Principles of Infrared Technology - PDF by jov15779


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									Principles of Infrared
                                                                         How do they measure temperature?
                                                                         All IRt/c’s have a proprietary infrared
IRt/c INFRARED                                                           detection system which receives the
THERMOCOUPLES –                                                          heat energy radiated from objects the
                                                                         sensor is aimed at, and converts the
                                                                         heat passively to an electrical potential.
TECHNOLOGY                                                               A millivolt signal is produced, which is
                                                                         scaled to the desired thermocouple
The IRt/c product line represents a                                      characteristics.
dramatic breakthrough in temperature
sensing technology. The IRt/c sensors                                    Since all IRt/c’s are self-powered
are unpowered, low cost, and can                                         devices, and rely only on the incoming
measure surface temperatures of                                          infrared radiation to produce the signal
materials without touching. They can                                     through thermoelectric effects, the
be directly installed on conventional                                    signal will follow the rules of radiation
thermocouple controllers, PLCs,                                          thermal physics, and be subject to the
transmitters, and other readout devices.                                 non-linearities inherent in the process.

                                                                         However, over a range of temperatures,
                                                                         the IRt/c output is sufficiently linear
                                         Actual IR/tc Signal             to produce a signal which can be
             millivolt                                                   interchanged directly for a conventional
             output                                                      t/c signal. For example, specifying a
                                                                         2% match to t/c linearity results in a
                                                                         temperature range in which the IRt/c                      OS36 Series
                                                                         will produce a signal within 2% of the

                                                                         conventional t/c operating over that
                                              Target Temperature
                                                                         range. Specifying 5% will produce a
            The actual signal generated by the IRt/c can                 somewhat wider range, etc.
            be approximated with a fourth order polynomial

            function of target temperature. This fourth power
            dependence is due to radiation physics, and                                                             Linear Range ***
            not a limitation of the IRt/c.                                                                  2%                            5%
                                         Conventional thermocouple                                                                             > 5%

      millivolt                          Linear region

                                                                                                                                         Actual IR/tc Signal
                                                Target Temperature
            The linear region matches the conventional
            t/c to a specified tolerance.                                                                            Target Temperature

                                                                                                            Temperature Selection Guide
                               90.00                                                                                                    Linear Range ***

                                                                                                   yy yy
                               80.00                                                                                          2%                      5%
                                                                                                                                                             > 5%
 millivolts of signal output

                                                                          Sensor - Type - Temperature         0°       50°       100°       150°      200°          250°    300° C

                               50.00                                         *   -   **   -   220C/440F
                                                                             *   -   **   -   170C/340F
                               40.00                                         *   -   **   -   140C/280F
                               30.00                                         *   -   **   -   120C/240F

                               20.00                                         *   -   **   -     90C/180F
                                                                             *   -   **   -     60C/140F
                                                                             *   -   **   -      27C/80F
                                0.00                                         *   -   **   -      10C/50F
                      –10.00                                                 *   -   **   -    37C/98.6F    Special Biomedical Calibration ±0.2 C (35.5-39.4), ±0.3 C (25-40C)
                           –50          50    150 250 350 450 550 650
                                                                                                       0°           100°       200°         300°    400°             500°     600° F
                                             Actual Temperature (C)           * Select OS36, OS36-2, or OS36-5                        Target Temperature
                                                                             ** Select Type J, K, E, T
                               The OS36-K-80 has its 2% linear range        *** Temperature Range in which IRt/c output is linear
                               centered at 80°F (27°C), but produces a          compared to conventional t/c, within stated % (of reading).
                               repeatable signal to 1200°F (650°C).

Principles of Infrared Thermocouples Cont'd

Each IRt/c model is specifically                        Long term accuracy is influenced by the      percentage of failure, the IRt/c has
designed for optimum performance                        same things that influence repeatability:    essentially unlimited long term
in the region of best linear fit with                   mechanical changes and metallurgical         calibration accuracy.
conventional t/c’s, but can be used                     changes. It is well known that
outside of those ranges by simply                       thermocouples can change calibration         QUICK INSTALLATION GUIDE
calibrating the readout device                          over time due to these effects.
appropriately. The output signal is                     Mechanical changes occur because             All infrared-based sensing systems
smooth and continuous over its entire                   conventional thermocouples are               must be calibrated for specific material
rated temperature range, and maintains                  constructed generally as small and           surface properties (for example, the
1% repeatability over its entire range.                 light as possible to enhance response        amount of heat radiated from the target
                                                        time, thus making them vulnerable            surface, environmental heat reflections,
The Temperature Selection Guide                         to deformations that can change              etc.). This calibration is performed by
is a summary of the linear range                        the thermoelectric properties. More          measuring the target surface temperature
performance of each IRt/c model. The                    importantly, the conventional                with a reliable independent surface
user selects the IRt/c model and type,                  thermocouple must operate at                 temperature probe. The easiest and
and the target temperature range for                    elevated temperature since it merely         fastest method of accurately calibrating
the application. The normal offset                      measures its own temperature.                out these effects is to use an OMEGA
adjustments on the thermocouple                                                                      OS91 hand-held Infrared Thermometer
readout device are used to calibrate                                                                 with a patented Automatic Emissivity
the installation for emissivity and                                                                  Compensation System to give a true
background effects.                                                                                  reading regardless of emissivity.

How reliable are these new devices?                                                                  The following procedure is recommended:
Of fundamental interest in temperature                                                               1. Install IRt/c as close as practical to
control is the ability of the measuring                                                              view the target material to be measured.
device to maintain its calibration under
service conditions, and over a long                                                                  2. Wire IRt/c to controller, PLC
period of time. The IRt/c is rated at                                                                Transmitter, etc. in standard fashion
1% (of reading) repeatability and to                           The IRt/c gives repeatable static     (including shield). As in conventional
have no measurable long term                                       and dynamic readings.
                                                                                                     t/c’s, red wire is always (–).
calibration change, which makes it well
suited for reliable temperature control.                The metallurgical changes which affect       3. Bring process up to normal operating
These attributes are inherent in the                    thermoelectric properties are a strong       temperature and measure actual
basic design and construction of each                   function of temperature, being negligible    temperature of target material with
IRt/c.                                                  at room temperature, and of serious          OS90 Series Infrared Thermometer.
                                                        concern at high temperature.
                                                                                                     4. Adjust “input offset,” “zero,”
                                                        The IRt/c solves both problems by            “low cal,” on the readout device to
                                                        its design and basic operation. Its          match the OS91 reading.
                                                        solid fully potted construction in a
                                                        mechanically rigid stainless steel           Installation complete.
                                                        housing, and operation at near room
                                                        temperature conditions, essentially
                                                        eliminates the classical drift problems of
                                                        conventional thermocouples. Every IRt/c
  IRt/c at Room Temperature
                                                        is double annealed at temperatures
                                                        above 100° C to ensure long term
 Thermocouple Probe at Product Temperature              stability, and tested 5 times prior to
                                                        packaging. Barring a very small
 Conventional thermocouple operating at elevated
 temperature is subject to long-term drift, while the
 IRt/c operating at room temperature is stable.
                                                          Quick Installation Guide

Repeatability is defined as the ability
of a measuring device to reproduce its
calibration under identical conditions.                                                                                       +
The IRt/c is a solid, hermetically sealed,                                                                                    –
fully potted system that does not change
either mechanically or metallurgically
during service. There are no active
electronic components and no power
source to produce the signal – only the                             3                         1                                       4
thermoelectric effects that produce a
thermocouple signal. The 1% rating
is a conservative value based on the
practical difficulty of demonstrating
tighter tolerances under test conditions,
rather than a true limitation of the device.

In many applications, heating elements                                                                            10,000 V
                                                                                                                   60 Hz
are employed to heat a product in an
oven, furnace, or with jets of hot air.
Conventional control devices using
contact thermocouples measure and
control the oven air temperature, IR                                IRt/c
heating element temperature, or air jet
temperature in an effort to maintain
product temperature and therefore,
quality; often with less than satisfactory
                   Heat Input                                                         1000 ft (300 m)
                                                                                Twisted Shield Pair t/c Wire

                                             IRt/c CAN BE USED WITH                               IRt/c’S ARE INTRINSICALLY SAFE
                                             UP TO 1,000 FT (300 M) OF                            WHEN USED WITH BARRIERS
                   Product Temperature       THERMOCOUPLE EXTENSION
                                             WIRE                                                 “Field Apparatus having energy storing
                                                                                                  or generating characteristics of <1.2V,
                                             With twisted shielded pair thermocouple              0.1A, 25 mW or 25 microJ shall be
                           Time              extension wire, an IRt/c can be mounted              considered Simple Apparatus (non-
                                             as far as 300 meters (1,000 ft) from                 energy storing). These general purpose
Replacing the contact thermocouple,          the readout device, even in a very                   devices may be used in a hazardous
(for example measuring oven                  fierce electrical noise environment. A               (classified) location without further
temperature) with a non-contact IRt/c        demonstration test was performed with                approval when connected to a certified
measuring product temperature directly,      a 300 m (1000 ft) coil of twisted shielded           intrinsically safe circuit.” – Quote from
will insure that product temperature is      pair of extension wire, with 30 m (100 ft)           R. Stahl, Inc. Comprehensive Product
maintained. Some readjustment of the         unwound, connecting an IRt/c to a fast               Manual On Intrinsic Safety Barrier
controller parameters is required            (100 msec. response) A/D conversion                  and Repeater Relays. Examples of
because of differences in sensor             module to a computer. As a noise                     non-energy storing Intrinsically Safe
response times (an IRt/c is much             generator, a 60 Hz 10,000 volt transformer           Apparatus are:
faster), and time required to heat the       and spark generator was set up to spark
product compared to the original sensor      within 15 cm (6 inches) of the wire.                 • Thermocouples • RTD’s • LED’s
(slower). After installing the IRt/c and                                                          • Dry Switch Contacts
calibrating the controller reading using     The test results showed less than 0.1°C              • NAMUR Inductive Proximity Switches
an OS91 Infrared Scanner (see Quick          of noise at any relative position of the             • Non-inductive Strain Gage Devices
Installation Guide below), initiate the      wire, spark, and transformer. The                      and Resistors
self-tuning cycle of the controller and      extraordinary noise suppression
check to see that the control is stable      characteristics designed into the IRt/c              The IRt/c falls into the category of
and accurate. If it will not self tune       make it possible to locate it at very long           thermocouples, since it generates its
properly, manually adjust the control        distances, without the necessity of a                signal by converting the radiated heat
coefficients to achieve stable control.      transmitter. The IRt/c housing is electrically       energy to an electrical signal via
Because the product temperature is           isolated from the signal leads and is                Seebeck effects, the basic driving
likely to change temperature more            connected to the shielded ground of the              force of thermocouples. Like all
slowly than the original sensor, start       extension cable. For long distances,                 thermocouples, it requires no power
with slowly increasing the “D” of the        twisted shielded extension cable should              source and generates signals measured
PID coefficients.                            be used, and the shield connected to a               in millivolts of voltage, microamps of
                                             good electrical ground.                              current and nanowatts of power. IRt/c’s
                                                                                                  have a small capacitance, but at one
IRt/c’s are intrinsically safe when used                                                          microFarad, the energy storage is
with barriers                                                                                     measured in nanojoules and is a
                                                                                                  thousand times lower than the
                                                                                                  25 microjoule criterion.

                                                                                                  Accordingly, the IRt/c qualifies as a
                                                                                                  Simple Apparatus for use in hazardous
                                                                                                  locations, and with the appropriate
                                                                                                  barrier, qualifies as Intrinsically Safe.



Principles of Infrared Thermocouples Cont'd

IRt/c                                         IRt/c RELATIVE HUMIDITY
                                                                                          CONTROLLING WEB ROLLER
APPLICATION NOTES                             IRt/c’s can be used to measure actual       The IRt/c infrared thermocouples have
IRt/c MONITORS                                relative humidity in many situations        quickly become the sensors of choice
TIRE TEMPERATURES FOR                         where there is a convenient source          for monitoring and controlling both web
                                              of water and flowing air, and measure       and roller temperatures. Tips on accurate
RACING PERFORMANCE                            it accurately and reliably.                 roller temperature measurement:
                                              An IRt/c aimed at a wet porous surface      1. Uncoated Metal or Chrome Rolls –
                                              with ambient air blowing across the wet     Shiny, uncoated metal rolls are difficult
                                              surface, can actually measure what is       for any infrared sensor to properly
                                              called “wet bulb” temperature for that      sense the true temperature (the sensor
                                              ambient area. (More precisely, wet          will see too many environmental
                                              bulb temperature is the equilibrium         reflections). The solution to the problem
                                              temperature of the air-water interface      is to simply: paint a small black stripe
                                              when a water film is evaporated. When       on an unused end of the roller. Aim the
                                              air is moved over a wet surface, the        IRt/c sensor at the black paint stripe.
                                              water cools by evaporation until it         It will then measure the temperature
                                              reaches wet-bulb temperature, then          accurately and reliably regardless of
Tire temperature is of critical concern in    the cooling stops, no matter how much       changes in the surface conditions of the
automotive racing for two reasons: the        more air is moved over the surface. The     rest of the roller.
tire temperature directly affects its         temperature at which the cooling stops
adhesion and its wear characteristics;        is the wet bulb temperature.)
and tire temperature patterns provide
valuable information on the set-up and        The IRt/c measures the temperature
performance of the suspension. For            of the air-water interface on a surface
example, excessive loading of a tire          directly. The quality of the water or of
caused by out-of-tune suspension will         the absorbing material does not affect                                 +
cause that tire to become considerably        the reading, since the IRt/c can directly
warmer than the others.                       view the air-water interface, and the wet
                                              bulb equilibrium temperature is not
The IRt/c is an ideal measuring device        materially affected by impurities.
for on-board data acquisition, due to its
small size, ruggedness, and low cost.         The highest precision method is to
It may be connected to standard               employ an IRt/c wired differentially        If there is very little space on the edge
thermocouple read-out systems.                with a conventional thermocouple            of the roller, move the sensor closer and
Installation should include connecting        to measure the quantity “wet bulb           paint a very small black stripe. The
the shield to a suitable ground in order      depression”. The differential pair          minimum spot size of the IRt/c is 8 mm
to avoid interference from the                arrangement guarantees high                 (0.3 inches) and for the OS36-2 it is
electrically harsh environment of a           accuracy, since RH is a strong function     4 mm (0.16 inches) when the sensor is
racing automobile. Mechanical                 of wet bulb depression and a weak           brought close to the surface.
installation should include attention to      function of dry bulb temperature.
air flow patterns to minimize dirt building   Standard psychrometric tables,              2. Dull Metal Rollers – Dull metal
on the lens. The OS36-2 or OS36-5 are         charts, and software algorithms             rollers can provide a reliable signal.
recommended due to their narrower             can be used with the data to obtain         It is best to test the surface for reliability,
field of view, thus allowing you to           accurate relative humidity for your         though, as the surface emissive
position it further away.                     environmental measurements.                 properties may shift via dirt, moisture,
                                                                                          cleaning, etc. It is best, if in doubt, to
                                                                                          simply paint a stripe to eliminate these
 Relative Humidity Measurement
                                                             +                            3. Non-metallic Surfaced Rollers –
                                                                    +     Wet Bulb        These will provide a reliable IR signal at
                                                         –               Depression       any point the IRt/c is aimed. No painted
                                                         –              Temperature       stripe is required.
                                                             +      –
      Air Flow

                                                                          Dry Bulb

CONTROLLING                                  IRt/c CONTROLS PRINTED                           2. The field-of-view: the preferred
VACUUM FORMING AND                           CIRCUIT BOARD PREHEAT                            method is to view the part between
THERMOFORMING PROCESSES                      DURING WAVE SOLDERING                            the coil turns or from the end. Select
                                                                                              the IRt/c model that best suits the
For forming plastics, an excellent           An excellent solution to the problem of          requirements.
combination of heating method and            proper heater control for PC board
control is radiant heat with an IRt/c for    preheat is an IRt/c. They work                   3. Part temperature: both the OS36-2
control. They work extraordinarily well      extraordinarily well together, since both        and OS36-5 models can be used to
together, since both the heating and         the heating and measuring occur right            target temperatures of 1100°C (2000°F),
measuring occur right at the surface –       at the surface – where the solder must           and have linear ranges to 260°C (500°F).
where the plastic is located. The IRt/c
reading is unaffected by reflections from
                                             flow. The IRt/c reading is unaffected by
                                             reflections from the heater, since the           ASPHALT TEMPERATURE
the heater, since the spectral response      spectral response of the 6-14 micron             MONITORING
of the 6-14 micron IRt/c lens filters out    IRt/c lens filters out any shorter
the shorter wavelengths of the radiant       wavelengths of the radiant heater energy.        Asphalt properties are particularly

heater energy.                                                                                sensitive to temperature, and it is
                                                                                              important that the asphalt is applied
                                                               Air                            at the correct temperature in order
                                                                                              to perform to its specifications.

                                                                        Ceramic Heaters       Accordingly, temperature monitoring
                                                                                              is a common requirement, but the
                                                                                              thermocouples normally used have
                                                                                              severe breakage problems due to the
                                                                                              harsh abrasiveness of the material, and
                                                                                              must constantly be replaced at high cost
                                                                                              and interruption of production.
                                             For this application, the IRt/c may
                                             be mounted identically to Vacuum
                                             Forming/Thermoforming (above).


The IRt/c may be mounted in between          INDUCTION HEATER CONTROL
ceramic heaters, or in the shroud or

reflector of the radiant heater, such that   The induction heating process can be                    Air
it can see in between the elements.          readily controlled by the temperature
Select the IRt/c standard, OS36-2            of the part as measured by an IRt/c
or OS36-5 model, depending on the            non-contact infrared thermocouple.
field-of-view required to see past the       Several issues should be considered
elements to the painted surface. Care        in an installation.                               OS36-5
should be taken in mounting the IRt/c in
such a way as to keep its temperature
below 93°C (200°F) and to keep the
lens clean. The OS36-2 is the preferred
model for this application because of                                                         The IRt/c solves this problem directly,
its small physical size with built-in air                                     Cooling Water   since the temperature is monitored
purge. It can be used in temperatures to           OS36-5                                     without contact. The normal thermocouple
121°C (250°F) environments when the                                                           controller can be used – simply calibrate
air purge system is used. Its narrower                OS36-2
                                                                                              offset if necessary. The OS36-2 and
field-of-view allows more leeway in                                                           OS36-5 models are recommended due
positioning, and thus more flexibility in                                                     to their built-in air purge, which will keep
installation. For still narrower fields of                                                    the lens clean by preventing vapors
view, use the OS36-5 with its 5:1 FOV.                                                        from condensing on the lens. The
                                                                                              OS36-2 can be mounted in the chute
                                             1. The effect of the field on the IRt/c:         to view the asphalt through a small
                                             since the measuring signal is electrically       hole, while the OS36-5 can be mounted
                                             isolated from the housing, the IRt/c will        some distance away due to its narrow
                                             operate in even a very strong field.             5:1 field of view.
                                             The shield wire should be attached to
                                             a proper signal ground. If there is
                                             excessive heating from the field,
                                             consider using the optional cooling              Reproduced with permission of
                                             jacket kit, with the same water source           Exergen Corp.
                                             as is used to cool the coil.

  OS36-2 and OS36-5
  models feature
  built-in air purge
  to keep these quality
  thermocouples functioning
  efficiently and accurately,
  even in dirty environments

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