# Parameterizing the pressure drop depending on airflow.

In this post we explain one of the methods that can be used to parameterize thepressure drop of an element, for example, an exhaust grill. In this example we chose the following item (TVB model 100): http://www.schako.de/project/cat_sp/pdf/TVB.pdf Anyone can understand the graphs provided by the manufacturer in its technical documentation, the pressure drop is an exponential curve that depends on airflow.We will represent this curve in an excel document, even though only takes 3 pointsto specify a curve, we will use 5 for refining and the result will be the next. NOTE: To see the formula of the curve in Excel, right click on an item -> Addtrendline -> choose exponential and check “display equation on chart” (Excel2010).

The formula will be displayed in the upper right corner of the graphics box: Y=10,439*e^(0,0156*X)

Where “X” is the airflow and “Y” the pressure drop.

Now that we have the formula we only need to enter into the parameters We reveal a little trick, Revit MEP assume all numeric values ​​written in pressure drop formula are values ​​with units of pressure drop ( Pascals in our case ), sowhen introducing in the parameter “caudal “we must divide it by,” 1 m3 / h “to override the default units dragged by the parameter” caudal “.

In the previous formula it follows that the value of The e Number is 2,718282.

LThe acoustic power of the exhaust grill it also depends on the airflow. We have done the same exercise using Excel and the resulting curve is a logarithmic curvewith this formula:Y=21,827*ln(X)-73,962. Podeis ver la fórmula en la linea superio de la perdida de carga (imagen anterior).

As revit does not have the natural logarithm function, we used an equivalent formula -> ln(x) = log(x) / log(e) The values given the bend are not exactly the same as those provided by themanufacturer, however, the margin of error is low enough to consider this systemas valid.

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