Sorry no one has helped U! You didn't mention the brand? Accurite I think is what walmart has I think. I use medical type thermometer. If U need one soon then go with medium priced one and compare the temps with another. Heck, take your temp with a medical one then take your temp with the one U buy.
Hydrometers measure the specific gravity of liquids (density) You will need a Hygrometer, that measures the relative humidity of air. U can find them in combos with a thermometer. Just remember the "d" in hydrometer measures density ie gravity.