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1)

I'm struggeling to understand the concept of the Cv or Kv value, also known as the flow factor. In SI units this is defined as the flow through a valve (m3/h) at a pressure drop of 1 bar, or 100 kPa. Fair, and the formula for calculating it is easy enough. I enter my flow and the desired pressure drop (in this case 5 kPa). So where does this "1 bar" enter the picture?

2)

To measure the flow through a valve one connects a meter to it (called a CBI, made by Tour & Andersson). I always hear people saying that the valve needs to have a certain pressure drop in order for the flow to be measurable. Why is that? I always considered pressure drop as something one wants to avoid...