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1 June 2005

Temperature price-points make cents

Wire is hot. Wireless is not.

By Ravi Jethra

No matter the type of power plant (coal-fired, oil, or gas-based), temperature measurement remains high on the list for operational excellence throughout the plant.

Implementation of some of the new technologies results in improved safety and lower installation and maintenance costs. Incorrect measurement information due to temperature effects, non-linearity, or stability can result in damage to major equipment.

Ensuring that instruments have minimal downtime, which in turn ensures instruments that provide safety integrity level (SIL) service in safety instrumented systems (SIS), is crucial.

Resistance with temperature

A resistance temperature detector (RTD) is a device that contains an electrical resistance source (referred to as a "sensing element" or "bulb"), which changes electrical resistance value depending on its temperature. This change of resistance with temperature is directly translatable to the temperature of a process or of a material.

RTD sensing elements come in two basic styles—wire wound and film. The measuring circuit consists of a combination of lead wires, connectors, terminal boards, and measuring or control instrumentation. The exact makeup of the measurement circuit is dependent upon:

• Temperature in the sensing area as well as the environmental conditions expected to exist between the sensor and instru- mentation.
• Distance between the sensor and instrumentation.
• Type of interconnections the customer prefers.
• Type of wiring system currently in place (if not new).

An RTD with two-wire construction is the least accurate of the three types since there is no way of eliminating the lead wire resistance from the sensor measurement. Two-wire RTDs work best with short lead wires or where close accuracy is not required.

One sees three-wire construction most commonly in industrial plant applications where the third wire provides a method for removing the average lead-wire resistance from the sensor measurement. When long distances exist between the sensor and measurement/control instrument, significant savings arise when using a three-wire cable instead of a four-wire cable.

Four-wire construction is primarily at work in the laboratory where close accuracy is required. In a four-wire RTD, the actual resistance of the lead wires can be determined and removed from the sensor measurement.

Although RTDs usually spec out as 100-ohm platinum sensors, other resistance's (200-ohm, 500-ohm, 1000-ohm, and higher) and materials (nickel, copper, and nickel-iron) are available. Since RTDs are resistors, they will produce heat when a current passes through them. The normal current limit for industrial RTDs is one mA. Thin film RTDs are more susceptible to self-heating, so usage should not exceed one mA. Wire wound RTDs can dissipate more heat so they can withstand more than one mA. The larger the sheath or the more insulation there is, the better chance there will be an error caused by self-heating.

Temperature coefficient for RTDs is the ratio of the resistance change per one degree change in temperature over a range of zero-100°C. This ratio is dependent on the type and purity of the material used to manufacture the element. Most RTDs have a positive temperature coefficient—the resistance increases with an increase in temperature.

Ranges of 1000 to 2700°F

Base metal thermocouples label as Types E, J, K, T, and N. These five comprise the most commonly used category of thermocouple. The conductor materials in base metal thermocouples are made of common and inexpensive metals such as nickel, copper, and iron.

The Type E thermocouple has a Chromel (nickel-10%, chromium) positive leg and a Constantan (nickel- 45%, copper) negative leg. Type E has a temperature range of -330 to 1600°F, has the highest EMF (electro motive force or voltage) versus temperature values of all the commonly used thermocouples, and works fine at sub-zero temperatures. Type E thermocouples function fine in oxidizing, or inert atmospheres are not for use in sulfurous atmospheres, in a vacuum, or in low oxygen environments where selective oxidation will occur.

The Type J thermocouple has an iron positive leg and a Constantan negative leg. They can be used in vacuum, oxidizing, reducing, and inert atmospheres. Due to the oxidation (rusting) problems associated with the iron leg, care is necessary when using this thermocouple type in oxidizing environments above 1,000°F. The temperature range for Type J is 32 to 1,400°F.

The Type K thermocouple has a Chromel positive leg and an Alumel (nickel- 5%, aluminum, and silicon) negative leg. Type K is for use in oxidizing and completely inert environments. The temperature range for Type K is -330 to 2,300°F.

The Type N thermocouple has a Nicrosil (nickel-14%, chromium-1.5%, and silicon) positive leg and a Nisil (nickel- 4.5%, silicon-.1%, and magnesium) negative leg. Type N is very similar to Type K but less susceptible to selective oxidation effects. Type N doesn't work well in a vacuum or in reducing atmospheres in an unsheathed condition. The temperature range is 32 to 2,300°F.

The Type T thermocouple has a copper positive leg and a Constantan negative leg. Type T thermocouples can be used in oxidizing, reducing, or inert atmospheres, except the copper leg restricts their use in air or oxidizing environments to 700°F or below. The temperature range for Type T is -330 to 700°F.

Types E, J, and T find widest use at temperatures above 1,000°F. Type K, like Type E, should not install in sulfurous atmospheres, in a vacuum, or in low oxygen environments where selective oxidation will occur.

Noble Metal Thermocouples are another category of thermocouples and are made of the expensive precious metals platinum and rhodium. There are three types of noble metal thermocouples: Type B (platinum/platinum-30% rhodium), Type R (platinum/platinum-13% rhodium), and Type S (platinum/platinum-10% rhodium).

Types R and S have temperature ranges of 1,000 to 2,700°F, and Type B thermocouples have a temperature range of 32 to 3100°F.

RTDs, thermocouples widely

RTDs have certain strengths and are good for applications where accuracy and repeatability are important. Common instrumentation wire couples the RTD to the measurement and control equipment making them more economical to install as compared to thermocouples, which must use special extension wire, much like the composition of the thermocouple itself, to extend the wiring to the control equipment.

RTDs also have weaknesses. An RTD in the same physical configuration as a thermocouple will typically be three to seven times the cost. RTDs are more sensitive to vibration and shock than a thermocouple and are limited to temperatures of approximately 800°F.

A thermocouple works at temperatures as high as 3,100° F. They generally cost less than RTDs and are smaller. Thermocouples will respond faster to temperature changes and are more durable allowing use in high vibration and shock applications. These are its strengths.

As to thermocouple weaknesses—they are less stable than RTDs when exposed to moderate or high temperature conditions. Thermocouple extension wire is necessary when hooking up thermocouple sensors to measurement instruments.

Both RTDs and thermocouples are widely used in power plant temperature measurement. Each has its advantages and disadvantages. The application will determine which sensing element is best suited for the job. An RTD will provide higher accuracy and more stability than thermocouples. They also use standard instrumentation wire to couple the sensor to the measurement device.

Thermocouples are less expensive than RTDs, are more durable in high vibration and mechanical shock applications, and tolerate higher temperatures than RTDs. They can be smaller than RTDs, generally, and they are adaptable to fit various and specific applications.

Over half of the temperature applications in the U.S., and most often in power plants, involve the direct wiring of a temperature sensor to the controls system. Despite the large installed base of direct-wired sensors, the trend is toward using transmitters in conjunction with temperature sensors.

Use transmitters over wire

For temperature measurement, engineers must decide whether they wire the sensors directly to the control system (PLC, DCS, and recording system) or if they use transmitters. Today, many engineers still wire direct because they mistakenly believe this is a cheaper and easier solution. The reality however, is different: transmitters allow an engineer to save time and money, improve the measurement reliability, and facilitate maintenance.

Why use transmitters? If you do not use a transmitter, you need sensor extension wires to the control system for a precise temperature measurement. These wires are expensive and sometimes fragile. By using a transmitter, you only need inexpensive copper wires. The greater the distance between the sensor and the control system, the more money the company saves. For applications using a Pt100 four-wire, only a pair of wires needs to run from the transmitter to the control system.

There are four typical wiring setups:

• Using extension wires to the transmitter
• Using compensation wires to the transmitter
• Using the thermocouple wires to the head-mounted transmitter
• Wiring directly to the control system

Extension wires are manufactured as stranded or solid conductors with various insulating materials and armoring. The conductors (the flexible strands or solid wires) consist of substitute materials. When a relatively flexible cable is required, flexible conductors are used. These conductor materials and the corresponding thermocouples have the same nominal structure and chemical composition.

Compensation cables are manufactured as solid conductors with various insulating materials and armoring. The conductors (the flexible strands or solid wires) are made of substitute materials, and therefore their chemical composition differs from the corresponding thermocouple material. Different alloys suffice for the same thermocouple type. The substitute material and the corresponding thermocouple have the same thermoelectric characteristics within the allowed temperature range.

Let's look at what distance it becomes economically sound to use transmitters over wire by looking at the numbers for a Type K thermocouple.

Assume these prices:

• Extension wires (twisted and shielded): $1.15/foot
• Compensation wires (twisted and shielded): $0.78/foot
• Copper connection wires: $0.10/foot

The breakeven price point using extension wires breaks out like this:

• PC Programmable DIN-rail transmitter (A): $175 Breakeven at 130 feet
• HART head transmitter: $245 (B) Breakeven at 200 feet
• HART field transmitter: $670 (C) Breakeven at 500 feet

The breakeven points leveraging compensation wires:

• PC Programmable DIN-rail transmitter: $135 Breakeven at 270 feet
• HART head transmitter: $205 Breakeven at 410 feet
• HART field transmitter: $510 Breakeven at 1,020 feet

Besides certain savings in wiring costs, other advantages exist that networking and communications engineers should consider.

Eliminate plant noise: Electromagnetic Interference (EMI) and radio frequency interference (RFI) are present in almost all types of plants, not just power plants. Their effects on the extension wires are important and obviously affect the measured value. By using transmitters, the temperature measurement is immune to EMI/RFI problems. EMC compliance to IEC61326 is necessary for use in noisy environments.

Make maintenance easier/advanced diagnostics: One can save long and unnecessary trips to the field. The smart diagnostics capabilities of the sensor indicate (via HART and upscale/downscale output signals) if the sensor is broken or if there is corrosion on the sensor input loop.

Increase accuracy: Temperature transmitters not only accept RTD inputs with two, three, or four wires. There are over two dozen different types of RTDs or thermocouples that can connect to a transmitter without the need for special programming.

Reduce control systems costs: If the system wires directly to the control system, one needs several different input cards for different sensor types.

• Price for a four-channel RTD input card: $399
• Price for a four-channel TC input card: $399
• Price for a four-channel 4-20mA Analog input card: $225

This also makes things simpler for power engineers since only one input type would be used (same 4-20mA card for flow, pressure, level ... inputs).

What about routine maintenance? You can switch from a thermocouple to a Pt100; simply reconfigure the transmitter, and send the output to the control system.

Allow sensor flexibility: If there is a need for a new sensor type, then just replace the existing unit and use the same transmitter. Transmitters accept universal inputs (12 different thermocouple types, six different RTD, mV, and Ohms). One can switch to another sensing element without worrying about the installation and wiring changes.

Avoid ground loops: In applications where fast response time is needed, customers use grounded thermocouples. This thermocouple type may cause a ground loop. This is avoidable by using transmitters with superior galvanic isolation (up to 2kV galvanic isolation). CP

Behind the byline

Ravi Jethra is a senior member of ISA. Has an engineering degree and an MBA. This article comes from his recent paper presented at the 15th Annual Joint ISA POWID/EPRI Controls and Instrumentation Conference, 48th ISA POWID Symposium.