05-28-2009, 08:01 AM
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Mine are all Hagen. $2.99 from Fosters & Smith. Due to shipping I always bought them with a lot of other stuff.
The stick on ones are unreliable for the simple reason that they don't measure the actual water temperature. They measure the temperature of the outer surface of the tank material. Well, the people who make the stick ons don't know a lot of factors such as what material your tank is made of, glass or acrylic which have widely varying thermal conductivity. They don't know the thickness of the material of the tank, thicker materials will be cooler on the outer surface. They don't know the temperature of the room you're in, colder rooms make the outer surface cooler. All these factors can impact what that outer surface temperature really is and the manufacturer can't know what they are, so they make assumptions. Maybe they assume it's a glass tank, 1/4" thick, with a room temp of 74 degrees (an example, not the actual assumptions). If you have that kind of tank and room conditions the stick on works great, all their assumption are correct. However the more you deviate from those assumptions the less accurate the reading is.
The only way to reliably measure the temperature of a material is to take a reading from the material itself. I don't trust a thermometer that isn't immersed in the water or a digital that doesn't put it's probe right in the water.