Old and in the Way?

The process variable temperature is often touted as the most measured variable in process control applications. If this is true (and it most likely is; see accompanying sidebar), it stands to reason that temperature measurement could easily be taken for granted. However, locating a thermocouple or RTD at the right spot, hooking up the required electronics to send a signal to controller, a...

02/01/2002


KEY WORDS

Process control and instrumentation

Process sensing

Temperature sensors

Temperature transmitters

Sidebars:
Is temperature the most-measured process variable or urban legend?
One is the loneliest number(when it comes to zone control)
What you should know about basic contact temperature sensing

The process variable temperature is often touted as the most measured variable in process control applications. If this is true (and it most likely is; see accompanying sidebar), it stands to reason that temperature measurement could easily be taken for granted. However, locating a thermocouple or RTD at the right spot, hooking up the required electronics to send a signal to controller, and on to a final control element is not always as simple as it sounds.

More often than not, details get in the way. And as any control engineer will tell you, once the problem is defined, the 'devil is in details' of the solution. Anything trivialized or overlooked has a way of sabotaging results, whether right away at commissioning or at a very inopportune time during a production run.

Temperature measurement has evolved with time into a trusted technology, so much so that most control engineers often take it for granted. According to Dennis Hablewitz, product manager for Eurotherm Barber-Colman (Loves Park, Ill.), part of the 'inertia in the market' is because control engineers trust the technology and automatically revisit the same one again and again.

Slow, steady progress

Thermocouples have made steady progress in accuracy and consistency. The gains have come largely from materials used to manufacture the bimetallic junction. Higher purity and consistency now available in the raw metallic wire has raised thermocouple accuracy without a specification change. And, as these materials became routinely available, thermocouple-based sensors have exhibited increased consistency from sensor to sensor. Thermocouple elements have exhibited better accuracy and evolution of newer alloys has allowed them to be used at higher temperatures.

Mr. Hablewitz points out that not all components of a thermocouple assembly have undergone change. 'Even though the element and the protection tube has seen material improvements, thermocouple insulation has not. Materials, such as high cobalt alloys now available for protection tubes, not only resist higher temperatures, but stand up to corrosion and carburization. The cost for these, however, is high compared to standard materials.'

Installation simplified

Thermowell depth is application specific, and, as such, the sensors that fit into them must also be specified to a correct length. In process applications with multiple and varying depth thermowells, stocking matching sensor lengths can be expensive. Additionally, with straight sensors, the connection head and thermowell assembly must frequently be removed to install the sensor.

Moore Industries-International (Sepulveda, Calif.) offers the Worm, series of flexible temperature sensors, which can be cut to fit different depth thermowells. The Worm can also be bent to allow it to fit through the top or face of an enclosure. It slides through the enclosure's entry port, and snaps into place without removing the enclosure, connection head, or any assembly components. The sensor is available in a variety of configurations including 3- and 4-wire platinum RTDs and J- and K-type thermocouples. They can handle up to 1,100°F.

According to J.R. Madden, temperature applications specialist at Moore Industries, these sensors have been used to advantage by ChevronTexaco (Sellows, Calif.) to populate multiple thermowells to monitor temperature in steam lines that maintain temperature on shipping and rejection crude oil tanks. Says Mr. Madden, 'While performance and reliability were major reasons for the Worm's selection in this application, it was chosen mainly because it greatly reduces inventory requirements. Previously, the company was stocking numerous types and lengths of RTDs and thermocouples.'

Flexibility counts again

Not all improvements to temperature sensor technology have occurred at the 'business' end of the transducer. In fact, improvements in packaging-especially miniaturization-and electronics have helped control engineers adapt this technology to a wider range of applications.

Nucor Steel Inc.'s Hartford, N.C., facility is a foundry that uses the continuous casting process to fabricate steel. To ensure that the mold receiving molten steel is properly and evenly heated, no fewer than 240 thermocouples are embedded in the walls of the mold to monitor its temperature profile. Proper flow of molten metal through the mold's die area can only be assured if no 'cold' spots exist.

The problem with having that many thermocouples in one area was not one of sensing, but rather of how to correctly connect (or disconnect, if sensor failure occurred) easily. High sensor density also required that wiring be flexible and easily connected to the control system.

According to Jim Adams, product manager for Wahl Instruments Inc. (Asheville, N.C.), finding the correct thermocouple for the job was not at issue. 'Nucor decided on the Italcoppie thermocouple probes exclusively for all its U.S. manufacturing process plants that used this type of equipment. The thermocouple probes are available in a variety of sizes with a bendable 316 stainless steel shank that couple using an IP67-rated threaded connection to extension cables. The probes are pre-wired to the keyed connector to avoid wiring mistakes-certainly a possibility in the close quarters on the surface of the mold. The temperature sensing system is also available using RTDs instead of thermocouples, should the application require them.

The bendable steel shank eliminates the need of custom conduit runs to each sensor. It also simplifies sensor change-out and parts inventory since one or two sizes can usually be adapted to a wide range of hook-up configurations.

All rolled into one

Not all temperature sensing applications can afford or even require elaborate electronics to perform the control function needed. Model TN temperature sensor developed by 'ifm efector inc.' (Exton, Pa.) offers single package convenience for these types of applications.

Cross Huller-North America (Port Huron, Mich.) uses the TN temperature sensor on its Specht machining line used to manufacture automotive engine heads. The sensor monitors temperature of the machine-spindle lubricant. The sensor is used to prevent spindle damage by providing an output to shut down the machine if lubricant temperature reaches a critical point.

According to Karl Klinger, product manager at ifm efector, 'The TN temperature unit is a gauge, switch, and transmitter in one compact, self-contained unit. The sensor's compact size and modular mounting made it simple to mount to the Cross Huller machine's manifold. Setup could be done on site using the device's LED and pushbutton progamming.'

TN sensor can be set up without additional instruments or temperature reference. Its integrated control monitor and probe are said to be highly accurate, and the unit offers high corrosion and thermal resistance. The sensor uses a PT1000 RTD with 1,000 V at 0 °C. Two models are available, dual output (high/low) or switched and analog output with adjustable zero and span.

To boldly go

Using temperature readings as indication of impending problems in mechanical systems has a long history. However with increasing temperature ranges, accuracy, and miniaturization, sensors are being put and monitored in places that they have never been.

Sensing engine malfunctions early can prevent unnecessary maintenance expense and costly downtime. In the case of internal-combustion engines, exhaust gas temperature can be an early indicator of abnormal conditions. However, sensing these temperatures was often left to manual checking and data gathering via handheld pyrometers-not always timely. In cases where automatic scanning could be done using contact sensors, a wide range of temperatures also meant a wide range of sensors.

Reinauer Transportation, a work boat fleet based in Staten Island, N.Y., originally used manual checking for each of 16 cylinders on every workboats. Because the fleet spent most of its time offshore, downtime had to be kept to a minimum. To guarantee accurate monitoring of exhaust gas and other critical parameters, such as oil and coolant temperature, a temperature scanner developed by the Barnant Co. (Barrington, Ill.) was adapted to each craft.

The scanner adapted to the fleet can be used to monitor up to 24 thermocouples including types J, K, T, B, R, S, or E. It features a rugged, lightweight ABS, NEMA-rated case-necessary in the hostile below-deck environment. Accuracy isostic system.'

Temperature sensing and temperature sensing systems may have been slow to evolve but, unlike other technologies that have proven more revolutionary, they have remained relatively uncomplicated and easy to adapt to a wide variety of applications. Old technology is not always in the way.




Is temperature the most-measured process variable or urban legend?

Temperature is the most measured process variable. Is this actually true or is this just that stuff of which urban legends are made?

Eurotherm/Barber-Colman's Dennis Hablewitz, addressed the question of temperature measurement superiority this way. 'It is hard to think of a process or, for that matter, area of industry in which temperature is not measured.' Almost all process industries including refineries, kiln operations, and chemical manufacture include hundreds of temperature control loops. All automotive, truck, and stationary engines require temperature sensing for cooling and lubrication systems. Motion control systems also employ temperature sensing as part of a 'smart' package of equipment 'health' monitoring. Sensors are now embedded in motors and power transmission components alike,' Mr. Hablewitz says. 'I guarantee I could prove that temperature is the number-one measured process variable if I could budget to do the research,' he adds.

Industrial and commercial HVAC control systems also rely heavily on temperature measurement as part of the control process, 'Almost exclusively,' says Bill Dove, technical consultant for Lennox Industries Inc. (Richardson, Tex.). 'Where other sensors, like pressure transducers, do appear in quantity are in the safety systems of these applications,' Mr. Dove adds. According to John Wetter, who works in technical support at Invensys Buildings Systems (Loves Park, Ill.), even though pressure and humidity are measured, temperature makes up 80% of the measurements taken in any HVAC system.

Whether is can be proven or not, the quantity of temperature measurements taken in industrial control situations seem 'staggering.' That's the stuff of which good urban legends are made.

One is the loneliest number(when it comes to zone control)

Most product developers envision applications before engineering on new product development begins. No matter what applications come to mind in the early stages of the design process, introduction of the product to the control community invariably brings other potential applications to the surface.

Integrated circuit manufacturing is one of the more interesting applications for the Love Controls, Div. of Dwyer Instruments Inc. (Michigan City, Ind.), Series 32DZ 1/32-DIN temperature/process controller. Based on original input for potential users, Series 32DZ was created to offer two independent PID temperature control loops with fuzzy logic and the developer's proprietary Self-Tune algorithm in the smallest available panel-mounted package, 1/32 DIN.

TGM Inc. (Richardson, Tex.) is a provider to the semiconductor industry of complete patented turnkey heater jacket systems for vacuum lines downstream from integrated circuit (IC) manufacturing tools. TGM Inc. uses the 32DZ in its Pyewch insulated heater jacket system. The Pyewch system is designed to deliver controlled, continuous, and even heating with no cold spots on vacuum forelines to the pump and exhaust line to the point of collection. Semiconductor manufacturers use this system in the IC manufacturing processes because many exotic and caustic materials often do not efficiently react in the process chamber. This inefficiency requires that the exhaust lines be heated to maintain the vapor state of the partially reacted byproducts until the materials exit the vacuum system where they can be properly abated. Heating the exhaust lines increases device yield, improves process yield, reduces downtime and maintenance costs, and reduces risk of exposure to hazardous materials during exhaust line cleaning.

Richard MacCracken, vice president of research and development for TGM, recognized that the two PID loop capability of the 32DZ could give TGM an advantage in achieving more consistent temperature control in the system. According to Mr. MacCracken, 'TGM can now provide two 1,000-Ohm platinum RTDs' inputs to one controller, and thus control two heaters with independent PID loops. Prior to the availability of this controller, TGM would have to use twice as many controllers or simply settle for using the same number of controllers but averaging the temperature by using one RTD input to control separate heaters. The method of controlling two heaters with a single zone control provides both the stable and uniform heating required.'

Since the piping installations can contain so many heaters along its length, panel space for controllers is at a premium. The 32DZ controller's ability to control two heaters and its small size come into play here. Significant savings in panel space and initial purchase and installation costs can be realized if only half the number of controllers is required per installation.

What you should know about basic contact temperature sensing

There are three commonly used sensors for measuring temperature in the industrial arena. None is considered terribly high tech, since each been around for many years, albeit some much longer than others. Nevertheless, they have all endured as viable methods-depending on the application-for measuring the most often-measured process variable. The three basic sensing elements-in no particular order-are filled systems, thermocouples, and resistance thermometers. Here is how each works.

The filled system is the most basic sensor. Filled system devices consist of a capillary tube that connects a bulb containing temperature-sensitive fluid to an element that is sensitive to pressure or volume changes. This element (either pressure- or volume-sensitive) may be a bellows, a helix, a diaphragm, or a Bourdon tube. The motion of one of these elements -coupled mechanically to an indicating, recording, or controlling device-is what makes this type of sensor work. Types of fluids used in these devices can vary greatly depending on intended application. Systems can be liquid filled (Mercury, organic fluids, etc.) or vapor filled using a volatile liquid.

A thermocouple is an assembly of two dissimilar metals (often wires) joined at the 'hot' end. At the other end, the 'cold' junction, the open circuit voltage is measured. This voltage, often called the Seebeck voltage or EMF-for the German physicist J. T. Seebeck, depends on the difference in temperature between the hot and cold junctions and the Seebeck coefficient of the two metals used. Simply put, a voltage measurement indicates the temperature of the hot junction when either the cold junction temperature is known or when measuring circuitry compensates for the cold junction temperature. Various types of thermocouples are available-Types T, J, K, R, S, etc-using a wide variety of dissimilar materials to handle various temperature ranges. Accuracy varies with application.

Resistance thermometers use a sensing element that has a predictable and stable relationship between its temperature and measured resistance. The specific resistance of the material used (commonly wire or film) must be relatively high so that measuring its resistance is easy. A Wheatstone bridge circuit is attached to the sensor and measures its resistance, translating it to a temperature reading. Just like with thermocouples, materials of the sensor (resistance bulb) are chosen depending on the temperature range of the intended application. Unlike thermocouples, resistance thermometers or RTDs provide excellent stability, repeatability, and sensitivity.

Thermistors are a form of resistance sensor that uses a specially prepared solid-state junction (usually a diode) that will change resistance with temperature. Thermistors are a niche product that produces a large signal over a very narrow temperature range. They are have evolved into stable, accurate sensors. They compete with RTDs in cost.



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