Terminology
Sample rate
This is how often the logger will record the temperature. For transport and storage applications a sample rate of between 1 and 30 minutes is typically required. When processing items sample rates down to 1 per second could be required.
If the sample rate is set too slow then it could possibly miss an event. For example, if it is set to 30 minutes then it could miss a 15 minute rise in temperature. If it is too fast then it will consume more memory and the results will take longer to check.
Memory
The total number of samples the logger can record is critical. It determines the fastest sample rate that is possible for an application. Once the memory is full the logger will either stop or write over the oldest readings. Either way information is being lost. The larger the memory, the better the device.
The maximum time the logger will record is the sample period times the memory size. The following graph shows the duration of a logger with the sample rate set between once a minute to once every 30 minutes. Four different memory sizes are displayed.
For example, an item is to be transported for 30 days. A unit with 2048 readings memory could sample with a period of 22 minutes. It is decided that a sample period of 15 minutes is required. A unit with at least 4096 readings is required.
Resolution
The resolution is an indication of how specific the answer is. It is measured in degrees. The smaller the value, the better the device.
For example, a resolution of 0.5° means that a reading of 3.5° could be between 3.25° and 3.75°. A resolution of 0.1° means that a reading of 3.5° could be between 3.45° and 3.55°.
The maximum permissible resolution depends upon the application. For example, when monitoring body temperature it is critical to have at least 0.1° resolution. For transport this degree of resolution is often not necessary. The change in temperature that has to be recorded is a couple of degrees and a resolution of 0.5° is often acceptable.
Accuracy
The accuracy indicates how close to the actual temperature a reading is. The smaller the number, the better the device.
The difference between accuracy and resolution can be seen in this simple analogy.Two men have a watch. One watch shows the hours, minutes and seconds. It has a resolution of 1 second. The other watch only has hours and minutes. It has a resolution of 1 minute. The first man sets his watch once a month and can be up to 5 minutes fast or slow. It has an accuracy of +/- 5 minutes. The second man sets his watch daily and has an accuracy of 1 minute.
In this example, the first man could state the time as “1:34 PM and 23 seconds” but could be up to 5 minutes out. His answer is specific (good resolution) but inaccurate. The second man can only say the time is “1:36 PM” but will be within one minute of the time. His answer has a lower resolution but is more accurate.
Construction
Like most items, temperature loggers break. Consideration should be given to the robustness of the logger if it is to be transported or regularly moved. Stainless steel loggers are available. Most loggers are plastic
Water resistance is another feature that should be considered if the logger is to be used in a wet or dirty environment. Water damage is a common occurrence.
Size
Loggers vary in size from the size of a coin to large rack mounted machines. For the larger sized units there is typically a probe that will be inserted into the area to be monitored.
Battery life
Most units have replaceable batteries. The battery life should be noted. If it is too short then you will be continually replacing batteries and you could also be loosing data mid-journey.
Starting options
Temperature loggers often offer a range of techniques to start the logging. A start delay is useful for programming loggers prior to usage. They can then be sent to the user or picked up later.
A trigger is another handy feature. For example, when the temperature reaches 6° then start logging. This means that memory is not being used while it is within the correct temperature range but will start as soon as extreme conditions occur.
Number of inputs
There are a number of single point loggers on the market. These devices record either their surrounding temperature or have a remote probe. They are very useful for monitoring individual trucks or items.
Larger units provide for multiple probes. The benefit is that there is only one unit that has to be connected to a PC. Retrieving the data is quicker and simpler. The installation is more expensive and difficult because cables have to be run from the probe back to the logger but once installed it saves time.
Outputs
Some loggers provide a display so that the current, minimum and maximum temperatures can be displayed. Indicators for alarms are also available on some units. A relay output is also available. This allows an external alarm to be activated if the temperature becomes too high or low. There are also dedicated devices that can achieve these functions.
Software
The great benefit of loggers is the ability to view the results and save them for later use. Each logger will typically have its own software. They vary in what the software can do and how easy it is to use.
One product on the market has two versions of software available. One provides a powerful database engine with flexible reporting capabilities. The other version has the same data display capabilities but is much simpler to use. For many customers the second version is the better option. It takes 2 minutes to learn and does everything that they require. So when testing software, first decide what you require and then test to see how hard it is to achieve.
Price
The biggest issue for many people is price. Temperature loggers range in price from under $60 to thousands of dollars. Money can be saved by not over specifying what is required and by shopping around.
Summary
If you store, transport, process or sell temperature sensitive items then temperature loggers can save you money, time and even your reputation. They are becoming increasingly popular due to falling prices but also increasing demands to sell quality products.