Temperature Measurement: Making Sense of It All This primer spells out the basics of the primary technologies at your disposal. With this information, choosing the right sensor should be easier. Arthur Volbrecht, Watlow Gordon

Photo 1. What temperature sensor should you use? Should it be a contact or noncontact device? What process, temperature range, and environment will you be working in? What response time and accuracy does your application require? There's no shortage of questions. What you need are answers. This primer examines thermocouple, RTD, thermistor, and IR technologies.

Selecting the right temperature sensor depends on the process being measured, the temperature range stipulated, the response time desired, the accuracy required, and the operating environment encountered. Another important factor to consider is price, which varies with the accuracy rate and the mounting style of the device.

Temperature sensors generate output signals in one of two ways: through a change in output voltage or through a change in resistance of the sensor's electrical circuit. Thermocouples and IR devices generate voltage output signals. RTDs and thermistors output signals via a change in resistance.

There are two methods of temperature sensing: contact and noncontact. Contact sensing brings the sensor in physical contact with the substance or object being measured; you can use this approach with solids, liquids, or gases. Noncontact sensing reads temperature by intercepting a portion of the electromagnetic energy emitted by an object or substance and detecting its intensity; you can apply this technology to solids and liquids.

To figure out which method you should use, just follow this rule of thumb: if the object or medium being heated moves, has an irregular shape, or would be contaminated by contact with a sensor, then you should use IR sensing.

  Thermocouples. These sensors have the widest operating range and are best suited for high temperatures. Thermocouples of noble metal alloys can be used for monitoring and controlling temperatures as high as 3100ºF. These devices are also best for applications requiring miniature sensor designs. The inherent simplicity of the devices enables them to withstand extreme shock and vibration. Thermocouples can be configured in small sizes to offer near-immediate response to temperature changes.

Thermocouple Advantages Work in high temperatures Rugged Fast response

A thermocouple is formed by joining two dissimilar metals. Proper alloy selection results in a measurable and predictable voltage-to-temperature relationship. One of the most common misunderstandings concerning thermocouples is where the voltage is generated. Many assume the voltage is generated at the connection point of the dissimilar metals. In reality, the output voltage is generated along its length as it passes through a temperature variation. The thermocouple voltage is the difference between the thermoelectric energies of the chosen metals as measured at the instrumentation connection. This predictable voltage can then be related to actual process temperature.

Thermocouples come in a variety of shapes and sizes. Here are the most common types.

Insulated wire products. These special wire-formed metal alloys are covered with insulating material, which provides both physical and electrical isolation between thermocouple wire alloys. The insulating materials are operative in temperatures as high as 2300ºF. This kind of thermocouple is cost-effective for short-term measurements. Junctions and instrumentation connections are easily fabricated on site.

Mineral-insulated metal-sheathed thermocouples. Specialty thermocouple alloys are encased in a metal tube containing magnesium oxide for electrical isolation. These are general-purpose products suitable for measuring many liquids, solids, or gases. A large variety of metal coverings is available to protect the thermocouple alloys from corrosive environments. The devices have a long life in applications involving rapid temperature cycling.

Protected-element thermocouples. An assortment of formed metal and ceramic tubes are used to protect the thermocouple sensing element from harsh process conditions. These thermocouples are long-lasting and durable, and you can replace them without shutting down the process.

  RTDs. These are precision temperature-sensing devices. They're the ones to use when applications require accuracy, long-term electrical

RTD Advantages Wide temperature range Repeatability and stability High output Linearity Lower system wiring cost Area averaging

(resistance) stability, element linearity, and repeatability. The devices can work in a wide temperature range—some platinum sensors handle temperatures from ­328ºF to 1202ºF.

The sensing element in RTDs is typically a fine platinum wire winding or thin metallic layer applied to a ceramic substrate. The platinum resistance thermometer is the primary interpolation instrument used by the National Bureau of Standards in applications with operating temperature ranges from ­436ºF to 1135ºF.

Precision thermometers can be manufactured with stability of 0.0025ºC per year. However, industrial models typically drift < 0.1ºC per year. RTDs with platinum and copper elements follow a more linear curve than thermocouples or most thermistors. Unlike a thermocouple, an RTD uses copper wire products for instrument connection and requires no cold junction compensation. As a result, system cost is often lower.

Although point measurements are often desirable, they can cause errors. An RTD element can be spread over a large area, improving control with area averaging, an impractical technique with thermocouples. The voltage drop across an RTD provides a much larger signal than thermocouple voltage output.

The drawbacks to this sensing technology are slower response time (due to large element size), sensitivity to shock and vibration, small resistance change (low sensitivity) for temperature variations, and low base resistance. Low base resistance and small resistance change for corresponding temperature change become a concern when long lead lengths are required because the leads create additional resistance. When added to the resistance of the RTD element, the lead resistance can result in measurement errors. To overcome lead-length problems, you should use 3- or 4-wire lead circuitry; this allows the effect of a bridge circuit to measure the resistance change based on temperature. Wire-length errors are minimized because the resistance change occurs at the RTD sensing point. Accuracy of the measurement is primarily dependent on the accuracy of the signal conditioning circuit in the controller or measuring device.

  Thermistors. These sensors are sensitive to small temperature changes. These devices are best for low-temperature applications over limited ranges. The element is small-thermistor beads can be the size of a pinhead

Thermistor Advantages Lower sensor cost High resistance Interchangeability Small enough for point sensing

for point sensing—and tends to become more stable with use. Depending on the grade and price of the thermistor, performance can range anywhere from low accuracy to accuracy matching high-end RTDs. The thermistor is a responsive sensor that permits control of a process variable to within one-half of one degree or better. Most thermistors in their basic form cost much less than RTDs. When provided with protective sheaths, the price difference narrows.

Base resistance can be several thousand ohms. This provides a larger voltage change than RTDs with the same measuring current, negating leadwire resistance problems. You must be careful, though, to limit measuring current because small thermistors are more susceptible to self-heating than RTDs. Many newer thermistor models are trimmed to tight tolerances over limited temperature ranges, but they are priced accordingly.

The drawbacks you will encounter when using thermistors are the result of the sensors' fragile nature, limited temperature span, initial element drift, and decalibration at higher temperatures. Thermistors are generally interchangeable and, unless additional instrument circuitry is added, will not provide a fail-safe condition if the element should open. Thermistors also do not have the same level of established industry standards as thermocouples and RTDs.

IR Advantages Can sense the temperature of a moving object Will not draw heat away from, contaminate, or deface the product Don't require slip rings Can be isolated from dangerous environments

Some IR sensors can also be interfaced with special IR temperature controls. These provide a closed-loop, noncontact temperature-control system with options for serial data communications and data logging.

• Does the application require contact or noncontact temperature sensing?
• How accurate must the temperature reading be?
• What temperature range is involved?
• What's the maximum temperature the sensor will be exposed to?
• How fast must the sensor respond to a temperature change and deliver an accurate reading?
• How long should the stability and accuracy of the sensor last?
• What environmental restraints exist?
• Are protective devices—such as thermowells and protection tubes—required to provide sufficient ruggedness?
• What's the budget?

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