blower motor pressure

[wfs]

I am in the process of making an ambient air filter for my woodshop. similar to the picture


i was hoping to be able to include a magnehelix or manometer in the build to indicate when the filters need changing.

but i have a couple of questions:

what is the typical air pressure a residential furnace blower fan gives - i was thinking in pascals or inches of water (this would help me get the right scale gauge)

also, does the pressure change with the blower speed? (I am trying to repurpose a two speed furnace blower i already have)

hopefully this makes sense - any questions let me know

thanks in advance