There are a few different ways I can think of to set up this experiment, but both ways will need a scale, preferably a kitchen scale, something which is pretty sensitive to small weights.
1. Take a diaper from each brand, and weigh with a scale and record the beginning weight.
2. Poor water on diaper until completely saturated- or immerse underwater.
3. Take the diaper out of the water and let it drip for a predetermined time, lets say 2 minutes. (the idea with the dripping is to remove the water which has not been 'absorbed' into the diaper- for instance, the water which is on the surface of the diaper)
4. Weigh the diaper once again, and record the saturated weight. The difference between the starting and ending weight will be the weight of water each diaper has absorbed. The diaper with the biggest difference, will have absorbed the most amount of water.
(*Note: you will have the weight of water, not the volume of the water. If you would like to also know the volume, try converting the measurements into metric, where one gram (g) of water is exactly one milliliter (mL). Liters are a measure of volume.)
Another way to run this experiment is to cut out a square from the absorbent part of the diaper, lets say 3 inches by 3 inches, and repeat the same process as above for each of the brands. The advantage of this is that it removes all the extra water that might have been absorbed by the fabric of the diaper from effecting in the results, or from one brand having a larger size absorbent area- however it may also require a more sensitive scale. This version will tell you which brand uses the most absorbent material in their diapers. Doing both these experiments is not a bad idea- they may not agree with each other.
The things to keep in mind as your doing this experiment is to do it exactly the same way for each diaper. For better results, you could do it several times for each brand and then average the results.
Other questions you may want to answer as your doing this experiment:
* Do certain brands drip more than others?
* Some claim that they 'lock in' the water, is this true? what if you press on them? How much comes out?
* Do the diapers have the same amount of absorbent material in them?