Vital Links to Process Control

The process variable transmitter (PVT) is a simple device with a big role. The link between process sensors and the control room in nearly every continuous process/batch operation worldwide, these devices must send accurate signals to the control network, ensuring that product quality and flow are not compromised.


The process variable transmitter (PVT) is a simple device with a big role. The link between process sensors and the control room in nearly every continuous process/batch operation worldwide, these devices must send accurate signals to the control network, ensuring that product quality and flow are not compromised. Control Engineering magazine and Reed Research Group, both part of Reed Business Information ( ), recently looked at trends in PVTs in an email/Web survey, the first one on this topic since 2002. Wireless, smaller form factors, and increased use of multivariable process transmitters are among key trends.

Those answering each survey use PVTs in a similar mix of applications, which is good for comparison. Continuous process applications accounted for 47.3% of the responses, both continuous and batch for 40.6%, and batch only for 12.1%, changing just a percentage point or two in each case from the 2002 survey. Transmitter types used remained concentrated among the 'big four' process variables: pressure, temperature, level, and flow.

On the horizon

Is the transmitter, then, being called on to become—like so many other control devices—smaller, faster, and cheaper? Constantly improving electronics have likely already achieved as much speed as needed for most process applications. Better electronics coupled with streamlined assembly techniques have led to more compact packaging and lower prices. One word, however, occurs in PVT product literature this year that was absent in 2002: wireless .

According to Lawrence Vanell, technical manager, field instrumentation for Honeywell Inc. (, efforts to add this feature to the transmitter's lexicon is considerable. Given the number of process sensing penetrations and the physical size of many process plants, it makes sense to eliminate costly wiring between transmitter and control circuit, which can cost as much as $1/ft.

But while wireless transmitters are available, they are not yet ready for control. Says Vanell, 'Today's wireless transmitters have been successfully used in a wide variety of monitoring situations within process plants. However, due to the fact that all radio devices can lose a signal occasionally, they have not been recommended for use in control functions as yet.'

Once wireless process transmitters can prevent signal loss or save data onboard during an interruption for rebroadcast once contact is re-established, the wireless transmitter will join the myriad other wireless devices available to the control engineer. According to Vanell, wireless transmitters for controlling processes are on the horizon in, 'as soon as three years, but not longer than five.'

Smaller, smarter

Miniaturized electronic components have led to smaller, smarter process transmitters. Multivariable devices let control engineers gather pertinent process data while lowering wiring costs and limiting process penetrations that are potential sources of leaks or compromise. However, these units have not been overwhelmingly adopted. Responses from the recent survey indicate only small numbers of process plants use multivariable units to any extent. (See accompanying chart for detailed breakdown.) For a great number of respondents (60.5%), less than a quarter of all transmitters used were multivariable. In fact, the use pattern is close to that of the 2002 survey.

process control

Only small numbers of process plants use multivariable units to any extent. For more than 60% of all respondents, less than a quarter of all transmitters used were multivariable. Use did increase from 2002, however.

In that survey, on average, 22% of all transmitters used by survey respondents were multivariable. In the current one, that figure rose to 26%, but hardly an assault on the total installed base. According to Sath Rao, research manager, automation and process control, Frost & Sullivan ( ), multivariable transmitters have only begun to make inroads in process control. 'Multivariable transmitters are increasingly being accepted not only for retrofit applications—where the cost of line modifications for multiple measurements with different transmitters is a major disincentive—but also for new installations. Use of multivariable devices reduces initial purchasing and installation costs. Overall system reliability also benefits just from the standpoint of having fewer transmitters on line,' Rao says.

Process Control

Michelle Hursh, global product manager, Emerson Process Management ( ), also sees increased market penetration. 'Rosemount has seen demand rise since the original 3095 transmitters were introduced over 10 years ago. Resistance to established plant practices aside, once control engineers saw the advantages of installing multivariable units, they began to specify them.'

Following protocol

Many communication protocols are available for process control systems and respondents to this survey made it clear that some of the 'old reliables' are very much alive. Analog communications still dominate. The greatest change from the 2002 survey was the increase in use of a FOUNDATION Fieldbus, which clearly eclipsed Profibus PA in this survey. (See accompanying chart of protocol use for detailed breakdown.)

Will digital protocols become the communications choice of the 21st century? According to Michael Cushing, product marketing manager, process instrumentation at Siemens Energy & Automation Inc. ( ), the answer is, 'not so fast.' 'To put this in historical perspective, when the smart transmitter was introduced in 1983, all potential users needed to do was remove the existing analog electronic device, install the smart one, configure it, and they were done. Only the transmitter was affected—no new wiring, I/O cards, power supplies, or system software were needed. Even under these ideal conditions, it took over 10 years before smart transmitters outsold analog,' says Cushing.

Replacing existing hard-wired devices with a bus system is not, in Cushing's view, analogous to replacing analog with smart. Going from hard-wired to a bussed system, such as Profibus or FOUNDATION Fieldbus, all components need to be replaced, including the transmitter, wiring (at least adding terminators), I/O cards, system software, graphics, and configuration software. Replacing the transmitter is the least expensive element. A better analogy, he says, is replacing pneumatic transmitters with electronic, because the entire architecture is affected.

'With new construction,' says Cushing, 'whether someone uses HART, FF, or PA depends on their familiarity and confidence with the newer technologies. As the longer-term benefits of improved asset management that bussed systems promise are realized and documented, eventually all new construction will be bussed. However, since the bulk of the current U.S. market is MRO, and sales of pneumatic transmitters (although obsolete 50 years ago) continue, it will be a long time before Profibus and FOUNDATION Fieldbus fully replace 4-20 mA/HART transmitters,' he adds.

Transmitter products

For more manufacturers, visit ; for systems integrators visit . For more on these products, see Websites listed below.

Process Control

Four measurements in one

3095 MultiVariable mass flow transmitter with FOUNDATION Fieldbus performs four measurements: pressure, differential pressure, temperature, and mass flow. One transmitter installation reduces process penetrations, inventory, and installation costs. Device provides fully compensated mass flow, reducing sources of traditional dp flow uncertainty. Mass flow is calculated by measuring process pressure and temperature to perform real-time calculation of all flow equation parameters, including density, viscosity, velocity, Reynolds number, beta ratio, discharge coefficient, velocity of approach, and gas expansion factor. Rosemount Measurement Div.

Process Control

Wireless monitoring

XYR 5000 wireless transmitters enable automated monitoring of variables where traditional hard-wired installations are too costly, difficult, or time-consuming. They monitor gauge, absolute, and differential pressure; temperature; and ultrasonic noise (for detecting steam and gas leaks). They include an analog input interface for adding wireless capabilities to 4-20 mA devices, a discrete input/output option, and optional external antennas to transmit measurements to a base radio connected to a control system or data acquisition device. Transmitters offer accuracy of Honeywell Inc.

Process Control

Multivariable sensing

Elite multivariable flow and density Coriolis meters have no moving parts, low pressure drop, and measure independent of fluid characteristics. Available in a wide range of geometries, they are suited for liquids, gases, slurries, and suspensions for nominal pipe sizes from 0.1 to 10 in. They can measure mass flow, density, and temperature as direct measurements while calculating volume flow, corrected volume flow, net solids, and concentration. Recent upgrades have extended operational range to temperatures to 400 Micro Motion

Level sensing transmitters

IMV31 density-compensated level transmitter is based on hydrostatic measurement and multivariable sensing technology. It uses differential pressure, pressure, and temperature measurements to compensate for liquid and vapor density changes, transmits a 4-20 mA output signal proportional to level, and has HART digital communications. IMV31 is suitable for open (vented) and closed (pressurized) tanks and for critical or high-performance applications. Invensys/Foxboro

Process Control

Easy-to-use transmitters

YTA Series includes YTA50, YTA70, YTA110, YTA310 and YTA320 temperature transmitters, all said to be easier to install, configure, and operate than traditional temperature transmitters. Options suit virtually any application. Universal input capability keeps inventory costs low. YTA Series, in particular the YTA310 and YTA320, feature D/A accuracy at 0.02% of span. Sensor matching capabilities reduce total measurement error by up to 75%. Yokogawa Corp. of America

Process Control

Transmitters for diverse uses

2600T Series HART-configurable pressure transmitters have sensors that measure differential and absolute pressure to meet diverse applications. An external process temperature sensor can be added to perform compensated volumetric and mass flow and level calculations. Certain models in the Series serve in safety instrumented system environments. Redundancy and diagnostics are in place to obtain a hardware fault tolerance of 1 and a safe failure fraction of 96%. ABB Instrumentation

Process Control

Bi-directional wireless interface

5605715-X-DRX interface is an integrated radio with I/O module designed for bi-directional interfacing of 4-20 mA and discrete signals in harsh industrial environments. Design lets users add multiple I/O channels to the transceivers in many combinations. The 902-928 MHz ISM band frequency hopping spread spectrum technology eliminates costly cable and conduit runs and guarantees a license-free, interference-free link between remote devices and a control room. Typical installations include monitoring control of pressure, level, temperature, and flow and switching and alarm functions. Ametek Drexelbrook

Process Control

Programmable transmitter

TMZ PC-programmable Modbus temperature and signal transmitter delivers high input resolution and exceptional digital measurement accuracy. Unit accepts a direct signal from a wide array of sensors and analog devices including RTD, thermocouple, ohms, mV, current, voltage, and potentiometer. It converts the inputs to the standard Modbus RTU (RS-485) protocol ready for direct interface with Modbus-based monitoring and control systems. Moore Industries-Int'l Inc.

Process Control

Pressure transmitter

Sitrans DSIII transmitter measures pressure, absolute pressure, differential pressure, flow, or level. It can be specified with a variety of metal process diaphragms for use in aggressive process materials. Two-wire device provides a standard 4-20 mA output signal proportional to input pressure. It can be programmed manually using three magnetic pushbuttons, or remotely using a HART communicator or its developer's PDM software. F OUNDATION Fieldbus communications are supported. Siemens Energy and Automation

Process Control

For temperature, humidity

HMT3300 Series Humicap humidity and temperature transmitter for demanding applications features a large, numerical/graphical display with multilingual interface that allows monitoring of measurement trends with a year of historical data. Six models cover the full humidity measurement range, a wide temperature range (-70 to 365 °F), and pressures from 0 to 1,450 psi. Features include multiple humidity parameters. Available outputs include three analog, a serial interface, and alarm relays. Vaisala Inc.

Process Control

IR wireless flowmeter

ST75 flowmeter for small line sizes now comes with a wireless communication option. With built-in wireless IR technology and a low-cost PDA, process engineers can obtain measurements, make setting changes, and read troubleshooting codes without opening the instrument. Device is intended for application in hard-to-reach locations or where opening instruments is inconvenient or labor intensive. ST75 is accurate to Fluid Components International

Process Control

Compact transmitter

626-CH pressure transmitter offers 0.25% full-scale accuracy on ranges from full vacuum, compound, 0-5 to 0-5,000 psig. Constructed from 316 and 316L stainless steel, device senses pressure of air, compatible gases, and liquids; then sends a current or optional voltage output signal. 626-GH comes in a compact (3-in. x 7/8-in. hex) NEMA 4X (IP66) housing and can withstand temperatures up to 200 8F. Several ranges are available. Dwyer Instruments Inc.

Process Control

High-temperature device

Optimass 8000/900c0 Series of high temperature Coriolis mass flowmeters provides mass flow, volume flow, and density measurement for applications with process temperatures from -292 to 662ng jackets. Flowmeters also incorporate dual redundancy electronics in the front end on the sensor and in the converter. Krohne Inc.

Process Control

Liquid analytical measurement

Solu Comp Model XMT transmitter family measures pH, ORP, conductivity, oxygen, chlorine, and ozone in a variety of applications. All feature enclosures that are weatherproof and corrosion resistant to NEMA 4X/IP65 specs and suited for panel, wall, or pipe mounting. Transmitters use HART and F OUNDATION fieldbus digital communication protocols to allow operators to configure the transmitter, read process variables, and troubleshoot from a network-attached computer anywhere in the plant. Emerson Process Management, Rosemount Analytical Div.

Process Control

Measuring energy, power

Signet 8900 multi-parameter controller with a Btu calculator provides high accuracy measurement of energy and power consumption. Designed to function as an all-in-one controller or as a transmitter for multiple parameters, it is suitable for virtually any application where more than one single-channel or single parameter instrument is required. Up to six sensor types, including flow, pH, ORP, conductivity/resistivity, pressure, temperature, and level, can be mixed and matched. Two base units, one with backlighted LCD and the other with a vacuum fluorescent display, can be user-configured in the field with many I/O options. George Fischer Inc.

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