Hello jz817,
I took a look at the pH test pens shown at the link you posted. From the information given I'm not sure of the differences among the pH testers listed there. The more expensive test pens may be more convenient to calibrate and use than the less expensive tester. I think the less expensive pH test pen should be OK for your experiment.
You will need to calibrate any of these by testing a solution of known pH. That means you'll have to purchase a calibration solution. Or perhaps your teacher has calibration solutions that you could use.
The “Get the Lead Out” project procedure suggests using pH test papers of the type shown at this link:
http://www.carolina.com/ph-test-papers/ ... scibuddies
The pH papers are less expensive than any of the test pens and no calibration is needed. I'm sure the pH papers will work for your experiment, so this may be a good option for your pH testing.
I hope this helps. Please post again if you have more questions
A. Norman