Contact Sensors: The Business End of Temperature Measurement

This is the third installment of a five-part series on Process Sensing. Pressure sensing appeared in March. Smart sensors appeared in May. Other articles include flow measurement (September) and level sensing (November).Unsung. Unremarked. Easily forgotten but definitely indispensable. They come in a myriad of sizes and packages.

By Dick Johnson, Control Engineering June 1, 1998
  • Process control & instrumentation

  • Temperature sensing and measurement

  • Transmitters

Sensor matching boosts performance
Why use direct wire temperature sensors?

This is the third installment of a five-part series on Process Sensing. Pressure sensing appeared in March. Smart sensors appeared in May. Other articles include flow measurement (September) and level sensing (November).

Unsung. Unremarked. Easily forgotten but definitely indispensable. They come in a myriad of sizes and packages. And they are often tucked away in a process penetration that defies access by all but the most dexterous instrumentation technician. If it can be said that temperature is the most measured process variable, then temperature sensors must be the most ubiquitous sensor. And as process industries multiply globally, the type can only increase its numbers.

Attesting to this fact, a newly published report titled World Temperature Sensor Market based on strategic research by Frost & Sullivan (Mountain View, Calif.) states, “Although the market for temperature sensors has been around a long time, several drivers continue to expand the marketplace, including the ongoing industrialization in underdeveloped countries, the alleviation of trade barriers, and the advancement of smart technology. The world market for temperature sensors is growing at moderate to high rates depending on the technology at hand. Currently it is valued at $1.3 billion, with an annual growth rate of over nine percent.”

The report continues, “The trend toward tighter control and measurement processes has, in general, assisted growth in many markets particularly in the United States, Europe, and Japan. However, developing nations in the Pacific Rim and Latin America are likely to experience annual growth rates five to eight percent higher than these markets.” Temperature sensor manufacturers, not satisfied with simply meeting demand for product, have introduced a wide variety of innovations to this humble device.

Little things mean a lot

Despite overall size of many process industry installations, areas where temperatures must be recorded are often inconveniently located. Although their locations in the original process layout may have been well thought out, thermowells can be become “buried” over the years as existing equipment is upgraded or new systems are layered over the old. The result can lead to technicians working in cramped, hard to reach, or elevated locations. These may also be located out-of-doors where exposure to weather or dangerous conditions such as smoke or fumes can be a problem.

In these situations, instrument technicians often depend on any advantage that a specific sensor can afford them. Convenience of installation can be a big factor when choosing one sensor over another of similar type or performance rating. Omega Engineering Inc. (Stamford, Conn.) has introduced several sensors intended to ease field installation. HTTC Series thermocouples (T/Cs) and the PR-10 Series sheathed resistance temperature detectors (RTDs) both feature reduced size and lead bending radius which allows them to be easily mounted in confined spaces.

The HTTP Series features hollow tube construction; J, K, T, E, N type T/C calibrations; Teflon-insulated lead wire epoxy potted into a stainless-steel sheath; a 450

Weed Instrument (Round Rock, Tex.) has addressed installation convenience by supplying both a line of Break-To-Length T/Cs and RTDs that can be trimmed in the field to any insertion length manually—no tools required. RTDs are available in all standard resistance curves. Thermocouple versions are available in types J, K, T, and E with grounded and ungrounded versions.

When accuracy is required

According to Frost & Sullivan’s research, RTDs have displaced T/Cs and other temperature sensors in a variety of areas requiring more accuracy. Specific examples of this abound. High accuracy and low drift, verified by the manufacturer’s in-house quality assurance testing, allows Burns Engineering’s (Minnetonka, Minn.) Series 200 wirewound RTDs to qualify for exacting pharmaceutical manufacturing operations. This specific sensor makes use of very stable coil-wound element suspended in a proprietary ceramic insulator.

According to a spokesperson for Alliance Pharmaceutical Corp. (San Diego, Calif.), Burns’ RTDs have been chosen for “one of the most critical applications, product sterilizers. Since Alliance’s products are injected into humans, it is imperative that they be sterilized under the strictest of conditions. One of the sterilizers uses three RTDs to measure steam, cooling water, and vent temperatures. All parameters are critical in ensuring that the product is sterile.”

Finding the right contact sensor for an application can involve tedious matching of hardware to specs. Because application requirements often go mismatched in the “standard” catalog, manufacturers have added custom sensor capability to their off-the-shelf lines. Total Temperature Instrumentation (TTI, Williston, Vt.) provides 800 number-access to application engineers who can recommend and custom-make sensor assemblies according to equipment and applications needs. Thermocouple elements are available from TTI in both base and noble metal types which can be sheathed in either metal alloy, ceramic, or composite protection tubes. RTDs are available for special applications in the plastics, rubber, and packaging industries.

Working with special needs of a specific industry often leads to development of unique sensors. Watlow Gordon’s (St. Louis, Mo.) RF probe thermocouple was originally developed to withstand effects of radio frequency (RF) energy that energizes chemical vapor deposition plasma used in the making of semiconductor devices and integrated circuits.

To accurately measure the high temperatures encountered while reducing noise susceptibility, Watlow Gorden developed a probe consisting of Chromel and Alumel alloys insulated using hard-fired ceramic. The purpose-designed tip provides thermal transfer to an electrically isolated sensor to prevent interference with conducted noise. Additionally, use of compensating alloy and twisted leadwires and a band-reject filter provides enough filtering and noise immunity to accurately control the temperature-dependent deposition process and raise wafer yields.

RTDs to the rescue

Keeping track of temperature often requires that a sensor be used in less than ideal conditions. Many electrochemical processes, such as printed circuit board etching and metal plating, require bath solutions to be accurately controlled. These solutions are often aggressive chemicals and immersion of the device is necessary.

To “harden” an RTD to withstand the rigors of a chemical bath, RdF Corp. (Hobson, N.H.) has developed a device that features a molded Teflon sensor with wide spectrum chemical sealing up to 260 °C. The device is available with a either a two or three flat wire design that fits through a 1.4 mm gap and is flexible for surface mounting. The 100 ohm and 1,000 ohm International IEC 751 or 1,000 ohm industrial grade sensors are refractory sealed for

When linear output vs. temperature is desired in an RTD and the application requires a really small sensor, thin-film RTDs can provide both. Once much more expensive than wire-wound devices, improvements in manufacturing technology have helped make thin-film devices competitive.

According to Honeywell Micro Switch (Freeport, Ill.), HEL-700 is the world’s smallest thin-film platinum RTD. The device, which measures 0.06 x 0.03 in. and is surface mountable, measures temperatures for–200 to 540 °C. The device, suitable for process control applications, is manufactured by depositing a thin layer of platinum on an alumina substrate and is laser trimmed to a resistance interchangeability of ±0.2%. Standard accuracy is the greater of ±0.5 °C or 0.8% of temperature.

Get my drift?

Thermocouples age after installation and drift at the beginning can be several degrees. The reason for this is the change in crystal structure of the thermocouple to a more stable condition. Drift, especially of this magnitude, can be detrimental to many process applications. On the other hand, T/Cs’ wide temperature range and signal resolution capabilities make them suitable for a wide range of industrial use.

To supply K-type sensors in this stable configuration and eliminate on-site aging and recalibration requirements, Bailey-Fischer & Porter’s Sensycon Div. (Warminster, Pa.) provides users with a mineral isolated K-type sensor that leaves the factory in stable condition. According to the company, the result is no drift after installation.

RTDs doing unusual jobs

Even though they can, not all RTDs are used as process temperature sensors. Heraeus Sensor’s (Philadelphia, Pa.) Microheater is a platinum RTD designed to operate at 200 mA or higher so a self heating effect takes over. The device is designed to heat up to a maximum temperature of 500 °C. This tiny heater features 10-20 msec response time, low power drain, and long-time stability.

According to its manufacturer, Microheater has applications as diverse as analytical and diagnostic equipment, air and fluid flow applications, and gas detection. Simple pump control, such as checking for “primed” condition before starting, can be done by measuring the time it takes to heat up a device placed inside a pump chamber. Time to heat up to a given temperature will be longer in liquid than in air.

Another unique application for a stand-alone RTD is sensing liquid flow speeds. According to the manufacturer, Kobold Instrument’s (Bedford, Tex.) KAL-A Series thermal flow transmitter is the only one to do this with a single RTD. All other designs require two RTDs, one heated and one reference.

KAL-A is rated for 1,450 psig and 176 °F. The standard device has a 1/2-NPT fitting and a nylon housing. The device features a 4-20 mA output and optional alarm. This “smart” RTD can be used in 1/2- to 20-in. dia. pipes and is available with an aluminum explosion-proof housing for use in hazardous areas. Because the device relates measured fluid temperature and probe cooling rate to fluid flow speed, its output signal can be used to provide temperature and flow rate. Navistar foundry (Milwaukee, Wis.) presently uses these devices to monitor the cooling circuits in its furnaces.

Improvements in materials, manufacturing techniques, and packaging have boosted overall contact sensor performance. And in the face of competition from other more complex sensing methods, this basic technology has held its own in an expanding process industry. Sometimes simple is better. .

Contact Temperature Sensors

Company T/C RTD Semiconductor Thermistor Other Circle No.
Bailey Fischer & Porter x x 202
Burns Engineering x x 203
Fisher Rosemount x x 204
Heraeus Sensor x 205
Honeywell Micro Switch x 206
Kobold Instruments x x 207
Omega Engineering x x x x x 208
RdF Corp. x 209
Total Temperature
Instrumentation x x x 210
Watlow Gordon x x x x 211
Weed Instrument x x 212

Sensor matching boosts performance

The single largest contributor to error in temperature measurement is the sensor itself. Progress on the electronics “front” of transmitters, and to a lesser extent of the input cards of control systems (DCSs and PLCs), have reduced errors due to ambient temperature effects, drift, and reference accuracy. In general, the error introduced by a sensor is larger than all other error sources combined.

According to Rosemount Measurement Div.’s (Eden Prairie, Minn.) Jose Rivera, temperature sensors product marketing manager, sensors are manufactured to meet industry standards that allow reasonable tolerance bands around an ideal sensor curve. For high accuracy applications, however, assuming that an actual sensor curve matches the “ideal” introduces an uncertainty, known as sensor interchangeability error. When an application requires the tightest measurement possible (lowest total probable error), any uncertainty must be eliminated. In these situations, sensor matching is required.

In the case of RTD technology, matching requires plotting device resistance change vs intended process temperature change under laboratory conditions to determine its Calender-vanDusen polynomial constants (R o plus three other constants). Resulting values identify the curve fitted to the actual data points.

Once determined, these constants can be used by software-configurable control systems or transmitters to compensate for the actual RTD’s deviation from the ideal curve and match sensor performance. If sensors must be replaced, sensor matching capability can reduce typical interchangeability error (1-6 °F) by as much as 75%.

Use of sensor matching technology translates directly into production cost savings. Mr. Rivera cites an example of a 100,000 bbl/day catalytic cracking process in which a 5 °F lower-than-optimal operating temperature reduced gasoline yield by 0.5 percent while increasing fuel oil yield by the same amount. Using matched sensors to reduce interchangeability error at changeout can keep the process running at optimum. With the current gasoline/fuel oil price diffential, loss of temperature optimization could result in as much as $380,000 in lost profits annually.

Why use direct wire temperature sensors?

On the surface, direct wiring of temperature sensors to a DCS or PLC looks cheaper and easier. But in the long run (especially with long sensor lead-wire runs) it may not be. Use of temperature transmitters to “shepherd” sensor signals along offers a number of advantages. According to Lori Risse, applications specialist for Moore Industries-International Inc. (Sepulveda, Calif.), when properly employed, transmitters can:

Protect signals for plant noise—transmitters convert a sensor’s “weak”low-level signal to a high-level RFI/EMI resistant signal (typically 4-20 mA) that will accurately withstand long distance transmission through a noisy plant.

Prevent ground loops—a transmitter’s built-in input/output/power signal isolation protects against signal inaccuracies caused by ground loops. Additionally, inexpensive 4-20 mA DCS and PLC input cards can be used instead of isolated thermocouple and RTD input cards.

Enhance measurement accuracy—DCS and PLC systems measure readings over the entire (very wide) range of a sensor. Temperature transmitters can be calibrated to any range within a sensor’s capabilities. Using a narrower range produces more accurate measurements.

Reduce hardware costs and stocking requirements—transmitters convert RTD, T/C, mV, and ohm signals to a standard 4-20 mA output allowing users can standardize on (and stock) inexpensive 4-20 mA DCS and PLC input cards.

Cut wiring costs—fragile sensor extension wires cost three times more than the common shielded copper wire used for a temperature transmitter’s 4-20 mA signal.

Avoid lead wire resistance imbalances—temperature transmitters that accept true 4-wire RTD inputs avoid lead wire resistance imbalances resulting from wire corrosion and aging.

Match the best sensor to the application—today’s universal temperature transmitters (PC-programmable and smart) take nearly any RTD and thermocouple input. This lets users specify whatever sensor is best for each particular process point. Direct DCS inputs aren’t nearly as flexible because they handle only one type of sensor input per card.

Lower maintenance time and expense—with “smart” transmitters users can remotely track sensor operation and find sensor failure.

Simplify engineering and maintenance and prevent miswiring—instead of numerous sensor lead-wire and DCS/PLC input board combinations, control design drawings need to show only a single wire (twisted pair) and input board type (4-20 mA). Simplified systems require easier maintenance and a lesser chance of loop miswiring.