What is a Descending Interface
The fire retardant agent is discharged from the ceiling –highest point) into the protected enclosure. An ‘interface’ with a constant concentration - known as the Initial Concentration) descends from the discharge level as gas leaves escapes from the enclosure via air leakage paths. The time it takes for this ‘interface’ to reach the minimum protected height such as the top of the server racks is defined as the Hold time. The time retention time for a Room Integrity Test Pass is usually set at 10 minutes.
What is Continual Mixing
The suppressant Agent is discharged at near-ceiling level into the enclosure, thereafter fans circulate gas throughout the room, resulting in a uniform agent concentration. This concentration begins at initial concentration and lowers until it eventually reaches a specified minimum concentration. The time it takes for the initial concentration to reach the minimum concentration is defined as the hold time.
What is an Extended Discharge
Typically extended discharges consist of an initial discharge for 10 to 60 seconds to get the concentration up to the design concentration. Then the extended discharge will continue at a lesser rate. The intention is for the extended discharge to replace the agent that leaks out of the room. It is assumed that the extended discharge will create enough turbulence to create a continual mixing situation throughout the extended discharge period.