2013-10-04 15:29:33 - Whether you are an engineer working out the details of your latest project or a buyer filling a requisition, the number of available options when choosing your data logger can often be overwhelming.
The majority of our conversations begin with this simple question: “What are you looking to accomplish?” In fact, placing a call to a solutions provider is the quickest way to get the information you need to proceed.
When choosing a data logger, consider your short- as well as long-term needs. Are you looking to solve a specific short-term problem or are you are looking for a multipurpose tool to handle current requirements as well as being flexible enough to accommodate future project needs?
In general, questions to help you determine which products are the most suitable including:
• How many inputs are required and what type?
• How often you need to take a reading?
• How much data needs to stored?
will the data logger be used?
• How you'll communicate with the device
• Any other requirements the application might have
Your first consideration is to determine the number of inputs you plan on logging. Since data loggers are available in configurations capable of handling anywhere from one to literally hundreds of inputs, knowing what you need now and possibly in the future will have a significant impact on your choice.
Since data loggers are designed for such a wide variety of applications, knowing the type of sensors or inputs you plan on using is critical. For instance, if you already have a thermocouple and you need to log temperature readings, the simple or inexpensive temperature data logger that caught your eye might not be suitable if it comes equipped with a dedicated RTD. If all you need is to record data from 4 individual 4-20mAmp current-loops, a simple 4-channel logger dedicated to process current loops might work fine. A single, 0-5 VDC? Again, a simple dedicated data logger might do the trick. On the other hand, if you are mixing inputs (current-loops, voltages, pulses, etc.) you'll need a more flexible, sophisticated data logger.
While some of these loggers are dedicated to a certain input type, others are user-configurable for different types and infinite combinations of signal types. Many models of data loggers are available on the market for the following types of signal inputs: AC Voltage, Process Current, Bridge, Strain, Load, Pressure, Dew Point, Event (or State), Frequency, Level, Process Voltage/Current, PH, Relative Humidity, RTD, Shock, Acceleration, Sound, Temperature, Thermocouple, Pulse, Serial and more.
Another important consideration is how often you need to record a reading. Most data loggers can handle recording at rates up to about 1Hz (once per second). If you need a faster recording frequency, be aware that as the speed of the data logger increases, the price of the data logger does as well. Make sure that the recording rate you are specifying is appropriate--in the case of a K-Type thermocouple, for example, the sensor/sample may take several seconds to change temperature. Recording such a temperature with the data logger at 5Hz would provide redundant or useless data.
Depending on your application, you may need to only capture a few minutes' worth of data or you may need to be able to store whole months of readings. You can determine the amount of data storage required by multiplying the number of channels by the sample rate and recording duration:
Total Number of Points = Number of Channels X Sample Rate X Recording Duration
Depending on the data logger, there may be a limit based on the total amount of internal memory, or the logger may offer the option of using external memory such as a USB memory stick to expand the available memory.
It also important to consider what is practical for the application and analysis. Many users initially state that they want to record multiple channels of data at hundreds of Hz. One problem with this is that this would quickly fill the available memory and necessitate more frequent downloads. Even worse, it becomes impractical to analyze all the data--with a high speed data logger sampling at 100 Hz, users can exceed the maximum number of rows in Excel in just over 10 minutes!
In this case you've got a handy guideline- if you're looking for trends, use the statistical capabilities offered by certain data loggers to summarize the data over an interval. If you're looking for anomalies, use the logger's triggering features to just capture a window around the event.
Naturally, there are data loggers that are designed for fixed installation and others that are intended for more portable applications. When considering how the logger will be used, keep these key issues in mind: how will it be powered--by battery, AC adapter, or solar panel? Will the logger be used in a lab or does it need to be protected from moisture and dirt? Does it need to be completely self-contained so that it can be used in the field? Checking the IP rating of these devices will provide you with a good idea of the punishment they can take, and some loggers such as the Grant Squirrels can endure full exposure to the elements!
Ultimately you have to retrieve the data from the data recorder somehow, so do you plan to bring the logger to the computer to download data, or would you prefer to handle it remotely? Communication with the data logger for set-up of monitoring and downloading data can be done in many different ways, including serial or RS-232 interface, USB or Ethernet interface, wireless capability including Wi-Fi and proprietary RF links, analog telephone (PSTN) modem, cellular, CDMA or GPRS/GSM modem, and satellite modem.
In addition to basic data collection, does your application require other features such as alarms? Does the data logger have to perform real-time calculations on the measured data? Do you need a local display or output signals? These are the details you'll need to determine to ensure your logger is a perfect fit for your specific application.
Established in 2004, NorthTree Associates (Waconia, MN) is a North American distributor that specializes in providing design engineers, test engineers, and production engineers the best protocol, bus analysis, and board-level testing and debugging tools available.