Theory of "Dry Isothermal" CV Determinations : Conventional Way
The CV (Calorific Value) of a substance is measured by burning it in a controlled environment. The resulting heat released by this combustion i.e. the net temperature rise, is proportional to the calorific value.
A problem, however, occurs in that during the determination period (-10min) some energy may be gained from or lost to the environment.
In the adiabatic system the environment is controlled so that no energy is lost or gained. To achieve this state, the Calorimeter Vessel (Bomb) and bucket are surrounded by a water jacket which is strictly temperature controlled by a system of circulators, heaters and coolers, so that it has the same temperature as the inner bucket.
The Isothermal Calorimeter measures the effect that the environment has on the bucket before and after the determination and corrects the result accordingly. This implies that the environment is stable during the determination. To achieve stability a large body of water surrounds the vessel and bucket assembly, which is thermally stable.
Modified Conventional Way
Prior to development of the CP500 (1990), over a 5-year period, it was demonstrated by DDS that a polystyrene jacket, which has very little thermal mass and a negligible heat conductivity, could replace the water jacket. This system was used in the AMPC.
In the conventional (wet) way a vessel is placed in a water container and the water temperature is measured. Since the water quantity and container are part of the effective heat capacity, great care must be taken not to change the water volume. Equally important is that the water acts as a heat sink and a temperature transfer medium to the sensor, so the water must be stirred.
In the dry vessel, the water and container are replaced by a highly temperature conductive aluminium jacket which is shrunk over the stainless steel body at a very high pressure and forms an integral part of the vessel. The temperature sensors are equally spaced around the vessel body in the aluminium jacket and the stainless steel body. This method eliminates all the problems with water and results in very fast temperature equilibrium, with a fixed heat capacity.
Since the vessel was that much faster, the environment could affect the measurement 5 times less conventionally.
The warm vessel is removed from the unit and placed in an air-cooler, which is measuring the vessel temperature and cools it to a ambient related temperature.
The vessel design was the result of a computer aided process and extensive computer simulations to predict its temperature behaviour. The vessel is tested and certified to international material and test requirements.
From the operator's point of view, the vessel is lighter, easier to handle and plated on the exterior, for easy cleaning.
The CAL3K has the electronics and sensors in the vessel (bomb). Effectively the whole calorimeter is placed in the vessel, and the vessel is "docked" in the CAL3K for supervision. The result is a faster and more stable temperature reading. The temperature stability is 0.000’010. The vessel's electronics are vacuum epoxy sealed, and require no adjustment or maintenance.
The vessel can be operated over a wide temperature range (+1 to +70 degrees C), but it is advised that the vessel starting temperature is the ambient (room) temperature.
The calibration of a vessel is performed in the calorimeter because the vessel surroundings make a difference. The calorimeter can hold 15 calibrations for each mode and a maximum of 4 vessels.