Which thermometer should I believe

hawk82 · Dec 31, 2020

I have two of the little Aquaneat digital thermometers, one says 77F, the other says 80F. I have an Eheim heater set at 78F. Which do I trust?

esther · Dec 31, 2020

I have like 6 thermometers on my tank. Bought a pack on amazon. Just in case. I recommend trying one more to see what it says. If you have three, usually you can figure out which one is the bad one.

Gtinnel · Dec 31, 2020

If I were you I would buy one of those old school cheap glass thermometers from a big box pet store for $2 and see which one it matches. Since all 3 temps that you get are acceptable then there shouldn't even be a rush to get one.

hawk82 · Dec 31, 2020

Do those old glass ones ever go bad? I have an old one, but its like 8 years old. Its hard to read, but it looks like its right around 79F

pseudorand · Dec 31, 2020

Are they in different parts of the tank? Is one just by the heater or under a MH light or in a very low flow part of the tank?

Gtinnel · Dec 31, 2020

hawk82 said:
Do those old glass ones ever go bad?

I'm not positive but I doubt they do. So you have reading of 77, 78, 79, and 80? Well at least they're all close to each other. I'd still probably get a new glass one since they're so cheap. I have several laying around so I can always find one when I want to compare to my inkbird

hawk82 · Dec 31, 2020

pseudorand said:
Are they in different parts of the tank? Is one just by the heater or under a MH light or in a very low flow part of the tank?

No they are all in the same spot, I even took them all out and put them in a bucket of water, and they all read different in that also.

vetteguy53081 · Dec 31, 2020

78 seems to be the mean average

hawk82 · Dec 31, 2020

I was just worried because its rather cold here tonight.

Tastee · Dec 31, 2020

hawk82 said:
Do those old glass ones ever go bad? I have an old one, but its like 8 years old. Its hard to read, but it looks like its right around 79F

It’s hard to imagine how they could ‘go bad’. They are in a closed system and the reading is due to basic physics of how much the coloured liquid expands due to heat. The only changes that are possible are reactions within the closed system (unlikely due to the choice of materials used) or chemical changes due to light (possible but hopefully also unlikely due to good choice of materials). I guess they have a shelf life but it is probably measured in decades not years.

Hugh Mann · Dec 31, 2020

Do you know if they can withstand 100 Celsius?
If so, boil RODI water and measure the temperature. Whichever is accurate will read the boiling point of water. Don't forget to account for elevation!

A fat simpler method would be as others suggest and compare them to other thermometers.

don_chuwish · Dec 31, 2020

Way back when I got started in the hobby I was dismayed by the variance in thermometers. So I got one I could believe, just to use as a reference. I calibrate my Apex temp probe based on it.
"H-B Instrument 3/1230 DURAC Plus General Purpose NIST Traceable Thermometer with Blue Spirit Liquid, Total Immersion, Length 305mm, Range 0 To 230°F"
I don't see that exact one on Amazon anymore but there are several from H-B that are similar around $15-$30.

Greg P · Dec 31, 2020

I use 5 el-cheapo floating glass thermometers to check/calibrate my other units. 5 because I used to use them in other situations.
Each one hits the same temp so I trust them.

Cory · Dec 31, 2020

Believe the one that says 0 c in ice water.

wonroc · Dec 31, 2020

inkbird

dmy535 · Dec 31, 2020

I had this issue when I calibrated my inkbird and my apex probe. When I was researching most digital thermometers say they have a variance from 1-2 degrees! So, I had one of those electronic infrared surface thermometers at my business and the accuracy is within .01 so I trusted that! It was able to take the surface temp and gave me my reading. I then calibrated my digital off that.