Volume of Voids for Dummies

Volume of Voids

The famous actor and comedian W. C. Fields advised freelancer writers that, "If you can't dazzle them with brilliance, baffle them with bullshit." Well, actually, he wasn't talking directly to freelancers, but many of them have nevertheless taken the advice to heart. One such Fields aficionado we've found is eHow.com's W D Adkins (William), whose favorite freelancing trick is to throw so much bull at his content editors that they don't realize that he's full of hooey¹. Not to worry, though, because the Antisocial Network's researchers aren't as easily fooled – like the time that Adkins attempted to explain "How to Calculate the Volume of Voids" for Sciencing.com.

Oddly, we'd already seen one example of half-witted freelancing on the topic from eHow, compliments of DotD Jennifer Fleming. What neither Fleming nor Adkins seems to know is that this is cut-and-dried calculation in soil science, based on the mathematical relationship between void ratio and total volume. Adkins would have his readers carry out a demonstration experiment (as opposed to an actual measurement) to show a void volume, using more than 400 words in the process (see what we mean by throwing lots of bull?). The problem we encountered is that this liberal arts graduate didn't know how to carry out a science experiment.

We say that because of his instructions:

Fill a 1,000 ml graduated container about half full of water... Record the weight and... volume of water... Add enough sand to bring the overall level in the container to about 3/4 full. Weigh the container again and record the weight and the volume of material now in the container. Subtract the original weight (water only) from the weight of sand plus water to find the increase in weight. Subtract the original water volume from the volume of sand plus water to find the increase in volume. Divide the increase in weight by the increase in volume to find the specific gravity of the sand. Empty and dry the container thoroughly. Weigh the empty container. Fill the container to the 1,000 ml mark with dray sand. Weigh the container of sand and subtract the weight of the empty container to find the weight of the sand. Divide the weight by the volume (1,000 ml) to find the density of the sand. For example, if the sand weighs 1,500 grams, the density is 1.5. Subtract the density of the sand from the specific gravity of the sand, then divide the result by the specific gravity to find the voidage (proportion of empty space in the dry sand).

Our science staff had so many things to say about that...

We have no idea why he wanted people to do all that crap.  We estimate that adding dry sand to 500 ml of water in a 1000-ml cylinder "to bring the overall level in the container to about 3/4 full" would result in a column of dirty water on top of a short column of wet sand – probably about 550 cc of sand² plus another 200cc of water We don't know why anyone would finally weigh the graduated cylinder halfway through the procedure instead of doing so first. We don't know why Adkins thought anyone would need to separately calculate both the specific gravity and the density of the sand, since they'll be almost exactly the same.  We think, but aren't sure, that Adkins wanted his readers to somehow compare the weight of saturated sand to to the weight of dry sand -- but we aren't certain.  We give up.

Our staffer suspected it would be much easier to place a known volume of sand in the cylinder, weigh that, then (slowly) pour in water until the sand is fully saturated to the top. Weigh the cylinder full of wet sand again and you know the mass of the water filling the void spaces, which is easily translated to the volume of the voids. You also have the total volume, so you can calculate the void ratio and the porosity, too! Easy-peezy, lemon-squeezy. Not terribly accurate, of course, but easy.

We got that suggested procedure from a scientist, though, as opposed to a self-described "professional journalist" with degrees in history and sociology. In other words, from someone who's constructed a scientific experiment or two, not to mention someone who actually knows what void spaces are and how void volume is measured. Here's a hint, dear Dumbass of the Day: it isn't "calculated" with "high-tech tools such as measuring lasers"! Whatever those are...

¹ Admittedly, it's not that difficult to pull the wool over the eyes of DMS content editors
² A rough calculation based on the maximum porosity of dry sand with hexagonal packing, about 36%

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