, which can be adjusted to a lid B supporting a stirrer C, a resistance D for electrical calibration, a thermistor E for temperature measurement, and a Teflon tube F for titrant delivery. The assembled Dewar and lid set-up is immersed in a constant... 

Figure 4.2 Left Dewar calorimeter equipped with stirrer and calibration heater. T thermometer, C calibration heater,...

The reaction is initiated by the addition of a reactant, which must be exactly at the same temperature as the Dewar contents, in order to avoid the sensitive heat effects. Then the temperature is recorded as a function of time. The obtained curve must be corrected for the heat capacity of the Dewar flask and its inserts, respective of their wetted parts, which are also heated by the heat of reaction to be measured. The temperature increase results from the heat of reaction (to be measured), the heat input by the stirrer and the heat losses. These terms are determined by calibration, which may be a chemical calibration using a known reaction or an electrical calibration using a resistor heated by a known current under a known voltage (Figure 4.2). The Dewar flask is often placed into thermostated surroundings as a liquid bath or an oven. In certain laboratories, the temperature of the surroundings is varied in order to track the contents temperature and to avoid heat loss. This requires an effective temperature control system. 

A catalytic reaction must be performed in aqueous solution at industrial scale. The reaction is initiated by addition of catalyst at 40 °C. In order to evaluate the thermal risks, the reaction was performed at laboratory scale in a Dewar flask. The charge is 150 ml solution in a Dewar of 200 ml working volume. The volume and mass of catalyst can be ignored. For calibration of the Dewar by Joule effect, a heating resistor with a power of 40 W was used in 150ml water. The resistor was switched on for 15 minutes and the temperature increase was 40 K. During the reaction, the temperature increased from 40 to 90 °C within approximately 1.5 hours. The specific heat capacity of water is 4.2 kj kg K 1. 

Frey s variant of the silvered vessel test has been in use in the Germany In its variant, different amounts of heat are supplied to the electric heating elements mounted inside the Dewar flask, and the temperature differences between the interior of the Dewar vessel and the furnace are measured by thermocouples. A calibration curve is plotted from the values thus obtained, and the heat of decomposition of the propellant is read off the curve. In this way, the decomposition temperature at a constant storage temperature can be determined as a function of the storage time, and the heat of decomposition of the propellants can thus be compared with each other. If the measurements are performed at different storage temperatures, the tempera-... 

Dewar flask (short, wide mouth, 1 pt) cork stopper with three holes coil-type stirrer stopwatch two 10-mL weighing bottles 50-mL burette burette clamp and stand two 100-mL volumetric flasks one 10-, one 25-, and one 50-mL pipette pipetting bulb battery jar wash bottle precision ciyoscopic thermometer and magnifying thermometer reader, or a digital resistance thermometer with 0.01°C resolution, or a calibrated thermistor. 

The experimental details have been reported elsewhere (X, X) Briefly, matrices are formed by codeposition of excess argon with atomic potassium on a sapphire plate mounted inside an ESR cavity which is itself attached to a variable temperature liquid helium dewar. Cluster formation occurs during deposition and is accomplished by warming the sapphire surface above a nominal deposition temperature of 4.2 K. For spectra shown here, temperature measurements were made with a calibrated carbon resistor and are judged accurate to within 5%. 

A Dewar flask, in which 400 cm of kerosene, one CA thermocouple to measure the Tug and a Nichrome wire of about 1.7 Q are contained, is set under conditions of no air circulation in an aluminium box settled in a fairly large thermostat, which in turn is placed in an air-conditioned room maintained at an almost constant (room) temperature. In this connection, the temperature of the thermostat is maintained at a constant temperature 2 3 K higher than the The other thermocouple to measure the T i-up is held outside the flask set in the aluminium box. The thermocouples used are calibrated in the range of 40 to 60 °C before the measurement (Figs. 48-53). 

In 1952 Dewar developed Perturbational Molecular Orbital (PMO) theory, a 7t-electron method calibrated directly on the energies of model organic compounds. The accuracy of this simple method is remarkable for 20 conjugated hydrocarbons the average error in the heat of atomization was 6.5 kcal/mol, and, if the worst case, biphenylene, were left out, the average error dropped to 3.33 kcal/mol. ... 

Microwave absorption measurements were made with a Bruker ER300 ESR spectrometer operated with 100 kHz magnetic field modulation where the derivative of the absorption part of the magnetic susceptibility with respect to magnetic field was detected (dx VdH). Temperature was varied with an Oxford Instruments ESR 900 helium flow system from room temperature to about 6 K. The temperature at the sample position in the flow dewar was calibrated with a separate thermocouple and found to be 2-3 K higher than the Instrumental... 

Solution calorimeters are usually adiabatic calorimeters. They are mainly used for the study of rapid reactions, for example, heats of solution, heat capacity of liquids, heat capacity of solids by a method of mixtures, or the enthalpy change of rapid reactions in solution. A schematic diagram is shown as Figure 3. The temperature sensor, plus a means of electrical calibration and a device for mixing reactants are all enclosed within a Dewar flask, or other adiabatic assembly. 

Temperature Control. Liquid nitrogen was supplied to the chevron panels and helium transfer line jackets from self-pressurized dewars. Cold helium gas was circulated in a closed loop. Temperatures at the helium cold plate were measured at two points by means of helium-filled gas thermometers equipped with a small cold-gas bulb and a large warm-gas bulb, giving an almost linear pressure vs. temperature curve in the 4.2 to 30 K temperature zone. The thermometers were calibrated at 4.2 , 20.4°, and 77.8°K. Liquid-nitrogen temperatures were not measured, but adequate provisions were made to keep the circuits flooded. Helium temperature to the cryopump was controlled by means of in-line electric heaters. 

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