Test Series 4 is used to determine additional material characteristics which are used to define in-process parameters, in addition to those examined by the DOT and BATF. If the process is designed within these parameters, the maximum credible event from a Class 1.3 material or process is a mass fire hazard. The series also includes tests to determine venting parameters for confined operations. If, however, the process parameters exceed the critical height or critical diameter, then explosion of the material due to self confinement (from a flame initiation), or propagation of a detonation is possible.

Process Simulation

Where the process is known, appropriate process simulations may be used. However, a change in the process may necessitate re-testing of some or all of the explosive substances used in the process. Appropriate process simulations may consist of modifications to the standard tests below, to represent worst-case confinement, configuration, and/or loading in the process.

Critical Diameter Test

The critical diameter of an explosive is the largest diameter at which steady-state detonation cannot be maintained. The test uses varying diameter cylinders and a witness plate. A Comp C-4 or similar booster is used to initiate the sample. The test results are considered positive if the witness plate indicates detonation. Normally, the test shall be completed after three "no-go" reactions are obtained at a diameter one increment below a diameter that previously yielded a positive result.

Critical Height (Mass) Test

The critical height (mass) test is used to determine the critical height at which a flame initiation transits to an explosive reaction (explosion or detonation). In this test, a flame initiator (bag igniter) is placed at the bottom of a pipe assembly filled with the test material. Pipes of varying lengths and diameters are used to contain the test material. The test is performed by selecting a diameter, and progressively changing the height of the pipe until the material transitions from burning to explosion or detonation. The diameter is then changed and the progressive height variation testing is repeated. Normally, a curve can be fitted using the data, to predict the critical height for other diameters as well. A "go" reaction, for explosion, is one in which the pipe is damaged. The test is concluded at each diameter by running a minimum of three successive trials that produce a "no-go" result at a height below a level that produces a positive reaction (explosion). This level is referred to as the critical height at that diameter.

Internal Ignition (10-Gram Bag)

This test is similar to the internal ignition test described earlier except that a 10-gram bag igniter is used instead of the 20-gram bag. As stated previously, either the pipe or at least one of the end caps must be fragmented into at least two distinct pieces for a positive result. Three trials are performed unless a transition from deflagration to explosion occurs earlier. The test determines if a material will explode or detonate when ignited under confinement.

Koenen Test

Th Koenen test is used to determine the sensitiveness of a material to the effect of intense heat under vented confinement. In this test, the material is placed in a steel container with an orifice plate. The test apparatus is then placed in a protective steel box, and heated at a specified rate. A series of trials is conducted using different sizes of orifices. A "go" reaction is determined by examining the container. Conducting three successive "no-go" reactions with an orifice plate size above that which produced a positive result concludes the test. This orifice is called the limiting diameter. The limiting diameter may be used to evaluate the degree of venting required to avoid an explosion in the process.