In my last post, I wrote about how we get our samples for moisture and density (MAD) measurements. In this post, I’ll discuss the measurements themselves. We measure three things for MAD: wet mass, dry mass, and dry volume. From these three measurements, we calculate a number of other properties, including porosity, grain density, porewater, and about 10 more. This may sound straightforward, but measuring mass on a boat is not as simple as on land because the boat is rolling!
In order to measure the mass of the sediment cylinders, we have to compare it against a standard of known weight (weights). Two balances are set up together so that we can put our sample of unknown weight in one, and little standard weights in the other (balance). We enter the reference mass in the computer and start the measurement. The mass of both the standard and sample are weighed 300 times over about a minute and a half to get a robust
average measurement. You can see the manifestation of the rolling boat on the computer as it measures the sample and standard! The sample weight is then calculated based on the difference to the known standard.
After measuring our fresh sediment cylinders, we put them in a convection oven to dry over night and weight them again. Next, we measure volume. For this, we use a pycnometer. This is a deceptively clever instrument. In order to measure volume, it uses a derivation of the Ideal Gas Law – P1V1=P2V2. (Pressure1Volume1=Pressure2Volume2.)
We place the sample of unknown volume into a cell of known volume (sample chamber). The pynometer then fills the chamber with helium gas and measures the pressure. It then opens up a second chamber of known volume – the expansion cell. The gas expands into the cell, and the pressure decreases because the volume increases. The pressure is then measured again. So to break the pycnometer measurement down into its Ideal Gas Law components, we have:
P2=Psample cell + Pexpansion chamber
V2=Vsample cell + Vsample +Vexpansion chamber
We have 5 variables, two of which are known (Vsamplecell and Vexpansion chamber) and two of which are measured (P1 and P2), leaving only Vsample left to be solved. All this machine relies on is changing the known volume enough to have a measurable change in
pressure and you can solve the equation. Theoretically, that means you could measure the pressure in the known empty sample chamber, place the sample in, and then measure the pressure with the new smaller volume, but there’s no way to do that without breaking the
seal on the chamber, so the machine simply adds an extra chamber for the gas to be displaced by the sample, so that the measurement can be made. Beautifully simple! Also, it’s calibrated with these shiny standard spheres of known volume.
If volume measurements aren’t your thing and you still made it this far, here’s your reward – another sunset picture and sushi night: