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 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 Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
Each year, a panel of Control Engineering editors and industry expert judges select the System Integrator of the Year Award winners.
Control Engineering Leaders Under 40 identifies and gives recognition to young engineers who...
Learn more about methods used to ensure that the integration between the safety system and the process control...
Adding industrial toughness and reliability to Ethernet eGuide
Technological advances like multiple-in-multiple-out (MIMO) transmitting and receiving
Virtualization advice: 4 ways splitting servers can help manufacturing; Efficient motion controls; Fill the brain drain; Learn from the HART Plant of the Year
Two sides to process safety: Combining human and technical factors in your program; Preparing HMI graphics for migrations; Mechatronics and safety; Engineers' Choice Awards
Detecting security breaches: Forensic invenstigations depend on knowing your networks inside and out; Wireless workers; Opening robotic control; Product exclusive: Robust encoders
The Ask Control Engineering blog covers all aspects of automation, including motors, drives, sensors, motion control, machine control, and embedded systems.
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
News and comments from Control Engineering process industries editor, Peter Welander.
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
This is a blog from the trenches – written by engineers who are implementing and upgrading control systems every day across every industry.
Anthony Baker is a fictitious aggregation of experts from Callisto Integration, providing manufacturing consulting and systems integration.
Integrator Guide

Integrator Guide

Search the online Automation Integrator Guide
 

Create New Listing

Visit the System Integrators page to view past winners of Control Engineering's System Integrator of the Year Award and learn how to enter the competition. You will also find more information on system integrators and Control System Integrators Association.

Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Control Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.