Take a small bottle cap and fill it with salt. Add distilled water to the salt. Put the bottle cap and the sensor into a ziploc bag and make sure you force all the air you can out of the bag before sealing it. Then put that bag inside a second ziploc and do the same,that makes sure there's very little air getting into the bag. Let it sit for about 24hrs. It should read close to 75%

I think that's right others can correct me if it's not.