The PHD-4 is a portable compact leak detector which includes a battery for autonomous use in the field and uses helium as a tracer gas. It allows detection of very small leaks in objects where a slight helium pressure has been introduced
The PHD-4 principle of operation is based on a Varian patented technology, Selective Ion Pump Detection (SIPD). The sensor incorporates a quartz capillary tube maintained under high vacuum by an ion pump. The quartz tube is heated with a platinum filament and becomes permeable to helium. As the partial pressure of helium in the ion pump increases, so does the current drawn by the ion pump, proportional to the pressure, indicating the helium concentration present in the test probe of the PHD-4.
Helium is a superior choice as tracer gas for a number of reasons: