A data logger is a small device that can measure, and record continuously, temperature, relative humidity, or both at the same time. There are also loggers available can record a whole range of other parameters, but for tortoise keepers or field herpetologists, temperature and humidity are typically the areas of greatest interest. The key advantage of a data logger is that you can put it in place, leave it, and can recover it days, weeks or even months later to find a complete set of measurements ready and waiting for you. These can provide really great insights into conditions within a housing unit, incubator, or when studying a particular microclimate or behaviour in the natural habitat.
An example of just how revealing and valuable a data-logger can be. A graph of substrate temperatures measured directly next to a brumating (hibernating) wild Testudo graeca graeca in Almeria, Spain.
Data loggers come in various shapes, sizes and price ranges. They might also offer various optional features. Some of the features that we like to see ourselves, and that can make them either easier to use or more versatile, include:
User-replaceable battery. Some loggers use a non-replaceable battery, and although battery life is often very long (many months or even a year or two) once the battery does eventually die the device has to be disposed of and replaced. Hence, a battery than can be easily replaced is a good feature.
Real-time visual display. This is not essential, and on the very smallest 'button' type loggers there is not space for it, but having a quick readout of current temperature and RH on a small LCD screen can be really useful. These screens do not usually offer a continuous display, as that would greatly add to battery drain, but are often activated by a quick button push and they then reveal the current measurements for 10-15 seconds.
Built-in visual and audible alarms. For some applications, especially when measuring and recording temperatures in tortoise egg incubators or when monitoring a tortoise or turtle that is brumating (hibernating) having an audible alarm or bright flashing LED lights if preset temperatures are exceeded can literally be a life-saver. Typically, alarm types and thresholds are set in special software that is compatible with that particular data logger. As we shall discover, this varies widely in quality, stability and user-friendliness! In principle, however, being alerted if things get too hot or too cold immediately is extremely useful in some situations. To function effectively, audible alarms need to be sufficiently loud, and visual alerts need to be bright enough and obvious enough to be easily seen.
Accuracy. It should go without saying that you need your measurements to be reasonably accurate. This is where some of the ultra-cheap 'no-name' loggers fall down badly. The good news is that after testing quite a large range of medium-priced devices we found that most performed surprisingly well. Many come with a certificate of calibration which improves confidence. Generally, you can expect temperatures to be recorded with a +/- 0.5C accuracy, and relative humidity with no more than a +/- 2% deviation (measuring humidity is technically more difficult than measuring temperature).
Remote sensing wired probes. Most loggers have built-in sensors, but for some applications it is essential to be able to separate the logger itself from the position where the data is being measured, Again, an egg incubator or refrigerator being used for brumation/hibernation are typical examples confronting tortoise keepers. In such cases you can site the probes INSIDE the incubator or chiller, while being able to read the temperature and/or humidity on the loggers LCD display conveniently sited OUTSIDE. Also, if you have low-high temperature alarms set, having the logger clearly visible and where it can be heard if an alarm is triggered is vital.
Weather-resistance. If you are using the logger in the field, then it needs to be able to withstand getting soaked or exposure to dust, etc. For other applications this may not be important.
The precise combination of the above features that you will need depends upon your intended use. For burying a logger alongside an estivating or brumating (hibernating) tortoise, for example, a good degree of environmental protection, a long battery life, and small size will be priorities. For attaching to the outside of a 'fridge or incubator, an easily-read LCD display, remote probes and a good alarm system will be most important. In fact, something as simple as an included magnet to attach to a fridge can be a useful feature.
Software compatibility and functionality.
This, actually, is where quite a lot of otherwise quite nice loggers fail badly. Physically, the loggers are fine, and plenty accurate enough. However, the manufacturer's software can be little short of a disaster zone. Some of it looks like a design from the 1980's, and even doing the simplest thing such as setting an alarm point or changing the frequency at which measurements are taken can be an exercise in frustration.
Worse, we found that some manufacturers own download sites had expired security certificates, incorrect links, and software that simply failed to install correctly. In other cases the languages used were a strange mix of English and Chinese and 'user manuals' that were very poorly written indeed . This included some quite well-known brands! The vast majority of loggers require use of a Windows-based computer, though a few do also offer a Mac OS version. As we use a MacBook Air when on the road and in the field, this is what we tested them with. Unfortunately, this was not exactly a smooth experience. It is worth knowing that some loggers are also available that interface by Bluetooth to both Android and Apple 'phones. We did test a few of these too, and they definitely have their uses. The more 'professional' grade loggers, however, do tend to require a USB connection direct to a desktop or laptop PC.
One function that is really useful is to be able to save the charts produced in different formats, either PDF, JPEG or TIFF - but very few actually offer this level of flexibility. For many years we used a (paid for) program that did permit this, but the cost was really high and it only ran under Windows.... to their credit, the software available for most loggers is free, and so some limitations are to be expected. Typically they save a chart in PDF format and you can then later grab a screenshot of this or open it in a suitable editing program to convert to a different format. Most also permit exporting the raw data points in Excel or comma-delimited format so you can import to your own analyses and graph-making programs.
Knowing what to measure
Having the most accurate logger in the world is of little use if you don't know how to use it properly. For example, in an egg incubator where do you position the temperature probe? Where do you position the humidity probe? Even a very slight change in position can result in misleading readings. In this case you should position the humidity sensor in free air (not touching anything) at the EXACT SAME LEVEL as the eggs. Higher, or lower can give false readings. Relative humidity sensors are also not the same as a MOISTURE METER which is a different parameter altogether. For measuring the moisture content of say, an incubation, substrate or soil then, a RH meter is - strictly speaking - the wrong tool. For this you really need a logger that records the moisture content of solids. That said, you can usually infer an approximate moisture content from a RH reading, and most advice on incubation cites a relative humidity reading from within the incubator. Dedicated moisture meters that read soils and similar solids are available if required, however.
In the case of temperature in an incubator there are two key options. 1) You can measure the free air temperature, again at the same level as the eggs, or 2) You can place the probe in physical contact with the substrate next to the eggs. Both readings can be useful, but in practice the latter is likely to give a more useful, stable and realistic readout of exactly what is happening to the eggs themselves.
With a tortoise in brumation/hibernation, you really want a fast response if temperatures start to fall to dangerous levels so in this case a FREE AIR measurement where the sensor is free of thermal 'drag' is the best option. Again, place the probe at the same height as the tortoise to avoid misleading readings due to thermal layering effects.
The usual 'rules' for using thermometers still apply, of course. If you want an accurate ambient air temperature, do not expose the logger (or the probe) to direct radiant or reflected heat, sunlight, for example, and keep it at a sufficient distance from other heated objects that could influence your result.
Some practice taking measurements and of observing exactly how different loggers and sensor positions affect your readings is highly advised. You can learn a lot from this and will be better placed to interpret your readings accurately.
Example Data Loggers
We have owned and used many different types of loggers over the years... dozens! Some were extremely expensive (many hundreds each) and others were 'cheap buys' off Amazon or eBay. Based upon this experience, we'd strongly advise avoiding the really cheap ones as they tend not to be very accurate and the accompanying software (or app) is frequently buggy and difficult to use. The possible exceptions to this are some of the lower cost 'phone-based Bluetooth devices some of which, despite being really cheap, are not bad at all. Yes - they have some limitations, but they are certainly good enough for casual applications, like checking conditions in a greenhouse, for example. For critical applications like monitoring egg incubators or hibernation conditions, however, we'd definitely suggest getting something better and preferably something that is certified and calibrated.
ELITECH GSP-6
This is a combination temperature-humidity logger that has a particularly large and clear LCD display and features remote, wired sensors. Build quality is not totally inspiring but probably adequate for the intended uses. One really good feature is that the humidity probe cable has a FLAT profile, so is easy to squeeze through a refrigerator door seal or into an incubator lid, while the temperature probe wire is sufficiently thin and flexible as to cause no problems. The cables are also a very good length.
The GSP-6 comes with an individual certificate of calibration and has a very high accuracy for temperature measurements, typically +/- 0.5 Celsius. Humidity measurements in 'normal' ranges also have very excellent accuracy at circa 1%. These are very good results indeed, especially for the fairly modest cost of the unit. We compared results directly to our own Fluke 971 Thermo-Hygrometer (€485.00) and found the GSP-6 to be surprisingly close.
The setup and graphing software (we used Elitech Log for Mac) can be downloaded from the manufacturer's site. This may bring up security warnings but we installed without issue. Using this parameters such as start delay, time between data point recording and alarm settings can also be adjusted quite easily. You can also produce graphs and export the raw data. There is also a very useful physical 'over' and 'under' alarm that provides both flashing lights and an audible buzzer alert if temperatures exceed the set thresholds. These are not super-high visibility or particularly loud, but should be noticed. This makes the GSP-6 ideal for monitoring egg incubators or brumation environments. At €45 (approximately) this is a great value, accurate and versatile logger that can be recommended for such purposes. It is not weather sealed and the build-quality is not up to demanding fieldwork, but otherwise this is perhaps the best low-medium priced data logger we have tested to date. Tortoise keepers will find many uses for it.
ELITECH RC-51
The RC-51 is a temperature-only logger that is designed to be weather-sealed. It has no remote probes. There is an LCD display screen but it is very small indeed and quite hard to read. There are also no audible or visible alarms. It is a basic logger, and sometimes, that is all you need. Something to 'set and forget' and collect all the data later. Unfortunately.... while it does connect to the Elitech Log software just like the GSP-6, we found many essential settings to be 'greyed out' and unable to be changed. This included such essentials as setting the intervals for recording and other configuration settings. It might be better on a Windows system, but its functionality is severely limited on a Mac, despite the manufacturer's claim of compatibility. We really cannot recommend this model based upon our experience. Disappointing, as the build quality feels very solid and the weather sealing looks good. A good, basic logger let down by poor software integration and support.
XETRON ELOG 10
This is another basic, simple and useful 'all in one' logger. It comes in two versions, One is temperature only (as tested) the other also records relative humidity. It is small, flat profile device with just two buttons 'Start' and 'Stop' and a small, but not ridiculously small, LCD screen. It plugs directly into a standard USB port for configuration and export of data. Fortunately, the XETRON LOG software is available for both Windows and Mac, and appears to work well, being quite clear and offering a good range of options. You can set measurement intervals over a wide range (hours to seconds), set start delays, Celsius or Fahrenheit readings, and several other parameters. Reassuringly, you can set it to emit a quick green 'flash' of the LED to confirm it is actually recording, Graphing options are basic, but functional. We have seen nicer looking graph output for sure, but the essentials are there. For a 'prettier' result you can export the data and use it in your own graphing program. Interestingly this is the only graphing software in this test to offer a highly useful direct JPG image file output. That can be very handy when you need a quick result you can email or share, for example. Overall, this is a good, simple to operate basic logger that has a decent build quality (IP65 rated) and is very good value at just under €30 each. Tested accuracy was excellent. There IS an alarm setting with very bright LEDS and a sound warning too. If you just want something to 'set and forget' and record for very extended periods (it can record 32,000 separate measurements and has a very long battery life) this might just be ideal. There are also other versions available, some with external probes. The real strong point of this logger is that it is easy to use, is never complicated or confusing (that matters a lot, especially out in the field) and is well enough built to take a bit of rough handling.
Inkbird IBS-TH1 Plus
This is the first of the 'phone based loggers. It connects to an app on your phone via bluetooth. There is a large LCD display and a detachable external temperature probe, though it also has built-in sensors if an external probe is not required. Accuracy is not the best, only 3% in terms of humidity and we also found the temperature readings to be somewhat unreliable, though we have seen far worse. The app itself is both overly complex and too simplistic at the same time, forcing you to choose between various 'scenarios' such as checking your cigar store (!) or inside a vehicle. It is clear that this intended for private domestic use and not for industrial or scientific applications. We would not advise relying on this for any kind of critical purpose, but it is probably OK for monitoring a general environment or greenhouse. A positive is that it runs from two common AAA alkaline batteries and the large LCD display is permanently 'on'. It is therefore good to have on a wall where you can get realtime visual results. It does feature alarms (called 'alerts' that show up on your phone, provided it is within Bluetooth range) and the build quality seems reasonably robust (it is not fully weather-sealed, however). We had no problems pairing via Bluetooth. The graphing capabilities of the app are very limited indeed however, better suited to cursory checking than any kind of detailed analysis. In conclusion, the more 'serious' loggers described above are much more accurate, more flexible, and definitely more appropriate for tortoise keepers than this device, although it is not bad value at around €30 for non-critical uses. The large real-time LCD readout of temperature and humidity is it's best feature.
ORIA 'Smart Hygrometer' (two pack)
Another 'phone based logger. These are sold under a wide variety of names, many of which you will never have heard of before. They are very distinctive in appearance though, and the published specifications are almost always identical, so it is a reasonable assumption that they are indeed all the same thing. You can find them via Amazon, eBay, etc. The main point going for them is that they are cheap! Very cheap! In this case only €20 for two. So - are they worth it? In our opinion no, and certainly not if you are going to rely on them to incubate eggs or keep a tortoise safe in brumation/hibernation. Most of the comments made about the INKBIRD (above) apply here too. The 'phone app is very basic and limited. The build quality here though, is terrible. Not only are the devices sensors exposed to the environment, but the entire internal electronics are too. This means that in a high humidity situation, all kinds of bad things will happen to the circuit board, battery (it is user replaceable, with a little difficulty) and other components. This is not good at all. Accuracy was quite poor too, and not even close to the published specs. Needless to say, there is no individual certificate of calibration here. We could not recommend these. Better to spend a bit more and get something with greater accuracy and reliability. The XETRON ELOG 10 is not that much more costly, but is greatly superior across the board.
Data loggers then, are genuinely useful devices that can provide you with a lot of valuable information if used correctly. You can observe minute-by-minute changes in temperature or humidity over very extended periods of observation, months in some cases. You can compare different environments, you can discover the effects of varying methods of keeping, and you can save all of your data in various formats for later reference. We have used them for years, and find them to be invaluable tools in our everyday work.