HEPA Filters

The velocity over the face of the filter will vary being higher in the center and lower at the edges. In theory the velocity at the edge of the filter face will be zero, but this is a very narrow area a few molecules wide. If the flow is laminar the velocity profile will be a parabola. If the flow is turbulent and the profile will be flattened in the center. The higher the velocity the flatter the profile. If you have not done an area weighted traverse of the filter face then the calculated value is going to be high.

As the hood gathers all of the air issuing from the filter the velocity through the vane will be higher than a spot reading. This will have 2 advantages over a spot reading. Firstly the accuracy of the rotating vane is likely to be better than at lower flow rates and secondly but more importantly you will be measuring the total airflow. However the extra resistance of the hood and vane will reduce the flow rate slightly so the true flow rate will be less than 343cfm and greater then 200cfm.

This is one of those examples of the process of measurement affecting what is being measured.

Just to prove it to yourself measure the face velocity in the corner of the filter face and compare it to that in the centre. If you are getting 45m/s in the centre with a 100mm dia vane then I would expect the velocity in the corner to be less than 0.08m/s.

Good luck

When a T&B person measures with a vane, they measure multiple points in a matrix over the surface of the filter. Then they calculate the average velocity. This average will probably be less than 0.45 m/s. Typically the professional T&B person uses a hood because it is more accurate. However, be advised that hoods need to be calibrated every year or so.