Radio Shack and Set & Forget Hygrometer readings

[Dobbs]

I have a Radio Shack hygrometer in my Aristocrat as a secondary unit. I did the salt test right after I purchased it. It was within 1%. The problem now is that the RS reads 74% and the set-and-forget reads 64%.......a 10% differance!!!!! I have the RS unit possitioned at the same level as the S&F unit, and about 12" apart.

My question are.....

Do either of these units loose calibration over time (4-6 months)?

How can I test the Set-and-Forget unit?

Is there a possibility that having the 2 units in the same cabinet may cause problems between the two?

Thanks again,
Dave