|Stacked filter unit with a very similar design to our own (except that we will have eight columns instead of just one).|
These filters are Nuclepore in spirit but without the eponymous brand name. They are both made of polycarbonate with etched holes to trap coarse and fine particulates, respectively. Notice how much more the 47 mm filters cost. This is due to the grease coating used to prevent bounce-back. I don't know how bad the bounce back is in a straight-shot filter yet it has been argued enough to necessitate the extra cost of the greased filters. I don't know if grease would benefit the smaller size fraction as well. Also the coated disks come in 47 mm diameters only, so we needed to cut them down to size. There were many email discussions about cutting filters in a replicable way, i.e. we need to pre weigh filters accurate to 1 microgram so piece can't be later falling off. The plan is proceed using a sharp steel punch (made courtesy of the workshop in the Dunn basement) sandwiched between two filter papers.
Otherwise the filters look good; the 25 mm filters allows an air flow rate up to 62.5 litres per minute and stable until about 140 degrees Celsius and the coarse filter is much the same. As we are operating these at 4.0 lpm and a temperature between 5-40 degrees, there should be no issue with these parameters.