**Thermal Temperature Measurement of a Plasma
Torch by Alexandrite
Effect Spectropyrometer **

The thermal
temperature of a 100kW dc plasma torch is measured (Fig. 1). The collimator of
the alexandrite effect spectropyrometer is directed toward the nozzle of the
plasma torch to measure the thermal temperature of the plasma jet.

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Fig.1. The
layout of the non-transferred 100 kW dc plasma torch and temperature measurement
setup.

Argon was used for
the plasma torch, and the power is kept
at 20 kW and
current at 100 A. The location of collimator, along the axis-direction of the
plasma, is 1.5 m away from the torch head. Fig. 2 shows a measurement window of
the spectropyrometer of the argon plasma. The temperature (11,329 K) and the
emission spectrum are simultaneously displayed. The average measured thermal
temperature is 11178 __+__ 382 K for 12 measurements. The spectral power
distribution consists of continuum underlying distribution and spectral lines.

##

##
Fig.2. A temperature measurement of the
plasma torch
by the alexandrite effect spectropyrometer.

The
spectropyrometer has a spectral correction function, which can be used to
correct the spectral power distribution by deleting the spectral lines in Fig.
3. The corrected spectral power distribution of the plasma is shown in Fig. 6,
and the thermal temperature is 9949K.

Fig. 3. The corrected spectral power distribution of the
plasma jet.

The very strong
spectral lines of the plasma jet can cause an error for directly measuring
thermal temperature by the spectropyrometer. However, the measurement error
caused by the spectral lines can be corrected using the spectral correction
function of the spectropyrometer. The thermal temperature calculated from the
corrected continuum underlying spectral power distribution of the plasma jet is
9949K with an estimated standard deviation of about __+__ 340K (see Fig. 3).

Fig. 4 shows the
further corrected spectral power distribution of the plasma jet according to
that of the relative spectral power distribution of a blackbody. The calculated
thermal temperature changes to 10106 K __+__ 345K, which is slightly higher
than the calculated thermal temperature of 9949K __+__ 340K in Fig. 6. The
thermal temperature calculated from the further corrected spectral power
distribution in Fig. 7 is the true thermal temperature of the measured plasma
jet, and it is more accurate than that calculated from the corrected spectral
power distribution in Fig. 3. However, the difference between the two corrected
thermal temperatures is about 1.5%, which is not significant for the thermal
temperature measurement of the plasma jet.

Fig. 4. Corrected spectral power distribution according to
that of thermal radiator.

In fact, the stronger the spectral lines, the less accurate the directly
measured thermal temperature by the spectropyrometer. The ratio between the
directly measured thermal temperature and the corrected true thermal temperature
can be used to indirectly estimate the thermal equilibrium state of the thermal
plasma jet. The ratio of the true thermal temperature to the directly measured
temperatures is a measure of how well the plasma approximates a blackbody, and
the ratio is defined as the blackbody level (BL) of a thermal plasma:

(1)

where TT is
the calculated true thermal temperature (in Kelvin) and MT is the directly
measured thermal temperature (in Kelvin). When the BL of a thermal plasma is 1,
such as that of the Sun, the thermal plasma is a blackbody in the state of
complete thermodynamic equilibrium. When the BL of a thermal radiator is less
than 1, it does not reach the complete thermal equilibrium. The smaller the BL
is, the less the thermal plasma reaches the complete thermal equilibrium. The BL
of the thermal plasma jet is:

In general, the directly measured thermal temperature is always higher than the
true thermal temperature calculated by the corrected spectral power distribution
of a thermal plasma, thus the BL is always less than 1.

Link:
Peir-Jyh Wang,
Chin-Ching Tzeng, and
Yan Liu, 2010. Thermal
Temperature Measurements of Plasma Torch by Alexandrite Effect Spectropyrometer,
www.hindawi.com/journals/aot/2010/656421.html.