Choosing sensors for the application: Answers to audience questions

The Control Engineering RCEP webcast, “Choosing sensors for the application,” resulted in more questions than speakers had time to answer in the allotted time. Here each speaker provides more answers.

08/29/2017


Frank Lamb and Christopher J. Thompson, PE, answered additional question on the Aug. 22 webcast Choosing sensors for the application. Courtesy: CFE MediaSensor selection was the topic of an Aug. 22, 2017, Control Engineering webcast; the two expert presenters answer additional audience questions below. The archived version of the webcast, along with a link to a quiz for RCEP professional development hour (PDH) is available for a year at www.controleng.com/webcasts.

The webcast provides advice about how to choose sensors for automation and control applications, including how details about the implementation are important for selecting the right type of sensors. Webcast learning objectives follow. 

  1. Define what is to be sensed and differentiate between the two major areas of applications for sensing, machine position and conditions and quality of product.
  2. Define where sensors can be located and constraints.
  3. List methods, technologies, and considerations for sensor applications.
  4. Outline fundamentals of specifying process sensors.
  5. Describe several process sensor technologies and their application.
  6. Provide lessons learned from installed applications.

Webcast speaker information follows.

Frank Lamb is founder, owner, and manufacturing and automation business consultant, specializing in controls, programming, machine building and design at Automation Consulting LLC. Lamb also is a Control Engineering Editorial Advisory Board member.

Christopher J. Thompson, PE, is department manager, process engineering, Process Solutions Department at Matrix Technologies Inc. Matrix Technologies Inc. was named System Integrator of the Year by Control Engineering magazine in 2008, 2012, and 2016. 

More answers on sensor selection

Below, Lamb and Thompson provide additional answers to webcast audience questions about industrial sensors.

Q: Can you discuss communications protocols used for sensors?

Lamb: Most communications protocols are common to control networks rather than sensors themselves. For instance, Allen-Bradley's Remote IO was one of the early options for connecting nodes in real time to a control system, but it required an intermediary device between the sensor and the controller. Later came DeviceNet [now managed by ODVA], which was an attempt at placing individual devices on a network. Profibus [PI North America, promoted by Siemens and others] and Modbus [Modbus Organization Inc., promoted by Modicon and others] also were early networks to handle I/O, though not on an individual device basis. HART protocol (carrying communications over a 4-20 mA linkage) was another early method.

Most sensors still are wired into a device that handles the communications rather than being connected directly, but almost every controls vendor has some sort of Ethernet protocol that they favor for devices. EtherNet/IP [also ODVA], Profinet [PI North America], and EtherCAT (EtherCAT Technology Group, championed by Beckhoff Automation and others) are all options for this. Sensor manufacturers usually make versions for the major platforms.

Q: Can you discuss application of radar and ultrasonic sensors and transmitters?

Thompson: Radar sensors are typically associated with measurement of liquid or bulk level in some type of container (e.g. tank, vessel, silo, or hopper). The sensor emits a microwave pulse that is either directed down a fixed probe (guided wave) or into the open area of the container (non-contact). In both cases, the transmitter then detects the reflection time of the microwave pulse as it bounces back from the fluid or build solid level in the container. Guided wave also can detect interface levels in liquid-liquid applications, and its measurement is largely independent of the property changes that occur at various operating conditions. Non-contact radar sensors can be inhibited by use with some solids, fluids with low dielectric, substantial surface turbulence, condensing vapor environments, and echoes from container walls or internals.

Ultrasonic sensors operate on a similar principal as the radar, with the sensor measuring time-of-travel or reflection time of the ultrasonic pulse emitted. The sensor correlates this time to the liquid or bulk solid level in a container, or flow through a pipe. The flow sensor is typically used with liquids and gases, and the emitted ultrasound must be capable of penetrating through or being reflected from the material being sensed. Ultrasonic level sensors are fixed, but flowmeters can be either fixed or portable, in-line or clamp-on. Ultrasonic level sensors can be inhibited by use with some solids, foaming, interference or echoes from container walls or internal, smoke and/or dust, and heavy vapors. Flow sensor applications are highly dependent on the fluid being metered. If there are any questions or uncertainties regarding the sensor, inquire into possible vendor testing, instrument trials or rentals, or an application warranty.

Q: Can you discuss sensor use in hazardous areas?

Lamb: To prevent explosions and combustion in hazardous areas, sensors are housed in an explosion-proof enclosure or intrinsically safe "NAMUR" rated sensors are used. These sensors operate on <8 V dc to prevent sparking and often are connected through an intrinsic safety barrier.

Q: Can you discuss accuracy considerations for gas flowmeters?

Thompson: With most gas flowmeters, accuracy will depend on the material being sensed, the process conditions encountered, the installation location, operating range, and the technology selected. Condensing vapors are a challenge for most applications, so operating in a range that will minimize the likelihood of liquid droplets forming is important. Depending on the technology, turndown could impact not only the accuracy but ability to use the meter (e.g. vortex, turbine, differential pressure). Particulate-laden gases also can pose a challenge for some meters, depending on the type and concentration of particulates.

Proper temperature/pressure corrections as well as proper flow distribution into the meter itself (e.g. sufficient conditioning or upstream/downstream straight runs) will impact differential pressure flowmeter accuracy, while thermal mass meters will struggle with streams containing variable composition, aerosols, droplets, etc. Flowmeter accuracy issues can be addressed by having an accurate description of the stream being sensed, an clear definition of the operating parameters, and an understanding of the potential technology limitations prior to sensor selection.

Q: Can you discuss electrical parameters (ac/dc, sink/source, voltage, and analog)?

Lamb: Discrete sensors are available typically in standard voltages; 24 V dc and 120 V ac, but they also may be available with a 12 V dc output. They may also use a contact closure, similar to relay contacts; you can use a variety of voltages with those. DC sensors come in 2 "flavors," sinking or NPN transistor-based devices, and sourcing or PNP transistor-based devices. Sinking sensors receive current flow from a sourcing-type input, while sourcing sensors provide current to a sinking-type input; they are therefore complementary. AC sensors general use a Triac (solid state ac device) output. Analog sensors generally are available in 0-10 V or 4-20 mA varieties; mA outputs are considered to be more immune to noise, but 0-10 V signals generally can be run over a longer distance.


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