Unlike folding quadrants, pleating a fluted filter is not that easy and require some training or some patience. We propose an 8 steps method using symetry to improve the consistency of the pleats.
Prepare a quadrant and start from the top half. Form an "accordion" by folding back and forth to make 4 or more pleats. Return the quadrant and do the same on the other (flat) side. This way is faster but you need to know the exact size of the pleat (different for each diameter) and stay consistent. You might waste a few dozen discs before you get the hand of it...
Well, there is no miracle solution as long as you consider manual pleating. But if your budget can accept a small increase, the much faster way is to buy pre-pleated filter paper.
]]>There are a few different methods to fold a filter paper disc into a funnel-ready quadrant.
We propose an easy 3 steps method bellow.
Why some manufacturers express the filter's permeability as pore size while other use retention? And what the difference between pore size and nominal pore size?
For membranes, defining a pore size is easier because they have a relatively consistant shape whereas filter paper don't really have pores in the sens of capillar holes. For paper filter, the "pores" are virtual 3D paths in the matrix of tangled fibers so we just can't measure them optically. We can still attempt to aproximate an average ideal pore size based on the behavior of the filter flow. This is the nominal instead of absolute pore size.
Retention is the capacity of the filter to retain particles of a given size. To keep it simple, let's not consider particle or pore shape. For a membrane, a pore size smaller than a particle size should retains the particulate. For a paper filter, depending where the particle land, it might be retained on the surface, or in the thickness of the filter or pass through. There is a distribution of "pore" size with a probability that a certain size of particle will be retained. Moreover as the filter gets loaded with more and more particle, its flow get reduced. Retention needs to be evaluated at a given efficiency, usually > 80%.
To summarize, pore size is the ideal diameter of an imaginary tube passing through the filter while retention size is the ideal diameter of the smallest particle blocked by the filter. In filter paper, pore size measure an average distribution while retention measure a minimum and is what matter if you want to retain a given particulate.
]]>Have you ever tried to compare filter paper between brands? Found a few cross-reference tables only to find out they contradict each others? You might have wondered how come nobody came up with a standard to describe grades and make sens of this mess?
When it comes to basic grades, the de-facto standard is Whatman, but what about the myriad of grades offered by other manufacturers that don't fit? The problem lies in the many parameters that define a filter paper, a lot more than what can be caracterized by flow or retention alone. Here is a non-exauhstive list of the main paper filter characteristics:
Both methods turn a flat filter paper disc into a conical shape fitting a standard 60° funnel so it can hold more solution and use gravity filtration more efficiently.
While a quadrant folded filter paper is a disk simply pre-folded in a quarter of a circle, a fluted filter paper has been pre-pleated to minimize the contact surface of the filter with the funnel, decreasing filtration time.
There are a few variants of the quadrant or quarter folded filter paper method, all turns a disk into a smooth conical shape that fir a standard funnel. Folding is fairly easy but it takes some extra steps to make sure the conical shape remains in place while pouring the solution.
Fluted filter paper is a disc that has been pre-pleated, forming a sort of accordion to maximize the effective filtration surface of the filter. Even though there are a few methods to fold a disk in 8 pleats, it's not an easy task and it takes some time. Most fluted filter paper sold by brands uses machines to ensure a higher consistency of the shape and a more accurate position and size of the pleats. Small discs (<100 mm diameter) often have 8 pleats, larger diameters can have from 10 to 16 pleats, maintaining a well defined conical shape. This prevent the filter from sticking to the funnel, increasing the surface area that can effectively filter the solution thus speeding up filtration.
Fluting of folding a disk increase gravity filtration efficiency compared to a flat disk, but it can't be used for vacuum (suction) filtration which requires a flat Büchner funnel.
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