Thermometer Accuracy: Why Not To Use Dial Thermometers

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Claudiusx Sicko
Staff member

Dials are wildly inaccurate

  • In low wattage testing inside the tank dial off 7 degrees.

    In high wattage testing, Dial was 40 degrees different than probe

    Dial side down on basking surface = 14 degree difference from probe.
Hello everyone..

Over the past few days, I have been doing a lot of testing on the accuracy of the highly recommended digital thermometer with probe, versus the dial stick on theremometer.

Many times unsuspecting pet owners will have a dial stick on theremometer, and believe they are getting accurate readings. Well hopefully after reading through this thread, you will understand why it is imperative to get a more accurate means of measuring temperatures.

Temperatures are just as important to a dragon as you remembering to feed it. This is why you can't be guessing what your temperatures are, which is basically what you are doing with the dial thermometers.

I am splitting this thread up into a few parts, for those who just want to see results, or want to see the data.

Anyways, table of contents: (all links lead to different parts of this thread, there are no outside links in the table of contents)


Claudiusx Sicko
Staff member
Original Poster
How the Tests Have Been Conducted

Thermometers used were the acu-rite digital thermometer with probe end and an All Living Things brand Dial thermometer.

The first five tests, were performed with the dial/probe placed on a ceramic tile, directly 12 inches underneath a 57 watt incandescent bulb. A square marked out with tape was placed in the middle of the tile, so that the thermometers were being placed in the same spot each time (leaving very small margins for any change in positioning under the light)

The first five tests each lasted 30 minutes each. After each test, the thermometer was given 30 minutes or more rest time to return back to normal room temperature before being used again in another test.

Here is what was done during the tests:

First test, the probe is placed flat side down

Second test, the probe is placed flat side up

Third test, the dial is placed dial side up

Fourth test, the dial is placed dial side down

Fifth test, sticky pad applied, dial side down

Test 6 was done in a 10 gallon tank, with the same bulb, and same light fixture.
The Dial thermometer was placed against the wall at the very bottom of the tank, and the probe was placed right next to it. I did it this way so that the dial thermometer has the best possible chance to be as accurate as it can. But please realize, the way dial thermometers are often used in the tank, they are usually 6+ inches away from the actual basking surface, and in this test, it was directly at it.

Test 7 was done exactly the same as test 6, but a different fixture was used, and a 75 watt halogen floodlight was used in replace of the 57 watt incandescent. Please note that the same cool down period was allowed for each thermometer between test 6 and 7 as was allowed between tests 1 and 5.

No tests started until the thermometers had reached a reading of back to room temperature.

For the first 5 minutes of each test, the temperature reading was recorded each 5 minutes. After the 5 minute reading, the next reading was recorded at 15 minutes, and then at 30 minutes. For tests that lasted longer than 30 minutes, the temperature was checked every 5 minutes after to determine when the thermometers would rest on their final reading.


Claudiusx Sicko
Staff member
Original Poster
Results and Conclusions

I've found that it actually can take up to an hour for the probe to reach its final resting temperature. This seems to be the same for the dial thermometer too.

In low wattage testing inside the tank, I found the dial thermometer to be reading a cooler temperature by 5-7 degrees.

In high wattage testing, the difference was much larger. During testing with a high wattage bulb, there was a 39-40 degree difference, even after 65 minutes!!

I found that the dial thermometer is just as ineffective when actually placed on the basking surface. When placed dial side up, the dial read 4 degrees warmer than the probe. When the dial was placed dial side down, it read 14 degrees hotter!!

Based on my testing for now, I think it is safe to say that dial thermometers are extremely innaccurate, and have no place in your reptiles tank.

Oddly enough, it seemed the cooler the temperature, the more accurate the dial became. At 78 degrees in my room, both the probe, and dial read 78 degrees. It seems that once the dial is exposed to true temperatures over 95 degrees, that its accuracy quickly falls.

Read below for more details on the results of each test.

Test 1 and test 2 (both the digital with probe) both read the same temperature (give or take 1 degree) after 30 minutes. As was expected.

Test 3 read 4 degrees hotter than the previous two tests with the digital thermometers, and seemed to climb much more slowly than the probe.

Test 4 read 13-14 degrees hotter than test 1 and 2, and read 10 degrees hotter than test 3.

Test 5 read 7 degrees cooler than test 4, 3 degrees hotter than test 3, and 6-7 degrees hotter than the digital with probe.

Test 6 (1) gave me a reading of 96-97 degrees.

Test 6 (2) gave me a reading of 90. 6-7 degrees cooler than the digital.

Test 7 : after 65 minutes, the probe read 149-150. The dial thermometer read an astounding 39-40 degrees cooler!!! at 110 degrees after 65 minutes.


Claudiusx Sicko
Staff member
Original Poster
Graphs of the Experiment

Chart 1
Chart 1 is a line chart of tests 1 - 6. These were grouped together as they were all under the same wattage bulb. Note how the probe readings all usually end in the same place (give or take a degree or two) after the half hour. But, the final dial readings are all over the place.

Chart 2
Chart 2 is of experiments 6 (1) and 6 (2). I felt the need to give these two tests their own graphs since its a more applicable setting, inside the tank. Please note that I did not in any ways try to lean the experiment into the dial being inaccurate. I placed the dial in the best possible position it could be in to give it the chance to get the most accurate reading it could. Refer to the pictures section to see a photo of this.

Chart 3
Easily the chart that shows the biggest difference in temperatures. This is a chart of test number 7 (1) and 7 (2)The same exact thermometer placement was used in this as in test 6, the only difference was a different fixture and a stronger bulb.

Data sheet for tests 1-6
(Right clicking the image and selecting "open in new tab" will show the picture much bigger.)

Data sheet for test 7
(Right clicking the image and selecting "open in new tab" will show the picture much bigger.)

(do note that 30 seconds after the 65 minute mark, the probe dropped back down to a reading of 149)


Claudiusx Sicko
Staff member
Original Poster
Pictures of the Tests

Setup for the tests 1-5

Test 1

Test 2

Test 3

Test 4

Test 5

Test 6

Test 7


Claudiusx Sicko
Staff member
Original Poster
Additional Information/Data

It has always been said to not use dial thermometers because they can be off by as much as 20 degrees. I always thought this was a bit extreme, but after these tests I see it was actually an understatement, and that these thermometers can be off by as much as 40 degrees.

It has always been said to leave your probe thermometer where you want to measure the temp for 30-45 minutes to get an accurate reading. My tests show that sometime between 30 and 45 minutes the dial thermometer will stop changing as much, but showed that it can take up to 65 minutes in some cases to reach its final temperature. I would say its best to leave the probe in place for a whole hour, as long as it seems.

Oddly enough in one of my tests (one that I had to start over because I accidentely moved the lamp, which would have caused inaccurate temperature readings) after I turned off the lamp, the dial thermometer continued to rise, rising an extra 3 degrees (from 79 to 81) in 5 minutes with the lamp off.

Another thing I noticed is it took significantly longer for the dial thermometer to return back to room temperature compared to the digital with probe, which makes sense.

Lets beat the dead horse some more


Beardednoob Addict
Great job Brandon this really shows the difference of accuracy between the two types of thermometers typically seen in reptile enclosures. Judging by the charts it is easy to tell if you go off from a dial thermometer you may be cooking your dragon.

Awesome post.

diamc Sicko
Staff member
Awesome job, that was a LOT of work. I'm gonna make this into a "sticky" so it will always be at the top of the Enclosure forum.

Claudiusx Sicko
Staff member
Original Poster
Don't confuse "has to much time on his hands" with "rocks" :laughing6:



Juvie Member
LOL Brandon - but you do rock! it could just be that you have a job where you're on the computer all day and can do anything BUT work - like go on and share your wisdom!
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