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"Ullage" is a useful name for the empty space that often remains in a container of liquid after it has been filled as much as is practical. (Think of the small space at the top of a sealed bottle of wine.)

Is there a name for the complementary situation, where a vessel has been emptied as much as is practical, or its design allows, but there still remains in the vessel a quantity of liquid that is inaccessible? I can think of "dregs" but wonder if there is another word that does not have the connotation of "the worst part" as "dregs" has. A word that could be used in the following sentence without disparaging the quality of the liquid that has been lost.

"The poor placement of the submersible pump made dregs unavoidable."

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    So, something way more specific than residue or waste, I assume. Maybe there is jargon for it. Good luck in your search, and welcome to ELU. – KannE Jan 29 at 8:15
  • Residue is usually restricted to solid matter. Dregs does not mean inaccessible. I too do not know a word for the distinct idea you describe, however. – David Jan 29 at 17:19
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    BTW, residue is the word they used for it (pubs.acs.org/doi/abs/10.1021/acsami.6b06958). I found that link here (physicscentral.com/explore/plus/to-the-last-drop.cfm) while searching "the last bit" online. Anyhow, this is interesting: "By further extension, in brewing and beer retail, ullage is the residue of beer left in a barrel that has been emptied" (en.wikipedia.org/wiki/Ullage), assuming that's accurate. – KannE Jan 30 at 2:27
  • I am imagining an oil sump, and the liquid at the top is the same as the liquid at the bottom. Ullage is distinct - it differs from the liquid. I'm also confused: You say "... a quantity of liquid that is inaccessible." and "...made dregs unavoidable." Is it unavoidable or inaccessible? I suggest "The poor placement of the submersible pump made a quantity inaccessible." – Greybeard Feb 23 at 20:42
  • I've heard UK pub landlords use "ullage" to describe the beer lost through spillage at the taps. This beer website says the word has a different meaning in the US and UK. beerandbrewing.com/dictionary/qJJRUFDEWB "It is also the amount of beer in a cask when it is not full or the data-volume of liquid and sediment in a cask after all the saleable beer has been removed". – Kevin Ryan Feb 23 at 21:49
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In trucking we call it the heel. In some cases it's referred to as "tank heel." As sited at (id.energy.gov/EIS/Chpts/ch_7.pdf), "A tank heel is the amount of liquid remaining in each tank after lowering to the greatest extent possible by use of the existing transfer equipment, such as ejectors."

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    Hello, Westerdale. Does this refer solely to what can't be removed from a fluids tanker? Can you please add either a dictionary reference, or an example sentence from the internet? – Edwin Ashworth Feb 23 at 19:38
  • In some cases it's referred to as "tank heel." As sited at (id.energy.gov/EIS/Chpts/ch_7.pdf), "A tank heel is the amount of liquid remaining in each tank after lowering to the greatest extent possible by use of the existing transfer equipment, such as ejectors." There are other examples as well. We just call it the heel. – Westerdale Feb 24 at 19:43
  • @Westerdale Please click the edit button on your answer and add the above info into the body of the answer. You don't have to indicate the edit, answers can be freely edited here. – Phil Sweet Feb 24 at 23:49
  • ... this may be old news, but in the vernacular, you may hear: "We loaded the tank to a 10-inch headspace (ullage), and then drained it down to a 2-inch heal." The 10-inch headspace refers to the distance between the capacity line and the top of the liquid. The 2-inch heal refers to height of the liquid line from the bottom of the tank. I like referring to the bottom remnants as "ullage" as suggested, but I don't think in the field it would be recognized specifically as the bottom remnants. Maybe "remnants", "bottom remnants" or "remaining product" may be better fitted to your example. – Westerdale Mar 18 at 10:38
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I think it's worth getting a little deeper into what ullage is. The word crops up in several seemingly contradictory usages.

Capacity A tank has exactly one capacity. It is the volume from bone-dry empty until stuff starts to pour out of the vents. There is only one answer to the question what is this tank's capacity?

Ullage refers to measuring the distance down to the liquid surface. It starts from the top and measures down to the surface. The manual method uses a table to convert ullage to the amount of liquid present. The zero point is usually the completely empty tank—no heel. The volume indicated is called—wait for it—the ullage.

"Ullage" is a useful name for the empty space that often remains in a container of liquid after it has been filled as much as is practical.

Yes, and for any other state of filledness as well.

Sounding refers to measuring the filled space from the bottom upwards to the surface. The zero point for sounding is the point of loss of suction of the draft pipe or pump. This makes sense if it's your gas tank, and is the norm everywhere. The volume indicated is called the sounding.

So a full tank, a half tank, and an empty tank all have an ullage and a sounding. But if the ullage table is zeroed at a completely empty tank, you still have nonzero ullage at loss of suction. So when converted to liquid volume, ullage and sounding don't yield the same number.

Hopefully, this explains why a valid answer to what is the complement of ullage is—ullage. As already mentioned by others, it is used in brewing and distilling. It is also used this way in shipping. Heel would be problematic on ships. Another term that isn't exclusive to the pumped-down scenario, but is used for that situation, is ROB, which stands for Remaining On Board.

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It seems the complement to "ullage" is "ullage" or "heel"! Thanks especially for the references, especially "ullage" which seems to gain a precise meaning only in context.

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