# How do I get accurate temperature readings from my tanks?



## Robbie X (1 Dec 2021)

Hi all.
I purchased a new Eheim fixed temp 25c for my new blackwater tank but both thermometers are giving different results.
The tank is quite cool to the touch and I’m worried the temp is much lower than the stated 25c.
What do I need to get accurate results when testing the temperature? 
Cheers


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## bazz (1 Dec 2021)

HI,
I have seven thermometers/digital thermometers/digital thermostats and no two are the same, there is a 2 degree difference between 6 of them and the ADA one is out on it's own a full 2 degrees cooler than all the rest.
So I don't know!


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## The grumpy one (1 Dec 2021)

Most aquarium thermometers are +/- 1 to 2°C. This means they can be out with each other by 2 to 4°C.
Also, I find that the temp difference between one end of the tank to the other can be 4 or 5°C
The heater accuracy will be about +/- 0.25°C this means that two exactly the same make and model could be out by 0.5°C to each other.
So a combination of the accuracy of the thermometer and the heater could give you a 4 to 4.5°C error.

The only way is to buy a accurate scientific thermometer and use that to find how far out your aquarium thermometer is. Very expensive.


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## Robbie X (1 Dec 2021)

The grumpy one said:


> Most aquarium thermometers are +/- 1 to 2°C. This means they can be out with each other by 2 to 4°C.
> Also, I find that the temp difference between one end of the tank to the other can be 4 or 5°C
> The heater accuracy will be about +/- 0.25°C this means that two exactly the same make and model could be out by 0.5°C to each other.
> So a combination of the accuracy of the thermometer and the heater could give you a 4 to 4.5°C error.
> ...


Would this do the job? RS PRO RS40 Wired Digital Thermometer | RS Components


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## Corbie (1 Dec 2021)

Robbie X said:


> Would this do the job? RS PRO RS40 Wired Digital Thermometer | RS Components


It is only accurate to +/- 2 degrees C, so not great. How about something like this? More pricey though. PT100 thermometer


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## jaypeecee (1 Dec 2021)

The grumpy one said:


> The heater accuracy will be about +/- 0.25°C this means that two exactly the same make and model could be out by 0.5°C to each other.


Hi @The grumpy one

You're unlikely to get the above accuracy with an aquarium heater. It will depend on factors such as:

1  Water flow rate

2 Heater construction - bimetallic strip or electronic temperature control.

3 Mounting - horizontal or vertical.

4 Heater power rating and tank dimensions

I would suggest that +/- 0.5°C to +/- 1.0°C would be more typical.

And I'm not talking about the 'cheap-as-chips' variety of aquarium heaters.

JPC


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## jaypeecee (1 Dec 2021)

Robbie X said:


> What do I need to get accurate results when testing the temperature?


Hi @Robbie X 

I have a Hanna _Checktemp_ electronic thermometer. It gives +/- 0.2°C accuracy and it has 0.1°C resolution. I believe this is the one:









						HI-98501 Checktemp Digital Thermometer with Stainless Steel Penetration Probe
					

The Checktemp delivers high accuracy temperature measurements over a wide range without concern for breakage or condensation.




					www.hannainstruments.co.uk
				




Note: It is not submersible!

JPC


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## Wookii (2 Dec 2021)

Robbie X said:


> Would this do the job? RS PRO RS40 Wired Digital Thermometer | RS Components



You are better off getting a full controller, then not only will you get an accurate temperature reading, but overheat protection if your heater thermostat fails. You can also add a fan into the cooling socket for the summer months.

Amazon product

There are non-wifi versions if you don't need that functionality or want to save some cost, but accuracy is stated as being +/- 0.1 °C - how true that is I don't know, but mine have worked flawlessly for various applications.


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## dw1305 (2 Dec 2021)

Hi all,
An Inkbird is a good idea. Personally I like a traditional <"spirit based thermometer">, they are cheap and don't have any electronics etc.

cheers Darrel


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## ian_m (2 Dec 2021)

Use one of these. After having been through loads and loads of in tank digital thermometers, finally settled on one of these. 





Advantages are:

Low cost.
Works 24/7.
No batteries.
Doesn't leak (that's looking at you Juwel, Maplin and cheapy Ebay).
In tank cable doesn't degrade and fall apart under tank lights. (That's you Maplin & RS).
Made of glass so no plastic to degrade under tank lights (that's you Juwel and Maplin).
Mine is years old and still works fine.


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## Andy Taylor (2 Dec 2021)

Robbie X said:


> Hi all.
> I purchased a new Eheim fixed temp 25c for my new blackwater tank but both thermometers are giving different results.
> The tank is quite cool to the touch and I’m worried the temp is much lower than the stated 25c.
> What do I need to get accurate results when testing the temperature?
> Cheers


What's the tank size and wattage of the heater?


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## mort (2 Dec 2021)

Another thing to remember is you need good water circulation to keep all the water heated to the same level but there is a simple test of accuracy with whatever you are using.

Fill a glass half full with cold water and top the rest up with ice. Leave it for a few minutes and put your thermometer in it. After a further few minutes it should read zero and since you know this you can use the margin of error for the thermometer to adjust what you read. Ie if it only goes down to 2c then you know it's 25c in the tank when it reads 27c.

I use an aquarium heat/cooling controller but you are still reliant on the thermostat being accurate (unless they can be calibrated) and most I've seen still have a accuracy swing. The same can be said for the glass thermometers like shown above. We used to order them by the dozen in the shop and very few would read exactly the same (we had one that read 40c at room temperature bizarrely). Unless you go for an expensive scientific quality one there will always be an accuracy swing.

I also wouldn't be too worried about getting the temperature absolutely precise. It is swings in water temperature that are best avoided but if we are a degree out but consistent then it tends to not be an issue for most of us.


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## dw1305 (2 Dec 2021)

Hi all,


mort said:


> Unless you go for an expensive scientific quality one there will always be an accuracy swing.


A "precision" scientific grade one, with certificate, is still only about ~£10 and you get them scaled from -10 oC to 50 oC


mort said:


> I also wouldn't be too worried about getting the temperature absolutely precise. It is swings in water temperature that are best avoided but if we are a degree out but consistent then it tends to not be an issue for most of us.


I'm the same, ever since I saw the fish <"swimming into the cold water stream">, (to grab _Daphnia_ etc), when I change the water. 

cheers Darrel


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## Angus (2 Dec 2021)

My fish always seem to prefer the cold water end of the tank at a waterchange, i usually drop about 2 degrees C when refilling without any problems. 

I dont know whether it is that accurate or not but i take my temp off my heating thermostat and compare to a food grade thermometer, my thinking is that those are the 2 most accurate measurements available to me.

Sent from my BV5500Plus using Tapatalk


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## jaypeecee (2 Dec 2021)

Hi @dw1305 

A "precision" scientific grade one, with certificate, is still only about ~£10 and you get them scaled from -10 oC to 50 oC

Please tell me more. Where would I get one of these?

JPC


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## jaypeecee (2 Dec 2021)

mort said:


> I also wouldn't be too worried about getting the temperature absolutely precise. It is swings in water temperature that are best avoided but if we are a degree out but consistent then it tends to not be an issue for most of us.





Robbie X said:


> What do I need to get accurate results when testing the temperature?


Hi @Robbie X 

I agree with the point being made by @mort. But if, for whatever reason, you want 'accuracy', there's plenty to choose from. 

JPC


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## dw1305 (3 Dec 2021)

Hi all, 


jaypeecee said:


> Please tell me more. Where would I get one of these?


A <"few companies"> sell them. All the precision ones used to use mercury (Hg), but there are spirit ones available now.  I haven't looked but I'd guess (in our brave new world) that £10 was hopelessly optimistic in price terms. 

cheers Darrel


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## Nick potts (3 Dec 2021)

dw1305 said:


> Hi all,
> 
> A <"few companies"> sell them. All the precision ones used to use mercury (Hg), but there are spirit ones available now.  I haven't looked but I'd guess (in our brave new world) that £10 was hopelessly optimistic in price terms.
> 
> cheers Darrel



You can pick them up for around £30 these days.

I agree with most of the above, absolute temp is not a major concern as long as it's in the ballpark, I'd pick 1 of your thermometers and go with that.


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## Robbie X (3 Dec 2021)

Andy Taylor said:


> What's the tank size and wattage of the heater?


100W in less than 60L can’t off the top of my head remember if it’s 54 or 58L.


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## Andy Taylor (4 Dec 2021)

I built myself one of these,

 Easy to calibrate and reliable.  I check it once a year using a well known brand of temperature probes.


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## jaypeecee (4 Dec 2021)

dw1305 said:


> A <"few companies"> sell them.


Hi @dw1305 

Many thanks for that. _Brannan_ have been making thermometers for donkey's years. It's good to know that they're still around.

JPC


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