The best way to measure diaper performance is to evaluate its most important attributes: Absorbent Capacity, Absorbent Retention, Speed of Absorption, Diaper Re-wet, Fit and Comfort. I will refer to the first three because they are more objective and easier to measure. It is important to use a saline solution in all of your experiments because that is the easiest way to simulate urine (think of it as “synthetic urine”). Salts in the water affect the performance of the super-absorbent (SAP) so much, that your data will be meaningless if you use plain tap water. And remember that babies’ urine too has minerals – 0.9% is the average minerals content in the urine of a healthy baby.
A performance gain of as much as 35% can be achieved just by using plain water instead of the “synthetic urine”, depending on the amount of SAP in a diaper. So if you think that using water will make the test fair to all diapers… STOP! Not all diapers are made with the same amount of SAP and it is only the SAP that is affected by the salts. By showing respect to individual properties of all the components, you are exercising a scientific criterion. Make sure that you record all relevant information in your “experimental record’s table“. This is what the Science Fair is all about – learning how to have and use a scientific approach. Just to confirm that this is not a simple theory but a hard fact, test one diaper with plain tap water and another with water after adding plenty of salt. You will be surprised by the huge differences in results. Another way to verify this phenomenon without damaging two diapers is to add a tablespoon of salt on top of a used wet diaper. You will see how much liquid is released due to the inability of SAP to retain liquids having highly concentrated dissolved salts. Grown-ups, please skip the following part because it was written just for kids.
A bed time story for little kids not to take too seriously:
“Once upon a time, many years ago, there was a nasty wicked old witch who visited supermarkets disguised as a “demo” lady in order to trick innocent mothers into buying low quality diapers. Luckily for us, a handsome prince watched one of her presentations and caught the old witch doing her wicked tricks. The bad witch owned a very large disposable diaper factory. That is why she was conducting diaper tests in front of the consumers to convince them that her diapers were the best in the land. But the trick was that she was using altered water; in one jar she had water with a lot of salt and in another jar she had plain water. She tried to convince mothers that her diapers were the best (using only plain water on her diapers and water with a lot of salt on all other diaper brands, including diapers made by the handsome prince’s little diaper factory). Guess which diaper performed better? You are right! Diapers made by the wicked old witch seemed to be better. But you know that it was just a wicked trick. The happy end of the story is that the handsome prince, whose name was Carlos, was able to uncover the fraud and later married the beautiful young manager-in-charge of the diaper department, Adriana, and lived happily ever after. The wicked witch never sold another diaper in town and later moved to New York where she was last seen driving a Taxi!”
Diaper Tests to do:
Absorbent Capacity: Prepare a few liters of saline solution, mixing water with regular table salt at 0.9% (This is the same as 9 grams per liter of solution, and simulates the minerals in babies urine). Find a large container in which you can immerse the diaper. Start by weighing the dry diaper, using a scale, and record the data. Put the saline solution into the container and place the diaper flat upside down at the bottom of the container, using a chronometer or stop watch set for exactly ten minutes. Do not apply any additional pressure to the diaper as it is soaking the solution. After the ten minutes have elapsed, remove the diaper, holding it from the corners, and let it drip for two minutes. Hang the diaper with cloth pins or something similar (or use your own hands), in such a way that it drips vertically. After the two minutes, measure the weight on the scale. The Absorbent Capacity will be the difference between the two recorded weights. The best diaper for this performance attribute will be the one with the highest Absorbent Capacity. Record the data in your table.
Absorbent Retention: I am going to explain a simplified test that you can carry out without elaborate equipment. You will need only a washing machine. This experiment lets you assess how much liquid a diaper will hold under pressure. First weigh a new dry diaper on a scale and record the data in your table. Then allow it to soak liquid for ten minutes, using the same saline solution and the container you used in the previous experiment. After ten minutes have elapsed, remove the diaper and let it drip for two minutes. Instead of just recording the weight difference (as in Absorbent Capacity experiment), this time we need to place the diaper flat on the inside wall of the washing machine. Important: You will use only the centrifugal cycle. If you are 13 years old or less, please ask permission of your parents before you use the washing machine. A very angry parent once sent me an E-mail complaining about this situation and the thing is that I agree with her – a washing machine can be a dangerous equipment. She made me realize that I could be responsible for anything happening to her child (it seems her kid made a mess with water all over the floor and she was angry with me); yes, even when I was just trying to offer a little help. Life many times is not fair and you should know this by now. It is up to all of us to make it a little better for everybody. Never give up an action when your intentions are good and honorable. It is much better to have your parents involved with the project; this way I am also “off the hook”… By the way, sometimes your parents can also help you with the writing. You will need to improve your negotiating skills. For example, in exchange for cleaning your room … (ups!!). Enough chat already, let us get back to the experiment ….
Select the centrifugal cycle in the washing machine and let it spin for two minutes. Remove the diaper from the machine and measure its weight. The weight difference will be the total diaper retention. Be consistent by always using the same washing machine with all the diapers you test. The best diaper will be the one with the highest retention. Record the data in your experiment’s data table.
Speed of Absorption: You will need a flat table and the means to hold the diaper (pins if you use a cork table, or adhesive tape). You will also need a plastic pipe (or equivalent) with an internal diameter of one inch, about ten inches long, though the length is not so important. You will need to prepare a stand for the pipe, by using a 10 CMS X 10 CMS (4″ x 4″) Plexiglas square, with a hole in the center where you can attach the one inch pipe. Start by measuring 100 ml of saline solution @ 0.9%. Place the diaper flat on the table, avoiding wrinkles by stretching the diaper before you apply the pins. Put the stand with the pipe on top of the diaper at the target area of the pad. With your chronometer (stop watch?) on hand, start the timer at the same time when you pour the solution into the pipe. Record the time required for all of the 100 ml of liquid to penetrate the diaper. This will be the speed of absorption or First Strike Trough Time. Repeat the test again after ten minutes with an additional 100 ml for a Second Strike Trough. You can also try a Third Insult if you please. The best diaper for this performance attribute will be the one with the least time. The second time is more important than the first, in terms of diaper performance.
These tests are fine for a school project but please be aware that if you want more precise results, you should not use tap water or regular table salt as they have an effect on the performance of the super absorbent due the the variable mineral content of the water. Please visit the following link if you want to download formal instructions on diaper performance: Diaper Performance
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