Adiabatic jacket

The energy released when the process under study takes place makes the calorimeter temperature T(c) change. In an adiabatically jacketed calorimeter, T(s) is also changed so that the difference between T(c) and T(s) remains minimal during the course of the experiment that is, in the best case, no energy exchange occurs between the calorimeter (unit) and the jacket. The themial conductivity of the space between the calorimeter and jacket must be as small as possible, which can be achieved by evacuation or by the addition of a gas of low themial conductivity, such as argon. 

A variation, which results in a more simple apparatus, is the drop calorimeter. The test piece is heated (or cooled) externally, dropped into the calorimeter and the resultant change in temperature monitored. For the simplest measurements, the calorimeter need not be surrounded by an adiabatic jacket but in that case, corrections for the heat exchange with the surroundings must be applied. A procedure using a drop calorimeter has been standardized for thermal insulation in ASTM C35l". It is possible to combine the adiabatic and drop calorimeter methods by dropping a heated sample into an adiabatic chamber and this has been used for plastics12. 

The Nemst calorimeter is used for low-temperature heat capacity measurements. The sample is contained in a small metal case equipped with a heater and thermometer and is placed in an isoperibol (isothermal) jacket of large heat capacity, which in turn is surrounded by an evacuated chamber surrounded by, for example, a liquid N2 or H2 chamber (Fig. 11.77). A variant is to use an adiabatic jacket. Of course, what is measured is not Cp, but a hopefully reasonable approximation to it ... 

Thermometer Scale Correction. An NBS thermometer was used for the adiabatic jacket and a Beckmann differential thermometer for the calorimeter water. The Beckmann thermometer was calibrated against the NBS thermometer. Corrections were made to the initial and final temperatures recorded on the Beckmann thermometer. 

One may interpose between system and surroundings an adiabatic jacket, reasonably well insulated from both. This jacket is so constmcted that its temperature can be adjusted at will from outside, as, for example, by supplying electrical energy to a heating circuit. In use, the temperature of the jacket is continuously adjusted so as to be as close as possible to that of the system, so that no significant quantity of heat will tend to flow between the system and the jacket. This corresponds to making T - veiy small. 

The use of an adiabatic jacket should in principle eliminate the need to plot and extrapolate temperature values. However, there are inevitable errors in adjusting the jacket temperature during a ran, and the adiabatic jacket on the Parr is more for convenience in routine... 

Bomb calorimeter (Parr design), shown with an adiabatic jacket, which may also be used empty as an insulating air jacket. The precision mercury thermometer can be replaced by a high-resolution resistance thermometer or a calibrated thermistor. 

Procedure. The successful operation of tliis experiment requires close attention to detail, as there are possible sources of trouble that might prevent the experiment from woridng properly. The basic procedure is very similar for both Parr and Emerson calorimeters. Necessary changes in procedure will be provided by the instructor if an Emerson calorimeter is to be used. Although considerable detail is given below, further details on the operation of the Parr apparatus (including the adiabatic jacket) are available in a manual pubhshed by the manufacturer. ... 

For the present experiment, the adiabatic jacket is to be used empty as an air jacket. The procedure is experimentally simpler and capable of high accuracy, provided that the ambient temperature in the laboratory is reasonably steady. 

The adiabatic control is performed between the temperature of 2 cm of the chemical and the Tam, not between the temperature of 2 cm of the chemical and that of the inside wall of the adiabatic jacket, in order to hold the chemical under true adiabatic conditions. 

An air bath is adopted instead of a liquid bath, considering that the heat capacity of the former is much smaller than that of the latter. That the heat capacity of the bath is small means that it is possible to heat the atmosphere in the adiabatic jacket, which is set in the air bath, at a rate high enough to follow a rapid increase in temperature of 2 cm of a chemical of the TD type, including every gas-permeable oxidatively-heating substance, in the two kinds of adiabatic tests. 

The structure of the adiabatic jacket is shown in Fig. 74 in Section 7.2, in particular, in connection with the performance of the adiabatic oxidatively-heating test. 

The power supplies connected to the cold junction, the air bath, the digital D.C. microvoltmeter, the digital D.C. millivolt recorder and the two-pen strip chart recorder are then switched on, respectively. The Tdijf pen of the two-pen strip chart recorder is, however, left on the short position until the thermal equilibrium state is attained nearly around the reference material confined in the closed cell and inserted into the adiabatic jacket maintained at the nominal T,. of the run. For the term, the Tdiff pen, refer to Subsection 4.5.4. 

It is always indispensable for almost all self-heating chemicals, irrespective of whether they are of the TD type or of the AC type, to determine the or the T, of each run by inserting the reference cell assembly into the adiabatic jacket in the adiabatic self-heating process recorder, or into the aluminium block bath in the isothermal storage testing device, prior to the insertion of the sample cell... 

Insertion of the reference cel) assembly into the adiabatic jacket set in the preheated air bath... 

Figure 37. Insertion of the reference cell assembly into the adiabatic jacket has now been finished.

The two pens of the two-pen strip chart recorder are composed of the temperature or T pen and the temperature difference or A Tj,ff pen. After the reference cell assembly has been inserted into the adiabatic jacket set in the air bath, which is now being heated up to the temperature, T the T pen records the... 

Insertion of the sample cell assembly into the adiabatic jacket maintained at the T, of the run and the start of the adiabatic self-heating test for 2 cm of a chemical of the TD type confined in the closed cell... 

The reference cell assembly is successively taken out of the jacket, and the sample cell assembly including a closed cell, in which a definite quantity of a chemical of the TD type, the volume of which is also about 2 cm, has been confined in the same manner as performed to prepare the reference cell assembly in Subsection 4.5.2, is inserted into the adiabatic jacket. The quantity of a chemical of the TD type tested depends naturally on the specific gravity. 

After the start of the adiabatic control, the Tam is raised automatically by the adiabatic control in order that the condition, A Tsjf = T - Ta, , = 0, may always hold, with the result that the heat generated in the self-heating chemical is all accumulated in it, and, it continues to self-heat at a very slow, but virtually constant, rate depending on the value of T/, in the meantime, the temperature of the space in the air bath, except that in the adiabatic jacket, is maintained at the Ts of the run throughout every run. 

In the case of the digital record of the self-heating process exemplified in Table 5, the digital D.C. microvoltmeter indicates that the exact value of the T, of this run is 3111.5 jUV. This value of T, is determined, based on the fact that the indication of the digital D.C. microvoltmeter has fluctuated constantly between 3111 and 3112 juV for 1 to 2 h after the thermal equilibrium state has been attained around the reference material charged in the open-cup cell and inserted into the adiabatic jacket maintained at the nominal T, of 76 °C in the adiabatic self-heating process recorder. 

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