If filter is set in position besides being pointed down an air lock can occur. It can occur in any position but your vent up allows u to bleed excess air do it isn't stuck only allowing liquids to flow around it to get to filter.
If this happens filter will not have equal pressure from fluids against going thru at equal rate throughout whole surface area.
If any small air leak on suction side occurs air will be sucked in, which is no big deal because prefiltered gets filtered, but this air will rise to highest point and create a pocket a build up. It can pressure or compress making back pressure or start beating liquids to filter from least resistance to flow thru and your flow rate of liquids filtered will slow down.
Your vent is to bleed air also to get liquids filtered rather than air.
If u didn't have any issues without venting then- u had good connected sealed connections, u had brand new filter with one sample without problem of it slowing allowing air build up in system. Now if u had no trouble with sample but say it was 100ml tren a and say next u will do 100ml of tren a and tren e blend because filter is full of what it's supposed to filter out, then next sample can require venting unlike first.
Alot of filters say they are good for 500 ml but depending on purity of raws they can get stopped up before at say 250 ml.
They will require venting more and if not careful air back pressure can break bust filter before u notice and decide to change filter. This would mean your whole sample would need refiltering instead of just changing filters.
These are some of reasons vent would be necessary and the using it would be better safe than sorry kind of thing.
I myself would just be sure all connections are secured preventing suction leaks or flow leaks.
I would use a beaker and tilt it to side with something under it to have suction hose able to get as much as possible at end.
I would definitely vent it from any air to be trapped, one simple reason besides back pressure, is the fact that if I am halfway done for example and I had no air in capsule that I already vented then all of a sudden I had air again then I would know my filter is getting stopped up slowing flow rate down and air is getting into system because least resistance will always occur with in any pressure system with air or liquids. If I still had flow I would vent air out and continue. If flow begins to stop I would tighten connects possibly with a clamp.
Knowing your filter max pressure capacity is important. Having a gauge inline between pump and filter us important and prevents wondering.
So if I bleed air out again tighten clamps and not much flow is going thru filter and I didn't have a gauge it would be important to bleed a little pressure once and while to not over pressure filter.
if a gauge was present and I wanted to let pressure out to go down some I can use vent to do this as well.
If there is an air leak on suction end it's possible to never see it because same amount of air could stay in capsule if enough bubbles are passing through making it appear as liquid flow except only air would be being filtered and not much liquid. The vent allows us to stop this illusion.
Anyway if u got it great and I only got stressed out because u asked and was confused. Because you were confused I rambled about it to explain possible reasons and hope u could have visualized it.
Alot of it isn't necessary but sometimes can be mandatory depending on circumstances thickness of liquids, how pure raw is, all of those things.
I have rambled on and on here but feel it's important because someone can be at risk if precautions aren't taking.
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