# MM H20 in regard to cooling fan specs

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Saw this rating as I was comparing different cooling fans. I don’t understand this definition at all. Millimeters of Water Column is a low range metric pressure unit derived from the hydrostatic pressure of 1mm head of water at 4 degC. 1 millimeter of water gauge equals 9.80665 pascals.

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static pressure of a fan, in other words how well can it push air through let’s say a radiator

Millimeters of Water Column (mmWC) is a unit of measurement used to describe air pressure or airflow resistance in ventilation, heating, and cooling systems. It is based on the hydrostatic pressure of a 1mm column of water at 4 degrees Celsius, and is commonly used in HVAC (Heating, Ventilation, and Air Conditioning) systems to measure the static pressure drop across ducts, filters, and other components.

When used in the context of cooling fans, millimeters of water column (mmWC) is used to measure the static pressure that the fan generates. This pressure is measured in millimeters of water column at the point where the fan is generating airflow. This static pressure is important because it helps determine how well the fan can push or pull air through an obstacle, such as a filter or radiator.

So, in short, the higher the mmWC rating of a cooling fan, the more pressure it can generate and the more effective it will be at moving air through obstacles.

You know how they measure atmospheric pressure in inches if mercury? Well, you can also use millimeters of water instead, for measuring much smaller pressures. In this case, it’s how much pressure the fan can generate.

Fill a hose with water, bend it into a U-shape so you have a connected body of water inside. Then attach the fan to one side (so it blows directly onto the water surface inside the hose, and measure how much lower that surface now is compared to the other side of the hose.

The more pressure the fan can generate, the more it will push the “water column” down on its side. If it’s for example 3mm of difference, that’s a 3mm H2O fan.