While many of the earlier chapters of this volume have explored the physics and technology behind non-contact temperature measurement, now it's time to delve into the wide array of products that are available to take advantage of radiation phenomena--and how they're applied to industrial use.
Non-contact temperature sensors allow engineers to obtain accurate temperature measurements in applications where it is impossible or very difficult to use any other kind of sensor. In some cases, this is because the application itself literally destroys a contact-type sensor, such as when using a thermocouple or resistance temperature detector to measure molten metal. If the electrical interference is intense, such as in induction heating, the electromagnetic field surrounding the object will cause inaccurate results in conventional sensors. A remote infrared sensor is immune to both problems.
For maintenance, no other sensor is able to provide long-distance, non-contact temperature measurements needed to find hot spots or trouble areas in distillation columns, vessels, insulation, pipes, motors or transformers. As a maintenance and troubleshooting tool, it's difficult to beat a hand-held radiation thermometer.
Although non-contact temperature sensors vary widely in price, they include the same basic components: collecting optics, lens, spectral filter and detector. For more detailed technical information on each sensor type, see the previous chapters.
The user can select among non-contact temperature sensors that operate over just about any desired wavelength range, both wide and narrow. Radiation thermometer sensitivity varies inversely proportionally with wavelength. Therefore, an instrument operating at 5 microns only has one-fifth the sensitivity of an instrument operating at 1 micron. This also means that optical noise and uncertainties in emissivity will result in measurement errors five times greater in the long wavelength instrument.
Radiation thermometer optics are usually the fixed focus type, although designs with through-the-lens focusing are available for measuring over longer distances. Fixed focus devices can also be used to measure at long distances if the target area is smaller than the lens diameter in the optical system.
|Table 8-1: Strengths and Weaknesses of Non-Contact Temperature Sensors
||Inexpensive (from $99)
No measurement drift
Plugs into standard thermocouple
display and control devices
Reaches into inaccessible areas
Susceptible to EMI
|Portable and convenient
Inexpensive (from $235)
Excellent maintenance tool
|Maximum probe cable length of 1 m
|Can focus on any target at almost
Portable or fixed-place operation
Camera-like operation (point and shoot)
Low to medium cost (from $350)
|Measures only a fixed spot on target
Accuracy affected by smoke, dust, etc.
in line of sight
Affected by EMI
||Works in hostile, high-temperature, vacuum
or inaccessible locations
Can bypass opaque barriers to reach target
Unaffected by EMI
|Fairly expensive ($1600-$2600)
||Sees through smoke, dust and other
contaminants in line of sight
Independent of target emissivity
|Fairly expensive (from $3600 for sensor,
and $5000 for display/controller)
||Only sensor that makes full-width
temperature measurements across product
Measures continuously as product passes by
Computer can produce thermographic
images of entire product and its
|Very expensive (from $10,000 for sensor
alone, $50,000 for complete system)
Non-contact temperature sensors range from relatively inexpensive infrared thermocouples, priced from about $99, to sophisticated, computer-based $50,000 linescanners. In between is a wide variety of hand-held and permanently mounted measuring systems that meet just about any temperature monitoring need imaginable.
|Typical fiber optic probe, transmiter, and bench top display
An infrared thermocouple is an unpowered, low-cost sensor that measures surface temperature of materials without contact. It can be directly installed on conventional thermocouple controllers, transmitters and digital readout devices as if it were a replacement thermocouple. An infrared thermocouple can be installed in a fixed, permanent location, or used with a hand-held probe.
Because it is self-powered, it relies on the incoming infrared radiation to produce a signal via thermoelectric effects. Therefore, its output follows the rules of radiation thermal physics, and is subject to nonlinearities. But over a given range of temperatures, the output is sufficiently linear that the signal can be interchanged with a conventional thermocouple.
Although each infrared thermocouple is designed to operate in a specific region, it can be used outside that region by calibrating the readout device accordingly.
Radiation thermometers, or pyrometers, as they are sometimes called come in a variety of configurations. One option is a handheld display/control unit, plus an attached probe. The operator points the probe at the object being measured--sometimes getting within a fraction of an inch of the surface--and reads the temperature on the digital display. These devices are ideal for making point temperature measurements on circuit boards, bearings, motors, steam traps or any other device that can be reached with the probe. The inexpensive devices are self-contained and run off battery power.
Other radiation thermometers are hand-held or mounted devices that include a lens similar to a 35mm camera. They can be focused on any close or distant object, and will take an average temperature measurement of the "spot" on the target that fits into its field of view.
Handheld radiation thermometers are widely used for maintenance and troubleshooting, because a technician can carry one around easily, focus it on any object in the plant, and take instant temperature readings of anything from molten metals to frozen foods.
When mounted in a fixed position, radiation thermometers are often used to monitor the manufacturing of glass, textiles, thin-film plastic and similar products, or processes such as tempering, annealing, sealing, bending and laminating.
Fiber Optics Extensions
When the object to be measured is not in the line of sight of a radiation thermometer, a fiber optic sensor can be used. The sensor includes a tip, lens, fiber optic cable, and a remote monitor unit mounted up to 30 ft away. The sensor can be placed in high energy fields, ambient temperatures up to 800°F, vacuum, or in otherwise inaccessible locations inside closed areas.
For use in applications where the target may be obscured by dust, smoke or similar contaminants, or changing emissions as in "pouring metals," a two-color or ratio radiation thermometer is ideal. It measures temperature independently of emissivity. Systems are available with fiber optic sensors, or can be based on a fixed or hand-held configurations.
A linescanner provides a "picture" of the surface temperatures across a moving product, such as metal slabs, glass, textiles, coiled metal or plastics. It includes a lens, a rotating mirror that scans across the lens' field of view, a detector that takes readings as the mirror rotates, and a computer system to process the data.
As the mirror rotates, the line scanner takes multiple measurements across the entire surface, obtaining a full-width temperature profile of the product. As the product moves forward under the sensor, successive scans provide a profile of the entire product, from edge to edge and from beginning to end.
|Hand-held IR thermometers include such options as laser sights.
The computer converts the profile into a thermographic image of the product, using various colors to represent temperatures, or it can produce a "map" of the product. The 50 or so measurement points across the width can be arranged in zones, averaged, and used to control upstream devices, such as webs, cooling systems, injectors or coating systems.
Linescanners can be extremely expensive, but they offer one of the only solutions for obtaining a complete temperature profile or image of a moving product.
Portable vs. Mounted
Non-contact temperature measurement devices also can be classified as portable or permanently mounted. Fixed mount thermometers are generally installed in a location to continuously monitor a process. They often operate on AC line power, and are aimed at a single point. Measured data can be viewed on a local or remote display, and an output signal (analog or digital) can be provided for use elsewhere in the control loop. Fixed mount systems generally consist of a housing containing the optics system and detector, connected by cable to a remote mounted electronics/display unit. In some loop-powered designs, all the thermometer components and electronics are contained in a single housing; the same two wires used to power the thermometer also carry the 4 to 20 mA output signal.
Battery powered, hand-held "pistol" radiation thermometers typically have the same features as permanently mounted devices, but without the output signal capability. Portable units are typically used in maintenance, diagnostics, quality control, and spot measurements of critical processes.
Portable devices include pyrometers, thermometers and two-color systems. Their only practical application limit is the same as a human operator; i.e., the sensors will function in any ambient temperature or environmental condition where a human can work, typically 32-120°F (0-50°C).
At temperature extremes, where an operator wears protective clothing, it may be wise to similarly protect the instrument. In shirt-sleeve manufacturing or process control applications, hand-held instruments can be used without worrying about the temperature and humidity, but care should be taken to avoid sources of high electrical noise. Induction furnaces, motor starters, large relays and similar devices that generate EMI can affect the readings of a portable sensor.
Portable non-contact sensors are widely used for maintenance and troubleshooting. Applications vary from up-close testing of printed circuit boards, motors, bearings, steam traps and injection molding machines, to checking temperatures remotely in building insulation, piping, electrical panels, transformers, furnace tubes and manufacturing and process control plants.
|Figure 8-1: Ambient Effects on IR Thermometer Accuracy
Because an infrared device measures temperature in a "spot" defined by its field of view, proper aiming can become critical. Low-end pyrometers have optional LED aiming beams, and higher end thermometers have optional laser pointing devices to help properly position the sensor.
Permanently mounted devices are generally installed on a manufacturing or process control line, and output their temperature signals to a control or data acquisition system. Radiation thermometers, two-color sensors, fiber optics, infrared thermocouples, and linescanners can all be permanently mounted.
In a permanent installation, an instrument can be very carefully aimed at the target, adjusted for the exact emissivity, tuned for response time and span, connected to a remote device such as an indicator, controller, recorder or computer, and protected from the environment. Once installed and checked out, such an instrument can run indefinitely, requiring only periodic maintenance to clean its lenses.
Instruments designed for permanent installation are generally more rugged than lab or portable instruments, and have completely different outputs. In general, systems that operate near a process are ruggedized, have NEMA and ISO industrial-rated enclosures, and output standard process control signals such as 4-20 mA dc, thermocouple mV signals, 0-5 Vdc, or serial RS232C.
For very hot or dirty environments, instruments can be equipped with water or thermoelectric cooling to keep the electronics cool, and nitrogen or shop air purging systems to keep lenses clean.
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