How do you "create neagtive pressure"?

[UncLeJunkLe]

When someone complains their tank is leaking or not wicking/feeding juice, someone usually inevitably comes along and says "you need to leave some space above your juice and create negative pressure in your tank", or something similar.

Explain to me like I am 5 years old how one goes about "creating that negative pressure" in a vape tank?

Also, in the case of juice wicking/feeding, how does negative pressure help with that?

 

[Territoo]

You close all the air flow vents and suck on the vape a few seconds without powering the vape. Then you open back up the af and vape. Sometimes it works, sometimes it doesn't. Supposedly the negative pressure created by sucking causes the o-rings to seal off, stopping any leaks. I'm really not sure how it would help with wicking.

 

[WorksForMe]

For years I've seen people talk about needing negative pressure in a tank to keep the juice from leaking out, but I don't believe it. Air can flow through your wicking back in the tank, so the pressure inside the tank will be the same as outside the tank.

If you put a straw in a glass of water, cover the top, and pick it up, water will stay in the straw. If a little air leaks into the straw at the top, a little water will leak out the bottom. Juice stays in a tank the same way.

I know people will disagree with this, but that's what I believe.


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[denali_41]

when you fill the tank you have zero pressure,once you start vaping you now have vacuum which then feeds the wick and if you're quick enough you will see a bubble or twp of air rise from the wick

 

[ppeeble]

'Negative pressure' is a term used by sloppy or ignorant people. It's either pressure or vacuum.....
If i use a top fill tank and take the top off for an hour or two then liquid will find it's way out through the air holes. This is gravity at work. Putting the top on will stop this (mostly) - therefore something has changed between the two scenarios. Since we cannot negate gravity simply by adding more mass to the tank (putting the top back on) then another force must be at work...... This is a vacuum seal. Leaving air space above your liquid allows the vacuum seal to work - not by allowing the air to form a seal but by equalising the forces between the vacuum and gravity. People often get the advice right but use faulty logic to get there.
I'm starting to bore myself now so will bow out

 

[cats5365]

I see it two ways. It might not really be negative pressure, just equalizing the pressure.

1. If I have a top fill tank and have just filled the tank and replaced the cap, I might be pressurizing the tank by screwing down the top and letting the air/bottom of the top compress against the juice. I relieve the pressure by taking a quick, unpowered hit to pull any excess air out of the tank through the drip tip. This might not be needed had I not been greedy when filling the tank.

2. Sometimes I think a bottom airflow tank can get a bit of siphon started if you have juice leaking out through the air holes and it starts pulling juice down the wick and out the holes. A quick, unpowered hit can sometimes temporarily stop this leaking by breaking up the wicking action. If I draw too hard, I have pulled the wick out of the juice pocket in the tank and opened the floodgates. Most of the time, the only solution to the wick siphon is to just re-wick the coil and start over.

^^^Actually, I think it's called capillary action where the juice follows the juice trail out of the tank.

 

[BillW50]

Negative pressure or more correctly a vacuum works a lot like those automatic bird water dispensers.

The water can't dump out because there is a vacuum at the top preventing the water from running all out. Your tank works the same way. Bottom fill tanks automatically gets the vacuum when you turn them right side up. Top fillers doesn't until you start vaping and the bubbles comes up. But if you tip them upside down for a few seconds (careful some juice can drip from the drip tip on some models) and then upright them, now they too have automatic vacuum.

 

[Z-Lee]

'Negative pressure' is a term used by sloppy or ignorant people. It's either pressure or vacuum.....
If i use a top fill tank and take the top off for an hour or two then liquid will find it's way out through the air holes. This is gravity at work. Putting the top on will stop this (mostly) - therefore something has changed between the two scenarios. Since we cannot negate gravity simply by adding more mass to the tank (putting the top back on) then another force must be at work...... This is a vacuum seal. Leaving air space above your liquid allows the vacuum seal to work - not by allowing the air to form a seal but by equalising the forces between the vacuum and gravity. People often get the advice right but use faulty logic to get there.
I'm starting to bore myself now so will bow out

This was more or less what I was going to say. It's a vacuum, not a negative pressure chamber. If seals are not properly sealing or there's alternative ways for the air to enter the tank (like exposing the refill port), then there's nothing preventing gravity from pushing the remaining juice in the tank, out through the coil into the chimney. "Negative pressure" in this case would be the air that displaces the draining of the juice in the tank, being a lower comparative pressure than the pressure needed to push the juice through the coils, out into the chimney. Once you expose the refill port, that lower comparative pressure equalizes with the rest of the ambient pressure allowing gravity to take over and juice to transfer to the area of lowest pressure - the area there isn't juice. The chimney. Viscosity and other factors play into this as well.

In regards to wicking, if air bubbles aren't displacing the juice that's being used up, then the osmotic exchange of pressure cannot occur which means juice will no longer feed into the wick.

I'm interested to look this up more when I get home. There's a better way to explain this if you were 5, lol.

 

[zoiDman]

...

If you put a straw in a glass of water, cover the top, and pick it up, water will stay in the straw. If a little air leaks into the straw at the top, a little water will leak out the bottom. Juice stays in a tank the same way.

I know people will disagree with this, but that's what I believe.

What you are Describing with the Straw/Bubble Analogy involves the concept of Displacement.

With your Finger over the Top of the Straw, the Capillary Action of the Water on the Straw keeps the water in contact with the Straw Walls. If the Straw is narrow enough, the Force of Gravity isn't enough to overcome the Capillary Action/Surface Tension and allow any Air to rise up the Straw.

But if say you Introduce a Bubble to the bottom of the Straw, then as you Mentioned, the amount of water Equal to the Volume of the Air Bubble will leak out. This is Displacement. And when it happens, the Straw/Water "System" returns to a Static State again. No More Displacement, No More Water coming out the Bottom of the Straw.

Lots of Words to Describe what Everyone Knows. I Know. LOL. But here is the Big Black Fly in the Frosting. What Happens if you Remove your Finger from the Top of the Straw? Simple. All the Water Comes out of the Bottom of the Straw. Duh.

So why Doesn't that Happen every time I unscrew the Top Cap of my Tank/RTA to Fill it? Because Isn't that like Taking the Finger off the Top of the Straw?

---

My Theory about why a Tank sometimes Leaks and Why they Mostly Don't is that is More than One thing going on.

I believe New Wiking Material plays a Big Role in things. Because whenever I have gotten Massive Flush type Leaking, it Generally Happen more when the Wicking Material was New.

I also believe that Hydrostatic Force plays a Role in things. Because a 1/2 Full Tank is (from what I've Seen) much Less Likely to Leak than a Tank that is Fully Filled.

I think your Straw Analogy/Displacement isn't Wrong. I just think it Is Part of what is going on. But Not Everything.

Sometimes when you Can't Figure Out why Something Works, figuring out why something Doesn't Work can Help. So maybe a Good Question we should Asking is... "Why e-Liquids Don't Leak from a Tank?"

 

[zoiDman]

^^^Actually, I think it's called capillary action where the juice follows the juice trail out of the tank.

I think it Plays a Significant Role in Massive, Flush Type Leaking.

 

[BillW50]

With your Finger over the Top of the Straw, the Capillary Action of the Water on the Straw keeps the water in contact with the Straw Walls. If the Straw is narrow enough, the Force of Gravity isn't enough to overcome the Capillary Action/Surface Tension and allow any Air to rise up the Straw.

That isn't how it works in a tank or even an automatic bird water dispenser. The liquid should go nowhere until you let air bubbles in. If no air bubbles can get in, you end up with vapor lock. Which means liquid starved (too much vacuum). So the wicking has to be loose enough to keep the wicking wet, but not loose enough to allow air bubbles to freely pass to empty the tank dry.

 

[Ed Brown]

I'll just stick with drippers and top air tanks.

 

[englishmick]

Negative pressure or more correctly a vacuum works a lot like those automatic bird water dispensers.

View attachment 963505

The water can't dump out because there is a vacuum at the top preventing the water from running all out. Your tank works the same way. Bottom fill tanks automatically gets the vacuum when you turn them right side up. Top fillers doesn't until you start vaping and the bubbles comes up. But if you tip them upside down for a few seconds (careful some juice can drip from the drip tip on some models) and then upright them, now they too have automatic vacuum.

That sounds like how I understand what goes on. Combination of gravity and capillary action try to pull the juice out the bottom of the tank. In a well sealed tank if juice flows out at the bottom the air pressure above the juice would be reduced relative to the air pressure outside the tank. Just a small difference in air pressure will hold the juice in place. Differential air pressure is a strong force, gravity and capillary action are weak forces.

We call it a vacuum but it's actually just slightly reduced air pressure.

I read a really good explanation here years ago by someone with expert knowledge. Including how taking a hit causes air bubbles to make their way to the space above the juice. I don't remember how that part of the process works.

 

[UncLeJunkLe]

how taking a hit causes air bubbles to make their way to the space above the juice. I don't remember how that part of the process works.

They look like they just float up, to me.

 

[Z-Lee]

They look like they just float up, to me.

I think he means the osmotic transfer of ambient air being pulled through the coil/wick to displace the juice.

 

[zoiDman]

That isn't how it works in a tank or even an automatic bird water dispenser. The liquid should go nowhere until you let air bubbles in. If no air bubbles can get in, you end up with vapor lock. Which means liquid starved (too much vacuum). So the wicking has to be loose enough to keep the wicking wet, but not loose enough to allow air bubbles to freely pass to empty the tank dry.

Should and Shouldn't seem to be Relative Terms. But Tanks do Leak sometimes. And Most of the Times they Don't.

I'm Not sure why "Vacuum", in the context you used, would pay any role in Wicking? A Air Vacuum, or Negative Atmospheric Pressure, would be Exerting a Force on the e-Liquid in the Tank? Not really on a Bubble introduced at the Bottom of a Tank.

BTW - How Much Vacuum are we talking about?

BTW2 - The Straw/Water analogy is Actually a Pretty Good Analogy as to what is going on Inside a Tank/RTA. Not 100% Complete. But Not Bad on a Conceptional Level. A Huge Difference, of course, is the Viscosity of the Fluids we Vape as compared to Water.

BTW3 - I just Re-Read what you Posted. When you said "Vacuum", are you referring to a Lowering of Pressure of the Air in the Tank. Or was Vacuum referring to the e-Liquid? Or More Specifically, the e-Liquid at the Bottom of the Tank our the e-Liquid Inlet Ports?

 

[zoiDman]

I think he means the osmotic transfer of ambient air being pulled through the coil/wick to displace the juice.

I Don't think Osmosis is the Process you are looking for. Because I believe that Osmosis deals with the Relationships of Solution/Solute Concentrations.

But your Concept of e-Liquid being Displaced by Air I think is Right on the Money.

 

[UncLeJunkLe]

This is why I prefer Juice Flow control and I think it should have been made a standard a long time ago. It doesn't solve the problem 100%, but almost...if you use it right and always shut it down when not in use for a while. Getting the wicking "just right" becomes a thing of the past as long as the juice control mechanism is variable.

 

[Z-Lee]

This is why I prefer Juice Flow control and I think it should have been made a standard a long time ago. It doesn't solve the problem 100%, but almost...if you use it right and always shut it down when not in use for a while. Getting the wicking "just right" becomes a thing of the past as long as the juice control mechanism is variable.

Exactly why the Taifun GTIV is my top favorite RTA, on top of its wonderful and luscious vape.

 

[Z-Lee]

I Don't think Osmosis is the Process you are looking for. Because I believe that Osmosis deals with the Relationships of Solution/Solute Concentrations.

But your Concept of e-Liquid being Displaced by Air I think is Right on the Money.

I think you're right - I don't think osmosis by definition was the word I was looking for, but the idea of a substance being transferred through a semi-permeable membrane. In this case, the coil/wick. I should have said "osmotic-like".