Temperature measurement basics

Tips from Omega Engineering Inc., Stamford, Conn.

Temperature can be measured using a diverse array of sensors, all of which infer temperature by sensing some change in a physical characteristic. Six types that engineers are likely to encounter include thermocouples, resistive temperature devices (RTDs and thermistors), infrared radiators, bimetallic devices, liquid expansion devices, and change-of-state devices. Here we briefly define each type:

Thermocouple temperature measurement sensors

Thermocouples consist essentially of two strips or wires made of different metals and joined at one end. Changes in the temperature at that juncture induce a change in electromotive force (emf) between the other ends. As temperature goes up, the output emf of the thermocouple rises, though not necessarily linearly.

Resistive temperature devices (RTDs)

Resistive temperature devices capitalize on the fact that the electrical resistance of materials changes with temperature. Two key types are metallic devices (commonly referred to as RTDs), and thermistors. As their name indicates, RTDs rely on resistance change in a metal, with the resistance rising more or less linearly with temperature. Thermistors are based on resistance change in a ceramic semiconductor; the resistance drops nonlinearly with temperature rise.

Infrared devices

Infrared sensors are non-contacting devices. They infer temperature by measuring the thermal radiation emitted by a material.

Bimetallic devices

Bimetallic devices take advantage of the difference in rate of thermal expansion between different metals. Strips of two metals are bonded together. When heated, one side expands more than the other, and the resulting bending is translated into a temperature reading by mechanical linkage to a pointer. These devices are portable and they do not require a power supply, but they are usually not as accurate as thermocouples or RTDs, and they don't readily lend themselves to temperature recording.

Fluid-expansion devices

Fluid-expansion devices, typified by the household thermometer, are generally one of two types — mercury or organic liquid. Versions employing gas instead of liquid are also available. Mercury is considered an environmental hazard, so there are regulations governing the shipment of devices that contain it. Fluid-expansion sensors do not require electric power, do not pose explosion hazards, and are stable even after repeated cycling. On the other hand, they do not generate data that is easily recorded or transmitted, and they cannot make spot or point measurements.

Change-of-state devices

Change-of-state temperature sensors consist of labels, pellets, crayons, lacquers, or liquid crystals whose appearance changes once a certain temperature is reached. They are used, for example, with steam traps; when a trap exceeds a certain temperature, a white dot on a sensor label attached to the trap turns black. Response time typically takes minutes, so these devices often do not respond to transient temperature changes and accuracy is lower than with other types of sensors. Furthermore, the change in state is irreversible, except in the case of liquid-crystal displays. Even so, change-of-state sensors can be handy when one needs confirmation that the temperature of a piece of equipment or a material has not exceeded a certain level, for example, for technical or legal reasons during product shipment.

Infrared probe devices

One of the most effective methods of making non-contact, high-temperature measurements in industrial applications is with a fiber optic infrared sensing device. This typically consists of a lens probe assembly that is aimed at the object to be measured, and a fiber optic interconnecting cable connected to an electronics package and the transmitter, which makes the temperature measurement and converts it into a useable output signal. Let's examine each of these components:

The probe assembly consists of a housing specifically designed for the conditions to which it will be exposed, a lens or an optical rod to collect infrared radiation from the target, and an optical fiber interface for connection to the fiber optic cable. The probe is usually placed within a few inches of the object being measured. Because of this, construction of the probe assembly can vary dramatically. For measurements in an open-air environment, it can be a simple metal cylinder. However, it is not uncommon for these devices to be used in very harsh environments, such as high-temperature chambers, under vacuum, in corrosive atmospheres, or immersed in molten plastic. These applications require specialized probes with threaded housings, ceramic or other durable construction, non-glass lenses, and even glass or quartz optical rods (tips).

The fiber optic interconnecting cable acts as a waveguide to bring the radiation to the infrared detector assembly in the electronics package. The quality of the fiber optic interfaces at each end is critical to overall system accuracy and repeatability. Because the signal is transmitted optically, it is immune to the often-substantial electrical and magnetic interference found in industrial settings.

The electronics package does the work of converting the infrared radiation delivered by the fiber optic cable into a temperature reading or a signal proportional to the temperature. It may include enhancements such as high and low temperature alarms, various output options, and even a computer interface connection.

Applications for non-contact temperature measurement

Although thermocouples are the most common temperature measurement devices in process control, they have limitations. They must be in contact with the measured object, have a slow response time, and are subject to electrical and magnetic interference. Fiber-optic infrared transmitters overcome these issues, but are generally limited to reading temperatures above 100° C. Why? Fiber optic cable cannot transmit infrared energy below a certain wavelength; this depends on the cross-section of the fiber optic strands and their optical properties. Following are some typical applications:

Annealing processes

The critical surface temperature of the metal can be monitored directly while it is inside an oven, rather than indirectly by measuring the ambient oven temperature.

Induction heating of metal

The strong RF field used can heat up conventional heating devices and interfere with their electronics; fiber optics are immune to RF fields.

Plastic extrusion and injection molding

Precise control of the melt temperature is essential for polymer formation. An infrared reading eliminates errors that are common for thermocouple-based devices immersed in the plastic flow.

Temperature monitoring of drill bits

For high-speed PC board drilling, wear can be determined by optically monitoring drill bit temperature.

Semiconductor doping, deposition, or sputtering

Because these processes are usually carried out in a vacuum or controlled gas atmosphere using induction heating, conventional temperature measurement devices cannot be used.

Any high-temperature application where a direct measurement of the part temperature is critical to success is a good candidate for non-contact temperature measurement.

Copyright Omega Engineering Inc. All rights reserved. Reproduced with the permission of Omega Engineering Inc., Stamford, Conn. 06907 USA. www.omega.com.

Demystifying resistance thermometry

Tips from Minco, Minneapolis

Understanding resistance thermometry helps engineers get accurate readings from the two kinds of devices that leverage this technology.

Resistance thermometers

These thermometers consist of a metallic element in which resistance increases with temperature. Benefits include accuracy and stability. Their designs range from helical-wound thermometers for laboratory use to industrial thermometers made to conform to surfaces being monitored.

Resistance thermometers are sometimes called RTDs (resistance temperature detectors), PRTs (platinum resistance thermometers), or SPRTs (standard platinum resistance thermometers). These thermometers operate on the principle that electrical resistance changes in pure metal elements, relative to temperature. The traditional sensing element consists of a small-diameter wire coil wound to a precise resistance. The most common material is platinum, although nickel, copper, and nickel-iron alloys are used in many applications.

A more recent alternative to the wire-wound RTD substitutes a thin film of platinum, which is deposited on a ceramic substrate and trimmed to the desired resistance. Thin film elements attain high resistances with less metal, thereby lowering cost.

Resistance thermometers offer the greatest benefits relative to other thermometer types in the following situations:

  • Accuracy and stability are the foremost goals of the application

  • Accuracy must extend over a wide temperature range

  • Area, rather than point, sensing improves control

  • A high degree of standardization is desirable

Resistance thermistors

These thermistors consist of a semiconductor material in which resistance decreases as temperature increases. Key benefits of thermistors are high-resolution measurements over limited ranges and low cost. Instead of tracking resistance in metals, thermistors track it in semiconductors. The base material is a mixture of metal oxides pressed into a bead, rod, disk, wafer, or other shape. The bead, with embedded leadwires, is sintered at high temperatures and often coated with epoxy or glass. Beads may be quite small, down to 0.01 in. diameter in some cases.

The design, construction, and characteristics of thermistors vary widely among manufacturers.

Typical properties of thermistors are as follows:

  1. Thermistors exhibit very large resistance changes, but usually in a direction opposite to resistance thermometers; resistance drops as temperature rises. This is called negative temperature coefficient of resistance (NTC).

  2. Base resistances, commonly specified at 25° C, range from thousands to millions of ohms. Thermistor sensitivity dwarfs that of resistance thermometers.

  3. Resistance/temperature curves deviate widely from linearity, except over narrow ranges.

  4. Thermistors tend to drift more than resistance thermometers, although they stabilize over time.

  5. Temperature ranges are moderate, with 300° C as the common upper limit.

The combination of nonlinearity, high sensitivity, and instability has generally limited thermistors to high-resolution measurements over limited ranges. A classic example is medical thermometry. Physicians are only concerned with a small range around 98.6° F and thermistors can be chosen to provide a large, fairly linear signal in this area. One-point calibration is simple and sufficient.

Some manufacturers of thermistors offer special models with these characteristics:

  • Positive temperature coefficient (PTC) models, which are used more for current limiting in electronic circuits than for temperature measurement; as current increases through the bead, self-heating drives up resistance dramatically, throttling the current.

  • Thermistors that are interchangeable over a specified range.

  • Linearized thermistors or thermistor sets that produce a highly linear output over a limited range.

  • Glass-coated specially aged thermistors that maintain high stability over moderate temperatures — for example, ± .005° C per year between 0 and 100° C.

As a general rule, a thermistor acts as a single-purpose sensing device. The designer must relate temperature ranges, resistance/temperature characteristics, and output circuits for each application. The reward: high resolution and accuracy from a relatively inexpensive system. Thermistors offer the greatest benefit when:

  • The application requires high resolution over a narrow span

  • Probes are disposable, or require frequent and easy recalibration

  • Low cost is a primary consideration

  • The application is an OEM device produced in sufficient volume to justify the design of special linearizing circuits

  • Point sensing or miniaturization is preferred for the application

The superior sensitivity and stability of resistance thermometers and thermistors, in comparison to thermocouples, gives them important advantages in low and intermediate temperature ranges. In addition, resistive devices often simplify control and readout electronics.

Resistance thermometers are specified primarily for accuracy and stability from cryogenic levels to the melting points of metals. They are accurate over a wide temperature range, may be used to sense temperature over a large area, and are highly standardized.

While normally less stable than resistance thermometers, thermistors offer lower cost and higher sensitivity over limited ranges. They are also relatively small and suitable for equipment applications where quantities of use justify the design of special readout circuits.

Finally, thermistor designs abound, so can be chosen to resist electrical noise, vibration, self-heating, leadwire resistance, and out-of-range temperature extremes.
Information courtesy of Minco, www.minco.com

A few FAQs about temperature sensing

Q: How can I choose between thermocouples, resistance temperature detectors (RTDs), thermistors, and infrared devices when measuring temperature?

A: Consider the characteristics and costs of the various sensors, as well as the available instrumentation. In addition: Thermocouples generally can measure temperatures over wide temperature ranges, inexpensively, and are very rugged, but not as accurate or stable as RTDs and thermistors. Conversely, RTDs are stable and have a fairly wide temperature range, but are not as rugged and inexpensive as thermocouples. Because they use electric current to make measurements, RTDs are subject to inaccuracies from self-heating.

Thermistors tend to be more accurate than RTDs or thermocouples, but they have a much more limited temperature range. They are also subject to self heating.

Infrared sensors can be used to measure temperatures higher than any other devices and do so without direct contact with the surfaces being measured. Using fiber optic cables, they can measure surfaces that are not within a direct line of sight. However, they are generally not as accurate, and are sensitive to surface radiation efficiency — more precisely, surface emissivity.

Q: What are the two most often overlooked considerations in selecting an infrared temperature-measuring device?

A: The surface being measured must fill the field of view, and the surface emissivity must be considered.

Q: What are the best ways to overcome electrical noise problems?

A: Use low-noise, shielded leads, connectors, and probes. Use instruments and connectors that suppress EMI and RF radiation. Consider using analog signal transmitters, especially current transmitters. Finally, evaluate the possibility of using digitized signals.

Q: If a part is moving, can I still measure temperature?

A: Yes. Use infrared devices or direct contacting sensors plus a slip ring assembly.

Information courtesy of Omega Engineering Inc., www.omega.com

Data loggers track paranormal activity

In recent years, interest in ghost hunting has increased. Television shows such as Ghost Hunters, Most Haunted, and Paranormal State have intrigued many viewers, and curiosity about the equipment used in these programs has grown. Confidential Paranormal Investigators (CPI), a Wisconsin-based team of ghost hunters, is just one example of paranormal researchers using modern-day technology to document and debunk paranormal activity. To ensure the integrity of their research, the CPI team relies on a variety of tools including temperature data loggers, which are used to monitor temperature fluctuations during an investigation.

Walt Baker is the technical manager and founder of CPI: “When it comes to paranormal investigations, we don't know what we're seeking, so we use a range of tools to provide background environmental data. We use technology to provide us with evidence that can support or demystify any paranormal experiences.”

During investigations, Baker and his team employ HOBO data loggers from Onset Computer Corp., Bourne, Mass. The loggers are mounted at different heights to detect any fluctuations in temperature.

“Non-contact IR thermometers are used to find the base temperatures, along with natural explanations for temperature differences, like a drafty window,” says Baker. “However, the thermometers are useless for locating columns of varying temperature, like a cold spot. For that, we use data loggers to measure and record the ambient temperature.”

During an investigation, each logger takes temperature measurements every second. “The loggers give us a date and time stamp,” explains Baker. “If someone on our team has a personal experience, or if there is a spike on the Electromagnetic Field (EMF) meter, we go back and check the dataset at that specific time to see if there was indeed a temperature change during the event, and then decide whether or not there is a correlation.”

After an investigation, collected data is downloaded onto a PC and analyzed using Onset's HOBOware Pro graphing and analysis software. The software features easy logger launch and readout functions, powerful data plotting capabilities, and an intuitive graphical user interface.

Baker emphasizes the importance of substantiating paranormal activity with environmental data. “First, we look at the data we've collected to see if anything significant sticks out,” he states. “For example, during one investigation, we caught an object on video; later, we examined concurrent EMF spikes and temperature changes.”

According to Baker, the video footage determined that the speed of one object captured on film was traveling at 60 miles per hour. “After reviewing the video, we were able to dismiss objects like bugs and dust,” he says. “The finding was even more interesting because the EMF field spike was not accompanied by temperature fluctuation.”

Baker concludes, “There is no ghost meter. We are using this equipment to correlate logged information with other techniques.”

Information courtesy of Onset Computer Corp., www.onsetcomp.com.