kilogram per square metre per second
One kilogram of material crossing one square metre of surface every second. It is the SI coherent form of mass flux — the rate at which a fuel surface burns, a wet solid dries, a membrane passes water, or a coating builds.
Watch out: Almost nobody works in this unit, which is exactly the problem it causes. A membrane engineer quotes litres per square metre per hour and a fire engineer grams per square metre per second, and both are describing this. The gap between L/(m²·h) and kg/(m²·s) is a factor of 3600 — near enough four thousand — so a figure moved between two sources without its unit is not slightly wrong, it is unrelated to the answer.
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It is what the SI produces when mass is divided by area and time, with no separate history of its own. Its importance is that it is the quantity that actually governs those processes: a total mass flow says nothing until you know the area it crosses, because the same kilogram per second through a square metre and through a square millimetre are entirely different physical situations.
About the kilogram per square metre per second
Mass flux is the honest form of a mass flow rate. Divide by the area it crosses and you get a number that can be compared between a laboratory coupon and a full-size vessel; leave it as a total and you have a figure that only means anything for the one piece of equipment it was measured on.
The conversions worth carrying: , and — the one that catches people — of water is , because a litre of water is a kilogram and an hour is 3600 seconds.
That membrane unit deserves a note of its own. L/(m²·h) is a volume flux wearing a mass unit's clothes. It converts to mass flux only because water is taken as one kilogram per litre, which is a convention the trade adopted and a fact about water rather than about the unit. Run the same membrane on a brine or a solvent and the conversion moves with the density.
In fire engineering this quantity is the mass burning flux, and it is what turns a fuel area into a heat release rate: multiply by the heat of combustion and by the area, and you have the fire size that every other calculation depends on. In drying it is the evaporation rate that sets how long a batch takes. The two fields have no shared literature and no shared notation, and the dimension is identical.
One kilogram per square metre per second in every mass flux unit
| 1 | kilogram per square metre per second (kg/(m²·s))this unit | |
| 1,000 | gram per square metre per second (g/(m²·s)) | |
| 3,600 | kilogram per square metre per hour (kg/(m²·h)) | |
| 3,600 | litre per square metre per hour (L/(m²·h)) | |
| 0.204816 | pound per square foot per second (lb/(ft²·s)) | |
| 737.338 | pound per square foot per hour (lb/(ft²·h)) |