Venting Above Flood Rim?

I know the plumnbng code in NJ and probably most other states require that the vent connection be at least 6" above the flood rim before continuing horizaontally. I am just curious as to why 6". What is the hydraulics behind this rule?

Also, is it 6" as measured to the centerline of the horizonal run?

Thanks

The rule is there so that there would never be a case where a clogged fixture would cause water. and presumably some waste..to back into the vent line and clog it. I think the 6" number was just chosen as a good number. I can't tell you right off hand if it is measured to C/L or to the weir of the fitting. If you are close, an inspector will probably not give you any grief.

vent

The other part of the sentence which you did not quote, is usually "or 42" above the floor whichever is HIGHER". This is because a 36" high kitchen sink might be attached at some later date and this would require it to be 42" to comply with the 6" rule. The 6" measurement is to the flow line of the horizontal pipe, and was probably determined in a laboratory using a vacuum pump to simulate some theoretical situation, such as a high rise building.