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Obesity is currently one of the largest health problems in America. Numerous studies positively link obesity with diabetes, heart disease, acid reflux, uncomfortable plane rides, being shunned from amusement park rides, etc. Normally the Body Mass Index (BMI) is used to determine if you are a healthy weight, but the BMI is really an inadequate system. It is not very difficult for someone to fall into the overweight category of the BMI because they have a higher than normal ratio of muscle to fat.
Body composition or your body fat percentage is a much more reliable indicator of health and fitness, but it can be difficult to measure. The three most common methods of calculating body fat % are:
-hydrostatic (underwater) weighing
-skinfold (caliper) measurements
-Bioelectrical impedance
All of these methods have their inherent strengths and weaknesses, but underwater weighing is the "gold standard" for accuracy. If you want to achieve the best possible accuracy you would need a much more elaborate set-up, but in this instructable I will show you how you can estimate your body fat % with fairly good accuracy using nothing but a spring-based bathroom scale and this instructable!
NOTE: I have a tendency to try and convey to much information. If you just want the big picture/to give this a quick try, read only what's written under "Essential Info". If you would like to improve accuracy and learn a bit check out what's written under "Additional Info".
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Essential Info
The two equations in this picture are the equations that will be used to calculate your body composition. The steps in this instructable will show you how to estimate or calculate the different variables in the equations. If seeing these equations gives you nightmares about high school/college math classes, have no fear. Step 7 has a spreadsheet which will automatically do all the calculations necessary, and all you have to do is plug in the appropriate figures: Gender, Dry Weight, Underwater Weight, Age and Height (or Vital Capacity), and Water Temperature (or density of the pool water).
I have also attached the spreadsheet to this step if you would like to open it up and play around with it as you go through; however, more explanation of how it works will be in Step 7.
Additional Info
This test is really to determine the density of an individuals body. By plugging the density of our body into a well studied equation (Siri Equation) we can determine an individuals body composition (%BF). You can determine our density (as I will show you in the following steps) by taking a ratio of our weight in the air to our weight underwater. The difference in our weights will result from what is known as buoyancy force. The buoyancy force is related to our density through Archimedes' Principle.
According to Archimedes' Principle things with a larger density than water will sink (in water), that with a smaller density than water will float, and things with the same density as water are what's known as neutrally buoyant (aka it will hover at any depth until you give it a push up or down);
FUN FACT: NASA has what's known as a neutral buoyancy lab where astronauts train for space missions because neutral buoyancy is very much like zero gravity...but anyway.
Unlike a piece of metal, out bodies are composed of different materials with different densities. Luckily our bodies are composed primarily of water, and thus the majority of our components (skin and organs) have a density very similar to that of water and won't contribute to our weight underwater. The parts of our body that have significantly different densities than water are our muscles, fat, bones, lungs, and GI tract.
These parts of our body contribute to our underwater weight (UWW) by either helping us to sink or float, and their densities are approximately as follows: fat floats (density approximately 0.9 g/cc), muscles sinks (1.1 g/cc), and the air inside out lungs and GI tract floats (0.0012 g/cc). The volume of a human being's GI tract is relatively small and constant across most sized people, so this factor is built right into the density calculation. After exhaling all of the air we can from our lungs they are not completely empty; what's left is known as the Residual Volume (RV) of our lungs. Our individual RV can be estimated based on size, age, and sex; this information is generally known to an individual and the attached spreadsheet will make it easy for you to approximate this. Alternatively you could estimate your RV as a certain percentage (contained in the spreadsheet) of your Vital Capacity (the maximum amount of air you can exhale in one breath) which I will show you how to measure in Step 6.
After accounting for the buoyancy of our lungs and GI tract, the only factors left are the amount of fat, bone, and muscle we have. We can then solve for them in terms of a ratio of the mass of fat we have, to the amount of fat-free mass we have (muscle and bones); this ratio is more commonly referred to as our body composition or body fat percentage (BF%).
This is a relatively brief overview of the science behind this process. If you are interested in a more detailed explanation of the process using a professional set-up check out the following link (it's a brilliant explanation actually):
http://nutrition.uvm.edu/bodycomp/uww/uww-toc.html
A lot more information about different methods of calculating body fat % as well as the sources for the equations I'm using came from the following source: http://www-rohan.sdsu.edu/~ens304l/uww.htm
You can also always check out Wikipedia:
http://en.wikipedia.org/wiki/Archimedes_principle
http://en.wikipedia.org/wiki/Hydrostatic_weighing
The two equations in this picture are the equations that will be used to calculate your body composition. The steps in this instructable will show you how to estimate or calculate the different variables in the equations. If seeing these equations gives you nightmares about high school/college math classes, have no fear. Step 7 has a spreadsheet which will automatically do all the calculations necessary, and all you have to do is plug in the appropriate figures: Gender, Dry Weight, Underwater Weight, Age and Height (or Vital Capacity), and Water Temperature (or density of the pool water).
I have also attached the spreadsheet to this step if you would like to open it up and play around with it as you go through; however, more explanation of how it works will be in Step 7.
Additional Info
This test is really to determine the density of an individuals body. By plugging the density of our body into a well studied equation (Siri Equation) we can determine an individuals body composition (%BF). You can determine our density (as I will show you in the following steps) by taking a ratio of our weight in the air to our weight underwater. The difference in our weights will result from what is known as buoyancy force. The buoyancy force is related to our density through Archimedes' Principle.
According to Archimedes' Principle things with a larger density than water will sink (in water), that with a smaller density than water will float, and things with the same density as water are what's known as neutrally buoyant (aka it will hover at any depth until you give it a push up or down);
FUN FACT: NASA has what's known as a neutral buoyancy lab where astronauts train for space missions because neutral buoyancy is very much like zero gravity...but anyway.
Unlike a piece of metal, out bodies are composed of different materials with different densities. Luckily our bodies are composed primarily of water, and thus the majority of our components (skin and organs) have a density very similar to that of water and won't contribute to our weight underwater. The parts of our body that have significantly different densities than water are our muscles, fat, bones, lungs, and GI tract.
These parts of our body contribute to our underwater weight (UWW) by either helping us to sink or float, and their densities are approximately as follows: fat floats (density approximately 0.9 g/cc), muscles sinks (1.1 g/cc), and the air inside out lungs and GI tract floats (0.0012 g/cc). The volume of a human being's GI tract is relatively small and constant across most sized people, so this factor is built right into the density calculation. After exhaling all of the air we can from our lungs they are not completely empty; what's left is known as the Residual Volume (RV) of our lungs. Our individual RV can be estimated based on size, age, and sex; this information is generally known to an individual and the attached spreadsheet will make it easy for you to approximate this. Alternatively you could estimate your RV as a certain percentage (contained in the spreadsheet) of your Vital Capacity (the maximum amount of air you can exhale in one breath) which I will show you how to measure in Step 6.
After accounting for the buoyancy of our lungs and GI tract, the only factors left are the amount of fat, bone, and muscle we have. We can then solve for them in terms of a ratio of the mass of fat we have, to the amount of fat-free mass we have (muscle and bones); this ratio is more commonly referred to as our body composition or body fat percentage (BF%).
This is a relatively brief overview of the science behind this process. If you are interested in a more detailed explanation of the process using a professional set-up check out the following link (it's a brilliant explanation actually):
http://nutrition.uvm.edu/bodycomp/uww/uww-toc.html
A lot more information about different methods of calculating body fat % as well as the sources for the equations I'm using came from the following source: http://www-rohan.sdsu.edu/~ens304l/uww.htm
You can also always check out Wikipedia:
http://en.wikipedia.org/wiki/Archimedes_principle
http://en.wikipedia.org/wiki/Hydrostatic_weighing
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I have considered using a fish scale, but I didn't have one on hand at the time. Plus I didn't have time to build a rig to hold it and still get it entered in the contest.
My last step does have a crude paint sketch on how someone could use a diving board to do it though!