How to Select a Portable Data Acquisition System
By: Stew Thompson, CAS DataLoggers
Portable data acquisition systems are used worldwide to capture data and conduct routine testing of vital infrastructures such as mass transit systems, power grids, and bridges, as well as heavy industrial processes and applications. These powerful yet compact data acquisition devices play an important role in the testing and monitoring of many critical systems, and selecting the most suitable device for a given application requires careful consideration. The ideal portable data acquisition system for most users is a compact, lightweight unit powered by either a self-contained battery or a single DC power source, requiring no other connection to function other than the sensors being monitored. Operating in remote areas, a user interface and means of communication with the device become vital features. Signal conditioning such as gain and filtering as well as high-capacity non-volatile data storage are other important considerations.
When selecting a suitable portable data acquisition system for their specific applications, users face a bewildering choice of available manufacturers, models and specifications. Each system has its own configurations that make some units more suitable than others for certain applications. Additionally, purchasers must first consider several demanding requirements not necessary for traditional laboratory devices. Any existing and potentially damaging environmental extremes including temperature, excessive humidity, liquids, dust, shock and vibration should be carefully considered. Other relevant questions to ask include whether the data acquisition equipment can support the particular mix of sensors that will be used, as well as determining if the device has adequate memory and storage to support the specific project.
Before deciding on a specific manufacturer or solution, users need to form a clear idea of the results they require of any system they’ll be using. Sampling rates, as just one factor, are available in a wide variety from as low as once per day to higher than a million per second. Anticipating the future project’s needs today will save precious time and money on installation. Data acquisition system designs range from the simple to the complex, with an attendant variety in performance, features, and cost. Fortunately, in the face of all these options, fundamental guidelines are available for portable data acquisition equipment users to consider before making their purchase.
Of course, the main function of data acquisition systems are their basic function to accurately record data, and here the wide spectrum of degrees of accuracy attainable by units available on the market can make this a more involved decision than expected. A reliable constant, though, is that the accuracy of field measurements is heavily based upon the sensors being used. For most sensors that have been calibrated in the laboratory and installed in the field, accuracies in the range of 0.01 % to 1 % of full scale are typical, with many other sensors having less accuracy. The particular system’s input types must also match project requirements–for example, an application requiring extensive analog measurements would benefit from the portable Delphin Expert Key 200 series of data acquisition system, which features a full 28 analog inputs.
Likewise, the sampling speed of DAQ systems must also be taken into account when determining accuracy. The device must acquire signals quickly enough to avoid any loss of the data. The necessary data acquisition speed is calculated by using Nyquist’s Sampling Theorem, which states that a signal must be sampled at twice the frequency of the spectral signal components which are of interest in order to accurately reconstruct the waveform. The Delphin Expert Key 200 series mentioned above has a 100kHz maximum sample rate.
Also vital to many users’ considerations are the environmental conditions which the device will be subject to. Portable data acquisition equipment is naturally susceptible to damaging environments, such that ruggedized packaging of the DAQ unit itself, as well as its electrical components, is an essential manufactured precaution to ensure both device and data integrity. For example, portable DAQ equipment used in heavy industrial applications often needs to withstand a broad temperature range. In the absence of ruggedized models, a portable enclosure may give adequate protection for the system. In these harsh environments, and to retain their portability, DAQ systems need to be as compact and lightweight as possible. Further, beyond withstanding these environmental extremes, portable data acquisition units need to be able to survive in high shock and vibration environments such as the trunk of a car or onboard an airplane, or just as a result of the occasional accident such as being dropped by its owner. Other sealing and packaging precautions such as watertight housing should be under review for aquatic applications such as flow rate measurement and wastewater monitoring which partially or completely immerse the device.
For remote applications where access to a standard 120V AC power outlet isn’t available, electric power to the system can be provided either through an internal battery pack, or the user can connect an external wire to a DC power supply. Additionally, in order to conserve power and avoid unnecessary processor loadup, users with minimal processing requirements can select a lower performance CPU and rely on a capable storage system.
When analyzing recorded data and exporting to other formats, many users rate software primarily by how user-friendly it is, which often depends on graphical interface style, menu navigation and help tips. Continuing with the earlier example, the Delphin Expert Key 200 series features ProfiSignal software for data storage, display and analysis.
Choosing a portable data acquisition system featuring internal signal conditioning capabilities can greatly improve system quality and performance. Different types of signal conditioning include amplification, attenuation, and filtering. As always, the specific application involved and the types of sensors used to make the measurements will decide the type of signal conditioning required—to measure temperature, a device will probably need to use thermocouples or thermistors. Thermocouples produce a voltage that varies with the temperature, but connecting a thermocouple to a data acquisition system creates a cold junction point at the terminals that acts as a thermocouple itself. Signal conditioning is required to compensate for this, or else the recorded temperature which is taken from the total voltage will be altered by the additional voltage of the cold junction point. Signal conditioning can also be used for signal amplification to mitigate any noise distortion, and is also useful when a DAQ device is connected to other transducers such as strain gauges, accelerometers, etc.
When designed to monitor remote unattended systems, suitable DAQ devices have communications capabilities utilizing telephone connections or wireless systems to download data to remote PCs. They also require ample built-in storage and user interfaces to enable setup and control. Advanced solutions use built-in testing capabilities allowing users to sit back as the system acquires the data.
Taking into account all these specifications when choosing the right portable DAQ device is certainly an involved process, but one made easier by always keeping the needs of the present and any future applications foremost in mind.
For further information on the Delphin Expert Key 100-200 series of portable data acquisition devices, other Delphin data acquisition devices, or to find the ideal solution for your application-specific needs, contact a CAS Data Logger Applications Analyst at (800) 956-4437 or visit the website at www.DataLoggerInc.com.