Science Buddies: "Ask an Expert"

I am attempting to compare the absorptive and evaporative rates of different materials in an outdoor environment. I would like to be able to use that data to predict how these materials would behave in different geographic locations around the country. But each location has so many variables like temperature and relative humidity that I don't know how to properly compare the locations to my data.

I can test the items in a controlled lab but how can I then compare that data to the real world? Is there a baseline or something used by engineers to compare the evaporation rates of different materials in different areas? For example, how long would it take the same piece of cotton cloth soaked in water to dry in NYC vs Dallas TX. It seems like there must be some constant to measure against? Or some other way for me to predict how each material will function if at different locations?

Derson44 - Interesting experiment in that you want to test various materials. Let's stick with water for just a moment and try to understand what variables influence it's evaporation rate. I assume we are also talking about evaporation into our atmosphere. Several variables come to mind. Relative humidity, that is the amount of water vapor in the air relative to it's saturated state. If the relative humidity is 100%, the wet cloth will not give up any water and be wet permanently. At 0% RH, evaporation will occur easily. Is the air still or moving? Moving air will facilitate greater evaporation because we are changing the immediate "moist air" around our cloth and replacing it with dryer air so diffusion is more efficient. Another variable, a little different from relative humidity (RH), is what is the atmospheric pressure? A lower atmospheric pressure will facilitate faster evaporation because the vapor pressure of water is higher relative to the medium that surrounds it. There are probably others, radiation and other variables, (temperature is probably governed by RH).

You can likely apply these variables to the other liquids you want to test, but I think these are some of the variables that you will mainly have to consider. Perhaps other experts have a few more inputs.

Rick Marz

To add on to what Rick has said, to replicate temperature and humidity (remember there are three humidity measurements) in certain regions, I would look for the historical high, low, and average temp and humidity for each region and replicate that. I would also pick a baseline set of parameters, like 10% humidity at 20 degree C at 1 atmosphere pressure. If you can, contact a clothing manufacturer like North Face and ask how they test their garments for extreme weather.

Thank you Rick and theborg,

We had no idea there was three humidity measurements and that atmospheric pressure would matter at all. We are learning a bunch and that is what makes doing this stuff so great! Excellent information and very helpful. Thanks again.

Our idea is that if we took an absorptive material like a sponge, foam or cotton and fully saturated the material until it equalized, could we then determine which materials dried the fastest, and at what rate so we could quantify the differences. Then we want to attempt to predict how that material would function in different geographic locations based upon NOAA weather data.

So, we need to get a baseline measurement. Say we take the difference in weight between dry and fully wet and then set up our test at 24C and 50% relative humidity. We could then measure the moisture evaporated by weight to determine the rate and amount of evaporation. We could then change the temp and humidity to get some data points. Then use that data to forecast how the materials will behave based upon the weather variables. Does that sound about right?

Also, is evaporation the only way water could leave the material? I read something about energy balance theory and it seems like we would be ok with stating that the moisture evaporated. It is a HS project but we want to use good science.

Then we move on to learning more about the three humidity measurements and how atmospheric pressure effects evaporation!

Thank you for your interest and contributions.

Best regards,
Bob and Matt

Bob and Matt,

I think you are really on the right track. Here are some suggestions that I hope help. The primary feature of any successful science experiment is to isolate just ONE independent variable. In your case, you have two; humidity and temperature. This can sometimes be the case, where you have two parameters you wish to characterize.

1) Your proceedure of soaking a material is good. Be sure to try and keep some parameters of the material the same...I suggest wrapping each sample in foil or something reasonable air tight and cutting a hole so that the surface area of the test material exposed is standardized (i.e. 1 square cm exposure) on a sample that all weigh the same (i.e. 200 g).

2) Rather than pick certain areas around the world and trying to replicate humidity and temp, I would pick at least three temperatures (T) and three humidities (H) to test. Per comment above about having one independant variable this means you would have at least 9 test cases per material you test. For example:
T1 = 5 deg C (low)
T2 = 15 deg C (med)
T3 = 25 deg C (high)
H1 = 10% (low)
H2 = 50% (med)
H3 = 90% (high)

so you have the test conditions for each material (M1, M2, ..., Mn):

1) M1/T1/H1 = Material 1 at low temp, low humidity
2) M1/T1/H2 = Material 1 at low temp, med humidity
3) M1/T1/H3 = Material 1 at low temp, high humidity
4) M1/T2/H1 = Material 1 at med temp, low humidity
5) M1/T2/H2 = Material 1 at med temp, med humidity
6) M1/T2/H3 = Material 1 at med temp, high humidity
7) M1/T3/H1 = Material 1 at high temp, low humidity
8) M1/T3/H2 = Material 1 at high temp, med humidity
9) M1/T3/H3 = Material 1 at high temp, high humidity

and so forth for each material type...

Once you have the parameters for low to high temp and low to high humidity you can graph each material on a two axis graph: one axis Temp and one axis Humidity and then relate to any reagion of the world. Good performance at high temp but low humidity = good material for desert gear.