There is a saying "many experts, many opinions". You may find the differences that can arise when measuring the temperature with different measuring instruments and methods, confusing. Most people tend to trust a digital device more than an analog device. Is this justified? A finely scaled display makes us feel that the result is precise.
This is why it’s important to explain what the terms measurement and display accuracy mean and how the usual measuring instruments for aquariums, terrariums and garden ponds work.
The display accuracy
You can see the display accuracy of analog thermometers by the scale depicted on them. In our hobby sectors this is usually in 1 °C steps. The display accuracy is thus 1 °C. Digital thermometers usually have a display scale and display accuracy of 0.1 °C. The display accuracy does not determine the actual accuracy of a measuring instrument. It only shows the precision with which the values measured can be displayed.
The measuring accuracy
The measuring accuracy describes the precision of the thermometer. It refers to the accuracy of the measured value, taking into account the deviations due to the measurement method compared to the real physical measurement value. In short, it means that if the physical measurement value is 25.0 °C, how much deviation above and below this value can be tolerated and/or by how much the measured result may deviate.
Practical examples
Digital thermometers JBL Aquarium Thermometer DigiScan (Never immerse in water!) and JBL Aquarium Thermometer DigiScan Alarm (Never immerse in water!) have a display accuracy of 0.1 °C and a measuring accuracy of +/- 1 °C with a display range of 0-40 °C. In extreme cases, this can result in differences of 2 °C between different measuring instruments, as the deviation from the physical measurement value, which we assume to be 25 °C, may vary from 24 to 26 °C. Strict quality controls ensure that these deviations are not violated.
Let’s now compare this to a clinical thermometer to get a clearer idea of things. Clinical thermometers have a display accuracy of 0.1 °C as well. The measuring accuracy and any associated deviations for this sensitive range are a maximum of +/- 0.3 °C for digital thermometers, as laid down in a DIN standard. Differences of up to 0.6 °C are therefore considered to be acceptable. Depending on the measuring method, 0.2 - 0.6 °C are the usual deviation (+/- 0.1 - 0.3 °C measuring accuracy) for devices available on the market. These clinical thermometers measure within a very narrow temperature range of 35-42 °C and therefore encompass an indication range of only 7 °C. Aquaristic and terraristic thermometers measure between 0 - 40 °C (sometimes up to 50 °C) and thus cover 5.7 and 7.1 times the range respectively. If one puts this in relation to the measuring accuracy, not even a clinical thermometer would have a measuring accuracy of +/- 1 °C, but instead one of +/- 1.15 °C within the mean value (max: +/- 1.74 & min: +/- 0.57 °C). A very high measuring accuracy is only possible with methods that measure within a very narrow temperature range. High measuring accuracy is nevertheless achieved in the large temperature ranges mentioned.
Effects & insights
The combination of measuring accuracy and display accuracy may result in a deviation from the true value of the physical quantity. This is the case with any measuring instrument and, kept within a reasonable range which is not enough to affect the recommendation for action or evaluation in any field of application, it is negligible. We instinctively expect to measure values with absolute consistency, on the finest possible scale, but if we are honest, we’ll admit that these deviations do not impact on our assessments or recommended actions for our animals. Clinical thermometers and common room temperature thermometers similarly incur deviations, which most of us are unaware of.
It’s much more important to carry out the measurement correctly. In the aquarium and pond, for example, the water temperature is not always uniform throughout, so that a higher temperature can be present near the heater than at the other end. That’s why heaters should always be installed at a point with a good water flow. In the terrarium, the temperature differences occur intentionally, so that animals can move from a sun to a shadow area. It’s best to observe your measuring location carefully before choosing it. A second measurement is helpful too.
Outline of display and measuring accuracies
The following gives you an outline of all the JBL products with their measuring and display accuracies. If you compare them with those of other manufacturers, you’ll see that ours are the most precise in their price segment.
| Display accuracy | Measuring accuracy | |
|---|---|---|
| JBL PROFLORA pH-Control Touch | 0.1 °C | +/- 0.4 °C |
| JBL CoolControl | 2.0 °C | +/- 1.5 °C |
| JBL PROTEMP S 100 | 0.5 °C | +/- 1.0 °C |
| JBL Aquarium Thermometer Slim | 1.0 °C | +/- 1.5 °C |
| JBL aquarium thermometer Hang-on | 1.0 °C | +/- 1.5 °C |
| JBL Aquarium Thermometer DigiScan Alarm (Never immerse in water!) | 0.1 °C | +/- 1.0 °C |
| JBL Aquarium Thermometer DigiScan (Never immerse in water!) | 0.1 °C | +/- 1.0 °C |
| JBL Aquarium Thermometer Digital | 2.0 °C | +/- 2.0 °C |
| JBL Aquarium Thermometer Mini | 1.0 °C | +/- 1.5 °C |
| JBL Aquarium Thermometer Float | 1.0 °C | +/- 1.5 °C |
| JBL hydrometer | 1.0 °C | +/- 1.5 °C |
| JBL PROTEMP e300 | 1,0 °C | +/- 0,5 °C |
Background knowledge
We’d like to end with some facts about JBL’s measuring methods.
In the aquatic field, analog thermometers are usually expansion thermometers, in which an alcoholic solution with dye expands and rises through a glass capillary. Mercury and other toxic substances are not used, so your fish are not in danger even if the thermometer breaks, once the glass splinters have been siphoned off. These thermometers are 100 % waterproof and do not require electricity. They are easy to clean and disinfect. There is also no danger for the user of an allergic reaction to nickel or any other metals.
Digital thermometers in aquatics and terraristics have either a sensor with a cable, or a contact point directly on the underside of the display unit. They measure the temperature at the surface of the sensor via a sensor point. Some devices immerse the sensor in the water, others use the surface contact to the aquarium glass.
Infrared thermometers, which measure the temperature on a surface, can be used in terraristics to measure the substrate or stones etc. In aquatics this method is not useful, because reflections on the water surface will lead to huge deviations.
