Unless the manufacturer specifies otherwise, use a quality Alkaline battery in your therms vs. "heavy duty" or "super heavy duty" batteries which are much cheaper. The reason being (not to get technical into the chemical composition, etc. of each) the Alkaline batteries discharge at a tiny rate until spent, then drop off to nothing, giving a consistent voltage during it's life whereas heavy-duty-type batteries discharge at a constant rate; at half life they are at half voltage which can cause havoc on accurate readings on your therm! See chart below to demonstrate (Heavy Duty and Super Heavy Duty batteries are commonly found at Dollar Stores because they're cheap. Cheap doesn't mean good. About the only application they are good in are 9V's for smoke detectors because the smoke detector will start beeping it needs a new battery well before the battery is spent, giving protection until it's changed; alkaline batteries once they go they're gone and give little protection for smoke detection):
