Articulo de Medicion Temperatura

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Metals producers experience yield andquality issues from inaccuratetemperature measurement due to theunusual optical behavior of metals.

Multi-wavelength spectropyrometryprovides accurate temperaturemeasurement and enables close controlof liquid metal processes.

Ralph Felice

FAR AssociatesMacedonia, Ohio

Pyrometry is based on the ideal theoreticalresult that all materials radiate the samecolor and amount of light at the same tem-perature. It is an optical technique analo-

gous to our vision: Humans see color and texturefor identification of their surroundings.

Pyrometers see these two attributes rolled intoone to identify the temperature of the target (Fig. 1).The measure of that single attribute is emissivity,and it governs how bright the target appears to the

pyrometer. Emissivity is basically the radiation ef-ficiency for each material: the ideal radiator has anemissivity of one, and everything else has an emis-sivity of less than one.

The two types of conventional pyrometer are theone-color or brightness type, and the two-color orratio type.

One-color: In these instruments, the amount oflight is translated directly into the temperature: themore light, the higher the temperature. But sincethe target materials emissivity controls the detected

brightness, the emissivity must be known for themeasured temperature to be accurate. However,emissivity depends on texture, wavelength, com-

position, and temperature, which means it is almostalways unknown. Further, these variables oftenchange with processing, and therefore so does theemissivity. In the case of molten metals, the emis-sivity also changes with turbulence. Huge errors,often over 100F, have been caused by incorrectemissivity settings.

Two-color: To address this difficulty of un-known and changing emissivity, instrument man-ufacturers devised the two-color, or ratio, pyrom-eter. These instruments include two detectorssensitive at different colors or wavelengths. Math-ematically, the division of the amount of light meas-ured by each detector, or the ratio, can be solved for

the temperature. In ideal cases, the emissivity is thesame at both colors and cancels out in this division.Unfortunately, metals are not ideal targets. Theiremissivity routinely changes with wavelength orcolor, and this non-greyness causes difficulty forusers of ratio pyrometers.

Therefore, instead of the emissivity, the operator

must know the non-greyness, or relative emissivity.Unfortunately, this is no better known than the emis-sivity itself. And like the emissivity, the non-grey-ness can change with processing. Non-greyness alsois affected by turbulence in liquids. All this change in

both emissivity and relative emissivity means con-ventional pyrometers are mostly wrong. Again, theerrors resulting are often large; inaccuracies greaterthan 100F have been routinely observed.

Multi wavelengthsSpectroPyrometers are expert system multi-wave-

length pyrometers. Anything that measures at morethan two colors is a multi-wavelength pyrometer.

SpectroPyrometers measure at hundreds of colorsand use advanced, patented algorithms to makesense of all that data. Asimple way to think of aSpectroPyrometer is to consider it as hundreds ofthousands of pyrometers in one unit, all measuringthe temperature simultaneously at different wave-lengths. Then, the instrument compares all thesevalues and decides where the true temperature lies.With the wealth of information it gathers, it can de-termine more than just the temperature.

For example, SpectroPyrometers report boththe tolerance and the signal strength. Toleranceis a measure of the accuracy of each temperaturedetermination; signal strength is a measure of the

ADVANCED MATERIALS & PROCESSES/JULY 2008 31

PYROMETRYFOR LIQUID METALS

Fig. 1 An immersion thermocouple shown in an induction-heated superalloymelt. The bright lines are areas of higher emissivity that would appear as artificiallyhigh temperatures to conventional pyrometers. Turbulence disturbs the liquidssurface and increases the emissivity.


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amount of emissivity for each measurement.To be useful, instruments must be robust and easy

to deploy. All SpectroPyrometers are modular, witha remote lens connected to the electronic console byarmored fiberoptic cable, Fig. 2. This design makesit easy to install sensitive optical instrumentation inharsh metal-producing environments.

Data collectionIn both conventional types of pyrometer the oper-

ator must either know the elusive emissivity ormake assumptions about its behavior. And since thevery things he must know are going to change withprocessing, inaccuracy is sure to result. In contrast,SpectroPyrometers approach each measurementwithout any built-in assumptions. They determinethe optical behavior of the target from the data theycollect for each measurement. In this way they canaccommodate targets that are constantly changingeither their emissivity, or relative emissivity, or both.

An extreme example of a target that changes itsoptical parameters constantly is a molten pourstream. The pour stream of Fig. 3 can be seen to beturbulent by the changing shape. Darker stream-lines indicate locations where either the materialmay be cooler or the emissivity lower due to lesser

turbulence. Solid material inclusions are seen at sev-eral locations. Before these pictures were taken, theoperators were completely convinced this furnacewas uniform in temperature throughout.

SpectroPyrometers have often analyzed pourstreams and have recorded the phenomena shownin the picture. Data taken from a titanium pourstream where the lens was focused on the mouth ofthe pour spout is shown in Fig. 4. It shows a bit ofsuperheat at the beginning of the pour, which van-

ishes with the lump of semisolid material detectedaround the five-second mark. (This lump was iden-tified independently on videotape of the pour.)After it passes, temperature remains exactly in therange of the known melting point of titanium, 3020 18F, until the pour is finished at about sevenseconds.

The measure of the emissivity here is called thesignal strength; this term is used since there areoptical materials of unknown transmission in theoptical path. The lump is even more obvious in theemissivity/signal strength trace, where it causesa momentary 50% increase. When the pour iscompleted, the value can be seen to trend upward

another 10 to 15% as the titanium freezes on thespout.

The emissivity of some metals has been observedto more than double as they freeze. The ultimatevalue of the frozen emissivity is related to the rough-ness of the frozen metal.

Storing dataSpectroPyrometers store the thermal spectra, the

data behind the temperature values reported.Analysis of the data from Fig. 4 shows that emis-sivity does in fact change with wavelength. Evenworse, it changes a different amount as time goeson and process conditions change. Here that changecan be explained by the turbulence of the pourstream. Two examples have been chosen, one in theearly superheat phase and one from around the timeof the lump. Their spectral emissivity (emissivityplotted against wavelength) is reproduced in Fig.5. This clearly shows that the emissivity is not con-stant with wavelength.

For the trace marked A, the emissivity variesabout 15% from the short wavelength side to thelong. A ratio pyrometer operating on this without

Theemissivityof somemetals

has beenobservedto more

thandoubleas they

freeze.

32 ADVANCED MATERIALS & PROCESSES/JULY 2008

Fig. 2 The lens assembly (arrow) and a bit of the fiberopticcable of a SpectroPyrometer. Its a simple matter to attach theassembly to the sight port of a vacuum furnace as shown.

Fig. 4 Data taken by a SpectroPyrometer on a titanium pour stream. Videoshowed a lump of material at the five-second mark, which the instrument saw as botha decrease in temperature and increase in emissivity.

Fig. 3 A pour stream showing turbulence, solidinclusions, and streamlines of material that are either lowertemperature or lower emissivity than their surroundings.


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