Thermometers

Hi, i was just wondering if people could give there opinions on which of the 2 aquarium thermometers they would trust.
As you can see there is quite a difference in temp being shown and I'm unsure which to go by.

And if the answer is non of them, could anyone recommend a reliable, affordable one for a 10 gallon betta tank. Tia

I have used both types. I bought a box of the smaller ones and of ten, seven had the same temperature. The other three were off. You'd probably get close to the same variation on any inexpensive thermometer.

Test them outside the tank, with a jar of room temperature water. If you know the room temp, see what one gets you that.

Go to a pet shop and line up all the glass thermometers next to each other. See if any have the same temperature and if most have a particular temperature and some have a different one, get one that has the same as the majority.

Personally, I'd trust a digital thermometer over a stick-on one since they're usually more accurate. Another option is to get a third thermometer to see which two match up.

aquamangandalf said:

Personally, I'd trust a digital thermometer over a stick-on one since they're usually more accurate. Another option is to get a third thermometer to see which two match up.

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This. I am not familiar with the Fluval, but it seems to be reading the temperature of the water through the glass in some way.... this could potentially be affected by the temperature of the glass and the room? The other thermometer has the probe directly in the water, so I would think it is more accurate.

Having said that, what I did with my thermometers was to stick them in a cup of ice water. Whichever read closest to zero celsius (32 F) is the one I decided to trust I don't think you can do this with the Fluval, but you can definitely do it with the probe on the other one and it might give you an idea of how accurate it is.

If you are thinking of getting a third thermometer, I use a Taylor waterproof precision digital thermometer to check the tank and to make sure that incoming water during water changes is approximately at the right temperature. The one I have (model 9842FDA) can be calibrated (using a cup of ice water, for example) so you can be more or less certain of its accuracy. I think it was about $20 on amazon. There are other much cheaper models from Taylor that you don't calibrate, but they are meant for use in the food industry for food safety, so they need to meet NSF specifications and should be very precise.

I wouldn't trust a thermometer that does not go into the water. The outside the tank thermometers cannot give an accurate reading because they have to consider the temperature difference between the outside of the tank and the inside of the tank. This is influenced by the type of glass, the thickness of the glass and the difference of temperature between inside and outside the tank. The sensor outside the tank type thermometers will read differently based on the outside temp.

Personally, I prefer the alcohol in glass tube type of thermometers. If you are real concerned order a referenced thermometer and use it to determine how close your working thermometers are to the other thermometers.

In most application if you are shooting for middle of the road temps and you are not doing advanced fish keeping such as conditioning your fish to breed, being off by a degree are two is not going to make a big difference.

aquamangandalf said:

Personally, I'd trust a digital thermometer over a stick-on one since they're usually more accurate. Another option is to get a third thermometer to see which two match up.

Click to expand...

Wellll lol, I did actually had a another of the probe ones and that was reading different again, it was 1 degree higher .

Uberhoust said:

I wouldn't trust a thermometer that does not go into the water. The outside the tank thermometers cannot give an accurate reading because they have to consider the temperature difference between the outside of the tank and the inside of the tank. This is influenced by the type of glass, the thickness of the glass and the difference of temperature between inside and outside the tank. The sensor outside the tank type thermometers will read differently based on the outside temp.

Personally, I prefer the alcohol in glass tube type of thermometers. If you are real concerned order a referenced thermometer and use it to determine how close your working thermometers are to the other thermometers.

In most application if you are shooting for middle of the road temps and you are not doing advanced fish keeping such as conditioning your fish to breed, being off by a degree are two is not going to make a big difference.

Click to expand...

Thank you.
I panicked because because the 1st probe one I had in was reading 28 Celsius which was higher than normal so I had turned my heater down slightly. The temp wasnt changing so I thought I would try the spare probe one I had. That one read 21.5 which is a massive difference.
I'm assuming the one I had been using for a while needed a battery change and that's why it was so off.
I'm going to leave to two on for the min then at least I have a better idea where its at.
Thanks again.

Get a glass thermometer at the LFS. They are inexpensive and reliable. I am surprised the digital thermometers are not accurate. I have a digital one like you show, but the replacement battery costs more than the unit, so I just use the glass analog ones.