Tutorial: Challenges of unlocking HART data

Finding all the information you need about your instrumentation devices may be more difficult than you would expect.

10/16/2008


The growing availability of devices to unlock HART data from instruments is causing new interest in putting this technology to work. That’s a good thing, but don’t be surprised if what you find is a little confusing.

Let’s back up a little and define what we’re talking about: HART communication is a mechanism that sends digital signals on top of the main analog (4-20 mA) process variable from an instrumentation device. Most transmitters have offered this capability for quite a while now. The digital signals provide a means to configure or calibrate a device, send diagnostic information, and deliver additional variables in addition to the primary reading. This capability has been discussed extensively in Control Engineering and elsewhere.

Translating the digital information into something readable requires some type of interface. Most companies have a hand-held reader for intermittent use by maintenance, and there are interfaces or I/O cards that allow for continuous readings of secondary variables. (Read two earlier articles on wired solutions and wireless solutions .) The technology that supports the process is thoroughly proven. The potentially confusing part is what you may find when you put it to work.

While the HART protocol describes how the information is transferred, it doesn’t say what is in the information. That decision is left to the vendor, and that’s where things get complicated. “Most customers and even some manufacturers are not schooled as to what‘extra’ information is hiding in the HART data in specific instruments,” says Steve Todd, director, corporate marketing for Moore Industries-International . “Most manufacturers don’t make this data readily available or available at all. Most customers traditionally, and still do, use HART only as a diagnostic and configuration tool, but this is changing. Due to the HART Foundation’s efforts, and efforts by manufacturer’s that provide HART interfaces (like us), the education situation has gotten much better in recent years. In fact, one of the most requested elements on our web site now is for our HART interfaces.”

So don’t be surprised if you don’t find the information you need for a device right away. Ed Ladd, director of technology programs for the HART Communications Foundation (HCF) suggests the first place you should look is in the DD (device descriptor). “As you’re installing a device, and you get a DD for that device. That information is going to be at your fingertips, either online or offline,” he advises. “Even if the information is buried in the manual, it’s in the DD.” The HCF Website maintains a database of DDs that can help you sort through what you have.

Moore Industries has a downloadable HART interface guide that provides a list of what secondary variables are typically available from various types of instrumentation devices. As an example, let’s say you find out that most capacitive pressure sensors have internal temperature compensation, and that the temperature reading can usually be accessed via the HART data as a secondary variable. Doing that will necessitate finding out what is available from a specific transmitter from a specific manufacturer.

Instrumentation suppliers vary widely as to how they document this data, if at all. A few do a good job, but more often you may have to comb through many pages of manuals to find it, or just give up and ask. Typically you will also have the capability to define how you see the information, but this varies from vendor to vendor. If there are multiple secondary variables available, you can usually assign the ones you want to specific output channels. “Certain variables in a device will be called‘device variables,’” says Ladd. “Those device variables can be mapped to the secondary or even primary variable. For example, you can have a Coriolis flowmeter output density rather than flow as its primary variable if you choose to map it that way.”

When putting this data to work, you will need to make sure you understand what you’re seeing. The temperature value your pressure sensor gives you may reflect the process fluid, or it could be the temperature of the sensitive electronics in the transmitter. That information is being collected primarily to correct the main variable, and reflects that need, so check this carefully. Companies are not consistent in what data they collect, and manufacturers that make similar devices may not use the data the same way.

These cautions do not reduce the validity of the process or the value of the information, but don’t be surprised if you have to work to see the benefits.

—Peter Welander, process industries editor, PWelander@cfemedia.com ,
Control Engineering Process Instrumentation & Sensors Monthly
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