What is a datalogger? A datalogger is an electronic instrument that records measurements of temperature, relative humidity, light intensity, on-off and open-closed state changes, voltage, and events over extended periods of time. Typically, dataloggers are small, standalone, battery-powered devices that are equipped with a microprocessor, memory for data storage, and sensor(s). Most dataloggers interface with a personal computer and utilize software to activate the logger and view/analyze the collected data.

Benefits of Using Dataloggers

One of the primary benefits of using data logging for systems monitoring is the ability to use digital technology to collect data on an around-the-clock basis. Upon activation, the loggers are deployed and left unattended to gather and record information for the duration of the monitoring period. This allows for a more complete and accurate picture of the target system’s overall performance. Because some condition changes take place over long periods of time or when no one is present, the use of dataloggers is much more efficient and feasible than performing “spot checks” to gather data manually.

Another benefit is the ability to collect data in hard-to-reach areas through the use of built-in or external sensors. Temperature, relative humidity, CO2, voltage, amperage, pressure, light, and many other types of data can be monitored in areas that would otherwise be too difficult or cumbersome to access such as electrical junction boxes, air return vents, water recovery tanks, motor and fan housings, to name a few. Loggers with external input capabilities may be wired to existing gauges and sensors that have voltage output terminals, thereby allowing these devices to be monitored and recorded, too.

They can also be easily removed and reused in other application studies because they require no extra wiring and are stand-alone devices.

Useful Applications

Dataloggers are used in many different applications to monitor and collect specific types of information. The following are just a few examples of “real world” applications where dataloggers can make the task of gathering information a lot easier and much more efficient.

Monitoring comfort-level complaints. One of the more common occupant complaints is that the heating or air conditioning system is not working properly. It’s either always too hot or always too cold. These types of comfort level complaints are very easy to monitor and verify with the use of temperature dataloggers. Temperature dataloggers employ a sensor that reacts to changes in temperature. The logger monitors and records these changes at preset intervals and stores the data with date and time information in its memory for later retrieval. Many temperature loggers are small enough that they can be placed in hidden, “out-of-the-way” locations to gather information without being seen or disturbed. Depending on the amount of built-in memory and the interval for taking readings, the loggers can realistically collect data for several months at a time before reaching their full capacity.

This temperature data, upon later analysis, offers a much more accurate and complete picture of the actual temperature activity that occurred throughout the entire monitoring period. “Spot checks” or manual readings would not provide this type of extensive coverage. The data can then be used to ascertain where problems in the heating system might exist, before making an informed recommendation to the customer or client. The savings in labor alone is enormous, especially if the study is being done in a multi-unit residential or commercial building.

Temperature loggers can also be used in many other applications. Here are a few examples:

  • Verify nighttime temperature setback strategies.
  • Monitor air handler temperature fluctuations.
  • Analyze gradient temperature changes.
  • Monitor equipment operating temperatures.
  • Compare outdoor conditions to indoor comfort levels.
  • Assess operating temperatures of motors.

Logging motor/compressor cycling performance. “State” loggers are another very useful type of datalogger that can be used to monitor the run times of motors, fans, and compressors. These loggers record the “on” and “off” run times, along with time and date stamps. This information is very useful in detecting short-cycling and recording the overall usage of the equipment being monitored.

Motor on-off loggers with built-in vibration sensors are often used to monitor the start and stop times of compressors and pumps. The adjustable sensor detects the vibration when the compressor is turned on and records this event with a time and date stamp. When the compressor or pump is shut down, the logger will also record this event with another time and date stamp. This data can be used to show the total amount of time the equipment was operational. Further analysis of the data can show where short-cycling may have occurred, the peak usage times and percentage of time that the equipment was on or off.

For electrical devices that do not produce measurable vibrations (small fans, motors, etc.), ac-field sensor loggers are used to monitor on-off state changes. These loggers detect the electromagnetic field produced when the device is turned on, through the use of a built-in sensor. The loggers can be placed directly on the outside of electric motor housings, and may also be mounted on one of the phases of power being supplied to the motor or device being monitored.

There are “state” loggers that also record “open” and “closed” events as well as contact closures. These loggers can be wired to passive relay-switches and dry contacts. When a contact is made, or when the relay switch is “tripped,” the logger records this data with a time and date stamp for each on-off or open-closed state change.

When evaluating motor, fan, or compressor cycling and performance, “state” loggers are an invaluable tool. These loggers can detect and log events that might otherwise go unnoticed, or would be difficult to monitor manually. They offer the capability to capture events, whenever they may occur.

Monitoring electric light usage. When performing baseline studies on power consumption in buildings, knowing how much electricity is being used for lighting is very important. There are dataloggers available that detect and record light usage.

Light on-off loggers, are similar to “state” loggers in that they record data only when a “state” change occurs. These loggers employ a sensor that detects changes in light intensity. There should be a sensitivity control so that the threshold between light and darkness can be adjusted by the user. This will allow the logger to know the difference between “on” and “off.” Lighting conditions vary from room to room, and having the ability to adjust the sensor is important.

Care should also be taken to make sure that the light sensor directly faces the source of light being monitored. Sunlight and other reflected light sources may trigger these loggers, giving inaccurate readings. When the lights are turned on, the logger will record this state change along with date and time stamps. When the lights are turned off, this information is also recorded.

This type of data is valuable in determining how often electric lighting is being used. Peak hours and periods of nonuse can also be analyzed. This information is very useful, especially when making recommendations for retrofits and estimating the electrical demand for lighting.

Power usage logging. Tracking power consumption and usage is essential in evaluating a system’s overall performance. The efficiency and energy demands of equipment can be monitored and recorded through the use of dataloggers.

Readings of current, amperage, and voltage may be taken manually with hand-held meters and probes. These methods of collecting data are common and work well, when short monitoring periods are required. However, when longer studies are necessary, dataloggers can gather information for extended, uninterrupted periods of time.

Loggers with the ability to accept external inputs can be programmed by the user to take readings at preset intervals. The external inputs on these types of loggers usually accept voltage inputs from external sensors and preexisting gauges. Current transformers, meters, and gauges can be used with external input loggers, provided that these sensors have voltage output characteristics compatible with the input voltage requirements of the datalogger. Current transformers are simply attached around the phases of power being monitored. (In most cases, no hard wiring is necessary.)

The logger, upon activation, will take readings at the preset intervals and record the amperage, voltage, or current information with time and date stamps. Some of these loggers have multichannel capabilities and can collect data from various different sensors simultaneously. Recording data from all three phases of power allows for a more complete picture of the total power being used as well as the peak hours of usage. By relying on manual readings, events such as power spikes, surges, and fluctuations may go undetected. Dataloggers with external current probes and sensors attached can perform unattended studies for days, weeks, or months around the clock.

Air quality monitoring. The performance of air quality systems can also be monitored and recorded using dataloggers. Temperature readings for supply and return vents can be recorded and compared to help identify air balance problems. CO2 levels and relative humidity information can also be monitored to obtain important air quality data.

Various dataloggers are available that have built-in temperature and rh sensors so that these types of data can be collected simultaneously in the same location. Other loggers with external input capabilities can be effectively used by attaching external temperature, rh, and CO2 sensors for placement in hard-to-reach areas. Hybrids of these two types of loggers are also available. They offer built-in sensors as well as the capability to attach external sensors, for even more monitoring possibilities.

External temperature sensors can be placed inside air vents, and attached to heating coils and pipes for accurate readings. Temperature and rh readings can be taken inside and outside of buildings for comparison. CO2 levels can be monitored when the building is occupied and unoccupied for later evaluation of the ventilation system.

By collecting data from various locations simultaneously, valuable information about overall air system performance can be obtained. Later analysis of the data will identify energy saving opportunities in overventilated spaces. Complaints about air quality or insufficient ventilation can be studied objectively. Performance of heating and air conditioning systems can be evaluated. Temperature changes can be analyzed to verify proper heating and cooling operation.

Through the use of dataloggers, more accurate and usable data can be obtained with much less expense and over longer periods of time, than can be obtained by manual, hand-held monitor readings.

Analyzing the Data

Now that this valuable data has been collected, what happens next?

Most dataloggers operate with some type of proprietary software. This software allows the user to set certain operating parameters within the logger. Sampling/reading intervals, delayed start times, alarm ranges, external sensor selection, and a descriptive header for the logger are a few examples. Information about the logger’s battery status, clock synchronization, and memory capacity may also be available. There should also be a software command to activate the logger. All of this is accessed through a serial port connection between the logger and the computer.

Upon completion of the study, the data must be off-loaded from the logger into the computer. The information can then be viewed and analyzed with the logger software or spreadsheet programs. Software features vary among datalogger manufacturers. At a minimum, the user should be able to view the data files with date and time information in graphical and tabular formats. The capability to export data files into other spreadsheet programs can also be useful. Other software features to look for are:

  • Capability to display multiple data files on one graph.
  • Zoom in on data of interest.
  • Copy and paste graphs into other applications.
  • Compare multiple parameters, from multiple loggers.
  • Multiple value axes on one graph. Set axes ranges.
  • Calculate on/off run times and percentages.
  • View and overlay data from successive deployments.
  • Capability to customize graphs, colors, legends, etc.
  • Good technical support.

Software packages with these features allow for easier analysis of the collected data. Specific data points can be filtered out for more accurate comparisons and study. Graphs and tabular data can be printed for use in reports and evaluation of long-term studies. In general, good, comprehensive analytical software is as important as the datalogger itself, when evaluating systems performance.

Features to Look for in a Datalogger

By now, the benefits and advantages of using dataloggers should be clear. Listed below are some of the basic features that you should look for when choosing dataloggers for use in your performance studies.

Ease of use. The logger should be user-friendly and easy to set up, activate, and off-load data. Size. The loggers should be small, easy to carry, and easy to install in hard-to-reach locations.

Self-powered. Dataloggers that are self-powered can be placed in areas and locations where electricity is not readily available.

Flexibility. Users should be able to define reading intervals, select external sensors, customize, and modify data easily.

Reliability. Choose loggers that have a proven track record for accuracy, reliability, and dependability.

Durability. Look for loggers that are designed for rugged indoor-outdoor use as appropriate to the project. They should be able to withstand the rigors of harsh environments (extreme heat and cold, excessive moisture, pressure, etc.).

Technical support. Even in the best situations, there will always be questions about software, whether or not the logger is suitable for certain applications, and how to change the batteries. Choose loggers from a manufacturer that provides good, prompt, informative, and useful answers to your questions. There may be unforeseen problems that crop up, and having good technical support is a must.

Comprehensive software. The data that has been collected is useless if it can’t be analyzed. Make sure that the logger software is easy to use. Users should be able to access and customize data files easily.

Data shuttle capability. Data shuttles are convenient, hand-held devices that allow the off-load and relaunch of dataloggers while in the field. There is no need to bring the loggers back to the computer. The shuttle holds the data and is easily off loaded to the computer instead. This allows the loggers to remain in place.

Cost. Collecting data can be expensive, especially if it is done manually. Dataloggers can range in price from less than $100 to over $1,000. Although factors like accuracy, flexibility, and reliability are very important, so is the cost of the logger itself. Memory and computer processor prices have fallen sharply over the past few years, making dataloggers more accessible than ever before. The technology is readily available and increasingly affordable for small companies and large firms alike.


Dataloggers are effective, inexpensive tools ideal for monitoring and measuring building systems performance.

They can be used to collect important data from many types of devices and equipment, whether someone is there or not. They take the guesswork out of trying to evaluate the efficiency of a particular system, by providing accurate information. The benefits obtained from being able to print and easily store graphic and tabular data for analysis and evaluation are numerous. By using dataloggers to collect the information, valuable time can be spent elsewhere, analyzing, and evaluating the overall performance of the system. ES