Operating Experience
with An Emissivity Measuring Laser Based Infrared Pyrometer
Non Contact Infrared
Thermometer Temperature Measurement
International Test and
Transducer Conference
Sensors & Systems
October 24 - 26, 1989
E. K. Mathews and G. J. Kilford
Introduction:
While in use in many industrial applications and research activities, the measurement of temperature utilizing infrared greatest deficiency is quite fundamental; IR thermometers measure radiance received from a target i.e. one variable, while integration of Planck’s law governing the relationship between the radiance and temperature requires knowledge of a second variable, the target surface emissivity. Heretofore, the solution has been a combination of operator estimates based on experience, or use of dual and multi-wavelength pyrometers, which sidestep the mathematical problem by ratioing-out the emissivity using two or more simultaneous measurements of the radiance. The assumptions here are that the surface is a grey body, with constant emissivity (e) or fixed ratios between emissivities at two or more wavelengths. In many industrial applications these assumptions are not valid. Emissivity varies with surface condition, temperature, wavelength and indeed, with time. Industrial processes have dynamic characteristics and the targets have varying properties; the simple emissivity assumptions are not valid.
The second chronic error in industrial infrared temperature measurement is that caused by background radiation irradiance or energy reflected by the target to the measuring instrument. The instrument receives infrared energy from the target composed of the emitted energy plus that source by furnace walls or a combustion front and reflected by the target to the measuring instrument. We thus have a two-fold effect of target emissivity:
Where (r ) is the target reflectivity and the subscript (l ) refers to wavelength. Thus, if industrial targets had emissivity values of unity, the problem of irradiance would not occur. Knowledge of the target reflectivity is mandatory if one wishes to eliminate the error caused by irradiance.
The third problem regarding the use of the infrared radiation detection of temperature is that of interfering gases. Typically CO, CO2, H2O, the sulfur and nitrogen (oxide) gases are present in industrial applications. As is well known but frequently ignored, these gases absorb and re-radiate energy in the infrared spectrum. They thus contribute to temperature measurement inaccuracies. This error has been eliminated in the PyrolaserŪ (1) technology by use of a narrow bandwidth plus minus (15) nm at the 865 nanometer wavelength. Exxon Research and Engineering Co. has shown this selection to be a virtually interference-free operating zone. In general, the wider the bandwidth of an infrared thermometer - the greater the possibility for the gas interference.
The technology discussed herein was first researched by
Exxon in the early 1980’s. An engineering survey of their worldwide pyrolysis
furnaces revealed problems of short-furnace run lengths reduced product yields and
excessive steam combustion. A major contributor to these problems was the uncertainty of
the tube metal temperatures, which dictate the daily furnace operating conditions. From
this, the need to measure the tube emissivities and to take into account the wall derived
background radiation irradiance and to avoid the interfering gas problem was established. Exxon developed and
patented (2) the concept of the laser-based infrared technology and licensed it to the
Pyrometer Instrument Company, Inc.
www.pyrometer.com in mid-1985. At this time, PYRO is pleased to report on
some of the industrial applications and some of the research activities where the
resulting product, PyrolaserŪ has been effective. We will also point out areas where the
technology needs further work and where significant problems remain.
Description:
In this section an overview of the laser-based infrared thermometer is presented; details are revealed in Appendix A including the specifications.
Classic infrared pyrometers are passive devices; they receive energy from a target, plus an emissivity selection by the user and calculate and present a temperature display. Assuming no other energy source, the radiance received by the instrument detector is independent of distance from the target as long as the target has a uniform temperature and emissivity. As the distance to the target increases, the target size enlarges (R2) while the collected energy per unit area decreases ( 1/R2). Thus no distance-to-target input is required; only the estimated emissivity value.
The new technology of PyrolaserŪ incorporates the passive characteristics of conventional infrared pyrometers but in the precise wavelength restrictions previously mentioned. However, the emissivity-determining feature of PyrolaserŪ is achieved via an active reflectometer technique. A low-powered pulsed GaAS is detected via the same optics as the conventional infrared signal: the laser signal being (AC) on top of the (DC) target signal. Having monitored the laser outgoing energy and knowing the geometry involved, the instrument can determine the reflectivity and thus the emissivity of the target-measuring zone. It should be noted that we must know the distance to the target since the laser energy is dispersed (1/R2) on the return path. For this reason, Pyrolaser Ū has a very sophisticated optical system which provides optical ranging to an accuracy of plus minus 0.5% of range. This system was developed by E. Leitz of Canada under contract to Pyrometer Instrument Company (3).
Fig. 1 PyrolaserŪ
The Exxon survey highlighted numerous other weaknesses in industrial temperature measurement practices. Notably, in hostile environments, operators spend unnecessary time handwriting or tape recording temperature readings of various tube targets. The handwritten field notes must be ‘re-written’ or entered into the process engineer’s computing system. For this reason, a data-logger was incorporated into the electronics of PyrolaserŪ This information can be immediately displayed or printed, or inputted directly to a computerized data base for furnace analysis and trending. All information in the data-logger is immediately available to the operator while making measurements via the 2x20 LCD display. In addition, the statistics of the run are also immediately available to the operator.
(3) Laser Assisted Remote Temperature Measurement, presented at the SPIE Conference, E.S. Cameron, January 1989
Major Applications:
This infrared technology was developed to provide an accurate temperature measuring capability for field measurements in large industrial furnaces. The information presented herein comes from plant tests and continuing plant uses of PyrolaserŪ in refineries, petrochemical plants, steel mills and non-ferrous metals processing the U.S., Europe, and the Mid and Far East.
A.
Petrochemical Furnaces:
Hydrogen and ethylene furnaces typically are very large
installations with multiple furnace viewing ports on each of the four sides and on
multiple levels. An example of a hydrogen furnace layout is shown in Fig. 2. The procedure
is to first make the wall extraneous temperature readings shown as Tx and then to make
true temperature measurement readings. Approximately (8-16) Tx readings are desired over each wall
facing tubes before the tube temperatures are measured in the TT , true
temperature mode. Furnace dynamics change every 15-20 minutes so the Tx values which are
used by PyrolaserŪ to provide the background radiation irradiance correction are only valid for ten minutes.
Figure 3 shows some typical readings of Tx, e, and TT from PyrolaserŪ. The
column TL from PyrolaserŪ. The column TL is using a [conventional1]
non contact IR thermometer instrument which relies on a manual (e) setting and which does not account for
reflected energy. The column (TT - TL ) clearly shows that net error
(emissivity and irradiance) from 8°C to 34°C (46°F to 93°F) in this test. It should be
noted that the tube emissivities varied from 0.91 to 0.99, yet all tubes were of the same
type and in-use for the same period of time. This is a striking example of why manual
values of emissivity get one into trouble. Figure 2. Typical Hydrogen Furnace Hydrogen Furnace Data: Tx (Wall Temp)
E (Emissivity)
TT True Temp
TL Conventional1
TT -- TL
896
0.95 0.91 0.93 761 779 780 795 790 790 -34 -11 -10 901
0.95 0.93 0.93 763 772 776 795 790 795 -32 -18 -19 920
0.93 0.95 0.99 782 777 760 812 806 789 -30 -29 -29 910 0.94 0.99 0.92 795 780 781 816 800 789 -21 -20 -08 890 0.94 777 787 -10 889 0.95 769 786 -17 890 0.96 761 778 -17 889 0.95 758 774 -16 Figure 3 Emissivity and True Temperature
Values comparison of PyrolaserŪ with a Conventional1 IR non contact
infrared thermometer instrument. In hydrogen and ethylene furnace tests, we generally see
tube emissivities ranging from 0.85 to 0.97; they increase with age. Typically, operators
report that this is of little concern to them since "that’s close to 1."
Figure 4 shows measured values of the temperature correction associated with varying (e)
values and varying wall vs. tube temperatures. AT ( e ) = 0.95 a 130°C wall-to-tube
difference causes a 15°C (27°F) error. In a 500M metric ton/year ethylene furnace a
10°C (18°F) error is values at $500,000 loss of yield per year. The following shows typical results in a European steam
cracker. Figure 5 Tube Metal Temperatures All mean values are based on 6-10 measurements PyrolaserŪ Cracker no.
5 Cracker no.
10 Emissivity E TX " (°C) TE " (°C) TT " (°C) 0.93 1059 1004 998 0.93 1072 1040 1036 [Conventional
1, 2 E = 1.00 E = 0.93 1015 1021* 1047 1056* On cracker #5 the PyrolaserŪ readings, taking into
account the irradiance is 23°C lower than the emissivity corrected value of the
[conventional instrument1]. For cracker #10 the difference is 20°C. In hydrogen furnace operation, as the catalyst activity
declines, tube temperature measurements rise and steam consumption must go up in order to produce the
yield. Ultimately, a shutdown is required. Accurately monitoring the "hot" tubes
is one way of prolonging a furnace run. A North American hydrogen furnace user of PyrolaserŪ reported: The benefits to date are from the extended run of a 96
MSCFD hydrogen unit operating with localized hot spots on 33 of 445 reformer tubes (total
number of reformer tubes is 456 of which 11 isolated).The PyrolaserŪ was used to monitor
the progression of tube metal temperature rise at the hot spots as the reforming catalyst
reached end of run. With the development of a serious problem on a parallel hydrogen unit,
a unit risk assessment was done using the PyrolaserŪ data to extend the current run
length by five months. Monitoring of the hot spots had been ongoing for nine months during
which six tubes were pinched. The pinched tubes all had hot spots operating above design
temperature, and were bulging. With continuous monitoring of the known problem tubes, the
risk of operating the unit for the additional five months was deemed acceptable. The
parallel unit was allowed to go into a major turnaround, and a potential production loss
was averted. The above knowledge, and experience with the PyrolaserŪ
has been of great benefit to us." B.
Chemical Plant Reactor:
It is interesting to present the use of this technology to
monitor the condition and temperature of an 8-foot diameter catalytic gauze on a chemical
reactor processing highly toxic gases. Feed gas is introduced vertically downward and
forced to pass through catalytic gauze. The temperature control process is exothermic so feed rate and
catalyst activity affects the gauze temperature and process conversion rate. In this case PyrolaserŪ was used to 1) obtain the temperature distribution of the gauze tray and to
relate emissivity to "aging" of the gauze. The data in Figure 6 are typical
data-logger "NOTEBOOK" outputs of PyrolaserŪ. "Old
Gauze" Location Date Time Distance Mode Nx/EM Tunc. Tcorr. 00-00-0075 87-06-08 12:01:50 6.90 ft E O 0.91 1095°C 1107°C 00-00-0074 87-06-08 12:01:48 6.90 ft E O 0.90 1071°C 1082°C 00-00-0073 87-06-08 12:01:44 6.90 ft E O 0.90 1086°C 1098°C 00-00-0072 87-06-08 12:01:36 6.90 ft E O 0.90 1081°C 1092°C 00-00-0071 87-06-08 12:01:34 6.90 ft E O 0.89 1092°C 1105°C 00-00-0070 87-06-08 12:01:32 6.90 ft E O 0.92 1049°C 1058°C 00-00-0069 87-06-08 12:01:28 6.90 ft E O 0.90 1089°C 1100°C 00-00-0068 87-06-08 12:01:26 6.90 ft E O 0.90 1089°C 1100°C 00-00-0067 87-06-08 12:01:23 6.90 ft E O 0.91 1078°C 1089°C 00-00-0066 87-06-08 12:01:22 6.91 ft E O 0.89 1088°C 1101°C 00-00-0065 87-06-08 12:01:19 6.90 ft E O 0.90 1097°C 1109°C "New
Gauze" 00-00-017 87-06-08 11:40:51 8.24 ft E O 0.61 1079°C 1136°C 00-00-016 87-06-08 11:39:59 8.24 ft E O 0.58 1085°C 1148°C 00-00-015 87-06-08 11:39:54 8.24 ft E O 0.65 1082°C 1131°C 00-00-014 87-06-08 11:39:52 8.24 ft E O 0.65 1080°C 1130°C 00-00-013 87-06-08 11:39:48 8.24 ft E O 0.65 1079°C 1128°C 00-00-012 87-06-08 11:39:44 8.24 ft E O 0.66 1085°C 1133°C Figure 6. Emissivity and Temperature
Measurement of a
Catalytic Gauze Looking at the average values we find: Ave E
Ave TUNC°C
Ave
TCORR°C
New Gauze 0.63 1081 1134 Old Gauze 0.90 1083 1095 This shows that the emissivity increased with use, i.e.
"aging." It also shows that using a fixed emissivity value would yield only a
2°C change between the new and the old gauze – whereas by having measured emissivity
values the corrected temperature values indicate a 39°C decrease. Use of a manual or
preset emissivity would not have revealed this drop in reaction temperature which would be
masked by the changing emissivity value. C.
Metallurgical Applications In this section a few examples of PyrolaserŪ use in
steel, aluminum and other non-ferrous metallurgical applications will be presented. But
first, realize that neither PyrolaserŪ nor other infrared instruments can be used
indiscriminately on metals particularly freshly generated metal surfaces. The problem is
the emission pattern of the energy from a surface. In general, freshly prepared metal surfaces do not have
the uniform emission patterns exhibiting either hemispherical or Lambertian distribution.
These specular targets may or may not be measurable. Even for diffusely radiating metal,
the energy-emitted normal (perpendicular) to the surface is typically less than at angles
off the normal. Additionally, we do not advise use of PyrolaserŪ within 7° of the
normal, for we are concerned with the possible "bounce-back" of the laser beam.
It is thus our procedure to examine the emissivity distribution as a function of incident
angle early in programs involving metals. In many cases, the specular emission becomes
Lambertian as the temperature of the metal increases and with exposure to oxygen.
PyrolaserŪ has a built-in cosine function allowing the user to enter the incident angle. 1.
Steel Industry Applications PyrolaserŪ has been used to measure ingot temperatures in
soaking pits, refractory and slabs in reheat furnaces, silicon and 301SS in rolling mills
etc. For example, in this progress report, we present information concerning a normalizing
process for 48" wide silicon steel sheeting. The process receives steel from a hot
rolling mill which is passed through a three-zone heating oven, a holding chamber for
controlled cooling and oxidation (to facilitate cleaning) and then to a cold rolling mill.
The PyrolaserŪ was particularly utilized at the exit of the heating oven and entry to the
holding chamber. Emissivity readings laterally across the strip varied form 0.91 near the
edges to 0.85 at the center. For a conventional pyrometer set at a constant (e) obviously
an error occurs. This amounts to about 8°F. In this case with PyrolaserŪ, the
differences in emissivity paralleled differences in grain structure (alignment and size)
which were vital to magnetic product quality properties. (Details are confidential to the
steel company.) 2.
Steel Fabrication One application of PyrolaserŪ has been in a critical
steel fabrication operating in West Germany. In this case, the producer is endeavoring to
shape an annular specialty steel having a rectangular cross-section. The temperature
distribution within the member is vital; it being derived from non-contact surface
temperature measurements. The company determined that neither the metals nor their oxides
were grey bodies, and in October 1988, acquired a PyrolaserŪ. Figures 6 and 7 show the
configuration of the equipment and Figure 8, some example data. Figure 6 Figure 7 Figure 8 3.
Aluminum Field trials of PyrolaserŪ in a major aluminum
reclamation plant have yielded some interesting data. Typically, aluminum processing
involves temperatures below the (1112°F/ 600°C) threshold of PyrolaserŪ and typically
aluminum surfaces have specular emission characteristics. In reclamation plants, some of
the thermal treating reaches within the temperature ranges of PyrolaserŪ so the remaining
question is specularity. Three cases are shown in Figure 9 along with two sets of
refractory data. In addition, Figure 10 is a plot of emissivity corrected temperatures of
a flowing aluminum film; the line is that of thermocouples in the stream. Over a
twenty-minute period when the (source) furnace operation was constant, the standard
deviation was 18°F while the average temperature was 1838°F. This coefficient of
variation of approximately 1% is encouraging in this case vis-a-vis little surface
specularity. Target
Conditions
Emissivity
Tunc
Tcorr
.8
- .95
1240-1260°F
1250-1280°F
.69-.72
1500-2000°F
1650-2300°F
.68-.70
1020-1040°F
1050-1070°F
.6
- .8
1400-1800°F
1450-1900°F
.4
- .55
1350-1550°F
1500-1750°F
Figure 9. Data From Aluminum Reclamation
Plant Specialty Material
– MONEL A major U.K. metals manufacturer has completed System
Evaluation Trials of PyrolaserŪ on MONEL samples. The evaluation included tests in a
three-zone creep furnace and use of platinum/platinum-rhodium control thermocouples. The
PyrolaserŪ was evaluated over a range from 750°C to 1250°C and parallel data obtained
with a [conventional2] infrared instrument. The maximum error of PyrolaserŪ
over the entire range was 3.2°C, which is within specification. The maximum error for the
[conventional2] instrument reached 9.8°C. Figure 11 shows a plot of the
percentage error and the values of emissivity as determined by PyrolaserŪ. It is evident
that the ability to measure emissivity at the same location, time, temperature, wavelength
and surface condition provides a significant improvement in the measurement of temperature
by infrared means Figure 10. Metal Fil Temperature as
Measured by PyrolaserŪ. Graph. Error percentage Monitor Temperature
Pyrolaser vs Conventional2 Zirconium A series of PyrolaserŪ tests were made at a large U.S.
zirconium processing plant. The data presented below are from a heat cycling of a large
zirconium billet. In this case, the target zone was initially a freshly cut surface which
was tracked during the cycle. Conditions
Emissivity Range Corrected Temperature °C Fresh
Cut 8" Face 0.50
– 0.55
T=
20°C Ambient
Heating 0.44
– 0.46 0.38 – 0.42 0.35 – 0.40 673
– 683 835 - 848 930 – 940 1028 – 1038 Cooling 0.37
– 0.40 0.36 – 0.38 0.32 – 0.33 943
– 948 798 – 808 678 – 690 Cold 0.26
– 0.32 T
= 22°C The PyrolaserŪ was hand-held in the tests and an attempt
made to survey the target area. The results for zirconium appear contrary to other
materials, which tend to increase in emissivity when heated while exposed to air.
Additional tests during which the white oxide is removed may be justified. Obviously, the ceramic industry, including that associated
with the electronic components business, uses high temperature processing. Some
manufacturing is done in open small tunnel kilns as well as vacuum furnaces. Due to
secrecy agreement(s) with the manufacturer, at this time only "coded" data can
be presented. It does, however, verify the large differences in target emissivities in
ceramics and therefore the need to measure directly this vital variable.
*Uncorrected
for background thermal radiation. This must be done manually from tables’ etc.)
Molten Aluminum tapping
stream
Mixed Surface; some
dross, some clear
Refractory in furnace
hearth
Clear and then obscured
by burner flame
Refractory lining of
crucible heater
No interfering
impedance
Molten Aluminum surface
in earth
Slag, dross
Molten aluminum in
surface hearth
Skimmed
Sample Code |
Emissivity |
Tunc °C - °F |
Tcor °C - °F |
"A" |
.88 - .89 |
716 – 1321 |
723 – 1334 |
"B" |
.50 - .52 |
784 – 1443 |
831 – 1528 |
"C" |
.73 - .75 |
601 – 1114 |
619 - 1146 |
The emissivity corrections for sample "B", for example (47°C or 85°F), are significant and cannot be ignored in the control of the manufacturing process.
Small Target Applications
While the PyrolaserŪ was initially designed for use in large industrial furnaces, soon after its introduction the need for small target measurements became evident. Most of the items were in R & D laboratories and they frequently involved specialty metals. To meet the small target requirements, modification of the optics and software were required. At this time, the following ranges are available in addition to the standard PyrolaserŪ.
| Target Size | Target Distance |
||
| mm | Inch | Cm |
inch |
| 1 | .039 | 20 |
7.9 |
| 2 | .078 | 40 |
15.8 |
| 5 | .195 | 100 |
39.4 |
The optical modification for each case is an add-on lens and the software change is now simply a PyrolaserŪ keypad entry by the user. A PyrolaserŪ can be outfitted with any two of the four ranges; if four ranges are needed, an EPROM substitution available from PYRO is required. This, too, can be installed by the user.
Titanium
The first application of the small target PyrolaserŪ was on titanium strips in a vacuum. The transmission losses due to the glass bell jar wre calibrated (out) prior to making the following readings.
Emissivity |
Tuncorrected |
Te (Corrected) |
||
0.84 |
857°C |
1575°F |
870°C |
1598°F |
0.85 |
850°C |
1562°F |
863°C |
1585°F |
0.85 |
850°C |
1562°F |
862°C |
1584°F |
Preliminary tests using PyrolaserŪ to monitor the temperature of a "material" being made (processed) for super conductivity evaluation have been run. The processing is accomplished in a vacuum and, in this case, the data are reported without correction for the viewport transmission losses.
Conditions |
Emissivity |
Tuncorrected |
Tecorrected |
||
Ambient |
0.65 – 0.67 |
||||
Time (minutes) |
°C |
°F |
°C |
°F |
|
0 |
0.88 |
594 |
1101 |
600 |
1112 |
10 |
0.91 |
597 |
1107 |
602 |
1116 |
20 |
0.84 |
628 |
1162 |
637 |
1179 |
25 |
0.83 |
635 |
1175 |
642 |
1188 |
It is hoped that PyrolaserŪ will provide the means to monitor and control the temperature during the processing of these materials.
ConclusionsAn effective precision infrared pyrometer has been developed which has application in many industrial and R & D areas. The ability to measure emissivity and to account for irradiance are important for accurate surface temperature measurements. Practical use of the technology in the petroleum, chemical, steel, aluminum, ceramic and other industries verify the usefulness of the PyrolaserŪ concept. The development is a major advance in infrared pyrometry. Additional work on lower temperature units, specular targets and on-line process control systems is foreseen.
Appendix A Details of PyrolaserŪ Optical SystemThe optical sensor system is of single axis configuration with the viewing focusing (distance measuring), laser transmit, laser receive and radiance receive, all being coincident through the objective lens. The laser transmit energy is introduces via a beam splitter and filters protect the operator from any laser energy. The field of view is 7 degrees; the measuring cone 1/3 degree with measuring range 6 to 30 feet ( 2 – 10m). Lenses for measuring zones of 1, 2 and 5 mm diameter are available.
The optical configuration of the PyrolaserŪ, comprising transmit optics, receive optics, radiance channel and sighting system are all operating through a common objective lens. The lay-out of the laser monitor channel is also shown.
The measuring instrument uses a solid state GaAS three stack laser with peak power of 25 w. The laser is pulsed to distinguish between reflected laser (AC-signal) and emitted radiance (DC-signal) from the target. The large changes of radiant laser power with ambient temperature are roughly compensated for by changing the supply voltage of the storage capacitor. The final precision is achieved by referring the laser return signal to the monitored laser output signal.
Electronics:Besides communicating with the operator via keypad, LCD digital display, in-view-finder LED-display, and/or RS232 communication line, the electronics measure the distance to the target by detecting
the lens position after focusing manually, provides power and fires the laser, monitors and the outgoing signal, and detects the returning laser signal and the radiance emitted from the target, finally calculates from these raw data the temperature and emissivity and stores the results.
In the electronic design and fabrication special care has been taken to protect the very sensitive detector amplifier circuit from microprocessor noise. The noise has nearly been brought down to
the level of the Noise Equivalent Power of the PIN-photodiode detector itself. The dark current of this detector diode is brought to zero by keeping the bias voltage across the diode smaller than1uV.
Specifications
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°F,°C | ||||||||||||||||
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(4) Ranges Available | ||||||||||||||||
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ą 5°F (3°C) | ||||||||||||||||
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ą 1°F (1°C) | ||||||||||||||||
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ą 1°F (1°C) | ||||||||||||||||
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0.865 mm ą0.015 | ||||||||||||||||
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0.055 mm | ||||||||||||||||
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0.01 -1.0 ( Increments 0.01 ) | ||||||||||||||||
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1ms - 2000ms Selectable | ||||||||||||||||
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4 Digit Corrected Temperature (Tt) | ||||||||||||||||
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40 Digit Readout of Target Distance Emissivity Value (E), Uncorrected Temperature (Tu), & Corrected Temperature (Tt) | ||||||||||||||||
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2-10 meters | ||||||||||||||||
Standard 2 - 10 meter Range |
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7° | ||||||||||||||||
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0.333° (1mm @ 20cm; 0.04" @ 8") | ||||||||||||||||
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1, 2, 4, 8, 21, 23, 37 Readings/sec Selectable | ||||||||||||||||
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32°F - 125°F (0°C - 32°C) | ||||||||||||||||
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Cast Aluminum | ||||||||||||||||
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12.5" x 8.0" x 3.0" (318mm x 211mm x 74mm) | ||||||||||||||||
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7lbs. (3.5kg) | ||||||||||||||||