Pressure Sensing: It’s Everywhere!

KEY WORDS

Process control and instrumentation

Process sensing

Pressure sensing

One of the most-used sensors, pressure instrumentation is found nearly everywhere now. The old process standby is still there-chemical change-of-state monitoring still requires tracking such mundane variables as pressure and temperature-but it now can be found, however tiny, in manufactured goods from automobiles to household appliances.

The signal emanating from any of these gages can be used to trigger a reading and eventually a control response. However, there are entire families of gages that only do things like turn off the fill cycle on an automatic washer, or help control automobile emissions by sensing fuel-system pressure parameters. This does not make them any less pressure sensors than any others, large or small, whatever their underlying technology.

In reality, supplying signal pressure sensors is a package deal. Most sensors consist of a sensing element or transducer (see accompanying sidebar) and some way to process the signal, so that it is usable by the control system. This part of the package is commonly known as the transmitter. Transmitters consist of a combination of electronic subsystems (amplifiers, signal conditioners, etc.) used to make the sensor’s signal usable. Ultimately, as electronics have miniaturized, more computation functions have been added to this mix, giving rise to the multivariable transmitter. In these devices, inputs could be acted on by using algorithms and preprogrammed functions to directly output computed variables.

Whither are ‘they’ drifting

Pressure transmitters (with or without integral sensors) are available in standard (single variable) and multivariable devices. Although standard devices are moderately priced compared to similar industrial devices, multivariable transmitters remain relatively expensive. However, a multivariable device can replace several units, reduce process penetrations and wiring requirements, hold down costs associated with installation and commissioning, and provide easy migration to alternate fieldbus technologies, if plant expansion or modernization make it necessary. Many multivariable transmitters are remotely rerangeable and provide remote diagnostics capability.

Where are these devices headed in the market and what will be the impact on users? According to ARC Advisory Group’s (Dedham, Mass.) ‘Pressure Transmitter Worldwide Outlook-Market Analysis & Forecast’ published in late 2000, ‘The significant benefits of the new multivariable transmitters are predicted to help sustain growth of the pressure transmitter, also with the increased acceptance of and migration to fieldbus capabilities.’ The final outcome, according to ARC, is that the pressure sensing market will become ‘a two-tiered arena-smart and low-cost.’ In the control engineers’ application-specific, cost-conscious world, this is undoubtedly considered a positive trend.

Tweaking process conditions

Technology often evolves at the behest of its users. Once microelectronics ‘arrived,’ it was only a matter of time before increased computing power was interfaced to simple sensors. Smart pressure transmitters moved a step up the diagnostic ladder by letting the control engineer know remotely if a sensor was functioning or not. However, onboard computing power can now supply users with more sophisticated data for process tuning/troubleshooting and preventive/predictive maintenance.

Ability to configure sensors on site can be a real benefit in process situations where a technician’s assessment of changing, but difficult to quantify, process conditions is the key to successful product throughput. For example, as a commercial bio-digester operation, NaturTech Composting Systems Inc. (St. Cloud, Minn.) receives all types of waste matter to be broken down. Technicians set process conditions within each digester depending on the waste mix added at the beginning of the cycle.

Because different types of waste matter react differently to the aeration that enhances the breakdown process, batches run at different process pressures. Hence, the technician adjusts process pressures on site, based on the mix observed. As with biological digestive systems, certain materials give off more gas in the breakdown process. Pressure sensors within a bio-digester must be set accordingly to properly monitor the breakdown process.

Tuning these processes without relying on handheld devices is a great advantage to NaturTech. ‘Technicians need to adjust transmitters on-site for optimum performance,’ says Jim McNelly, NaturTech’s president. ‘Because of the remote areas in which NaturTech’s equipment is located, not having to deal with handheld communicators or laptops to get the job done is a tremendous benefit. Not having to remember to bring extra equipment along or go back to get a forgotten handheld unit is a real advantage.’

Size matters

There are also advantages to being small in the sensor world. Capturing meaningful pressure values depends on where in the process system they are taken. Though this may seem obvious for some applications, such as internal combustion engine performance monitoring (oil, exhaust backpressure, fuel injection pressure, etc.), it is also true for much larger process systems. Monitoring pressure rises in chemical mixing processes, determining local vessel pressures, and accurately determining vent pressures requires location-dependent pressure sensing capabilities. In these cases, smaller may be better.

Futek Advanced Sensing Technology Inc. (Irvine, Calif.) has developed the P1943 thin-film, strain-gage pressure sensor, which fits, along with its amplifier and signal conditioner, in a 3/8-in. diameter x 11/2-in. long package. The standard sensor features a titanium housing, which is compatible with most media. It is also highly vibration resistant-20-10,000 Hz, 30g in three axes. Also, the device is not torque sensitive, so that torque forces applied during installation do not translate to the sensing element. The tiny package has an accuracy of 0.35% full scale. It is available in 10 ranges from 2 to 250 bar (29 to 3,625 psi), gage or absolute, and has an operating temperature range of -40 to 302 °F (-40 to 150 °C).

That sinking feeling

Pressure sensor placement is not always a size-related issue. Sometimes just getting them into the right place to take a needed reading is the problem. This can occur often in utility operations, where pressure readings are needed to determine water levels in wells, aquifers, liftstations, tanks, or reservoirs.

Brownsville Utilities (Brownsville, Tenn.) has incorporated Honeywell Sensing and Control’s (Freeport, Ill.) SGS601 submersible pressure sensor into its municipal water systems. The device senses pressure using a strain gage, precision-etched from single-crystal silicon and isolated from the process by a stainless-steel diaphragm. It also features a vent tube in the sensor cabling, which exposes the sensor to atmospheric pressure. Integral signal conditioning rationalizes the diaphragm sensor’s signal, then compensates for barometric pressure and temperature to measure hydrostatic pressure and determine water level.

The SGS line is available in customer-specified custom calibrated ranges. It also is made with custom cable lengths and specially sized filterless vent tubes that prevent condensation from entering. They are rerangeable at the sensor and remotely. Brownsville Utilities uses the custom range feature to maintain tighter tolerance in measuring reservoir water level. Holding tighter tolerance on level ensures that service pumps will never ‘run dry’ and accurately controls the amount of water supplied to the distribution system.

Brownsville’s water system uses Honeywell Sensing and Controls’pressure sensor transmitter to optimize system performance.

Economics, performance

Low-cost pressure sensor technologies can suffer application shortcomings. Some of these are thick-film bonded gages, piezoresistive, and bonded semiconductor technologies. Thick-film bonded devices are very reliable, but can suffer instability and drift due to aging of the bonding epoxy. Bonded semiconductor devices can suffer the same fate. Devices using piezoresistive technology typically offer a limited temperature compensation range. Maintaining sensor accuracy, as well as thermal and long-term stability, are requirements for an optimally tuned control system.

To produce a low-cost pressure transducer with accuracy and stability for use in harsh industrial applications, Omega Engineering Inc. (Stamford, Conn.) has designed the PX906 pressure transducer. Its modular-designed pressure sensing subassembly uses heat-treated, stress-relieved stainless steel. Thin-film resistors are sputter deposited onto this pressure-sensing diaphragm, and then encapsulated under a thin layer of glass. In operation, pressure is applied to the diaphragm through a pressure port to strain the gages. Resistance change is proportional to measured pressure.

Eliminating adhesives brings long-term stability to the system. Thin-film sputter technology directly fixes gages to the diaphragm, eliminating the adhesive interface that can move or degrade with time and exposure to process conditions. Modular design has also lowered costs because flexibility in port configurations can be added to the sensor at final assembly.

Modularity, a good thing

Another way to bring increased functionality to pressure sensing devices is to build in convenient mounting. In the case of PSI-Tronix’s (Tulare, Calif.) MB-10000 pressure transducer, the device is built around a 1-in.

The pressure-sensing device is available with either 0-10 V dc or 4-20 mA output. It offers pressure ranges of vacuum to 10,000 psi (gage or absolute), all stainless-steel construction, and accuracy of

Pressure sensing devices have now found their way into many arenas of manufacturing. What was originally thought of a process-only sensor has been harnessed as a control component for discrete manufacturing operations, and become a standard accessory in many pneumatic- and hydraulic-based motion control systems. Wherever fluids are flowing, performing the work in pneumatic or hydraulic systems or ‘being worked on’ as media in a process control system, pressure is the variable du jour .

For more suppliers, go to www.controleng.com/buyersguide; for more information use the following circle numbers, or go online at www.controleng.com/freeinfo.

ARC Advisory Group www.arcweb.com .209

Dwyer Instruments Inc. www.dwyer-inst.com .210

Futek Advanced Sensor Technology www.futek.com .211

Honeywell Sensing and Control www.honeywell.com .212

Omega Engineering Inc. www.omega.com .213

PSI-Tronix www.psi-tronix.com .214

Pureron USA www.sensorguys.com .215

Siemens Moore Process Automation

www.smpa.siemens.com .216

As all these transducers evolved from the earliest incarnations, they have undergone remarkable changes. According to Dave Snyder, president of Pureron USA Inc. (Northridge, Calif.), the greatest changes have come in physical size and cost reductions of pressure sensors.

Miniaturization has been made possible by micromachining, the adaptation of silicon chip manufacturing techniques to the transducer/transmitter fabrication. And, because this technique is adaptable to volume manufacture, costs have fallen considerably in the technologies to which micromachining has been adapted, namely piezoelectric, strain gage, and capacitance types. ‘Thirty years ago, you could expect to pay $100 for just about any pressure sensor. Now, you can get the transducer/transmitter combination on one chip for about 20 bucks,’ Mr. Snyder adds.

Pressure sensors used in the consumer market have specialized functions. Unlike process situations that can require sensing of dynamic and/or changing pressures, consumer applications often need only a repeatable reading in a given range-a pressure switch. The prevalent technologies in these applications are strain gage and capacitance devices. Potentiometer, variable reluctance, and LVDT types have not been miniaturized, in size or price, to the extent of the others, and play a larger role in retrofit and replacement situations.

Review transducer basics

A transducer is a device, which when activated by energy from one system, supplies energy to another system. Its essential feature is an elastic element that converts energy from a system under pressure to a displacement in a mechanical measuring system-the underlying principle of the analog pressure gages. However, if an electrical element is added to this system, displacement of the mechanical system can be converted into an electrical signal.

The basic operating principle of conventional electrical pressure transducers represents a broad range of technologies. Among the commonly encountered technologies, only the piezoelectric type is considered an active transducer, one that generates its own electrical output. It is suited for measuring dynamic pressure changes. All other types are considered passive . Passive transducers require auxiliary electrical input that they modulate as a function of the mechanical displacement to supply electrical output.

Common types of passive devices include:

Strain gage;

Potentiometric;

Capacitance;

Linear variable differential transformer (LVDT); and

Variable reluctance.

Small sensor puts pressure on industrial fumes

In heavy equipment manufacturing, in-place welding must often be done in place on large metal assemblies. Problems associated with removing weld fumes and smoke from a cavernous assembly floor are often best solved by removing pollutants ‘at the source’ using an air recirculating fume hood.

FSX Corp. (Monroe, Wa.) uses a DCT1000 Dust Collector Timer Controller supplied by Dwyer Instruments Corp. (Michigan City, Ind.) to measure and control the differential pressure across dust cartridges in its reverse-pulse jet air cleaners. During operation, air cleaner filters clog and lower the efficiency of the unit. To clean and adjust the system’s differential pressure back to normal operating conditions, DCT1000 provides the timing sequence, which energizes the solenoid valves, supplying compressed air to clean the filters. Operation of this system can be done continuously, manually, or on demand at a predetermined pressure drop, sensed by the device’s on-board pressure module.

The module, which contains a plug-in miniaturized piezoresistive-type pressure sensor/transmitter combination, provides a digital interface to the controller assembly and a 4-20 mA signal for external monitoring. The controller, in turn, can provide up to 22 output channels for dust collector control requirements. The entire controller accepts 85 to 270 V ac/V dc through its universal power supply. Pressure modules are available in 10- and 20-in. w.c. full scale. Operating temperature range is -40 to 140 °F (-40 to 60 °C).

According to Mark Fisher, product manager at Dwyer Instruments, the flexibility of the firmware-based controller and its ease of interfacing to mechanical systems make it ideal for filter monitoring and cleaning applications. Pressure modules can be ported to either end of a filter using metal or Tygon tubing. With everything on one board, inexpensive and reliable turnkey control of the system is assured.

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