Eliminate Signal Gibberish

Several steps can help maintain the integrity of measurement and control signals.

By Greg Livelli, ABB Measurement Products

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Flowmeters based on the Coriolis effect emerged in the 1970s. Steady improvements in technology and pricing since then have greatly increased their acceptance. No other flow instrument is more versatile and capable. Besides measuring mass flow rate, they can provide simultaneous outputs for volumetric flow rate, total flow, density, temperature and percent concentration. They're unaffected by flow profiles or viscosity.

Temperature sensors. For process plants, this generally involves selecting between a thermocouple and a resistance temperature detector (RTD); both yield voltages that infer temperature. A thermocouple consists of two dissimilar metal wires joined together at one end. The voltage between the unjoined ends varies with the temperature of the joint. An RTD usually comprises a wire-wound rod or thin-film metals through which a current is passed. The resistance the current encounters varies with the temperature of the metal, usually platinum.

Thermocouples as a class have a wider operating range. They can measure temperatures up to 1,800°C (3,272°F). Most wire-wound RTDs measure temperatures below 500°C (932°F) while thin-film models usually are restricted to below 200°C.

Thermocouples generally are cheaper though less accurate than RTDs. If an application doesn't require particularly tight temperature control, an inexpensive thermocouple and a well-tuned control loop should do the trick. But for processes that only will work correctly at highly specific temperatures, pay for the greater accuracy an RTD affords. The cost of scrapping a batch of ruined products would eventually dwarf any savings in equipment.

A fast sensor also can be worth the extra cost. If a process requires a rapid succession of heating and cooling cycles, the temperature sensor must be able to generate a reading before it's too late to be of any use. Thermocouples tend to respond faster than RTDs.

In situations where thermocouples and RTDs are impractical or conditions too harsh, sensors that detect infrared radiation from a surface may work. You aim these radiation thermometers at a surface from a distance. Major factors affecting accuracy include the surface emissivity as well as water vapor, dust, smoke and suspended matter in the space between the sensor and the measured surface.

Pressure and level transmitters. Process plants essentially have standardized on DP transmitters with internal diaphragms. These sensor/transmitters can measure absolute, differential and gage pressures, and infer liquid tank levels and flow rates.

For direct pressure installations, selection largely involves choosing the right connection as well as the diaphragm material, seal and coating for protection from the process medium. Special diaphragm materials include variations of tantalum, super duplex stainless steel and high nickel alloys. Coatings that resist abrasion, hydrogen penetration, sticking and corrosion also are available.

For remotely mounted transmitters, impulse lines can transfer pressure from the process to the transmitter input ports. Some vendors offer transmitter algorithms to detect when impulse lines get plugged. Additionally, impulse lines consisting of sealed capillaries combined with remote process seals can isolate the process medium (Figure 1).

The trick is to find the right technology for the application or to choose instruments that span a broader range of solutions.

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