During air measurement restriction plates are still often not used in the return air opening. The argument given is that the air inlet will still go to the position where the CO2 content and RH reach the required levels. This is correct, but if the air inlet indicates relatively low positions this means that a lot of return air is (unnecessarily) being used. Return air creates extra exchange with the bed (which is not always desirable) and in principle doesn't extract anything from the growing room (only if the cooling is on).
With the exception of periods when outside air cannot be used (and therefore the restriction plate should be removed), using a lot of return air only makes sense if the outside air is unsuitable (e.g. very warm and moist). In all other situations it's best to use as little return air as possible. To begin with, set the fan position so low that the air inlet virtually fully opens and only takes very little return air. Such low fan positions can't be set with every type of installation, as this also effects the air distribution and air movement over the beds. In this case, you will have to use a higher fan position and restrict the levels of return air by placing a restriction plate in the return opening. It's not exceptional in this case to have to cover 2/3 of the opening.
Jan Gielen, Specialist climate and energy, C point