Choosing a Data Logging SystemOctober 1, 2005 By: Simon Wyre, Lascar Electronics Ltd. Sensors
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.
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