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.Contact Sensors
The three basic types of contact temperature sensors are thermocouples, RTDs, and thermistors.
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.
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
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
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.Noncontact Temperature Sensors
An IR device intercepts heat energy emitted by an object and relates it to the product's known temperature. An IR sensor offers many advantages and can be applied where contact sensors cannot be used. For example, IR
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.Making the Right Selection
When it comes time to select a sensor, consider these questions:
Arthur Volbrecht is a Product Manager at Watlow Gordon, 5710 Kenosha St., Richmond, IL 60071; 815-678-2211, fax 815-678-3961.