Analog Transmitters Lead the Pack

When working on a PC, you've probably come to appreciate that antivirus protection icon at the bottom right-hand corner of your screen. As that software works to protect your work and your investment, so do process variable transmitters (PVTs) work to protect your in-plant work and investment. In fact, they may be the savior of every engineer who uses them.

By Staff January 1, 1999

Process Variable Transmitters

4-20 MA communication still king

Sensor-transmitter integration

Raw materials processing leads applications

When working on a PC, you’ve probably come to appreciate that antivirus protection icon at the bottom right-hand corner of your screen. As that software works to protect your work and your investment, so do process variable transmitters (PVTs) work to protect your in-plant work and investment. In fact, they may be the savior of every engineer who uses them. They are often the first indication of potentially catastrophic situations, and in the case of volatile environments like chemical/petrochemical or utilities, they can even help save lives.

Though they’ve been around for decades, the role of PVTs is as crucial as ever according to the Control Engineering Process Variable Transmitter Study. The study polled 1,500 Control Engineering readers this past October and received 415 responses for a 27.7% response rate. Of the 415 who responded, 377 (91%) specify, recommend, or buy PVTs for their in-plant requirements.

Continuous applications were the slight favorite in the survey, accounting for 41% of the responses while 13% said their primary application involved just batching. The rest, 46%, said their primary function involved both continuous and batch.

Raw materials processing was the hands-down favorite for primary end product, with over a two-thirds share of responses (67.6%). The next closest category was other fabricated metal and miscellaneous manufacturers. Among discrete applications accounted for were transportation; communications; and aircraft, guided missile, and ordinance manufacturers.

Purchasing and type

When it comes time to specify or buy a PVT, probably to nobody’s surprise, the respondents want to buy sensor and transmitter all in one. Instead of trying to match sensor with transmitter, which takes time, engineers can usually walk into a distributor with specs and have it done for them. John Wynne, product marketing manager with Analog Devices (Bridgewater, Mass.), says the convenience of having a distributor match components stems from proprietary technologies’ incompatibility. Nobody has the time to try to mix and match because proprietary technologies don’t allow integration easily.

Certainly another factor in purchasing is whether to go with analog or one of the myriad forms of digital communications (fieldbuses). While fieldbus supporters race to create a digital standard, the nearly undisputed king of PVT communication is the still-reliable, standard-supported 4-20 mA signal. Ninety-four percent of PVTs communicate using this signal. HART was a distant second with a 35% share (result exceeds 100% due to multiple responses).

Mr. Wynne says yet a third reason accounting for 4-20 mA’s preference is the difficulty trying to convert transmitters from an analog to digital signal. Conversion most often involves integrating the sensor with an analog-to-digital converter (ADC), which isn’t as easy as it may sound. “Very many sensor technologies simply don’t allow themselves to be easily integrated onto the same silicon as ADCs.” The physical limitations can be prohibitive enough when trying to fit all the components into a very small area. The engineer could replace the analog transmitters with digital ones, but considering the 4-20 mA signal is backed by a standard, has such a large installed base, and is still very reliable, it’s not likely engineers will start ripping out their analog transmitters just yet.

Process Variable Transmitter Top Ten

Total Dollars Purchased by Survey Respondents

Source: Control Engineering Process Variable Transmitter Study, November 1998.




Micro Motion

Endress + Hauser

Yokogawa Corp. of America

Analog Devices

Moore Process Automation Solutions



Measuring multiple variables

As convenient and practical as multivariable transmitters (MVTs) are, their presence has yet to catch on widely. Respondents say that of the total number of transmitters in their plants, 21% are multivariable.

Some early misgivings about MVT’s are being allayed. When MVTs were first introduced, cost and difficult installation requirements deterred many buyers. Cost has since dropped in the last few years and companies are working toward making installation much easier. The communications issue, however, may be the biggest reason MVTs haven’t found their way into the control engineer’s pantheon. Multivariable transmitters use digital communications almost exclusively, so presumably MVTs won’t be used more until digital communications begins to replace 4-20 mA as the communication of preference.

Of the variables measured in respondents plants, at least 90% of those polled use pressure (97%), temperature (94%) and flow (90%) transmitters, with level transmitters not far behind, coming in at 87%.

What the future holds

It looks as though the 4-20 mA signal will continue its stronghold on transmitter communication for the next year. In the next 12 months, respondents say they plan to add 4-20 mA for transmitter communication by a ratio of more than two to one over its next two closest competitors, HART and Ethernet. Dinesh Neelay, software business manager with Micro Motion (Boulder, Colo.) doesn’t expect the analog communication dominance to continue for long. “In the future,” he says, “the drive to improve performance will dictate information requirements down to the transmitter level. Those increased information requirements will require a digital network or protocol.”

As far as purchases of the transmitters, 45% say transmitter purchases for their companies will at least remain the same, 26% say their companies’ purchases will increase, and only 9% expect their companies to reduce the number of transmitters they will purchase in the next 12 months. Twenty percent are unsure about their companies’ transmitter purchases in the next year.

Process variable transmitter products

For more information about process variable transmitters, visit

Versatile temperature transmitters

Newnan, Ga.— The YTA Series of temperature transmitters includes several types of transmitter styles: “hockey puck”; a smart single input transmitter: a high-end style that includes curve matching; and a multivariable model capable of dual inputs, hot sensor backup, differential, and average temperature measurements. All models handle up to 20 inputs covering most standard RTD, thermocouple, DC mV, and ohm inputs. The YTA310/320 model can also handle up to 40 points of custom curve input. Yokogawa Industrial Automation

Process measurement line expands

Boulder, Colo.— Micro Motion has expanded its Altus Applications Platform. New software offerings include process monitoring and totalizing capabilities, a frequency output, and expanded density. New density measurement software is available for the Model 3500 and 3700 transmitters allows food and beverage manufacturers measure 8Brix and net sugar flow. Model 3350 is a field-mount version of the original 3300 peripheral with Class I, Div. 2, and Cenelec Zone 1 approvals. Fitted with a NEMA 4x (IP67) housing, the Model 3350 retains the same user-friendly interface as the Model 3300 with a backlit LCD and keypad, the large tactile feedback buttons. Micro Motion

Nonintrusive ultrasonic flow device

Greenwood, Ind.— Prosonic Flow ultrasonic flow measurement device mounts onto existing piping without cutting into it and stopping the process. The device carries worldwide hazardous area approvals, including FM approval for Class I, Div. 1 or 2. The standard mounting and sensors are made of stainless steel. The robust NEMA 6P sensor housings can withstand demanding environmental conditions. The HART interface permits flowmeter configuration with either the HART hand-held or the installation of Endress + Hauser’s Commuwin II operating program on a PC. Prosonic Flow is well suited for liquid applications in the chemical, food, energy, wastewater, and mining industries. Endress + Hauser

Expanded functionality for transmitter family

Alpharetta, Ga.— Siemens’ family of process instruments for measuring flow, pressure, and temperature is now fully compatible with Asset Management Software (AMS) version 1.4 from Fisher-Rosemount. Compatible products from Siemens are the Sipart PS2 smart valve positioner, the Sitrans P pressure transmitter, and the new Sitrans TK H compact temperature transmitter. Full functionality is accomplished through HART protocol. Users are supported by a context-sensitive help function when operating these field instruments. All process variables are displayed on a product specific face plate and configuration properties are grouped in a logical order. Siemens

Critical transmitter

Spring House, Pa.— XTC critical transmitter is said to be the first pressure transmitter developed expressly for critical processes. Inherent features include hardware and software redundancy, a comprehensive self-testing system, and primary and secondary current sources to ensure safe shutdown if a failure occurs. As a member of the XTC family it integrates with the Apacs+ process automation system and the companion Quadlog safety PLC, thereby enhancing the system’s ability to deliver a comprehensive plant solution by improving total safety and availability for critical processes. Moore Process Automation Solutions

Smart analog transmitter family

Foxboro, Mass.— The I/A Series pressure transmitters use microprocessor-based electronics modules for users who want functionality and performance of intelligent transmitters but do not need digital communications. The new modules can be used with any I/A Series pressure transmitter. The module can be interchanged with other modules in the line, enabling customers to upgrade instruments to use a digital communications module for FoxCom, HART, or Fieldbus Foundation communications. Transmitters come equipped with a robust sensor and packaging, an LCD indicator, pushbutton field configuration, and re-ranging without pressure. Foxboro

Multivariable transmitter

Phoenix, Ariz.— SMV3000 measures differential pressure, absolute pressure, and temperature to calculate compensated flow rates. Measuring these variables with one instrument reduces the number of pipe intrusions and the chance of leaks. SMV 3000’s self diagnostics indicate impending problems before they affect performance. Remote diagnostics save time when the instrument is installed in several distant locations. Honeywell IAC

Pressure transmitter for pulp and paper

Eden Prairie, Minn.— Fisher-Rosemount’s model 2090P Smart Pressure Transmitter is compatible with the company’s 1 in. PMC process connection and can be installed in any existing 1 in. flushmount fitting without the cost of replumbing. The process connection is also available with a weld-on nipple assembly option for new installations. Smart electronics using HART communication compatibility, a 0 to 150 psi pressure range with 20:1 turndown capability, and three-hole rotational mounting flexibility are all standard. Fisher-Rosemount

Analog-to-digital converters

Norwood, Mass.— The AD7705 and AD7706 16-bit sigma delta analog-to-digital converters offer programmability, low power consumption and multiple channels for dc and low frequency ac measurement applications. Low power consumption allows these converters to be loop, battery, or locally powered. Programmability of these devices is achieved through a standard serial interface. The three wire serial interface suits them for use in microcontroller- or DSP-based systems by reducing the number of interconnect lines and optocouplers required in isolated systems. Analog Devices

Smart transmitter

Rochester, N.Y.— 621T Series smart transmitters feature full draft range and all-welded remote seals for full vacuum and temperature service. Output choices are linear, square root, cube root, fifth root, (for open channel flow) and fifth order polynomial curve fit for a linear output proportional to vessel volume. Dual ASIC locations allow quick field repairs and replacement of electronics without need for recalibration and performance loss. The unit provides real-time correction of temperature (sensor and electronics) and static pressure effect for the lowest total probable error under any application conditions. ABB Industrial Systems