Measuring and testing – Fluid pressure gauge – Electrical
Patent
1995-09-13
1997-03-04
Williams, Hezron E.
Measuring and testing
Fluid pressure gauge
Electrical
7320419, G01L 2112
Patent
active
056081685
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a process for operating a regulated heat conduction vacuum gauge with a Wheatstone bridge powered by a controllable supply voltage and a gauge filament and a resistor as two of its components among others, designed to be temperature-dependant to compensate for interference effects of the ambient temperature on the gauge filament. Moreover, the invention relates to circuits suitable for implementation of this process.
Heat conduction vacuum gauges utilize the effect, that from a temperature-dependant resistance element, more heat is lost at high gas pressures, i.e. at higher particle densities, compared to lower gas pressures. In the heat conduction vacuum gauge after Pirani, the temperature-dependant resistance element is a gauge filament which is part of a Wheatstone bridge. In the unregulated Pirani vacuum gauge, a change in the resistance of the gauge filament unbalances the bridge whereby this imbalance is taken as a measure for the pressure. In the regulated Pirani gauge, the supply voltage which is applied to the bridge is continuously regulated in such a manner, that the resistance and thus the temperature of the gauge filament remains constant, irrespectively of the heat loss. The current required to maintain the resistance value at a constant level is a measure for the heat conduction and thus for the pressure of the gas. Commonly, the Wheatstone bridge is aligned for minimum imbalance by readjusting the supply voltage applied to the bridge accordingly. The bridge supply voltage thus represents the primary electrical quantity which corresponds to the pressure.
The ambient temperature of the gauge filament has an interfering effect on this measurement principle, since it also has an influence on the thermal equilibrium of the gauge filament and its surroundings via thermal radiation and thermal conductance. In order to compensate this interfering influence of the ambient temperature, it is known to include in one of the arms of the Wheatstone bridge a temperature-dependant resistance having a suitable characteristic. However, this kind of temperature compensation is inadequate, since it would have to depend with different characteristics on the pressure of the gas. Commonly, the characteristic is selected in such a manner that it is optimized for atmospheric pressure. At low pressures an incorrect compensation is thus unavoidable.
SUMMARY OF THE INVENTION
From DE-PS 32 30 405 it is known to assign to the temperature-dependant resistance in the Wheatstone bridge, a further temperature-dependant resistance which is not part of the bridge and which is supplied with a constant voltage. Besides the additional temperature-dependant component in the measuring cell, this solution requires an additional constant voltage power supply which necessitates a further connecting line between the measuring cell and the measuring instrument.
The present invention is based on the task of improving or simplifying the temperature compensation during the operation of regulated heat conduction vacuum gauges.
BRIEF DESCRIPTION OF THE DRAWINGS
In an operating process of the aforementioned kind, this task is solved by linking various electrical measurement quantities of the bridge to determine the value or the temperature of the temperature-dependant resistance and take it into account in the formation of the measured pressures. The absolute value of the temperature-dependant resistance which was not accounted for in previous solutions, permits a continual formation of correction signals which take account of the temperature-dependence and thus can be utilized in the formation of precise measurement values. Additional components in the measuring cell and an additional constant voltage power supply for the measuring cell are not required.
Further advantages and details of the present invention shall be explained on the basis of drawing FIG. 1 to 7, of which drawing FIG. 1 shows a circuit according to the present state-of-the-art and where FIGS. 2 to 7 show design ex
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patent: 4492123 (1985-01-01), Reich
patent: 4729242 (1988-03-01), Reich et al.
patent: 4787251 (1988-11-01), Kolodjski
patent: 4920793 (1990-05-01), Djorup
patent: 5069066 (1991-12-01), Djorup
Amrozowicz Paul D.
Leybold Aktiengesellschaft
Williams Hezron E.
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