What thermometer is more accurate? Mercury vs digital? - The Planted Tank Forum
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post #1 of 24 (permalink) Old 10-02-2015, 09:36 AM Thread Starter
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What thermometer is more accurate? Mercury vs digital?

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.


Does anyone know which thermometer are more accurate?

Thanks.
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post #2 of 24 (permalink) Old 10-02-2015, 01:32 PM
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post #3 of 24 (permalink) Old 10-02-2015, 01:45 PM
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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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post #4 of 24 (permalink) Old 10-02-2015, 03:28 PM
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Quote:
Originally Posted by miogpsrocks View Post
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.


Does anyone know which thermometer are more accurate?

Thanks.
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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post #5 of 24 (permalink) Old 10-02-2015, 03:42 PM
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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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post #6 of 24 (permalink) Old 10-02-2015, 04:49 PM Thread Starter
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Originally Posted by lksdrinker View Post
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

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Quote:
Originally Posted by GraphicGr8s View Post
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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post #7 of 24 (permalink) Old 10-02-2015, 05:13 PM
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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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post #8 of 24 (permalink) Old 10-02-2015, 05:23 PM
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@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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post #9 of 24 (permalink) Old 10-02-2015, 05:36 PM
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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?
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. :P
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post #10 of 24 (permalink) Old 10-02-2015, 06:54 PM
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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. :P
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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post #11 of 24 (permalink) Old 10-02-2015, 08:15 PM
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Originally Posted by GraphicGr8s View Post
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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post #12 of 24 (permalink) Old 10-03-2015, 12:40 AM
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Originally Posted by WaterLife View Post
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?
Mercury is a metal, silver in color and no longer used because it is toxic. Typical mercury thermometer.
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post #13 of 24 (permalink) Old 10-03-2015, 01:56 AM
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Originally Posted by latchdan View Post
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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post #14 of 24 (permalink) Old 10-03-2015, 04:38 PM
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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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post #15 of 24 (permalink) Old 10-03-2015, 04:47 PM
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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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