Science Buddies: "Ask an Expert"

Hi,
I'm doing the above mentioned project for my science fair and I'm having a bit of an issue.

So I did the three trials for each of the SPF (30,50,100) that I tested and this occurred. For my control(plastic wrap with no sunscreen lotion), the uv index reading on the meter was 4. Then for each of the SPF the uv index reading on the meter was a 0. So if I did the subtraction-like what is listed in the procedure-each SPF yielded a -4.

I'm wondering if the data I collected was incorrect. Should there be a difference between the SPF data?

I did the total experiment 2 times and yielded the same result for each trial each time. Things that I noted and observed during the experiment: I used the exact uv meter that was listed in the material list--bought from Amazon, it was a partly cloudy day with some breeze. Also when I did the control test- the uv meter showed an uv index number and a uv irradiance number. But when I tested each SPF the uv index number was a zero and the uv irradiance number was also a 0. Is there an issue with my meter.?

I know the uv meter is working because when I turn on the meter no numbers where shown for both uv index and irradiance and when I took the meter outside in my yard and turned it on an I'd index and irradiance number appeared.

I'm planning on redoing the experiment again, when it's a more sunny day with no cloud cover and less breeze. Hopefully this will help or change data .

I also want to know how long should I make the sunscreen lotion dry for? When I initially did the experiment I made it dry for about an hour-- but it was still a bit sticky. Should I dry it over night? Must the sunscreen lotion not feel sticky on the plastic wrap to be considered dry?

When I tested the sunscreen lotion on the plastic wrap after drying for about an hour-it was a bit sticky but it was transparent on the plastic wrap and not white like when I first spread it on.


Please give me any answers you have to my questions or any suggestions. I would truly appreciate it

Hoanu,

Thank you for using the Science Buddies fora.

Pertaining to your negative results: the procedures described by the Science Buddies resource seem flawed. The experiment instructs the experimenter to subtract the control reading from each experimental reading. This would work in most cases, but in this case, the sunscreen is expected to yield a lower reading than the control. As such, the experiment will always yield negative results.

As for your data being too consistent, – you got a reading of four for each experimental group, correct? – remember that this is an experiment. Ideally, a researcher should begin an experiment without being completely sure of the outcome. If the researcher only publishes the data that he or she wants to see, then data would be very biased!

Perhaps you conducted your experiment correctly, but the sunscreens are actually identical. It is possible.

I have two suggestions for you.

A) Accept your current results and try to explain them. Are sunscreen companies making false advertisements? Even better, science fair typically asks about any potential errors you might have made by accident. In this case, you could explain that you might have somehow misapplied the sunscreen (or what you believe may have been the case).

- or -

B) Conduct your experiment again on a sunny day. When asked about errors, you might reference the first two attempts at the experiment.

Lastly, concerning the drying period, I would imagine that your idea of waiting until the sunscreen is no longer white is correct. You should pick an amount of time (such as, perhaps, an hour) and allot each application of sunscreen that amount of time to dry. Be sure to keep the time constant.


I hope that I have answered your questions. If you have further questions or if I misconstrued one of your questions, feel free to ask.

Good luck with your science fair!

- Ultra