Overview

Ordinary oxygen bomb combustion calorimetry is used to measure the heat of combustion or reaction of materials in oxygen or inert atmospheres.  Even for high strength vessels, such as the Parr 1104 Oxygen Combustion Bomb, the conditions necessary to detonate small amounts of highly reactive materials are often difficult to achieve and can result in unpredictable consequences.  For example, the conventional heat of combustion of pentaerythritol tetranitrate (PETN) [C₅H₈N₄O₁₂] in oxygen is 1957 cal/g while the heat of detonation in vacuum is 1490 cal/g (a 24% difference).

Additionally, it is well known that the degree of confinement of explosive materials significantly influences the released energy.  For unconfined or lightly confined charges, the released energy is largely retained in the products.  When the charge is heavily confined, the detonation energy, for the most part, is converted to kinetic and internal energy of the confining case.  For example, the conventional heat of combustion of 2,4,6-trinitrotoluene (TNT) in oxygen is 3590 cal/g.  The heat of detonation for TNT at a charge density of 1.53 g/cc is 1093 cal/g and at a charge density of 0.998 g/cc is 870 cal/g.  In contrast, an unconfined reaction yields approximately 600 cal/g.

Precise fundamental information about the detonation process can be obtained by combining calorimetric and dynamic pressure measurements.  These measurements can be used ultimately to predict explosives performance.  The Parr Detonation Calorimeter has been designed to aid research in this area.

Parr Instrument Company’s new heat of detonation calorimeter accepts up to a 25 g high explosive charge with a nominal total energy release per charge of ~160 kJ.  The detonation is initiated using a small commercial EBW style detonator incorporating 80 mg of PETN and 450 mg of RDX with a binder.  Detonators are fired using a one-microfarad – 4000 V capacitance discharge firing set.  A complete calorimetric measurement can be made in a few hours with a precision of several tenths of a percent.  The bomb can be optionally fitted with a high-speed pressure transducer that allows the user to gain further insight into the dynamics of the detonation process.

Major Subsystems

Detonation Vessel

The distinctive spherical bomb is made of 3.2 cm thick stainless steel and has an inside diameter of 21.6 cm providing an internal volume of 5.3 liters.  The 9 cm opening is covered by a lid 3.8 cm thick at the center.  The lid is secured by 12, 1.3 cm diameter, high strength bolts inserted through the lid flange and sealed with an o-ring.  The lid incorporates a bomb lift fitting at the center, surrounded by an inlet / outlet valve and two high-voltage style insulated electrodes.  An optional port can be provided on the lid for a fast responding pressure transducer.  The mass of the bomb (cylinder and lid) is 55 kg.

Calorimeter Stand

The detonation calorimeter uses a classical isoperibol design incorporating a static jacket.  The static jacket approach has been used successfully for many years in two generations of Parr calorimeters (Models 1356 & 6100).  The calorimeter stand houses the calorimeter jacket and also includes a bomb lift hoist as well as a conveniently placed bomb support and bomb head stand.  The calorimeter lid is stored on the lower shelf, below the bomb workstation, when not in use.

6772 Precision Calorimetric Thermometer