I use the digital coralife with the temperature probe, cheap and seems to be pretty accurate.

 

I bought three Coralife digital thermometers for a research project. Trying them out, I placed them in my 55 gallon (with the probes within 1" of each other). Result: I had three different temp readings ranging between 76.2 to 78.5F.
Actually, that's about as accurate as a generic off-the-shelf thermometers get (+/- a degree or so), even the large (12"+) alcohol thermometers used in College Biology or Chemistry labs. The trick is to measure some known temperatures with the digital thermometer, careful not to damage the probe with too high a temp, and note the correction factor unique to the individual thermometer. I note the correction factor (example: +0.7) on the front with a silver Sharpie marker. The trick is finding something with a precise, known temperature! freezing is easy (use ice water), boiling is over the limit for most aquarium digital thermometers.
A difference in 1-2 degrees F or 1C is pretty insignificant, as far as fish and plants are concerned as long as the temp is consistent. Obviously, you don;t want the temp bouncing up and down rapidly!

 

In my previous post, I never recommended anything - sorry. I'm quite happy with the Coralife digital thermometer also, and they are inexpensive, as already noted.

 

Thanks to all! I've managed to find a battery for my old Coralife digital - and I've been comparing it to a digital food thermometer I have - it's seems to be a little on the cold side - if the food one is correct of course... Someday I'll get the Pinpoint one - but at 36$+ it's expensive!!

 

WOW! I never realized my thermometers were probably so inaccurate till seeing the replies on this thread. I thought cheap, accurate thermometers were EASY to come by. 2 degrees F may not be much, but it is more than I want to be off. What about the glass ones with the red liquid in them? Doctors have crammed these under our tongues for decades. Wouldn't they be deadly accurate, seeing as what they are used for? You would think they would be cheap enough by now too. Maybe these from a medical supply place is the way to go?

Thoughts?

 

I always use the good ole glass thermometers. I have tried the Corallife digital ones and found they were off by as much as 5 degrees F (the ones I had anyway). Also, when the battery starts going they would act weird. The glass ones are cheap, only a couple bucks each, and very accurate.

 

Mercury thermometers are probably the most accurate off the shelf, but increasingly hard to find and hazardous to health if they break. Some digitals have a calibration option that make them very accurate (but more $$$). The range on mercury thermometer (you can tell because the liquid is silver) used for human temperatures may not go low enough for aquarium use? Alcohol-based thermometers typically have a red or green color added. Even in our university chemistry labs, mercury thermometers may be up to 1C off from each other. Again, the only way to be absolutely certain of a thermometer's accuracy is to compare it to a known temperature, preferably performing multiple comparisons to determine the precision (repeatability of readings) also. More expensive (non-digital) thermometers that are touted as super-precise hopefully (!) are more accurate because you are paying someone to "pre-check" and calibrate the thermometer before it's sold.
Always remember: "precision", the ability to give you the same reading repeatedly, is NOT the same thing as "accuracy!

 

thanks to all for their input. I have found out that "pinpoint" thermometers are rock solid - though they are not cheap!!