Wednesday, February 23, 2011

Specific Gravity

A few days ago, I posted a graph. I poured a measured mass of sand into a beaker that was designed to allow any displaced water to flow into another container. I then compared the displaced water to the mass of the sand I added. This gave me a quick way to measure how much heavier than water was the sand. This value is termed "Specific Gravity." It's dimensionless and allows us to understand how much a given amount of something will weigh (or mass, if you aren't factoring in gravity). It compares the mass of an object to an equal amount of water - it's comparable to density.

Instead of taking a few measurements and then averaging all the results, I graphed the mass of displaced water versus the mass of sand. The "trend line" represents a linear best-fit of the data. It's a way to calculate a variable that minimizes the inherent uncertainties with individual measurements. From the line, it appears that the sand is a little more than 2.7 times as dense as water (SG=2.7).

Steve Gough of Riparian Rap (Little River Research & Design) mentioned that their group was trying to figure out a way to measure mass flux in their river models. This got me thinking about how sedimentologists use the specific gravity of a fluid with suspended silt and clay as a method of figuring out how many particles of a given size are in a sediment sample (I talked about Stoke's Law previously here). Over time, the larger particles will fall faster - so the change in specific gravity of the fluid will start at some value and then gradually decrease as the large particles fall to the bottom.

This should work for the material coming out the drain on the river models, too. Although the particles are settling out of suspension much faster, the plastic grains will still cause a small change in the specific gravity as they push the water out of their way on their descent to the bottom. So I ran a trial - sure enough, the sand falling through the column of water created a small increase in specific gravity (about 4%). I also had issues with the sand interfering with the hydrometer as it fell. But, with a little work, one could probably translate change in SG to an amount of sediment moving through the column.

Incidentally, this is one reason why it's nearly impossible to get completely "sucked under" by quicksand. The suspended sand particles in water actually become more dense than regular water. Since the human body is about the same density as water, you are even more buoyant. Although you could still get stuck and starve to death...