Data Logging Primer

July 01, 2010
July/August 2010
A version of this article appears in the July/August 2010 issue of Home Energy Magazine.
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There are many reasons to measure things in the building science world. For instance, you may need to commission a commercial building to guarantee temperatures, air flows, or other comfort parameters. You may need to measure humidity levels in a house to diagnose comfort complaints. Or you may need to diagnose mechanical equipment problems that are not readily observable. These are just a few examples.

Steve Mann is a HERS rater, GreenPoint rater, LEED AP homes, Certified Energy Analyst, serial remodeler, and longtime software engineer. (Image credit: April Wise Photos)

The idea behind data logging is that you get a bunch of sensors that are capable of measuring the parameters of interest. You can get sensors that measure just about anything—temperature, humidity, voltage, current, air velocity, CO, and so on. The range of sensors is really quite astounding. You hook the sensors up to a data logger that records the sensor measurements over a period of time. You then download those data into a computer and analyze the logged data to (hopefully) solve, or at least define, your problem.

That’s a gross oversimplification of the process. Sensors, loggers, and computers come in a wide variety of choices. Some of this equipment can cost thousands of dollars—and usually looks like it belongs in a space shuttle. Some is quite inexpensive. Some loggers are full-blown computers that need only a monitor. Some systems can display data in real time; some can transmit data wirelessly to a remote computer or an Internet-based server. The possibilities are endless.

Sensors, loggers, and computers come in a wide variety of choices. Some of this equipment can cost thousands of dollars—and usually looks like it belongs in a space shuttle.

Recently I had the chance to try out some modestly priced data equipment from Onset Computer Corporation. Their HOBO series of inexpensive, battery-operated data loggers is well known. In addition, they offer a wide range of sensors, and their loggers are compatible with a variety of third-party sensors as well. To round out their offerings, they have Hoboware, a Windows- and Macintosh-compatible software application you can use to analyze the data that you log.


Configuration and Setup

The hardware I tried out included the following components:

  • one HOBO U12-006 four-channel data logger;
  • two TMC50-HD air/water/soil temperature sensors; and
  • one CTV-A 20-amp split-core AC current transformer, or CTV.

The U12 has four channels that can handle 0–2.5 volts DC with a 0.6 millivolt resolution. Smaller than a pack of cigarettes, and weighing just 46 grams (1.6 ounces), it has a built-in battery with an estimated one-year life, a 12-bit analog-to-digital converter, and 64K bytes of memory capable of storing up to 43,000 measurements.


The TMC50 stainless steel temperature sensors measure -40°C–50°C (-40°F–122°F) in water, and -40°C–100°C (-40°F–212°F) in air. The CTV measures 0–20 AC amperes. You can also get U12-compatible sensors that measure AC voltage, CO2, and DC voltage. All these sensors convert their measurements into 0–2.5 volts DC.



For my test project, I wanted to measure the supply and return temperatures on a radiant heating boiler and correlate them with the pump running and electronic ignition firing. The boiler was cycling in one zone, and it wasn’t clear to me why that was happening. I figured if I tracked these three items, I might get some insight into the problem. To measure the pump and ignition, I attached the CTV to the hot wire of the 120V outlet that provides electricity to the pump.

Setting up the U12 is a snap. First you load the Hoboware software onto your computer and start it up. A Setup Assistant greets you the first time you use the application. It walks you through some basic preferences settings. You then connect the U12 with a supplied USB cable (serial connections are supported, but USB is preferred). Hoboware automatically detects the U12.

Enlarge Image

Figure 1. The Launch window shows a summary of your current configuration and sampling setup.

You then select the Launch command from the Toolbar or the Device menu, making the Launch window appear (see Figure 1). Launching is the term Onset uses to describe starting a data logging session. You use this window to tell the logger basic session information, such as what types of sensors you will attach (or have attached) to each channel, the sampling interval, and when you want to actually launch the device. In my case, I indicated that I wanted temperature sensors on the first two channels and the CTV on the fourth channel, and that I wanted to start the launch after pushing and holding a small button on the U12 for three seconds. I then took the U12 into the mechanical space, hooked up the sensors, and pushed and held the Logger button for three seconds. A small light on the U12 blinked at me and logging started.

Hoboware comes with a 28-page Getting Started guide. It walks you through the initial setup and installation quickly and simply. You should be able to get through all the steps I describe above, from unpacking the hardware to pushing the Logger button, in 20 to 30 minutes.

Collecting and Analyzing the Data

Once you’ve logged the data you think you are interested in, you retrieve the U12 and hook it back up to your computer. (You can also take your laptop to the data logger and retrieve the data there). Start up Hoboware and select the Load command, which loads the data from the logger onto the computer. The software asks you for a file name for storing the samples and then displays the Plot Setup window. You use this window to select the channels you want to save, plus some optional preprocessing steps, and then hit the Plot button. The data get saved in the file you requested and are then displayed in a graph.

Baxi Luna Radiant Cycling: Initial Plot

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Figure 2. Initial plotted data, right out of the logger, may contain too much detail, requiring too much cleanup and adjusting.

Figure 2 shows my boiler data as they were displayed right out of the logger—it’s kind of messy. Once you see the raw data, you have to start zeroing in on the samples you really want to see. You can select a subset of the data points to display, zoom in or out of the graph, color code and label parts of the graph, change labels, and make many other modifications. Realistically, the first time you use the software, if you work your way through the Getting Started guide, it will probably take you an hour or two to get the data in a format that can be useful. Figure 3 shows my cleaned-up plot after color coding the data plots, closing some of the unnecessary windows, reducing the period of time being viewed, and a few other tweaks.


The hardware I used is not very expensive. The logger retails for $105; the temperature sensors with 50-foot cables are $42 (shorter-cabled models can cost as little as $29); and the current transformer retails for $90. Total cost for a data logging hardware collection that can be used to analyze a lot of different scenarios—under $300.

Hoboware comes in two different versions, Lite and Pro. The Pro version, which costs $89, has lots of features in addition to the ones I’ve already described. For instance, you can import and export data sets, combine multiple data files into one graph, use data assistants to modify data while loading them, and even access remote loggers using an Ethernet connection. It works with a wide range of data loggers in addition to the U12. All in all, Hoboware Pro is a sophisticated analysis tool. The Lite version, which costs $35, doesn’t work with all the Onset loggers, and has a reduced set of analysis tools, but it’s still very capable.

Baxi Luna Radiant Cycling: Final Plot

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Figure 3. Tweaking the raw lagged data plot can give you a more useful graph.

But Wait, There’s More

What I’ve described in this article barely scratches the surface of Onset Computer Corporation’s offerings. It has a wide range of data loggers, sensors, weather stations, and more. It has products designed for indoor and outdoor use. It has wired and wireless sensors and loggers. Their Web site is chock full of application notes, white papers, video support, and more. The company also offers free tech support. If you’re interested in data logging, you should check out its Web site.

In this article, I just wanted to give you an idea of what’s involved in basic data logging, a process that can be beneficial to many different types of energy-related projects. It’s simple, relatively inexpensive, and interesting.

Did it help me solve my problem? Well, not yet. It did give me some insights—such as realizing that I have to figure out from the data when someone is taking a shower, or use only data from periods when everyone is asleep. You see, it’s a combination boiler that supplies both domestic hot water (DHW) and radiant heating. DHW takes precedence, disabling the radiant pump when a fixture is turned on. I also learned that the electronic ignition does not spike the current consumption the way I thought it would, so I can’t really figure out when the boiler turns on (although the supply and return temperatures can provide clues, barring the shower issue). Oh, and it looks like the pump may be a two-speed pump—another monkey wrench.

I have more data to look at, and I have to bone up on the equipment specifications and spend some time just thinking about how the equipment works. That just goes to show you—you can’t necessarily solve a problem by throwing hardware and software at it. It helps to understand what you’re dealing with, too.

For more information:

Learn more about Onset Computer Corporation’s data logging hardware and software.

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