Choices for logging systems are expanding all the time, and defining your application and finding a logger to match it, rather than fitting your requirements to a given logger, dictates the best selection. Take care not to over-specify because this will reduce your choices and increase your expenses. Cost of ownership is not limited to the purchase price of the data logger; you also need to take into account the time and expense needed to maintain the logger on site.
But cost is just one of the factors to consider when selecting a data logging system. There are many others, and the significance of each depends on the application. In general, though, it's necessary to look at the way data are handled (including the software), the type of sensor you'll be working with, and the location of the logger.
Collecting and storing temperature and humidity data are vital functions of the data logger in the increasingly regulated food and food distribution industries.
The first and most important decision for the systems integrator is to determine how the data will be retrieved. Will the information be taken directly from the logger itself or collected via a remote link? Remote links offer maximum flexibility, but they're found only in the more expensive loggers. If you're going to collect the data directly, do you want the logger to stay in place while the data are collected, or can the logger be removed and plugged directly into a PC?
There are other factors to consider as well:
- 1. Will it be necessary or an advantage to have a display on the logger itself? There are considerable cost benefits to having one without a display, but then you can see the readings only when the data are downloaded. If you are performing an intermittent logging application, such as checking the performance of a refrigerator, a simple logger without display can be used (see Figure 1).
Figure 1. The Lascar Electronics EL-USB-1 battery-powered data logger can be left on a shelf in the refrigerator and then removed after a suitable logging period has passed. The EL-USB-1 fits directly into the computer's USB port, and it offers local LED status of any breach in a preset alarm level so an immediate indication can be seen before further analysis on a PC is carried out.
- 1. What sort of enclosure protection do you need? Will the unit be used indoors, outdoors, or in an environment where it might get damp, such inside a refrigerated unit?
- 1. How many parameters do you want to measure at one time?
- 1. What is the resolution of the input? There are two things to consider here: If the sensor cannot read more than 10-bit accuracy, do you need to specify a 16-bit logger? And even if the sensor is accurate, there is no point in over-specifying the logger if you need to read only to the nearest 1°C.
- 1. How many readings must the logger be able to hold before the data are recovered? Internet data loggers provide unlimited capacity. Even so, most sensible logging systems should offer at least 16,000 readings as the standard capacity.
- 1. What sort of power supply will it need? Some data logging systems use long-life batteries, so they can be used as stand-alone equipment in almost any application. Other loggers use a universal mains supply, but they can be powered by an external 12 V battery for remote applications.
- 1. How easy will it be to use? Can it be installed by nontechnical staff?
- 1. Is data storage nonvolatile? You don't want to lose data if power goes down.
- 1. How fast do you need to take readings? Don't over-specify logging rates. Remember that natural events such as temperature vary slowly, and you may need to log only once every 5 or 10 min., not every 5 s. The faster you log, the sooner the logger will fill up.
- 1. If you are using an Internet-enabled data logger, how do you want it to connect to the Internet and/or download its information? Do you need an Ethernet connection all the time, or do you need a gateway PC running continuously in the background for your logger to get online?
- 1. Can you set alarms and/or control outputs? How do you want to be notified when there is an alarm? Most data loggers offer you a way to set alarms specific to the application. Most give a local, latching indication of alarms and will show any breach of alarm level on a downloaded graph. For applications requiring immediate alarm information on critical processes, Internet data loggers generally have the ability to send alarms via e-mail or short message service (SMS) text (see Figure 2).
Figure 2. For applications requiring immediate alarm information on critical processes, use an Internet data logger that can send alarms via e-mail or short message service (SMS) text.
Most loggers need some form of software to configure the system and process the data. This can be in the form of a PC application or, in the case of an Internet logger, a Web browser (to go online and configure remotely). The application must be easy to use and let you match the sensor to the logger with simple commands.
Normally, you give the application two input values and their corresponding readings. For example, if your sensor gives an output of 1 V for a reading of 20% RH and 4 V for 80% RH, you should be able to input the figures and let the configuration software do the rest. For instance, with Lascar's EL-1 and EL-2 data loggers, you include the sensor in the calibration loop by applying a known signal and telling the logger that the input relates to a specific value. In the example of the humidity sensor, you would place the sensor in an atmosphere of 20% and, after allowing the sensor to settle, type in 20%. You would then repeat the process for 80% humidity.
The logger's data should also be easy to manipulate and store. If you want to include the data in reports or other applications, it should be in a form that spreadsheets and word processors can use (see Figure 3). One of the most flexible formats is .csv (comma separated variable), which can be read by most spreadsheets and text editors.
With Internet loggers, you don't need to install any programs (except a browser). All configuration is done via the Internet. With this functionality, you can change the data logger's settings or download data even if you are miles away from the unit (see Figure 4). If you receive an SMS text alarm on your mobile phone, you may want to be able to see if the unit has sent you an e-mail with more details and look at the events on the Web site. One advantage of using the Internet to operate the logger is that you can specify more than one level of user access. There may be cases where employees or customers need access to the data but don't require the ability to change system settings or delete data. Only the administrator should be able to do this.
Figure 3. Data loggers should make it easy to view, manipulate, and store data. Often, a simple Windows-based interface provides this functionality.
In some instances, you may want to use a sensor that was installed with a previous measurement system. If so, consider the sensor's power requirements and how far it will have to be from the data logger. For example, thermocouples don't need a power supply, but they are sensitive to interference over long cable runs. Sensor/conditioning units with 4–20 mA or 0–10 V outputs need power, but they are the most readily used because they have a linear response and you don't need to add your own interface circuitry. This makes setup easier, and many of the sensors are directly interchangeable.
A further consideration is the lifespan of the sensor and how long it can be used without recalibration. This is most important with chemical sensors, such as gas sensors and pH probes. If you are planning a remote logging installation, you'll want the sensor to operate unattended for at least as long as your regular service interval. Some sensor systems have interchangeable sensor elements. This may mean that some form of recalibration will be required even if the logger itself can be recalibrated because you will not otherwise know the response of the sensor. Some loggers can get around this by recalibrating the system as a whole.