PTI-10 Mass Spectrometric Leak Detector
- Tutorial
Hello. Today I would like to tell you about a wonderful profession - vacuum cleaner. We will also get acquainted with a very curious and complex device designed to localize micro-leaks.
The PTI-10 helium mass spectrometric leak detector was designed for leakproofness testing of various systems and objects that allow pumping out of the internal cavity (evacuation), as well as those filled with pure helium or a gas-air mixture containing helium.
By systems is meant, for example, a vacuum jacket of a cryogenic pipeline, a vacuum chamber for testing spacecraft or a spraying device.
The PTI-10 leak detector is a universal device designed for all types of tightness control, using helium as a test gas. (Why helium will be described below)
Since the error in determining the amount of leakage with this device depends on the directness of the operator’s hands, the geometric shapes of the test object, pumped means, humidity or temperature, then it is not standardized.
In order to understand why it is necessary to fence a whole cabinet on wheels to search for leaks, we consider the dimension of the measured values.
Technical characteristics:
The minimum recorded helium flow without throttling is not more than 1 * 10 -11 m 3 Pa / s , and with throttling - 6.6 * 10-13 m 3 Pa / s.
By throttling is meant a decrease in the pumping speed of the mass spectrometer chamber for the accumulation of test gas in the analyzer.
A flow of 1 m 3 * Pa / s means that in a volume of 1 m 3 for 1 s the pressure changes by 1 Pa, while with us at 1 * 10 -11 m 3 Pa / s.
Let us calculate how much it is:
1 m 3 = 100cm * 100cm * 100cm = 1,000,000 cm 3
P = 100,000 Pa (atmospheric pressure in pascals)
Therefore, at a pressure of 1 atmosphere 1Pa in 1m ^ 3 will occupy 1,000,000 \ 100,000 = 10 cm 3 in volume fractionbut we don’t have the whole Pascal, but his 1 * 10 -11 therefore, 1 * 10 -11 * 10 cm 3 = 1 * 10 -10 cm 3 .
It is worth remembering that the measurement is also performed in seconds, that is, such a small volume also flows in per second. How can it be recorded and measured?
The answer is simple and complex at the same time. On the one hand, the principle is to reduce the density of other gases (the “heavier” the gas, the easier it is to pump out) and the exact direction of helium ions to the analyzer (the target of the mass spectrometer), and on the other hand, the implementation is complex and contains a decent amount of not obvious underwater stones.
Let's start with an analysis of the principle of work.
To understand whether an object is sealed, it is pumped with helium and placed in an industrial vacuum chamber, such as this or home use .
Or vacuum the sample (if possible) and blow it with a helium stream. In any case, in the presence of leaks, helium enters the pumping system consisting of a cascade of various vacuum pumps.
Finally, the sample gas at the entrance to the leak detector, what happens next? But nothing, because first you need to open the inlet valve and smoothly equalize the pressure in the leak detector and pump nozzle. If it is opened too quickly, and the vacuum in the nozzle is significantly worse than in the leak detector, the nitrogen trap will boil and the mass spectrometer chamber will temporarily fail. But first things first.
Remove the back cover and look inside. In a schematic form, it looks like this. The residual atmosphere from the studied object passes through the inlet flange and ends up in a nitrogen trap, where the residual water vapor is frozen out of the vacuum (they are almost always present). A nitrogen trap is a place where vacuum and cryogenic temperatures come into contact (usually it is a sealed container into which a flask for pouring cryogenic liquid is welded) After this, helium generally goes to a steam-oil pump (since the vacuum is better there, which means the pressure is slightly, but less). A steam-oil vacuum pump pumps out the atmosphere with a supersonic jet of hot oil vapor, capturing and not releasing. P after that to the rotary vane pump and to the street:
The rotary vane pump, due to rotation, seizes and compresses the residual atmosphere by squeezing it out into the street.
But the rest of the helium enters the mass spectrometer. Here the fun begins.
Mass spectrometry in the PTI-10 assumes the presence of a charge carrier and the ratio of mass to charge of the ions resulting from the ionization of the test gas. Helium acts as a test gas, since it has a low primary ionization energy and its natural distribution in the atmosphere tends to zero.
Consider the camera of the mass spectrometer. As you can see, there are only 3 functional elements, but each of them is cunning and dangerous. Electron gun
- This is a small accelerator of charged particles (sorry for the quality, there are no other photo samples left). Above, a tungsten cathode is installed on the slit, heating up to 700-1000 C during operation, when it is heated to this temperature, it forms around itself a stream of electrons partially falling into the slit. Under the slit is the chamber of the electron gun, in which the mystery of impact ionization by the electron of the helium atom takes place (others too, but to a lesser extent). Cathodes Well, we have ionized helium - what's next? Then it accelerates through the “barrel” of the gun with a potential difference of up to 400V (the cathode is the camera, and the anode is the frame in front of the camera) and flies out of the electron gun.
Now, in theory, he should fly forward and die ingloriously crashing against the wall, but he is saved by the guiding magnetic system of the mass spectrometer. More precisely , the Lorentz force saves him , forcing him to move in an arc to the separator.
The separator is a metal plate with a rectangular hole located on the calculated arc of helium ion motion. It turns out that only helium flies through the hole in the separator (the rest do not have time to turn and die because of the larger mass).
Sometimes the magnetic system gets confused and you have to make an adjustment, changing its position relative to the camera of the mass spectrometer and thus directing the ion flow.
Next, the ion hits the target, creates an ion current, is amplified by tube and transistor stages and is displayed on a voltmeter in the head display unit (there are also blocks that control valves, chamber power, pressure indicators in different parts of the system).
Do not be discouraged, not only Soviet models work for us (they are simply unkillable and are being repaired on the go). Here, for example, is a sample of a St. Petersburg office.
Everything is automated (protection against the fool delights) and a convenient display system has been added (sorry for the quality, they worked without light).
Instead of a steam-oil pump, a turbomolecular one lives there .
Ask and correct if you need to supplement the article photo.