The technique is based on a two-step energy balance on the cooling-water circuit of the enthalpy probe. The first step is a tare measurement of the heat load to the probe in the absence of gas sampling conditions. The second is a measurement of the heat load under gas sampling conditions. The difference in heat load between the second and first steps represents the energy associated with the extracted gas sample. For a known amount of gas extracted from the plasma, the specific enthalpy associated with the sampled gas can be calculated. Based on the enthalpy and the composition of the gas mixture, the temperature and the specific density can be determined.
The system is automated with a Programmable Logical Controller (PLC), which performs periodic data acquisition and interfaces the computer with the instruments. It also allows continuous operation of the system even if the computer link is interrupted.
Tekna's control console monitors and controls a closed-loop deionized cooling-water circuit and a gas sampling system. They are equipped with high-precision instrumentation for the accurate measurement of the specific enthalpy and the velocity of the plasma. A quadrupole mass spectrometer can be easily attached for complete analysis of the local composition of the sampled gases. The console is linked to a computer for data processing and to a user interface.