Detector for helium leaks

An improved unit for detecting helium leaks comprising a cylindrical tube, internally coated by a reflecting material, concentrically located inside a cylindrical housing and defining a first sampling chamber between the inner surface of the cylindrical tube and the outer surface of a capillary membrane, as well as a second thermally insulating chamber between the outer surface of the cylindrical tube and the inner surface of the housing, said first chamber being equipped with a sintered filter with an extended surface and adapted to cool the gas flow exiting from the chamber and directed towards the outside environment, and such as to form a flame barrier;

• a free portion of said capillary membrane partially protruding through into a flange provided at the suction inlet of an ion pump;
• said unit further comprising:
• means for transforming a signal I.sub.a that is proportional to the current drawn by the ion pump into n signals Out.sub.i, . . . Out.sub.a the voltage of which is proportional to the current of the signal I.sub.a, said signals being activated one after the other when signal I.sub.a subsequently reaches n current thresholds, and being deactivated when signal I.sub.a subsequently reaches as many current thresholds;
• means for evaluating the level of the current drawn by the ion pump;
• means for selecting said means for evaluating the level of the current drawn by the ion pump on the basis of the voltage level reached by the n signals Out.sub.l, . . . Out.sub.a from said means for transforming the signal I.sub.a ;
• means for calculating the value He of the helium concentration in a gas mixture contained in the sampling chamber on the basis of the evaluated value I.sub.c of the current drawn by the ion pump, on the basis of the value T of the temperature of the capillary membrane, on the basis of the level I.sub.o of the bottom current drawn by the ion pump, on the basis of the permeability of quartz to helium, exponential term, and on the basis of the time constant .GAMMA. of the system that depends upon the volume of the capillary membrane and the pumping rate of the ion pump, according to the following relationship:
• ##EQU4## two stages, a pulsating feeding stage and a sampling stage respectively, adapted to cyclically switch from a phase in which the first stage that pulsatingly feeds the heater filament of the capillary membrane is activated to a phase in which the second stage which measures the voltage and current values of the filament is activated, said voltage and current values being used by a subsequent processing stage for calculating the filament temperature;
• a stage for processing the value t.sub.x of the capillary membrane temperature in which the filament voltage value V.sub.f and current value I.sub.f are combined with the temperature value t.sub.o and the resistance value R.sub.o at known conditions in accordance with the following law:
• ##EQU5## where Q is the proportionality constant of the material used for making the filament;
• a stage for a timed raising of the threshold of the maximum admissible current drawn by the ion pump, adapted to increase the depletion speed of the ion pump that becomes 10 times greater than that of the normal working conditions, thus allowing for a quicker removal of the helium accumulated;
• a stage for measuring and comparing the power drawn by the ion pump both in presence and without the flow of gas from the sampling pump;
• a check-trigger stage for the ion pump adapted to check that the triggering of the ion pump has taken place.