Tutorial: Understanding multivariable sensors

Many smart instrumentation devices can provide more than just one process variable. Is it information you can really use?

07/14/2009


One of the benefits of the growing sophistication of instrumentation devices is that they can often give you more than one variable. These bonus measurements are essentially free in that they don't require any additional sensors or penetrations into your process. All they require is a way for you to extract the information.

Multivariable approaches fall into three categories depending largely on the needs of the primary variable.

Corrective measurements

Most electronic sensors are influenced to some extent by more than one variable. For example, pressure sensors that use capacitive or strain gage technologies are affected by temperature. Consequently, the transmitter for such a device takes a temperature measurement and uses that data to correct the primary reading. Since that measurement is there in the transmitter, it is usually a simple matter to add a way to send it to the control system.

The caution of using corrective measurement data is making sure you understand where it comes from. The temperature in this example is taken where it is needed for the sensor correction and may not reflect the process at all; it just may be reflecting the ambient temperature around the transmitter or the electronic devices. Make sure you understand what it is before you use such data.

Multiple measurements

One of the most common flow measurement methods is using an orifice plate and differential pressure gage . There are many implementation variations, but the basic concept calculates flow based on pressure readings on both sides of a known obstruction. While the flow measurement only needs the differential pressure value, it isn't difficult to extract the line pressure measurements as well.


Another example is a corrosion measuring sensor . These typically provide outputs for both general corrosion and local or pitting corrosion. It is also simple to get a conductivity reading with such devices.

Calculated measurements

Coriolis flowmeters provide a combination of calculated and measured values.

With the growing sophistication of transmitter electronics, adding calculated values to measured process variables has become far simpler. Coriolis flowmeters use this technique, and can calculate a range of variables from the three that are actually measured (see diagram). Probably the most common example of this is setting your Coriolis device to read in gallons per minute, since the device does not provide a measured volume. It calculates volume based on its measurements of mass flow and density. The transmitter can be setup to provide whichever of the available values you need most as the primary variable.
Bear in mind that all the measurements from a multivariable device are connected in one way or another. These are not clusters of unrelated instruments. You won't likely find a spool section of pipe that has a magnetic flowmeter, temperature sensor, and pressure sensor all inserted. The specificity of process needs precludes the possibility of making something like that with wide enough applicability to be commercially viable.

 

Extracting the data

Most devices are designed to provide the primary reading via an analog signal (4-20 mA) or a digital output. However, if more information is available, you have to find a way to get at it.

• A few devices offer multiple (usually two) analog outputs. This approach certainly works, but requires as many cables as variables.

• The most common method for sending the secondary variable is via a HART signal on top of the primary analog variable. If you use a HART interface or have HART I/O with your control system, you can capture the secondary measurements and use them in any way that's valuable to the process. Complex devices such as Coriolis flowmeters, allow you to choose which output comes over the analog signal and which others are overlaid. There are various types of HART reading devices. Some translate the secondary variables into their appropriate engineering units for display. Others convert them into a second or third 4-20 mA signal for input into a DCS. There are even wireless approaches for capturing the information.
Getting your HART interface setup properly can be a challenge since manufacturers use this mechanism differently. You'll probably need to use the device description to sort through the data available for any given device. If you are counting on using some specific information, make sure it is available, since there is little consistency between different manufacturers even for similar devices.
(Opinions differ on using secondary HART information for anything important. Control system builders generally agree that it is valuable for monitoring, but are more cautious when using it for critical measurements. At the same time, any measurement that is truly critical will likely have its own device anyway.)

• Fieldbus protocols make multiple variables very simple, if you use that networking approach and have suitable devices. Using fieldbus requires minimal setup since all variables, primary and secondary, will be available in the appropriate engineering units. Moreover, they can all be handled with the same importance.

Multivariable sensors can be very useful in the right contexts, but using them to your best advantage does require some homework. As your best first step, make sure you know what your process needs.

-Edited by Peter Welander, process industries editor, PWelander@cfemedia.com ,
Control Engineering Process Instrumentation & Sensors Monthly
Register here to select your choice of free eNewsletters .





No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
2016 Engineering Leaders Under 40; Future vision: Where is manufacturing headed?; Electrical distribution, redefined
Strategic outsourcing delivers efficiency; Sleeve bearing clearance; Causes of water hammer; Improve air quality; Maintenance safety; GAMS preview
World-class maintenance: The three keys to success - Deploy people, process and technology; 2016 Lubrication Guide; Why hydraulic systems get hot
Flexible offshore fire protection; Big Data's impact on operations; Bridging the skills gap; Identifying security risks
The digital oilfield: Utilizing Big Data can yield big savings; Virtualization a real solution; Tracking SIS performance
Getting to the bottom of subsea repairs: Older pipelines need more attention, and operators need a repair strategy; OTC preview; Offshore production difficult - and crucial
Applying network redundancy; Overcoming loop tuning challenges; PID control and networks
Driving motor efficiency; Preventing arc flash in mission critical facilities; Integrating alternative power and existing electrical systems
Package boilers; Natural gas infrared heating; Thermal treasure; Standby generation; Natural gas supports green efforts

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
This article collection contains several articles on the vital role that compressed air plays in manufacturing plants.
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.
This article collection contains several articles on strategic maintenance and understanding all the parts of your plant.
click me