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issue: February 2007 APPLIANCE Magazine

Sensors
Choosing a Temperature Sensor


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by Merle Tingelstad, global marketing manager for Flexible Circuits and the Medical Implant Industry, Minco

The following provides an overview of how to select the appropriate temperature sensor for a specific engineering application.

When selecting a temperature sensor for an application, there are many variables to consider, such as the sensor’s proposed location in the application and the temperature it will be used to measure.
Of all environmental parameters, temperature is the most commonly measured. While our general concept of temperature is clear, the science behind temperature is less understood and can be challenging to apply.
Temperature is the measured amount of heat in an object linked to its average kinetic energy. An increase in kinetic energy in a system is observed as an increase in the heat.
The transfer of kinetic energy between objects can occur through:

Conduction—direct thermal contact between objects
Convection—transfer to a fluid medium, such as liquid or gas, creating current that flows to another object
Radiation—objects emit rays or waves of particles that strike and are absorbed by another object

The different types of temperature sensors draw upon these methods of transferring kinetic energy. Several options are described below and are outlined in Table 1.

Contact Thermometers

The common methods of measuring temperature involve contact between the sensor and the object being measured. Every contact sensor is only capable of measuring its own temperature and must rely on the thermodynamic principle of achieving equilibrium with the object to be measured in order to infer the temperature of the object. Most temperatures being measured in real applications are not static (heat is constantly being added or taken away from parts of the object), not isothermal (temperatures are different across the object), and equilibrium is never really achieved. Outside of controlled laboratory environments, temperature measurement is often a snapshot of a localized condition.
The type of sensor used and its location in the system can have a significant impact on the ability to measure the temperature of greatest interest:

- A sensor construction with the lowest internal mass and the least thermal resistance between the sensing medium and the object measured will provide the most responsive tracking if the system temperature is changing rapidly.

- A very small sensor construction will be capable of sensing a point of interest, but not capable of providing a representation of the average temperature of the system being measured in dynamic environments. Temperature averaging is often desirable.

- If the mass of the sensor is large relative to the object being measured, achieving equilibrium will shift the energy state of the object being measured.

- Sensors mounted to an object’s surface may be interacting with external environmental factors and steps should be taken to minimize the interaction.

- Lead wires and sheaths that connect to the sensing element are a conductive path to the external environment. In slower responding sensors with little penetration into the object being measured, these paths can impact the measured temperature, particularly when the gradient is large between the system and environment.

- The most responsive constructions may not provide sufficient ruggedness and protection from environmental conditions (i.e., thermowells) and metal cases may be required for strength. Insulations are needed for electrical isolation of elements.

- Larger mass sensors, particularly those with insulating materials that impede heat transfer, will lag the change in temperature in the system, becoming more pronounced if temperature swings of the system are faster.

- It may be desirable to deliberately slow down the sensor response to eliminate false alarms in systems that may be subject to sudden environmental changes with slow impact on the system. For example, doors opening and closing in cold storage bring in warm air that has little impact on items stored, but if a door remains open for too long, you would want to sound an alarm before any food items perish.

Non-Contact Thermometers

Non-contact thermometers are useful for measuring beyond the temperature limits in which contact thermometers can operate reliably, or when measurements need to be made remotely due to distance of separation. Other uses include instances when there is movement of the sensed system relative to the monitoring location, or when fast response measurements of low mass systems are required.
Various infrared (IR) sensing systems use optics to focus the radiation on photoreceptors sensitive in the IR spectrum. The object being sensed should be larger than the field of view of the optics. Otherwise, other background radiation will be sensed and can create error. The field of view can be selected in the instrument to be narrow for small objects, distant objects or when only an isolated measurement is wanted within the system. A wide field of view is selected for large or close measurements and to represent the best average temperature value within the system. Typical recommendations are that the object be 50 percent larger than the field of view of the optics to provide a good average temperature.
Fiber optic sensors can use a glass fiber for long distance temperature readings and can actually have multiple fiber Bragg gratings along the same fiber. Each grating location may be exposed to different temperature points in the system and transmit each as a unique temperature measurement back to the control center through the glass fiber.

Thermocouples

A thermocouple consists of two wires of dissimilar metals joined together into a junction (welded, brazed, soldered, or twisted). At the other end of the sensor wires, usually terminated in the reading instrument, is another junction called the reference junction. Heating or cooling the sensing junction generates a thermoelectric potential (millivolt-level emf) proportional to the temperature difference between the two junctions. The reference junction is normally within an isothermal block containing a calibrated sensor (usually RTD or thermistor) to determine the actual reference junction temperature.
Published millivolt tables assume the reference junction is at 0°C but are valid for any other reference junction temperatures, if the reference junction differential from 0°C is added to the table temperature. Voltage polarity indicates whether the sensing junction is at a higher or lower temperature than the reference junction. Sensing junction temperature is a simple addition of the reference junction temperature to the differential temperature represented by the emf. For example, if the reference junction sensor indicates it is at 25°C and the millivolt readings indicate the sensing junction is 15°C cooler, the instrument would indicate the temperature as 10°C.
A thermocouple is naturally a point sensor at its junctions. When it is desirable to average temperature over a surface, the thermocouple will average the output across multiple junctions of the same metals added in parallel or in a thermopile network.
Thermocouples are simple and familiar. Designing them into systems, however, is complicated, as they require special extension wires and reference junction compensation. Thermocouple junctions other than precious metal wires are generally low cost, making them advantageous for some disposable sensing applications where the sensing material is sacrificed. Fused wire junctions having a single electrical connection in the sensing tip are inherently rugged and resistant to shock and vibration damage. Thermocouples can be used reliably at much higher temperatures than RTDs, thermistors or IC sensors, making them the preferred contact sensor for higher temperatures, particularly those higher than 850°C.
Thermocouples work best when distances between the sensing junction and the instrument are small, as cost to deploy them in longer runs becomes significant. They are commonly found in applications within the chemical/petrochemical, metals, process control, food/beverage, rotating equipment, and semiconductor equipment markets.

Resistance Temperature Detectors

An RTD sensing element consists of a wire coil or deposited film of pure metal. The element’s resistance increases with temperature in a known and repeatable manner. RTDs exhibit excellent accuracy over a wide temperature range. The platinum resistance thermometer is the primary interpolation instrument for temperatures defined by the ITS-90 international standard from -260°C to 962°C.  Even ordinary industrial RTDs typically drift less than 0.1°C per year. Their repeatability, stability and wide temperature span are key factors in their wide adoption for use as industrial temperature sensors.
RTDs can be any metal or metal alloy conductor with a repeatable change in resistance. Many early commercialized RTDs employed copper, nickel or nickel-iron alloy fine wires for their low cost and ease of manufacture. Each of these was suitable over limited temperature spans that would not oxidize the fine element wires. Platinum wire elements were employed in higher end applications requiring extended temperature capability or laboratory performance. Wire-element RTDs are most stable when the element wires are larger in diameter because they are less susceptible to effects of oxidation or the mechanical strains of packaging. On the other hand, it is desirable to have the element wires very fine to increase the resistance (more sensitivity in ohms/degree) and keep cost of precious metal like platinum to a reasonable level.
RTD constructions vary widely at the simplest element level to complex packages. The most stable and accurate laboratory platinum resistance thermometers (PRTs) use element constructions of the purest platinum wire wound into coils that are strain-free (free to expand and contract with temperature without affecting their supporting structure) and surrounded with inert gas. This type of construction provides the widest temperature capability, but is quite expensive, fairly delicate and susceptible to shifting with mishandling.
The positioning of a sensor can greatly impact your process. Heaters or changing thermal loads impact the temperature of the system, requiring proper sensor positioning to best characterize the system. A sensor mounted very intimately with a heater will respond quickly to changes in the heater to prevent overshoot, but may not achieve the desired temperature in the process. A sensor that is too isolated from the heat source may be measuring the wrong part of the process and will not adequately control the heater cycles, subjecting the system to wide temperature swings. It may be that no single point is the best representation of the system temperature and that an averaging ability is needed.
RTDs are a common choice for applications in chemical/petrochemical, process control, pharmaceutical, rotating equipment, critical environment HVAC/R, aircraft, and automotive markets.

Thermistors

A thermistor is a thermally sensitive resistor device consisting of metal oxides formed into a bead, rod or disc and encapsulated in epoxy or glass. A typical thermistor shows a large negative temperature coefficient of resistance. Resistance drops dramatically and non-linearly with temperature. Sensitivity is many times that of RTDs, but useful temperature range is limited to generally 150°C (300°C for some designs). Some manufacturers offer thermistors with positive coefficients. Linearized models are also available.
Thermistors have many material families and base resistances that generate a multitude of different resistance versus temperature curves. There are wide variations of performance and price between thermistors from different sources. One of the difficulties that users of thermistors must face is that it is difficult to exactly match the curves of one source to those of other suppliers, although in most cases other suppliers can provide something close.
Basic thermistors are quite inexpensive. However, models with tighter interchangeability or extended temperature ranges often cost more than equivalent spec RTDs. A thermistor may change resistance by tens of ohms per degree temperature change, versus a fraction of an ohm for many RTDs. This allows practical use of a two-wire connection for measuring temperature at longer distances. A thermistor bead can be made the size of a pinhead for small area sensing and fast response measurement. Thermistors also are available in small surface-mount configurations for placement on printed circuits.
The lowest cost thermistors are coated with epoxy to seal the metal oxides from the environment. This is acceptable for many applications, but the best hermetic seal is a glass coating. Glass-coated thermistors are much less susceptible to shifting the resistance of the metal oxides due to environment; however, it is important to exercise care in the installation of any thermistor bead. If the hermetical seal is broken through damage to the epoxy or glass at the lead exit, it will be subject to moisture intrusion and shifting of resistance.
Thermistors work best when one is trying to control or measure temperature within a narrow span. Thermistors are commonly found in applications within automotive, telecommunications, medical devices, HVAC/R, appliances, computing, and aerospace markets.

IC Sensors

Integrated circuit (IC) sensors are manufactured in silicon chip form where semiconductor diodes exhibit a voltage to current relationship that is temperature sensitive. Because these are manufactured as ICs, they are manufactured with various integrated signal processing or signal conditioning circuits in the chip. With the versatility of on-board signal processing, the outputs of IC sensors may be generated as analog millivolt (scaled 10.0 mV/°C or 10.0 mV/°F), analog current (10 mA/°C), logic (“on” or “off” state) or digital (8- to 16-bit data packets representing temperature). The signal processing also ensures that the output is linear with temperature change.
IC sensors are fairly low cost, especially when the cost of associated signal conditioning circuits is added to other contact sensing technologies. They are available with pins for placement into probe cases or surface-mount configurations for packaging on printed circuit boards.
IC sensors have a limited operating temperature range and are generally not able to operate above 150°C. They are not as accurate as RTDs or thermistors, typically ±1.0°C at calibration temperature. Because the IC sensor contains an active electronic circuit that is generating heat, an offset is built in to attempt to compensate for self-heating errors, but this is not consistent across the temperature range and all environments. Self heating can become more of an issue when ICs are built into plastic chip packages or when they are assembled into insulated probe packages.
IC sensors are commonly found within computing, cellular communications equipment, medical devices, and industrial controls markets.

About the Author: Merle Tingelstad is Minco’s global marketing manager for Flexible Circuits and the Medical Implant Industry. He has more than 30 years of experience in electronic engineering and has a B.A. in Psychology from the University of Minnesota. If you would like to contact Tingelstad, e-mail editor@appliance.com.

Non-Electronic Thermometers

If all that is needed is a temperature indication or record that a temperature has been reached, there are many more forms of temperature sensing. Among these are labels, crayons, pellets, and varnishes that can be applied to the object.
When the correct temperature limit is achieved, materials undergo a visible phase change (melting). Crayons, pellets and varnish change physical appearance. Printing and coloring of labels will appear when the overcoat melts and absorbs into the underlying label material. Liquid-in-glass thermometers push fluid from a reservoir up and down a calibrated channel in the glass. Liquid crystals align to reflect different light spectra as temperatures move through their sensitive zone, going from black to red to green to blue and back to black. Filled systems push a dial gauge through a calibrated arc as the temperature rises or falls. Bimetals utilize the differential expansion of bonded metal strips to move a dial through a calibrated arc related to temperature. Each of these provides a temperature indication without the need for line power or batteries.

 

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