Leak detection
This is always a problem with vacuum systems. We can look for the biggest holes using alcohol. It quickly evaporates on the surface, and if the leak is large enough, then the penetration of these vapours into the vacuum vessel will result in an increase in pressure. But the gauge for measuring pressure must be sensitive enough so that changes in tenths of a Pascal are reflected on the record.
But if our leaks are getting smaller, then we usually grab a helium leak detector and the search battle begins. But this requires great knowledge and systematicity. After all, as soon as we release the helium from the blow gun, its penetration is so high that soon we will be reported with leaks, however completely at different position than we expected.
Fig. 1 –Quadrupole mass spectrometer PGA 100 from Leybold
Fig. 2 – Quadrupole mass spectrometer QMG 511 from Balzers
Fig. 3 – Helium leak analyser Leybold Quadro
Another option is a quadrupole mass spectrometer, but few people have one. I personally worked with the PGA100 device for the Leybold company and the QMG 511 from the Balzers company. But it’s been so long that I can’t even look back. But it is a very elegant measurement, only for it we have to create a vacuum at the level of a turbomolecular pump, i.e. 10-4(-5) mbar. I remember that in order to reduce the pressure used in plasma nitriding in the order of 1-10 mbar, we lasered a screen with a dia 30 µm hole. It is complicated, but if we have such a device, it is not a problem to control the partial pressures of all gases inside the device. This is how Dr. Munz on his Leybold Z700 device with two pairs of magnetron cathodes already in 1985, when the partial pressure of nitrogen in an argon atmosphere was controlled from the mass spectrometer.
Yesterday an article was published on HeatTreatToday about a brand-new device that will help us solve a number of problems with leaks or desorption of vacuum systems. Seems like a tech bomb to me. But maybe it’s not news at all, I just didn’t know about it.
Fig. 4 – OPTIX vacuum furnace atmosphere analyzer from Gencoa
It is an OPTIX device from Gencoa Ltd from the UK. The device works on the principle of optical emission spectrometry. It is enough to connect it directly to the vacuum vessel, and as soon as we reach a negative pressure of 0.5 bar, the device activates itself. Inside the device is a small plasma generator combined with optical spectroscopy. It evaluates the spectrum of gases that are inside the vacuum chamber. Evaluation software in various versions for Windows 7 to 10 is also supplied with the meter.
Fig. 5 – The spectrum of the residual atmosphere measured by the OPTIX instrument
Fig. 6 – The spectrum of the residual atmosphere measured by the mass spectrometer
From the spectrum we can see what we have in the vacuum chamber. Usually, water vapor and oxygen. If we do desorption, the so-called burning of the furnace, we can see the residual gases in direct transmission. Once they drop below a certain level, we can stop the desorption cycle and thus reduce the time of this unproductive cycle. If we want to look for leaks, then we blow the furnace casing from the outside with, for example, nitrogen or argon and observe the movement on its spectral line. I don’t know how reliable it is, these gases will not have the same penetration as helium, because their molecules are much larger. But if the inlet valves for the gas supply are not tight, it is an ideal choice. We will see more nitrogen or argon than is needed and usual.
Fig. 7 – Operating pressure range for OPTIX
Not so long ago, a company was dealing with the problem of not achieving vacuum for several months, because although the parts were clean, the furnace did not achieve the necessary vacuum for aerospace job. In the end, it ended up with a complete disassembly of the heater chamber and heat exchanger, only to find that it was not that, but that nitrogen was entering the space through the inlet valve seat.
I don’t know how much it costs, but it will definitely be cheaper than a leak detector.
Jiří Stanislav
April 24, 2024
