Leak detector

Claim 1

A leak detector of the type that detects helium, the detector comprising a sniffer connected via a flexible hose to an inlet duct which is itself connected via a nitrogen trap and a molecular diffusion pump to a main vacuum pump whose inlet is connected to the molecular diffusion pump and whose outlet exhausts into the atmosphere, the nitrogen trap being further connected to helium measuring means; the improvement comprising: firstly providing the main vacuum pump in the form of a two stage pump having a low pressure stage connected to pump from the molecular diffusion pump and to exhaust, via a link duct, into a high pressure stage which itself exhausts to the atmosphere; and secondly providing a bleeder arrangement connecting the flexible hose both to the inlet duct and to an auxiliary duct which leads to said link duct between the pump stages to pump air and any leaking helium along the flexible hose at a speed greater than that which is attainable when the hose is directly connected only to the inlet duct, the bleeder arrangement ensuring that a proportion of the gas pumped in through the sniffer is fed via the inlet duct and the nitrogen trap to the helium measuring means.