Measuring and testing – Fluid pressure gauge – Electrical
Patent
1995-09-13
1997-12-02
Brock, Michael
Measuring and testing
Fluid pressure gauge
Electrical
73756, G01L 902
Patent
active
056938888
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a process for operating a controlled heat conductance vacuum gauge with a measuring cell comprising a Wheatstone bridge with supply voltage and measurement voltage terminals, a power supply and measuring instrument and a connecting cable containing several conductors. Moreover, the invention relates to circuits suitable for implementation of this process.
Heat conductance vacuum gauges utilise 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 conductance vacuum gauge after Pirani, the temperature-dependant resistance element is a gauge filament which is part of a Wheatstone bridge. In the uncontrolled 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 controlled Pirani gauge, the supply voltage which is applied to the bridge is continuously controlled 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.
In heat conductivity gauges there is often the necessity to separate the locations of the measuring cell and the measuring instrument. For this it is required to connect measuring cell and measuring instrument by a cable of suitable length. In the case of longer cables, the conductors which are part of the cable and which generally have the same electrical properties, have resistances which can no longer be neglected. Therefore, the voltage which is generated in the measuring instrument and which is employed as the supply voltage to balance the bridge and which is also used as the measurement value, does no longer correspond to the true bridge supply voltage because of the voltage drop across the conductors of the connecting cable. From this there results a measurement error which increases with the length of the connecting cable. Temperature and thus resistance changes in the conductors of the connecting cable are the cause for further measurement errors.
SUMMARY OF THE INVENTION
In heat conductivity vacuum gauges according to the state-of-the-art, the length of the cable is accounted for by way of a manual alignment after having established the connection between the measuring cell and the measuring instrument. For this either a voltage which is proportional to the bridge supply voltage and the cable length or--in the case of microprocessor controlled instruments--the cable length itself, is entered. The entered values are then taken into account in the formation of the measured pressure value. A cable length alignment process of this kind must be repeated--manually--each time the cable is exchanged. The measurement errors which are caused by temperature-dependant changes in the resistance of the connecting cable and the bridge or which are caused by the operating conditions, remain unaccounted for here.
The present invention is based on the task of avoiding said measurement errors and moreover to automate the cable length alignment process.
According to the invention this task is solved by a process of the aforementioned kind so that the voltage of one of the supply voltage terminals of the Wheatstone bridge located in the measuring cell is recorded without current via one of the conductors of the connecting cable and by taking this into account in the formation of the measurement value. Recording the voltage of one of the two voltage terminals without current permits the calculation of the resistance of a conductor in the connecting cable o
REFERENCES:
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patent: 4373387 (1983-02-01), Nishimura et al.
patent: 4492123 (1985-01-01), Reich
patent: 4715003 (1987-12-01), Keller et al.
patent: 4729242 (1988-03-01), Reich et al.
patent: 4787251 (1988-11-01), Kolodjski
patent: 4872349 (1989-10-01), Espiritu-Santo
patent: 4920793 (1990-05-01), Djorup
patent: 5069066 (1991-12-01), Djorup
patent: 5135002 (1992-08-01), Kirchner et al.
Enderes Rolf
Schoroth Anno
Brock Michael
Leybold Aktiengesellschaft
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