Redesign Of The Eyedropper System

[de_pen_dent]

Well, realistically, the ink inside doesnt really heat up to body temperature.

 

So the actual pressure increase (not volume increase - volume remains pretty much the same) is likely closer to 1-2%.

 

Whether this is sufficient to force ink out or not, I cant say.

[hari317]

Well, realistically, the ink inside doesnt really heat up to body temperature.

 

So the actual pressure increase (not volume increase - volume remains pretty much the same) is likely closer to 1-2%.

 

Whether this is sufficient to force ink out or not, I cant say.

The air ink combo will reach a steady state temperature which will depend on some factors, easily modelled, provided you hold the pen long enough.

 

Since the feed is also a vent, the ink is forced out (if nib down), in the process equalising the inside barrel pressure to outside ambient pressure.

Edited August 28, 2013 by hari317

[a_m]

 

Essentially, there would be a barrel that holds ink within the outer barrel of the pen. The air pocket might reduce the tendency for the air inside the inner barrel to heat up. Even though this reduces the size of the capacity from a traditional eyedropper, it would still hold a pretty decent amount and have the advantage of being virtually maintenance free (having no moving parts.)

 

Thoughts?

 

 

That system already exists in Reynold Grippy - very commonly available in India

 

Has even been reviewed by SBE Brown

 

[Vintagepens]

Let's put a couple of numbers into this from a physicist's perspective.

Assume a 2mL capacity barrel with 1mL of ink in it and let's warm this up from 60F to 80F. The Gas Laws tell you that this will cause a 4% increase in the air volume. (Ink is incompressible compared to air). Without a feed in the design, 4% of 1mL will cause quite a big ink blob as it comes out.

As Brian and others point out, today's feeds are able to adequately buffer these sort of ink volume ranges.

If you follow Brian's example of stepping out of a meat freezer at -20F into a hot area at 90F then the Gas Law will tell you what ink volume your feed design must buffer for this temperature swing. Anyway, there is always a long time constant involved in real life for these temperature swings to additionally buffer ink movement.

 

Sorry to jump in so very late into this discussion, but it seems no one has pointed out that the scenario above doesn't exactly conform to the situation where blobbing typically becomes a problem in actual practice.

That is to say, we should be looking at a model where the reservoir is only 10-20% full, not 50%. That would increase the volume of expansion by 60-80%.

 

The comments about modern feeds with high capillary absorption capacity are on point, regarding the physics of contemporary fountain pens. But for those of us with a strong interest in the design and function of older pens (or contemporary pens made in old-fashioned style), the role of hand heat in blobbing should not be dismissed out of hand.