NRC 925-20 Mass Spectrometer Leak Detector
The function of the Leak Detector is to detect and indicate the rate at which helium passes through a leak in the object under test. The 925-20 consists of a spectrometer tube, main electronics unit, leak indicator, interconnecting cables, voltage regulator and the pumping system described below.
I purchased this unit a while back with some other industrial equipment at a local auction. Upon inspection, the large pump appears to be stuck/seized. Additionally, the leak rate instrument that sets on top of the unit appears to have been dropped. The leak rate gauge sticks and the removable sensitivity scale above dial is missing (I believe they could be made easily), but no internal components appear to have been damaged. See pictures for exact condition of the unit and components. I don't know much about these, but if I omitted any info or if you have a question, e-mail me. As with all items I sell, this unit is being sold “as-is”.
The spectrometer tube ionizes residual gasses by electron bombardment from a hot filament. The ions thus formed are accelerated into a magnetic field at an energy level which causes the mass 4 (helium) ions to be deflected 90 degrees, while other gas ions are deflected more or less than 90 degrees, depending on mass. Suitable electrostatic “lenses” are placed along the ion path to maximize the number of helium ions reaching the collector plate.
Helium ions entering through a leak cause a voltage to build up on the grid of an electrometer tube which is proportional to the helium leakage rate. The included manual describes in detail exactly how all functions work, as well as show schematics, diagrams, and operation/maintenance instructions.
The included pumping system is designed to be used with the 925-20 basic mass spectrometer leak detector whenever your vacuum system pressures are not low enough to permit direct coupling of the 925-20 spectrometer tube. A test port with 1-1/8” ID quick o-ring coupling and an isolation valve are provided for attaching to any system or part to be tested. A 1-1/8 OD tabulation, extending from the cold trap housing, is provided for mounting the spectrometer tube. An isolation valve between the cold trap housing and diffusion pump and forepump to remain running when the cold trap and spectrometer tube are vented.
Although intended primarily for use with the 925-20, the portable pumping system may be used for any application which high pumping speed is not essential (port speed is 10 liters/second), but large trapping capacity is useful (over 150 square inches of liquid nitrogen cooled surface). A separate roughing pump (not included) may be used or, if the time required to start and stop the diffusion pump is not of critical importance, the forepump may be used for rough pumping through the cold diffusion pump.
