The Ultimate Aquarium Thermometer Test

Just how accurate are aquarium thermometers? Results will surprise you.

Thermometers acclimating for 30 minutes prior to testing. Photo by Stephen G. Noble

Thermometers are one of the basic components of aquarium keeping yet, unlike filtration and lighting, they are rarely examined for features and performance. We all strive to provide our fish and organisms with the highest quality care but thermometers are often relegated to an afterthought. It is not uncommon to assume that thermometers are about the same and that they are accurate. To prove the point, visit any pet shop and compare thermometers hanging on the display rack. You will find everything from inexpensive, floating glass to quite sophisticated digital thermometers. Additionally, thermometers often show temperature variations up to as much as six degrees. This is especially the case with floating glass thermometers where the cardboard scale is incorrectly attached to the temperature sensor.


Thermometers acclimating for 30 minutes prior to testing.Photo by Stephen G. Noble

Such inaccuracy can result in excessively high or low aquarium temperatures, which could be detrimental to the well being of the tank’s inhabitants. Some fishes are tolerant to a wide range of temperatures and although unhappy, manage to survive in inappropriately heated water.  However, the Otocinclus for example, clearly lives longer in cooler water. The long-term health of this fish could be adversely affected by a thermometer erroneously indicating 74°(F)/23°(C) when the actual temperature is 80°(F)/26°(C) degrees.

The goals of this article are to examine the features of readily available aquarium thermometers, priced under $30, which seems to be preponderance of thermometers sold. I will also show how to evaluate the accuracy and ways to calibrate your thermometer. Those who have followed my articles over the years know I strive to follow scientific principles; in this case, replication. Therefore, I present my findings in a manner that the reader can reasonably reproduce.

I purchased a special calibration thermometer for my baseline comparison; this Thomas Scientific thermometer, model 1212P58, uses environmentally friendly liquid and ensures an accurate and scientifically-based comparison. This Thomas Scientific thermometer, however, is not recommended for hobbyist use. Quality thermometers such as this one come with a traceability statement indicating they conform to the National Institute of Standards and Technology (NIST) which certifies their accuracy to a certain level.

Test and Evaluation Conditions

A 10 gallon aquarium was used for conducting the test. A small power head was used for water circulation. The thermometer wire probes were grouped inside a floating Styrofoam ring that was positioned in the center of the tank.  The thermometers which are intended to be completely submerged were installed against the glass just under the water’s surface. Care was taken to ensure none of the thermometers or thermometer probes were in contact with each other.  The calibration thermometer was also positioned, vertically inside the ring.

The evaluation criteria includes subjective and quantitative data including price, viewing ease and accuracy.  Three trials were conducted comparing each thermometer against the calibrated thermometer.
Trial one accuracy of 84.2°F/29°C.
Trial two the accuracy of 74°F/23.34°C.
Trial three the accuracy of 64°F/17.77°C.
See results table at end of article.
The results of this evaluation are most interesting. My hypothesis was that we would find greater variation from our calibrated thermometer but that was not the case.  In everyday use, a degree of variance is probably negligible.  Correction factors are the difference between your thermometers reading and that of a calibrated thermometer. More than a degree makes it worthwhile to affix a correction factor sticker on or near the thermometer.

I recommend purchasing a calibration thermometer and verifying your thermometers. Mine was very inexpensive and came with a handy protective case. The calibration sheet accompanying the thermometer does not state this but the thermometer must be kept in a vertical position in use and storage. Flat storage can create a situation where the fluid separates requiring a heat process to restore its usefulness.

Someday, I hope to evaluate the accuracy of aquarium heater settings.  In the meantime, enjoy your fish!

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Article Categories:
Fish · Freshwater Fish

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