Buoyant weight

Buoyant Weight SOP, K. Cayemitte, updated Nov 2025

Background:

Archimedes’ principle states that the upward buoyant force on an object submerged in a fluid is equal to the weight of the fluid it displaces. The buoyant weight technique utilizes this principle to make observations about the mechanical properties of solid materials like shells and skeletons. Measures of weight using the buoyant weight technique can be used to calculate skeletal parameters, including micro density, bulk density, and porosity. This protocol is adapted from “Skeletal micro-density, porosity and bulk density of acroporid corals” (Bucher et al. 1998).

Equipment & Supplies Needed:

  1. Gloves/Lab coat/Safety goggles
  2. Datasheet or notebook
  3. 10% Bleach Solution (Bleach + H20)
  4. Container(s) for soaking samples
  5. Graduated Cylinder
  6. Container for soaking skeletons or shells
  7. Vacuum pump (and tubing)
  8. Large filtering flask
  9. Stopper for a large filtering flask
  10. 2 three-prong stop cocks
  11. Desiccator
  12. Carboy
  13. 1 Analytical Balance
  14. Milli-Q Water
  15. Kimwipes
  16. Density Kit for analytical balances A. Beaker stand B. Screw C. Weight D. Weigh bucket
  17. Beaker
  18. Forceps

Reagents & Standardization:

  1. Bleach Solution (10%, amount will depend on the volume needed to submerge samples, ex-100 ml of bleach, 900 mL of DI water, use a higher concentration of bleach if organic material strongly bound)- Measure out bleach using a graduated cylinder and add to the soaking container. Then measure out DI water using a graduated cylinder and add it to the soaking container.

Sample Preparation:

  1. In the lab, add samples to the soaking container with the bleach solution. Soaking samples will allow any biological material and debris to detach from the solid material.
  2. Materials should be kept in solution for 3-5 days, depending on the amount of debris on the samples. Multiple rounds of soaking may be needed if organic material is tightly bound to solid material.
  3. Samples should be removed from the solution after all debris and organic matter have detached, then set to dry until completely dry.
  4. Drying may take 1-3 days.

Sample Measurements (Dry Weight):

  1. Once the samples are dry, take 3 consecutive weights using an analytical balance for each sample; these are your “dry weights”. Record all weights on a datasheet.

Sample Vacuum and Saturation:

  1. Set up the system for vacuum conditions.
  2. Attach the vacuum pump with appropriate tubing.
  3. Tubing should run from the desiccator through a three-prong stop cock (a) to a filter flask, then to the vacuum, to avoid liquid moving into the pump.
  4. Attach a carboy filled with Milli-Q water with appropriate tubing; the carboy should be elevated above the desiccator so that the flow of water moves from the carboy to the desiccator.
  5. Tubing should run from the desiccator through a three-prong stop cock (a) to another three-prong stop cock (b) to the elevated carboy.
  6. Fill the Carboy with Milli-Q water.
  7. Check the desiccator to make sure it is dry and the O-ring is intact. You may need to add grease to hold a sufficient seal. Turn both 3-prong stop-cocks for vacuum conditions.
  8. Add dry samples to the desiccator, attach the lid, then turn on the pump to initiate vacuum conditions.
  9. Wait 1 minute, then tug on the lid to ensure there is an adequate seal. If you are able to break the seal, then there is a leak, or pressure conditions are not holding.
  10. Vacuum samples for at least 3 hours to ensure all the air has been removed from the samples, then turn the vacuum off.
  11. Set up the system for saturation conditions.
  12. First, adjust the three-prong stop cock (a) to saturation mode.
  13. Second, adjust the three-prong stop cock (b) to saturation mode
  14. Third, turn the carboy stopper (c) to the “on” mode
  15. At this point, water should be flowing into the desiccator quickly.

Make sure water is not leaking into the filtering flask. This can damage the pump if water enters the pump.

16.When all samples are saturated (aka the samples are completely submerged in water), turn the carboy nozzle (c) to the “off” mode, and let the rest of the water in the tubing drain into the desiccator.

  1. To release vacuum conditions, pull out the plug on the three-prong stop cock (b). You will hear a rush of air into the system. Once this sound has ceased, vacuum conditions have been released.
  2. Twist and pull the lid of the desiccator; you may need to use a bit of strength to break the seal. If the seal is not breaking, wait longer so the pressure difference can even out between the inside and outside of the system.

Density Kit Set Up:

  1. Remove the analytical balance weight plate.
  2. Obtain the holder, screw, and weight that fit into the analytical balance. Place the weight at the bottom of the holder and attach it to the holder using the screw.
  3. Place the weight attached to the holder into the analytical balance. Then add the beaker holder to the balance.
  4. Place the beaker filled three-fourths of the way with milli-Q water. Place the weigh bucket in the holder so that the bucket is submerged in the water.

Sample Measurements (Buoyant Weights):

  1. Zero the analytical balance and wait for it to read 0.000.
  2. Use forceps to quickly and carefully transfer one sample from the desiccator to the weigh bucket.
  3. Record the weight in the data sheet. This is the buoyant weight.
  4. Repeat this process (steps 1-3) until you have 3 buoyant weights per sample.
  5. To maintain a state of saturation for the samples in between buoyant weight measurements, keep the samples submerged in the desiccator water.

Sample Measurements (Saturated Weights):

  1. Zero the analytical balance and wait for it to read 0.000.
  2. Use forceps to pick up one sample, gently blot with a Kimwipe to remove large drops of water. The goal is to leave a thin layer of water covering the sample so water remains in any pores or crevices, but excess water that may drip off is removed.
  3. Weigh the sample on one of the outer weight plates.
  4. Record the weight in the data sheet. This is the saturated weight.
  5. Repeat this process (steps 1-4) until you have 3 saturated weights per sample.
  6. To maintain a state of saturation for the samples, as evaporation will act on the thin film of water on the outer surface of the sample, take repeated weights as quickly as possible.
  7. Do not place the sample back into the desiccator/submerge the sample between weights.

Mechanical Properties Analysis

Utilize the Mechanical Properties Spreadsheet to input the dry, buoyant, and saturated weights. This spreadsheet will then automatically calculate averages and standard deviations for each type of weight. Automatic calculations of mechanical properties will be performed, which will be used to calculate average microdesnity, average porosity, and average bulk density.

Mechanical Properties calculations:

Terms

Dry Weight (DW) = Sample’s weight in air Buoyant Weight (BW) = Sample weight when partly supported by buoyancy BW = DW – (mass of water displaced) Mass of water displaced = DW - BW

Based on Archimedes’ principle Saturated Weight (SW) = Sample’s weight in air when all pores are saturated with water

Mechanical Properties

Matrix Volume (cm3) = DW - BW
Pore Volume (cm3) = SW – DW Total Volume (cm3) = Matrix Volume (cm3) + Pore Volume (cm3)

Microdensity g/cm3 = mg/mm = DW/Volume of the Matrix Porosity % = (Pore Volume/ Total Volume) * 100 Bulk Density (g/cm3 = mg/mm3) = DW - Total Volume

Written on March 2, 2026