Laura,

You have found what I believe to be the most common reason for people thinking they have lower or higher cage humidity than they really do. Low cost hygrometers are notoriously inaccurate and many can easily be off by 10% to 25% depending on the brand. Unless you are willing to spend 150 to 200 dollars for a NIST traceable with a certificate, you can never be sure if your hygrometer is reading accurately. However, you can test your own quite easily to see if it is reading properly, and here is the procedure:



1. Place a teaspoon of salt in a bottle cap or small cup and dampen it with a few drops of water (without dissolving it) to make it the consistency of wet sand.



2. Carefully place the wet salt and the hygrometer, or its probe if that type, inside a see-through container and close tightly. Don't allow any of the salt to directly contact the hygrometer. You can use a zip lock bag provided it seals good and you leave as much air as possible inside as well, or a plastic food storage container as long as it seals airtight. It may be better with the probe types to place the entire unit, probe and all, within the container so as to insure that everything is air tight.



3. Let it sit for at least 8 hours at room temperature and note the reading on the hygrometer without opening the container. It should read 75%, and the difference is how much your hygrometer is off.



4. If it does have an adjustment screw or other adjustment device, adjust to 75%, through a small hole made in the bag if necessary.



I have an expensive certified dial hygrometer I purchased many years ago and have checked it periodically using this method, and it shows to be a very accurate way to check hygrometers as when done correctly, the humidity within the container or bag is always within a percent or two of 75%.



Mike is correct in his dew point analysis, however the difficulty with that method is determining the exact temperature at which the condensation begins to occur. I would be curious as to the reading of your hygrometer when checked with the above method. When done correctly, it is extremely accurate and foolproof.



Kelly

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