Selecting voltage-based pressure sensors

While most process instrumentation uses 4-20 mA current loops, there are situations where pressure sensors using voltage loops can operate with much lower power levels.

05/28/2013


While the bulk of process instrumentation has standardized on 4-20 mA current loops, this approach depends on having 24 Vdc with a few amperes available. Where power is on short supply, pressure sensors that use one of a variety of voltage output options can reduce consumption significantly. While the differences between the various voltages may seem minor, each option offers its own unique features and benefits for specific use cases and different user groups. The key is to match output with available system operating voltage.

Where power is on short supply, pressure sensors that use one of a variety of voltage output options can reduce consumption significantly. While the differences between the various voltages may seem minor, each option offers its own unique features and benefits for specific use cases and different user groups. The key is to match output with available system operating voltage.Lower-power systems are becoming more common, particularly in wireless environments thanks to lower installation cost features, along with remote device clusters that depend on solar panels and/or lithium batteries. In these situations, the need to conserve power takes on paramount importance. In such systems, voltage availability typically ranges from 7-12 Vdc, with currents in the 2-3 mA range to power the transducer. The only solution here is to use a voltage output approach, such as 0.5-4.5 V ratiometric with 5 Vdc supply, or 1-5 V sensors with 7-12 Vdc systems. Upstream and midstream oil and gas applications are driving this today where power is not freely available in remote areas.

For battery-powered and pulsed systems, the sensor unit is often energized for a short time between sleep periods for monitoring, such as a tank level application. Such systems operate at 3.3 V, so the sensor uses less than 1 mA excitation with 0.5-3 V output. Here, the need to maintain battery life for two years or more is essential.

The next most popular powering approaches are fixed 5 V or unregulated 6-12 V systems. These are typical with lithium and solar cell combinations. For these, sensors use 0.5-4.5 V outputs with maximum current limited to 2 mA or less so that the system can work for many years. This is particularly important in higher northern or southern latitudes where sunlight is limited for several months of the year.

Finally, land-based systems with generator power or some other permanent supply run at 8-28 V. These situations allow several options for output signals, including  0-5 V, 1-5 V, 1-6 V, and 0-10 V. Here current consumption is less than 10 mA, so it is well below a 4-20 mA loop. The downside of a voltage loop is the limit on cable length between the transducer and controller such as a plc or computer.

Low level or no signal?

The main disadvantage of any zero-based output signal is that there is no signal with zero pressure. If the transducer has a cut wire, broken sensing element, or electronics that received an over-voltage, the sensor will produce no signal, thus, no way to provide an output. The controller can’t tell if pressure is actually zero or if the unit is simply inoperative.

For example, if used in water pressure measurement, the controller might signal a pump to act when the sensor detects pressure has crossed a threshold. If there is no pressure in the line, the transducer will produce a 0 V signal. Similarly, at fault conditions, the sensor continuously provides a 0 V signal. Since the reading is the same at actual zero pressure and fault conditions, there is no way for the controller to distinguish between the two. In a worst-case scenario, the pump would not know to run and could cause a flooding condition.

As industrial pressure transducers become smarter with advances in electronics and microprocessors, sensors are available with a factory-set fault condition. Transducers can be programmed to rail or send output below the lowest point or above the highest point to indicate to the controller that there is an issue. For example, if a pressure spike in the system causes the sensor diaphragm to break, the output signal on a 1-5 V output signal can be programmed to drop the output below 1 V or above 5 V by about 10%. In a pump application, it can help prevent flooding, the pump from running dry, or extra wear. 

Karmjit Sidhu is vice president of business development and Greg Montrose is marketing manager for American Sensor Technologies.

Go online:

www.astsensors.com

Subscribe to the Process Instrumentation & Sensors eNewsletter at www.controleng.com/newsletters



Anonymous , 05/29/13 03:09 PM:

Please note: you loose the very good CMR and noise tolerance of the 4-20 loop going to voltage, the loop DOES NOT require "amps" and the standard has lasted this long because IT WORKS ! This post is misleading at best.
Anonymous , 05/30/13 07:52 AM:

Thank you for your feedback. AST's intent was not to dispute that 4-20mA is a reliable signal, but rather to show that it is not always the best available option. For short distance transmissions with limited power, a voltage output such as 1-5V is perfectly acceptable. 4-20mA output signals have greater noise tolerance for longer transmissions. We typically suggest limiting voltage outputs to 20 feet. For a well site that is pumping oil at the full scale output signal, the 20mA signal is going to consume more battery life than a voltage signal using 2mA.

The comment above suggests that an output like 1-5V does not work or have noise tolerance. AST uses a ferrite cage to protect both voltage and 4-20mA output signals from 100V/m, and in special applications 300V/m, in noisy environments. If you're interested in learning more about noise tolerance for pressure sensors with voltage outputs, AST has independent test reports conducted by UL and TUV to industrial and automotive standards to further show the signal's capability.
Anonymous , 05/30/13 09:59 AM:

What is normal in 24V supply systems is that the system integrator selects a power supply with enough current, typically 2 Amps so that they can run 4-20mA sensors and there may be several of them running from the same supply plus all the indicating lamps/LEDS and other control circuits. In wireless systems, one do not have this luxury.
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...
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
Your leaks start here: Take a disciplined approach with your hydraulic system; U.S. presence at Hannover Messe a rousing success
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
Putting COPS into context; Designing medium-voltage electrical systems; Planning and designing resilient, efficient data centers; The nine steps of designing generator fuel 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