Note: Descriptions are shown in the official language in which they were submitted.
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2270
A MULTIWAVELENGTI~S PYROMETER
The invention relates to a multiwavelengths pyrometer
for measuring the temperature and emission rate of a surface
above 900 K comprising several radiation detectors which are
sensitive to different wavelengths ~.1...~.i...~,n and a data
processor which receives the output signals of the radiation
detectors after digitalization, and deduces therefrom, by
means of the Wien-Planck law, the temperature, assuming that
the surface is an ideal black body, the emission rate being
then computed from these temperature values according to an
approximation law as a function of temperature and the
wavelength, the desired temperature being deduced therefrom.
From the journal "Temperature", vol. 5, 1982, pages
439 to 446, a rapid pyrometer of the type mentioned above is
known. An optical system is directed onto the surface to be
measured, which system splits up into six channels by means of
a glass fiber bundle, and is led to the photodiodes via narrow
band filters. The detector signals are then digitalized and
evaluated in a processor.
The evaluation is based on the Wien-Planck equation
for black bodies
L = C1.~. 5[exp(C2/~,T)-1)] 1 (1)
where L is the beam density at the wavelength ~., C1 and C2 are
constant terms and T is the temperature of the black body.,
Since the surface to be examined is normally no ideal
black body, the emission rate E must be taken into account,
which represents the ratio between the beam density of the
black body and the real body.
This emission rate is a function of temperature and
wavelength and can be expressed by a Taylor series of the
following kind:
In E = a0 + a17~ + a27~2 + . . . ( 2 )
According to experience, the dependance on wavelength
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in limited wavelength ranges is a steady function, so that the
series (2) can be cut off after a few terms.
In the cited article, it is thus proposed to choose a
linear approximation of the function (2) and to evaluate res-
pectively pairs of wavelengths out of the six measured values
of the beam density according to the six wavelengths of the
pyrometer and then to find out the temperature by the analysis
of the squares of the deviations of the different results.
It has been found out that in difficult cases this
method leads to results which do not permit a reliable state-
ment as to their precision.
Thus, pyrometrical measurements of highly reflective
surfaces, where the emission rate is very low and very un-
stable due to possible surface reactions (for example aluminum
during metallurgical treatments), are reputed to be difficult.
It is thus the aim of the invention to improve a mul-
tiwavelengths pyrometer of the kind cited above in such a way
that the computation complexity and the residual error are
diminished and that usable results can be obtained even under
very unfavorable measurement conditions.
According to the invention, this aim is attained by
the fact that the differences between the pyrometer signals
and the pyrometer signals to be expected due to the assumed
emission rate and the desired temperature deduced therefrom
are computed for several of the approximation laws and the
different wavelengths, and that then that approximation law is
selected which represents for all the wavelengths the lowest
sum of the squares of these differences and the highest preci-
sion of temperature and emission rate.
Preferably, the processor contains a memory in which a
data bank is established for the emission rate of certain
materials as a function of temperature and wavelength, the
processor also using this data bank for computing the tempera-
ture when the same materials are subjected to a pyrometer
measurement.
20~'~2~~
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The invention will now be described more in detail
with reference to two figures.
Figure 1 shows a flow diagram for the operations to be
carried out by the processor.
Figure 2 shows schematically a pyrometer according to
the invention.
A six wavelengths pyrometer 1, as it is described in
the above-mentioned essay in "Temperature", delivers simul-
taneously six radiation intensity values of a body, or its
surface respectively, observed by the pyrometer, the wave-
lengths used in practice lying between 400 and 2000 nm. The
bandwidth of a measurement channel lies under 100 nm.
The measurement values which are proportional to in-
tensity are obtained in a known way in photodiodes and then
applied to a processor 2 in digitalized form. The latter
firstly finds out whether the signals are sufficiently stable,
i.e. whether the noise level is sufficiently low. Only if this
is the case, the temperature can be computed with a suffi-
ciently small error (signal standard deviation: So). Then the
law for the determination of the emission rate E is chosen
according to equation 2. It must be differentiated between a
model of zero order, in which In E is a constant a, indepen-
dent of the wavelength, a model of first order, in which In E
linearily depends on the wavelength (the law is defined by the
determination of a0 and al) and models of higher order, in
which further members of the Taylor series must be evaluated.
First, a model of first order is taken as basis and a0
and al and thus the emission rate for the six wavelengths are
determined, a0 and al having the same value in all six deter-
mination equations. Basically, the evaluation consists in a
sub-routine which minimizes the sum of the squares of the
deviations between the measured signals and the beam density
value computed by means of the emission value defined by a0
and al, and which evaluates the resulting standard deviation
SK of the fitting procedure.
20~~~~~
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The expected temperature and emission rate errors are
computed as the differentials which are obtained by successive
incrementation of the signals at the respective error and by
repeated computation of temperature and emission rate. It is
recommended to control whether a model of zero order would
also be applicable, since i-t offers a smaller absolute error
in the temperature detection. This is the case when the con-
stant al from equation 2 lies below a given value, i.e. when
the emission rate practically does not depend on the wave-
length. In this case, six independent temperature measurements
are obtained for the different wavelengths.
The choice of models of higher order leads to a
reduction of the standard deviation SK, but not forcibly of
the temperature error. On the contrary, when SK reaches the
value of S~, any following increase of the model order (over-
fitting) results mostly not in a lesser, but in a higher im-
precision of the temperature. When during error evaluation it
is found out that the error increases, the optimal model has
been found and the constants al, a2,...a~ have been deter-
mined.
If the error analysis has shown that the error is
especially small, then it is recommended to memorize for later
utilization the group of curves connecting the wavelength and
the emission rate. Thus, a data bank organized according to
the kind of materials of the surface to be examined is estab-
lished which can be made use of lateron. This is especially
valuable when during a later measurement there are very unfa-
vorable measuring conditions, for example colour differen-
tiated vapour development in the optical path of the pyrometer
or instabilities in the electronics due to high environmental
temperature. In this case, the pyrometer measurement values
are simply compared to groups of curves evaluated at earlier -
times and the temperature can be directly computed therefrom.
Such a data bank fed by the least disturbed signals is shown
in Figure 2 with the reference 3. Also other unicolored pyro-
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meters can be operated with the data of this data bank in
parallel.
With the pyrometer according to the invention, the
desired evaluations can be carried out during a millisecond
even under unfavorable conditions, so that on a screen 4 tem-
porary evolution of the temperature or the emission rate can
be shown practically in real time also for rapidly developing
processes, such as for example the pulse heating by means of a
laser. This opens new possibilities for the analysis of rapid-
ly developing processes in the temperature range above 700 K
and up to 10.000 K.
