Dumb question about ED and piston fillers

[amk]

Okay, I know that eyedroppers blob and blurt and barf ink all over the place when they're getting empty-ish.

I know that pens with a sac, cartridge, or converter don't, because there is a gap between the barrel which you're warming up with your fingers and the bit that holds the ink.

But why don't piston-fillers burp, then?

You may have guessed I was a humanities student so the science may be very simple for all I know, but I like to know why my Pelikans and my eyedroppered Nauka behave differently.

[fountainpenkid]

Burping is caused by air expansion in the ink reservoir pushing ink out in the only direction it can go--the nib/feed. So a large part of the difference you're wondering about is because a piston filler's capacity is less than you might think: the average 1.5-2mL (today often less) pales in comparison with the 5mL or more an eyedropper might hold. The less volume, the less air-space there will be as the pen becomes empty. With less air-space, there is less air to expand upon heating and push out ink. Another way to address the air expansion problem is, as you were thinking with sacs and converters, through insulation--making the heating process itself slower.
Obviously, modern feeds, with their many fins, are designed to mitigate the burping effect by trapping the ink that does get pushed out using capillary action (the higher viscosity and surface tension also has a role here). If you're talking vintage eyedroppers, many were made before this advancement in feed design, so the burp tendency created by their large air-space is exacerbated by their burp-incompetent feeds.

[Ole Juul]

I have two pens with sacs that will happily burp if I don't watch it. ... Just sayin'.

[Empty_of_Clouds]

You know, if ebonite pen makers - like those in India - put a nipple on the rear end of the section, then users would have the option of fitting a rubber ink bag instead of using as an ED. For what it is worth, I've had lever fillers where it has been more convenient to pull the section and fill by manually squeezing the ink bag. Just a thought.

[SlowMovingTarget]

So... Piston-fillers do, it is simply far more dainty, as fountainpenkid mentions. It's also more of a blurt than a burp. You get a fat line for half a letter and it calms right back down again. At least, with my TWSBI Ecos, that's been my experience.

Also, not all eyedroppers are created equal. The Namiki Emperor, for example, has a valve that can close off the larger reservoir, limiting the air pressure interaction to whatever is in the tiny secondary reservoir just atop the feed. Visconti pens have this, Opus 88 eyedroppers also have this. (I have an Opus 88 Demonstrator, and it doesn't burp at all, for this reason.)

[lsmith42]

ED is no laughing matter... happens to everyone at some point


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[Ray-VIgo]

Sac pens will burp, particularly if you are down to the last drop of ink and the rest is air inside. I have lever fillers that will do it, but it's not all that severe. It tends to happen most on pens with the old-style "spoon" feeds that are unable to pick up the extra flow.

I've had Pelikan piston fillers burp to some degree as well. The older feeds aren't as good at taking the extra flow as the newer ones, but none are as bad as having an old Waterman with a spoon feed blob, for example.

The eye droppers, especially the oldies with plain feeds will blob because of the extra pressure of all that warm air, and the old feed's design not being able to take much extra flow.

But that is to say, they all will do it, it just is worse on some versus others. The best solution is to keep the ink level above a certain point so long as you plan to use the pen. It's not always easy, but just part of what you have to do sometimes.

[amk]

Thank you! Turns out it may have been a dumb question but I got some very clever and informative answers.

[Ron Z]

I keep posting this.....

Many pens (though not every pen) tend to flood, or burp if you will, or write with a very wet line when they are close to running out of ink. Even the modern Parker repair manual attributes flooding to a pen that is nearly empty. The modern repair manual trouble shooting page under "Complaint: pen floods" has as the remedy "Tell customer to fill the pen." With the manual sitting down in the shop, maybe not a verbatim quote, but accurate enough.

This is why I say you have to fill a pen when you test it, not just dip it, to see how the pen really writes.