Some examples on different mixes of retardants

Manufacturers of retardants, foams, and water enhancers have recommended ratios or formulas for mixing their product with water. But I have learned that these ratios are not necessarily set in stone and different retardant mixes can be used depending on the fuel, steepness of terrain, etc. I asked TL Stein for some clarification on these ratios or formulas:

The mix ratio is never a set number, at least in my experience.  There is the manufacturers recommended formula and then there is the actual field numbers.  Retardant and foam are tested in a variety of fuels and conditions and the manufacturers mix ratio is generated using the average effectiveness for all fuel types and conditions.
 
In the field, under actual conditions, sometimes a thinner mix is better than the recommended mix and sometimes a heavier mix is needed.  I'll give you a few examples:
 
A. On a gently sloping hill, with light fuel (short grass and brush) and nominal wind, a lighter mix of either foam or retardant will be more effective as the penetration factor is minimal.
 
B. Same hill except fuel is heavy (old growth sagebrush w/ a 12 foot canopy) and moderate wind, both a light mix and a heavy mix will be used.  The light mix, being less gelled, will penetrate the canopy to affect the undergrowth.  A heavier mix will only be effective on the upper parts of the sagebrush because the gelling action is more sticky, therefore only adhering to the upper parts of the canopy and not reaching the bottom layers.
 
C. On a steep hill (45 degrees or more) with medium to heavy fuel and higher wind conditions, a heavy mix is best as this fire type will burn fast, usually burning just the tops of the fuel (crowning).  On initial attack in a forest, crowning is a major issue.  It allows the fire to spread more rapidly and creates additional hazards for ground crews (snags and widow makers). By treating the tree tops and upper parts of the fuels involved, it will slow the fire down allowing ground crews to be more effective in their efforts with hot spots picked up by helicopters.
 
D. New Jersey's conditions and fuel types are different than, say New Mexico's.  Fire will be fought differently in each place.  Tactics in one place will not always work in the other. 
 
Think of retardant and foam like dish soap.  Depending on how heavy the grease is will depend on how much soap you use.  I guess that was a simpler way of explaining all this.  In any case, retardant and foam mixing ratios recommended by the manufacturer are only guidelines.  Consideration of the local fuel types, weather conditions and past trial and error usage will generally dictate what mix ratio is the best.

 
Glossary (Fire terminology from National Wildfire Coordinating Group)

Crown Fire (Crowning): The movement of fire through the crowns of trees or shrubs more or less independently of the surface fire.
Initial Attack: The actions taken by the first resources to arrive at a wildfire to protect lives and property, and prevent further extension of the fire.
Snag: A standing dead tree or part of a dead tree from which at least the smaller branches have fallen.