Thank you I didn't have a large baggie. So I put the mix in a Tupperware bowl then put it into a larger bowl then covered the whole thing . Its been in there for a couple of hours and it is reading 99% Humidity. Will it go down or should i get a new one?
Another reference that can be used to calibrate a hygrometer uses calcium chloride. That's the stuff used to melt ice that comes as little white balls. You can also sometimes buy it where ingredients for pickling (or even making tofu) are found. Place the calcium chloride in a cup, it will get very HOT when you add water. Some, but not all of the calcium chloride should dissolve. Seal it along with the hygrometer in a bag or other container. After 12 hours at room temperature (25 C, or 77 F) the relative humidity should be 28%. The relative humidity will vary depending on temperature, (the same is true for the salt method!). In a refrigrator at about 4 C (40 F), it should read about 35% RH.
It's really important to use more than one standard measurement for calibrating any measuring device. I'll post the temperature/RH chart on my BYC page sometime in the future. Just because a hygrometer is off by 6% at room temperature doesn't mean it will be 6% off at incubating temperature. Same goes for thermometers.....the error isn't the same at all temperatures.
There are many other substances that can be used to calibrate a hygrometer at different RH levels, but they use materials that aren't commonly available unless you have a laboratory. Is hygrometer calibration a big enough issue that an info page would be useful to others? If so, let me know what would be useful (or not!), and I'll work on it as I can. I do have a laboratory, and could work to identify common materials that would be useful.
edited to clarify that it will get HOT when you combine calcium chloride and water. As in, hot enough to give a low-grade burn. You can add the calcium chloride bit by bit to the water (instead of water to the calcium chloride) to help with heat dissipation, but it will still get quite warm.