Giant Displacement Tank



In order to answer this question we need to determine each person's MASS and their VOLUME. Mass we can determine easily with a weight scale, but Volume? To figure out a person's volume we have to use the "displacement" method -- in other words, dunk the person into a tank of water and then measure how much water flows OUT of the tank. The amount of water that flows out is their volume. Mass divided by volume will give us density in grams per milliliter (g/mL).

This is our set-up for the Giant Displacement Tank lab. A large trash can on wheels has been fitted with an overflow pipe and placed next to a counter in the classroom.

Two smaller trash cans are positioned below the overflow pipe on one side of the tank to collect water that flows out of the tank.

A ladder is placed to the left to assist students in getting up to the counter top level and back down to the classroom floor. 

With the tank filled to the overflow pipe level with clean tap water, 30 or so students volunteer to "dunk" themselves in the tank and thereby pushing water OUT of the tank in order to measure their "volume."

Measuring that many students takes two or three days of lab time. It's great fun and SO much entertainment!

Water that pours out of the tank is carefully measured and recorded. We measure displacement volume in milliliters.

Here you can see water flowing out of the displacement tank into a receiving trash can. 

For most students it takes two moderately full small trash cans of water to handle their displaced volume.

Water from the two smaller trash cans is measured in large milliliter cylinders, reported to our record-keeper, and then poured into blue and red receiving buckets.

Once a student has had their displacement volume measured, the water is returned to the giant displacement tank and the process begins again for the next student.

Since the process takes several days, we replace the water in the tank each day with clean water and use the "used" water to water the trees outside the classroom.

The final step in our data collection is to tabulate each student's MASS and their VOLUME.

Mass is converted from kilograms to grams.

Volume is measured in milliliters.

After ALL of the data has been collected, it is compiled for calculation. On a separate day from the tank measuring, students are given the entire data set from our displacement tank measurements and asked to determine the average DENSITY for boys and the average DENSITY for girls in the class.