The Physics Behind Eyedropper "burping"

[k3eax]

My eyedroppers will begin to burp when the ink supply is low. I can stop this annoyance by unscrewing the section to admit air. What is the physics behind this?

Edited June 26, 2014 by k3eax

[WayTooManyHobbies]

You are equalizing the air presure behind the ink with the ambient pressure. As a result - no burps!

[Mr.Rene]

Use a Breather Tube in the feed and burpi will stop...

Regards

[k3eax]

Why the need to equalize the pressure by removing the section? This should be accomplished by the breather passage. I should assume that there is some deficiency in the operation of that passage.

 

One would assume that the burping results from an higher than ambient pressure in the ink-supply chamber. Yet it would seem that poor venting through the breather passage would result in a lower than ambient pressure in the supply chamber which would cause a stoppage of flow rather than burping. .

[Jimmy Joe]

As temperature goes up, the product of volume and pressure has to go up for a fixed quantity of gas, i.e., the air inside the eyedropper. If volume is constrained, the pressure difference between the inside of the pen and the outside creates a force that drives ink out of the pen. The smaller the starting volume of air--the more full the pen is--the smaller the absolute volume change required to equalize the pressure for a given temperature change. The feed can absorb a certain amount of ink flowing out of the reservoir. If the volume change is less than this, no burp. This is why the pen burps when the ink level is low, but not when it is high. Burping can be reduced by a feed that can handle larger surges of ink, or by giving the air an alternate path to equalize the pressure without pushing ink through the feed, such as a breather tube or by unscrewing the section. For example, I have several Wality 69Ts. With the simple feeds they come with, they burp. If the feeds are replaced with modern Sheaffer ones, I can write the pens dry without a single burp.

[Scribblesoften]

You may not have to unscrew you section. Sometimes you can just hold your pen nib up for a while. This allows the ink to run back into the pen and the air to escape out the nib/feed.

[k3eax]

Thanks to those who took the time to reply,

 

It's been suggested that a Scheaffer breather tube might be fitted. Are these or other breather tube available separately? Is it possible to adapt other tubing to this ise?

 

By the way, the pen in question with the burping problem is an Indian-made "Doctor-Deluxe". And, I do have several other Indian-made pens, yet to be inked, that I'm quite sure will exhibit the same problem..