Process Variable Transmitters Keep Pace with Customers' Needs

Traditionally, process variables are considered to be units of flow, level, pressure, or temperature, and the media measured include gases, liquids, slurries, powders, and solids (see table).Transmitters are devices that accept the output of sensors and transducers designed to detect process variables and produce an output which directly or proportionally alters as process variables change.




  • Data communications

  • Fieldbus

  • Self-diagnostics

  • Year 2000 compliance

Traditionally, process variables are considered to be units of flow, level, pressure, or temperature, and the media measured include gases, liquids, slurries, powders, and solids (see table).

Transmitters are devices that accept the output of sensors and transducers designed to detect process variables and produce an output which directly or proportionally alters as process variables change. The sensor/transducer may be either remotely located from or integral with the transmitter.

Almost 37% of our readers specify, recommend, or purchase process variable transmitters. Control Engineering conducted a random survey of our readership involved with process variable transmitters. Here are some highlights from this survey:

  • 68% of the total respondents are involved with specifying, recommending, or buying process variable transmitters for in-plant requirements; 17% are involved with OEM requirements; 5% are involved with both; and the remaining 10% are involved with miscellaneous requirements.

  • Continuous processing is the primary application for 47% of the respondents.

  • 42% prefer to specify or buy integrated units (sensor/transducer with transmitter).

  • Over 80% of the respondents are using these transmitters for liquids (see table).

  • The average respondent reported that 18% of their total transmitters are multivariable transmitters.

  • HART (highway addressable remote transducer) was the most popular communications protocol as reported by 52.5% of the respondents (see pie chart), while 3.8% of the respondents said their applications don't use any communications protocol.

  • 48% of the user respondents supplied complete purchasing details. A total of $8.1 million was reported for process variable transmitter purchases in the last 12 months.

  • 25% of the respondents expect expenditures to increase in the next 12 months, while 47% expect such purchases to remain about the same as the past year.

Manufacturer's viewpoints

We also contacted several manufacturers of process variable transmitters to discuss what trends they see and to learn their views and experiences.

Communications leads the list

"I'm seeing our customers placing a greater emphasis on communications capabilities," notes Brooks Pollock, senior product manager at Elsag Bailey (Wickliffe, O.). "We see this trend as customers focus more on the cost and conditions of production, and these are two key reasons why it's happening. Communications gives management the means to take an in depth look at its operation and see just where the cost savings might be. Additionally, regulatory agencies are requiring that more process data be acquired and stored."

Mohd Alam, applications engineer at Smar International Co. (Houton, Tex.) says, "Unlike traditional distributed control systems, there is two way communication between the field devices and the operator, in a system where smart field devices are networked. The smart transmitter sends diagnostic information to the operator along with the process variable. This information is used to check the reliability of data and to take corrective action. The operator sends corrective actions to the transmitter over the same communications network."

"Now that intelligent process variable transmitters and digital field communications are coming into more widespread use," says Tom Kinney, director of product development, The Foxboro Co., (Foxboro, Mass.), "end-users are demanding even greater functionality and benefits from their intelligent instrumentation; functionality that goes beyond the basics of digital measurement communications, remote instrument configuration, and remote diagnostics."

Mr. Kinney reports that Foxboro, in conjunction with Oxford University (Oxford, U.K.) and a number of users, is working to bring fieldbus capability to the next level by leveraging the power of on-board microprocessors and digital communications to provide users with true self-validating intelligent transmitters. (For more on self- validation of sensors, see CE , Aug. 1994, p. 63 and May 1995, p. 81, both by Dr. M. Henry, Oxford U.).

Process Variable Transmitter Applications





(Totals exceed 100% due to multiple responses.) Source: Control Engineering


























Applying diagnostics

Bob Harvey, product line manager, Honeywell IAC (Phoenix, Ariz.) comments, "Enhanced diagnostic information from smart instruments will provide users with a new level of field information to better manage their process and plant assets. Instrument users are looking for ways to cost effectively capture multiple process variables from fewer process intrusions. In many cases these additional measurements will be used by customers to capture previously unattainable incremental process efficiencies, yield increases, or raw material savings."

According to Yokogawa Industrial Automation Inc. (YIA, Newnan, Ga.), some of the most costly problems are those repeated failures which occur when the root cause of the problem has not been determined.

Using YIA's DPharp transmitters, when process variables exceed the limitations of the transmitter, an error message is generated. This message will alternate every three seconds with the process variable the unit is programmed to display. This is intended to alert the technician to the existence of an out-of-tolerance situation.

Repeated events are often the sign of a process disturbance, while single events usually indicate the possibility that an oversight occured during installation. If the error message is not responded to within two hours, the event is logged and stored as a permanent record.

Mr. Kinney adds, "While the diagnostics available from current intelligent transmitters only tell you if an instrument is functioning or not, self-validating intelligent transmitters also transmit the accuracy (or uncertainty) value of the process measurement. This measurement accuracy value, calculated right in the intelligent transmitter using rigorous and fundamental methods, can be used to allow alternative measurement or control methods to be employed and/or to trigger maintenance activity.

Hostile environments are increasing

A marketing department spokesperson for Endress + Hauser (Greenwood, Ind.), sees a trend for measurements in higher temperatures and severe services. Its Dewpro MMR 101 transmitter features an operating temperature up to 302 °F and a dew point range from 32-302 °F.

Year 2000 problems

The Year 2000 (Y2K) software problem arises from the use of a two-digit field to identify years in computer programs (for example 85 = 1985), and the assumption of a single millennium, the 1900s. Any software so created will read "00" as the year 1900. Programs that use dates (including programs within machinery such as process control systems or PLC-based systems, HVAC systems, etc.) will fail or malfunction if these errors are not corrected. As if the year segment of a date were not enough of a problem, there is another year 2000 problem to consider. Many computer systems are designed to divide a year by 4 to determine if it is a leap year. This is only part of the rule for determining leap years. Years divisible by 100 are not leap years unless the year is also divisible by 400. Thus, the year 2000 is a leap year whereas the year 1900 was not.

Viatran Corp.'s (Grand Island, N.Y.) parent, Eurotherm Controls, has put into place a management system designed to identify and take remedial action to ensure that its products don't fail on Jan. 1, 2000. Viatran reports that owners and users of discrete process transmitters should not experience any sort of problems as these instruments do not have any date-related real-time clock functions. Any intelligent transmitters that contain internal clocks should be tested for Y2K compliance.

Temperature Transmitter Offers Two Protocols

Phoenix, Ariz.— The Smartline STT250 smart temperature transmitter offers users a choice of either Honeywell's digitally enhanced (DE) or HART communication protocols. Configuration is accomplished by using a hand-held communicator or Honeywell's SCT3000 PC-based configuration toolkit. The transmitter mounts directly to the sensor, which reduces installation costs and supports integral spring loading of the sensor for fast, accurate measurement. Alternate mountings include 2-in. pipe, wall, and DIN rail.

Honeywell IAC

Small Hazardous-Duty DP Transmitter

Grand Island, N.Y.— Model 574 differential pressure transmitter's small size allows it to be used in limited spaces. Its all-welded 316 stainless-steel construction allows the airtight and watertight transmitter to be used in corrosive environments.

Viatran Corp.

Coriolis Flowmeter for High Temperatures

Wickliffe, O.— Model MC1 Trio-Mass Coriolis mass flowmeter simultaneously measures mass flow rate, density, and temperature of process liquids. It measures flow rates up to 7,050 lb/min., features operation where the process temperature may reach a maximum temperature of 356 °F, and is suitable for clean-in-place applications.

Bailey-Fischer & Porter

Fielbus Pressure Transmitter

Houston, Tex.— The LD302 is a transmitter based on the capacitive sensor technology that measures absolute, gage, and differential pressure using digital sensing. As an intelligent device, it can perform control functions, flow totalization, and drive final control elements through the bus. The LD302 intelligent pressure transmitters now offer users a choice between ISA fieldbus and FOUNDATION fieldbus protocols.

SMAR International Co.

Level Transmitter Measures Solids

Greenwood, Ind.— Time domain reflectometry technology is employed by Endress + Hauser's Levelflex to detect solid levels. Levelflex is claimed to be immune to influences such as dusty filling, angled surfaces, tank obstacles, and bin construction—all problems for other noncontact technologies. Levelflex is positioned to compete with capacitive technologies. E+H claims it is accurate when changes occur in the bin materials or the material's characteristics—a weakness E+H perceives in capacitance technologies.

Endress + Hauser

DP Transmitter Measures Mass Flow

Spring House, Pa.— Model 3340S SteaMeter is a diffential pressure type flowmeter that has built-in lookup tables for steam properties to produce a 4-20 mA output representing mass or energy flow. Standard features include a 14-segment characterizer, both a mass and energy totalizer, measurement of differential pressure, absolute pressure and sensor temperature, HART communication protocol, and on-line diagnostics.

Moore Products Co.

Noninvasive Conductivity Measurement

Foxboro, Mass.— Foxboro 871FT is a noninvasive conductivity sensor available in an industrial flanged version or a sanitary Tri-Clamp version. Unlike conventional sensors, the "flow-through" toroidal sensor mounts directly in-line as a continuation of process piping, eliminating the need to construct separate sampling runs.

The Foxboro Co.

Ultrasonic Flowmeter

Alphretta, Ga.— Sitrans F is an ultrasonic flowmeter that offers greater measuring accuracy and range for both laminar and turbulent liquid flow measurement. The Sitrans F can replace conventional products such as vortex meters, positive displacement meters, orifice plates, electromagnetic flowmeters, and Coriolis flowmeters in a variety of applications. Its patented, helix-shaped ultrasonic guidance system provides a higher degree of measurement accuracy than one or two-dimensional signals in instances where smaller pipes, low flow rates, or high viscosities are a factor.

Siemens Energy & Automation Inc.

Four Variables in One Product

Eden Prairie, Minn.— Rosemount Inc. and Dieterich Standard combined the Model 3095 MV mass flow transmitter and the Annubar to create the Mass ProBar mass flow measurement instrument. Mass ProBar provides measurement of four process variables in one device, DP, static pressure, temperature, and mass flow.

Rosemount Inc.

Improved Specs

Newnan, Ga.— Known as the A-Series of DPharp digital pressure transmitters, these transmitters are claimed to offer a 40% improvement in general specifications for ambient temperature and static pressure effects over previous transmitters from the manufacturer. The A-Series replaces the "high accuracy calibration" option which was available at extra cost.

Yokogawa Industrial Automation Inc.

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