# Thermometers



## Manny (Oct 8, 2013)

Hey guys. So I have a dilemma here. I have two thermometers in my tank right now. I have a digital and a float in the tank thermometer. When I bought the digital I read reviews saying that it was usually off by 1 degree. I figured as long as I kept my water temp constant, not a big deal. Well here's the thing though. They are off from each other by almost 4 degrees  my digital is reading 74.9 and my floating thermometer is reading 78. My house is set at 73 degrees. So now I am stuck trying to figure out which is closet to being correct. I don't have any fish in there yet but I sure would like to get the temperature figured out before I go buy some fish. Any advice on which one I should be following or which one might be closet to being accurate. I hope the room temp helps a bit. Thanks in advance.


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## emc7 (Jul 23, 2005)

Unless you have a heater plugged in in the tank, the tank should sit at room temp when the light is off unless the filter is making a lot of heat, then it will sit a bit higher. You can also have a temp. gradient in the tank, with the warmest water directly under the light or the coolest water at the surface do to evaporation. If you take the thermometers out of the tank and let them sit in the air, what do they read? How are you measuring the room temp? Do you have a good cooking or weather thermometer you can use for a tie breaker? 

I would tend to trust the digital over one with a red liquid in it, but a certified, calibrated mercury over a cheap digital one.

The first lab in every chem book is "calibration of a thermometer", over a small range, you can usually just add or subtract a correction factor. For wider ranges it gets to be like the F to C conversion, multiply by A and add B.


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## TheOldSalt (Jan 28, 2005)

I suppose the logical thing to do would be to get a third thermometer and see what it says. It should match one of the other two, and then you'll have your answer.


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