I ran into an issue on a project where I'm needing to be sure of the accuracy of airflow measurements being taken. There are two basic ways to measure the airflow of an air conditioning system: 1. First is to traverse the main supply duct with a pito-static tube array and a differential pressure gage and measure the whole system airflow at once. These measurements have an accuracy of 2% with a 95% confidence factor. This is a typical rated accuracy for such sensors/gauges. 2. Second is to measure the airflow at each airflow control device, which has a very similar piece of equipment (a pito-static array with a pressure transducer) permanently installed - and add-up the numbers. The accuracy of each device is then about the same as in #1. For this example, I have somewhere around 100 of these devices. What is the accuracy of this airflow measurement? It seems to me that method #2 should yield an accuracy far better than of 2%, but I'm not sure and I don't know how to calculate the accuracy. Can anyone help?