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November 2007

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Trends in Process Temperature Measurement
An Evolving Technology Segment Changes Focus to Meet End-User Needs

By Mike Cushing

The process temperature measurement market continues to evolve as the user industries and technology change. There are several key trends to be aware of:
• Shift from thermocouples to RTDs;
• Shift from wire-wound to thin-film RTDs;
• Continued price erosion due to global competition;
• Increased use of transmitters versus direct wiring;
• Increased use of transmitters versus temperature switches in safety applications; and
• Increased availability of integrated assemblies.

The following discusses these trends and the impact they will have on the end-users involved in process temperature measurement.

From Thermocouples to RTDs

Figure 1. The voltage (V) across a thermocouple circuit is proportional to the temperature difference between the hot and cold junctions (V1 and V2, respectively).

Since temperature is the most commonly measured variable in process control, with over five million RTDs (resistance temperature detectors) and thermocouples sold annually, an abundance of sensors and physical construction options has emerged over the years. However, while the options are numerous, the optimum choice for a particular application is not always clear. There continues to be a steady shift in the use of RTDs in lieu of thermocouples. Before discussing the reasons for this trend, it is important to examine the basic theory behind Thermocouples and RTDs.

Thermocouple Operation: The industrial thermocouple is based on the principle that when two dissimilar metals are joined together, an electromotive force (EMF) will exist. If the circuit is closed, a current will flow as a result of the EMF. This principle of operation has made it possible to construct thermocouples capable of measuring a wide range of temperatures using different metal combinations.

Figure 2. Two-, three-, and four-wire units are the most common RTDs on the market today.

A thermocouple circuit has two sides called “junctions.” The process side of the circuit is called the “hot junction,” while the opposite side is called the “cold junction.” The voltage (V) across this circuit is proportional to the temperature difference between the hot and cold junctions (V1 and V2, respectively).

Figure 1 shows how this is implemented with a temperature transmitter. The hot junction is at the thermocouple, and the cold junction is at the transmitter’s terminal strip. The voltage produced at the cold junction is proportional to the temperature difference between the thermocouple’s hot junction and a transmitter’s terminal strip. A reference junction sensor is typically used to measure the temperature of the terminal strip. By measuring the reference junction (V2) and the voltage (V), the temperature at the hot junction (V1) can be determined.

RTD Operation: An RTD is based on the principle that the resistance of a conductor varies with temperature. The most common conductor used in RTDs is platinum, which provides a resistance of 100 Ohms at zero C. Platinum is the most repeatable and stable of all metals. Nickel and copper are also commonly used conductors.

An RTD is connected to a transmitter that supplies a sense current and measures the RTD’s resistance. The sense current is a small current passed through the RTD to generate a voltage that can be read by the transmitter.

There are several types of RTDs available, with the most common being two-, three-, and four-wire RTDs (Figure 2). The three- and four-wire versions provide an additional wire loop to compensate for connection wire resistance and, in turn, improve accuracy.
   • When accuracy is not critical, a two-wire RTD is the least expensive choice. The potential for poor accuracy from a two-wire RTD stems from its inability to compensate for lead-length resistance that changes the ohm value of the original signal. A two-wire RTD should only be used when the receiving device connects directly to the sensor.
   • Three-wire RTDs compensate for resistance resulting from length differences by adding a third lead to the RTD. This requires that the wires match exactly. Any difference in resistance between the lead wires will cause an imbalance, which will compromise the accuracy of the RTD. Lead length variance, work hardening, or corrosion and manufacturing irregularities are errors to avoid. Quality manufacturing is critical to ensure balance of all three leads.
   • Errors caused by resistance imbalance between leads are cancelled out in a four-wire RTD circuit. Four-wire RTDs are used where superior accuracy is critical or if the sensor is installed far from the receiving device. In a four-wire RTD, one pair of wires carries the current through the RTD while the other pair senses the voltage across the RTD. Two- and three-wire RTDs require heavier lead wire, which creates less resistance to the measured signal and thus reduces measurement distortion. Four-wire RTDs, on the other hand, employ less expensive, lighter gauge wire.

RTDs are also defined by their accuracy and are known as Grade A or Grade B performance. A new generation of highly accurate, highly stable Grade A+ RTDs are also emerging on the market. These A+ devices are being used when temperature accuracy and stability are most critical.

RTD vs. Thermocouple Selection
The primary reasons for the shift from thermocouples to RTDs are maintenance, cost, and accuracy. More and more users are discovering the value of the accuracy and stability offered by RTDs exceeds the lower initial cost of a thermocouple.

The two main situations where a thermocouple still makes sense are when the process temperature exceeds the limit of an RTD (1,200 F), or when a fast response is needed. There are some fast-response RTD designs that may also negate this need.

From Wire-Wound to Thin-Film RTDs
Within the RTD category, wire-wound RTDs are the more traditional manufacturing method used. The RTD wires are wound onto a ceramic mandrel. Thin-film elements are etched onto a circuit and can be produced more economically than wire-wound devices. Thin-film technology was originally less reliable than wire-wound, but current-generation thin-film devices have made significant progress in this regard.

Thin-film RTDs are typically limited to temperatures of 500 F, while wire-wound elements can withstand up to 1,200 F. Due to the construction of the sensing element, thin-film RTDs also do not perform as well as wire-wound elements in high-vibration and severe mechanical shock environments.

The shift away from thermocouples is also reinforced by a shift to thin-film RTDs since the cost difference is less than with wire-wound. The benefits of an RTD can now be obtained at a lower price premium than in the past.

It is recommended to use a thin-film RTD whenever possible. Applications where they cannot be used will be limited to high temperatures above 500 F, where high vibration is expected, or where a very fast response is needed. Otherwise, thin-film RTDs are generally recommended.

Price Erosion Due to Global Competition
The very large unit volume of sensors, in excess of five million per year (all with mature technologies), has driven this segment of the market to be as much like a commodity market as exists within the instrument and controls industry. All the thermocouples and RTDs, excluding the thermowell and other accessories, generally sell for less than $100.

Since there are so many sensor and process connection options available — threaded, tapered, straight, or flanged, in any length or material — it has become very difficult to be all things to all people in such an environment and maintain cost and delivery competitiveness. What separates many of the over 200 suppliers of temperature sensor technology is that each operate in a niche based on industry, application, or geography. Also, since the technologies used for process temperature sensors are all mature, there is a low market entry barrier, with only relatively simple welding, machining, and testing capabilities required.

This is not the same for temperature transmitters. There are far less variations necessary to participate in the market, but more technology is needed.   Today, there are transmitters available that support only RTDs, only thermocouples, or both (universal). The only other significant difference (other than input accuracy specifications) is whether the transmitter has one or two inputs and which comm. protocol it uses.

Transmitter vendors try to reduce their unit costs by increasing the production volume. With excess global capacity available, the rules of supply and demand drive the market price down. This has also happened since several vendors have improved the capability of the lower-cost products. End-users, also under cost pressure, can now buy a transmitter for under $250 to accomplish what once cost $700 or $800.

Increased Use of Transmitters vs. Direct Wiring
The majority of process temperature measurements are still wired directly to the control system or recorder in use. This is usually done when the controller is relatively inexpensive and is close to the measurement point. Transmitters are used for several reasons:

1. Cut wiring costs. Standard 4-20 mA wiring is far less expensive than RTD or thermocouple wiring. Test the difference by calculating the distance of the wire runs and the cost of the T/C wire versus standard signal wire plus the transmitter. (The availability of inexpensive head-mounted transmitters reduces the cost hurdle necessary to overcome the wire cost.) The cost of three- or four-wire RTD cable makes the decision easier as well, although a four-wire design allows the use of lighter- grade wire.

2. Protect signals from noise and grounding problems. The benefit here is the added cost of operating with bad input data that causes off-spec product or increased material or energy costs. Transmitter circuits are designed to minimize the impact of noise. This is particularly useful when the temp. measurement is near electrical equipment generating high RFI or EMF levels. The transmitter also provides grounding.

3. Reduce hardware and stocking costs. The DCS or PLC direct temperature input card is usually more expensive per point and sometimes has a lower density (fewer points per card) that can increase the overall I/O cost. Using a universal input transmitter also reduces the variety of transmitters needed in-stock as spares.

4. Enhance accuracy and stability. Transmitters can also be ranged to only view a narrow span (say, 50 F-250 F), while a direct input must be able to process the full range of sensor used. The smaller range provides better resolution and accuracy.

5. Simplify engineering and prevent miswiring. With only standard 4-20 mA input cards on the system, there is less need to group and segregate the temperature inputs. Maintenance of the I/O system is also made easier and fewer spares will be needed.

6. Ease future upgrades. When a temperature measurement has been upgraded from a thermocouple to an RTD, it is much easier to make the change when only a short run of T/C wire needs to be replaced.

Transmitters vs. Temperature Switches in Safety Applications
For safety applications, there is a growing recognition of the value of using a transmitter instead of a switch. This is because a transmitter has a “live zero,” i.e., the 4-20 mA signal range indicates the unit is working at a zero percent output (4 mA). For a switch, the output possibilities are zero or one, with the one value normally indicating the switch trip point has been exceeded. A zero output could result from a good measurement OR a failed switch. This means the information supplied by this measurement is less reliable than that provided by a transmitter. The improved diagnostics available with smart devices makes this position even more obvious. Users are increasingly using the new international standards for designing their safety systems (IEC 61508, IEC 61511, and ISA S84). This methodology is recommended.

Increased Availability of Integrated Assemblies

New integrated designs provide a thermowell, RTD, transmitter, and local LCD display within a single device.

One trend in the market that seems to be good for both suppliers and users is the growing use of integrated assemblies for process temperature measurement. An integrated assembly provides a thermowell, RTD, transmitter, and local LCD display with a single model number, with some models available for less than $300. Flexibility is provided by such units that can be configured locally from the LCD display, and these models are recommended. This reduces cost for both suppliers and users by simplifying order processing and reducing installation costs. Via an integrated assembly, the user gets a fully tested system, calibrated to match the sensor.

Although temperature is arguably the simplest of the primary measurements used today, being aware of the latest options can yield significant savings. It is recommended to use thin-film RTDs for applications below 500 F. Use transmitters to reduce installation costs for long wiring runs and improve signal quality. And, finally, use an integrated temperature assembly whenever possible to reduce purchasing costs.

Mike Cushing earned a bachelor’s degree in Chemical Engineering from the New Jersey Institute of Technology and an MBA from Rutgers Graduate School of Management. He has over 25 years experience in the instrumentation and controls industry in various positions in engineering, sales, product development, marketing, and product management. Mr. Cushing manages pressure and temperature product lines for Siemens Energy and Automation, Inc. He can be reached at
michael.cushing@siemens.com.

www.siemens.com

References
1. “Temperature Measurement, physical principles underlie the four common methods.” Ernest Magison, Intech, November 2001.
2. “So, what is an RTD?” Doris Garvey, Sensors, August 1999.
3. “Choosing RTDs and Thermocouples” Jim Sulciner; Control Engineering, February, 1999.
4. “Hot Issue: RTDs versus Thermocouples”; Robert Waterbury, Intech, March 1994.
5. Technical Reference Manual, Smart Sensors, Inc.

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