# What thermometer is more accurate? Mercury vs digital?



## rragan (Jun 2, 2015)

trust the mercury


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## GraphicGr8s (Apr 4, 2011)

Check the accuracy for yourself. Take a glass of ice water, about a 50-50 mix will be 32 degrees and put the thermometers in it. For the high end a steadily boiling (not a rolling boil) pot of water is 212.

And mercury thermometers are not mercury.


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## Seattle_Aquarist (Jun 15, 2008)

miogpsrocks said:


> My Mercury thermometer said that my tank it about 72 degrees however my digital thermometer said 75.
> 
> My heater is keeping the tank set at 75 when I ask it to keep the temperature at 72.
> 
> ...


Hi miogpsrocks,

The simple answer is .......... the one that was calibrated the best. That said I have an old mercury thermometer that was used in industrial applications and it is accurate to +/- 1 degee F.


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## lksdrinker (Feb 12, 2014)

Unfortunately the cheapo thermometers used in this hobby are not all that accurate. They do a good job of telling how much hotter or colder the water might be now compared to what it was before; but whether or not the actual temperature reading is correct is always a guess. Here are various digital thermometers sitting only inches apart from one another on a table in my fishroom. Each was brand new and I had just opened the packages and let them sit out for a while before taking the photo:


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## miogpsrocks (Sep 3, 2015)

lksdrinker said:


> Unfortunately the cheapo thermometers used in this hobby are not all that accurate. They do a good job of telling how much hotter or colder the water might be now compared to what it was before; but whether or not the actual temperature reading is correct is always a guess. Here are various digital thermometers sitting only inches apart from one another on a table in my fishroom. Each was brand new and I had just opened the packages and let them sit out for a while before taking the photo:


Those are the same ones that I got from Ebay! 

My mercury thermometer was also purchase from Ebay(hong kong I think) so I don't know if that is much better. 

I guess I am going to have run some scientific experiences and label how far off they are. 

Thanks

Bump:


GraphicGr8s said:


> Check the accuracy for yourself. Take a glass of ice water, about a 50-50 mix will be 32 degrees and put the thermometers in it. For the high end a steadily boiling (not a rolling boil) pot of water is 212.
> 
> And mercury thermometers are not mercury.


Good suggestion. I think I will do this.


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## WaterLife (Jul 1, 2015)

I did a search before on many digital thermometers and everything says they are inaccurate.
Go with the Mercury ones (2 if possible, opposite ends of tank, one higher up, one lower down). 
Your finger is a good rough temperature estimate as well to ballpark verify. Your fish (and even plants) behavior/responses are good indicators as well if things are getting drastic (too hot or cold).
Don't get the "LCD" sticker thermometers, they just suck and don't last very long. I would get a digital over the sticker ones.
@GraphicGr8s I never knew that. You mean there is no mercury inside the thermometers? Then what is exactly?


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## mistergreen (Dec 9, 2006)

WaterLife said:


> @GraphicGr8s I never knew that. You mean there is no mercury inside the thermometers? Then what is exactly?


As you know, mercury is bad for people & the environment. The stuff in thermometer is red dyed alcohol, I think. Mercury is silver.


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## latchdan (Sep 7, 2007)

WaterLife said:


> I did a search before on many digital thermometers and everything says they are inaccurate.
> Go with the Mercury ones (2 if possible, opposite ends of tank, one higher up, one lower down).
> Your finger is a good rough temperature estimate as well to ballpark verify. Your fish (and even plants) behavior/responses are good indicators as well if things are getting drastic (too hot or cold).
> Don't get the "LCD" sticker thermometers, they just suck and don't last very long. I would get a digital over the sticker ones.
> ...


I wouldn't knock the LCD ones I've had mine for years and it matches identically to my "mercury" one. Or maybe they are both wrong.


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## GraphicGr8s (Apr 4, 2011)

latchdan said:


> I wouldn't knock the LCD ones I've had mine for years and it matches identically to my "mercury" one. Or maybe they are both wrong.


Have you checked them in ice water?

Bump: Within maybe a degree or two high level accuracy isn't needed with fish. We don't need to know down to the 1/10000 of a degree.


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## latchdan (Sep 7, 2007)

GraphicGr8s said:


> Have you checked them in ice water?
> 
> Bump: Within maybe a degree or two high level accuracy isn't needed with fish. We don't need to know down to the 1/10000 of a degree.


well i can't take the LCD one off the glass and test it, but if both thermometers read the same I'm assuming they are accurate, at least as accurate as I need.


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## Steve001 (Feb 26, 2011)

WaterLife said:


> I did a search before on many digital thermometers and everything says they are inaccurate.
> Go with the Mercury ones (2 if possible, opposite ends of tank, one higher up, one lower down).
> Your finger is a good rough temperature estimate as well to ballpark verify. Your fish (and even plants) behavior/responses are good indicators as well if things are getting drastic (too hot or cold).
> Don't get the "LCD" sticker thermometers, they just suck and don't last very long. I would get a digital over the sticker ones.
> ...


Mercury is a metal, silver in color and no longer used because it is toxic. Typical mercury thermometer.


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## GraphicGr8s (Apr 4, 2011)

latchdan said:


> well i can't take the LCD one off the glass and test it, but if both thermometers read the same I'm assuming they are accurate, at least as accurate as I need.


Your last part pretty much sums it up. But you could check the other one in an ice bath and see if it's accurate.


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## Oso Polar (Apr 22, 2015)

Usually these cheap digital ones (exactly like on the photos above) are good enough for our purposes, just don't expect ±0.1 °F accuracy from them. From my experience with them I believe their real accuracy is about ±0.5 °C (that is 0.9 °F) and even the photos above show this: 5 out of 6 show the same temperature around 69 °F (±0.9 °F) and only one thermometer is a bit off.


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## latchdan (Sep 7, 2007)

I had one of those thermometers and it was accurate for several months then 1 day it was off by 7 degrees, changed batteries it never went back... I think the sensor went bad or something. I don't know why they even put decimals when its not even that accurate.


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## Argus (May 22, 2013)

Oso Polar said:


> Usually these cheap digital ones (exactly like on the photos above) are good enough for our purposes, just don't expect ±0.1 °F accuracy from them. From my experience with them I believe their real accuracy is about ±0.5 °C (that is 0.9 °F) and even the photos above show this: 5 out of 6 show the same temperature around 69 °F (±0.9 °F) and only one thermometer is a bit off.


I bought a couple of those $5 



 on Amazon. CNZ only claimed them to be actuate to ±1.5°F.


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## GraphicGr8s (Apr 4, 2011)

97.78% of the time that's more than enough. But as has been stated how long are they accurate for? I've got 3 in the closet that started out great. I have no idea why I still have them.


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## Oso Polar (Apr 22, 2015)

±1.5 °F makes sense - after all, the majority of the world uses °C and 1 °C is 1.8 °F, so accuracy is better then 1 °C (= good enough for all Celsius world). The oldest such thermometer I have is from 2005 and it is still working good. 10 years ago it had Coralife brand name attached and had a price around 8 dollars - now you can get a noname one looking exactly like this (except for branding) for about $3. :smile2:


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## Mxx (Dec 29, 2010)

I bought a second mid-range digital thermometer for use on my reef tank and when I checked it against my first one I found the new one measuring 7 degrees less! (I bought two more cheap digital ones, and found those to both measure a temperature of 5 degrees less than my original. So now I'm still not sure which are correct, but am seriously peeved about stressing for a year that my reef tank was running too hot... I better buy an expensive alcohol thermometer and try to figure out what is correct, and keep an extra one in the tank at all times as well, sheesh...


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## WaterLife (Jul 1, 2015)

Is it just the digital ones have their probes get covered in hard water deposits and other things (biofilm, algae, etc.) and that is what makes them read inaccurately? Or are they just badly inaccurate to begin with? Can they be recalibrated? I've never used a digital one.


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## catchthecarp (Nov 22, 2014)

I've been using a 



 digital thermometers for over three years now. It's accurate, it has always registered the same temp as my expensive Therm-o-pen. Easy to read, mountable, and has some nice features like a high-low temp alarm. I've had no issues with the probe getting covered with deposits or algae. I would buy one again.


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## Mxx (Dec 29, 2010)

catchthecarp said:


> I've been using a General Tools AQ 150 digital thermometers for over three years now. It's accurate, it has always registered the same temp as my expensive Therm-o-pen. Easy to read, mountable, and has some nice features like a high-low temp alarm. I've had no issues with the probe getting covered with deposits or algae. I would buy one again.


That's the one I had been using which I then realised was several degrees off, as per my post above... I suppose the lesson is to check them against a more accurate thermometer now and then.


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## catchthecarp (Nov 22, 2014)

Mxx said:


> That's the one I had been using which I then realised was several degrees off, as per my post above... I suppose the lesson is to check them against a more accurate thermometer now and then.


 Totally agree. A high quality pen type digital thermometer is convenient for doing water changes and verifying that the one in your tank is accurate. They aren't cheap but the peace of mind is worth every penny to me.


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## fulljionslly (Jul 20, 2017)

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