Calorimetry Methods and Modes
Calorimeters and Related Lab Instrumentation
An isoperibol calorimeter is one where the surrounding jacket is maintained at a constant temperature while the temperature of the bomb and bucket rise as heat is released by the combustion. The Model 6400 and 6200 Calorimeters are true isoperibol calorimeters. In these implementations, a water jacket, maintained at a fixed temperature, completely surrounds the combustion bomb and its ‘bucket’. A microprocessor-based controller monitors both the temperature of the bucket and the jacket and performs the necessary heat leak corrections that result from differences in these two temperatures. These corrections are applied continuously throughout a test rather than as a final correction based on pre and post test measurements.
Continuously Compensated Calorimetry
The Parr 6100 and 6050 Calorimeters take advantage of the real time, continuously corrected method developed by Parr in its implementation of the isoperibol method. No attempt is made in the Model 6100 or 6050 Calorimeter to establish the constant jacket temperature required for isoperibol calorimetry. Instead, the temperature of the jacket is continuously monitored and real time heat leak corrections are applied based upon the temperature difference between the bucket and the actual temperature of the jacket. While this method is not truly an isoperibol method, its real time correction procedure achieves the same purpose with nearly equal results. What it can not do is match the temperature uniformity of a circulating water jacket.
The Parr 6772 Calorimetric Thermometer, serving as a controller for the 1341 or 6725 Calorimeters, uses yet another approach to emulate the isoperibol calorimetric method. In these calorimeter systems, the heat leak is precisely measured during the calorimetric pre-period. This evaluation results in an estimate of the effective, average temperature of the calorimeter surroundings. This temperature value is then used throughout the test interval to provide the calorimeter heat leak correction. While not as robust as either of the other two methods outlined above, it harnesses the computing power of the controller, with no additional hardware costs, to provide heat leak correction capability that is almost identical to the approach used when non-electronic thermometry and manual calorimetric techniques are employed.
Removable bomb calorimeters are the more traditional design most users will recognize. In this design the oxygen bomb and bucket are removed from the calorimeter for loading the sample and filling the bucket with the carefully measured amount of water which absorbs the energy released in the combustion.
The choice of bomb style may affect the calorimeter chosen. Bomb choice is dictated by sample size and alloy of construction. These bombs range in sample size from 500 to 12,000 calories per charge, and are offered in different alloys and designs for a variety of applications.
Alloy Selection – Parr oxygen combustion bombs are typically made of Alloy 20 which is richer in chromium and contains three times as much nickel as series 300 Stainless Steels. Alternatively, Alloy G-30 is offered for chloride service as the metal contains cobalt and molybdenum to resist the corrosive effect of the chloride ion. The fixed bombs of the 6400 Calorimeters are available in either alloy.
In the fixed bomb and bucket design used in the 6400 Automatic Isoperibol Calorimeter, the bomb and bucket are not removed from the calorimeter during routine operations. This design concept has made it possible to offer unique levels of automation for the entire calorimetric determination not just the data collection and reporting steps. The result of this automation will save approximately five minutes of operator time per test when compared to any removable bomb calorimeter.
Oxygen Charging and Release – The fixed bomb and bucket design allow the oxygen supply to be directed into the head of the bomb at the beginning of each test. The head of these bombs incorporate a check valve which dynamically seals when the bomb is pressurized. At the end of the test, the gases in the bomb are automatically released while the calorimeter is being returned to its starting temperature.
Isoperibol Jacket vs. Isoperibol Mode
The most advanced oxygen bomb calorimeters use an Isoperibol water jacket. This a water jacket that is kept at a constant temperature at all times. This results in a consistent and predictable heat exchange between the bucket and jacket throughout the test.
There are two generally accepted methods for calculating the correction for heat gain or loss from a non-adiabatic oxygen bomb calorimeter. The first is the Dickinson method while the second is the Regnault-Pfaundler method.
Some manufacturers claim an Isoperibol Mode even though their calorimeters do not have an isoperibol jacket. This claim is based on using the above methods to calculate the result.
Parr makes the distinction between an isoperibol jacket and these calculations. All of Parr’s bomb calorimeters use these calculations to determine the test results, but only the 6400 Automatic Isoperibol Calorimeter and the 6200 Isoperibol Calorimeter are true isoperibol calorimeters.