How do direct-flow and counter-flow leak detectors work?
Figure 5.12 shows the vacuum schematic for the two leak detector types. In both cases the mass spectrometer is evacuated by the high vacuum pumping system comprising a turbomolecular pump and a rotary vane pump. The diagram on the left shows a direct-flow leak detector. Gas from the inlet port is admitted to the spectrometer via a cold trap. It is actually equivalent to a cryopump in which all the vapors and other contaminants condense. (The cold trap in the past also provided effective protection against the oil vapors of the diffusion pumps used at that time). The auxiliary roughing pump system serves to pre-evacuate the components to be tested or the connector line between the leak detector and the system to be tested. Once the relatively low inlet pressure (pumping time!) has been reached, the valve between the auxiliary pumping system and the cold trap will be opened for the measurement. The Seff used in equation 5.4b is the pumping speed of the turbomolecular pump at the ion source location:
In the case of direct-flow leak detectors, an increase in the sensitivity can be achieved by reducing the pumping speed, for example by installing a throttle between the turbomolecular pump and the cold trap. This is also employed to achieve maximum sensitivity.
The smallest detectable partial pressure for helium is:
pmin,He = 1 · 10-12 mbar. The pumping speed for helium would be
SHe = 10 l/s. Then the smallest detectable leak rate is
Qmin = 1 · 10-12 mbar · 10 l/s = 1 · 10-11 mbar · l/s.If the pumping speed is now reduced to 1/s the unit is l/s, so 1 l/s, then one will achieve the smallest detectable leak rate of 1 · 1012 mbar · l/s. One must keep in mind, however, that with the increase in the sensitivity the time constant for achieving a stable test gas pressure in the test specimen will be correspondingly larger (see below).
In Figure 5.12 the right hand diagram shows the schematic for the counterflow leak detector. The mass spectrometer, the high vacuum system and also the auxiliary roughing pump system correspond exactly to the configuration for the direct-flow arrangement. The feed of the gas to be examined is however connected between the roughing pump and the turbomolecular pump. Helium which reaches this branch point after the valve is opened will cause an increase in the helium pressure in the turbomolecular pump and in the mass spectrometer. The pumping speed Seff inserted in equation 5.4b is the pumping speed for the rotary vane pump at the branch point. The partial helium pressure established there, reduced by the helium compression factor for the turbomolecular pump, is measured at the mass spectrometer. The speed of the turbomolecular pump in the counter-flow leak detectors is regulated so that pump compression also remains constant. Equation 5.5b is derived from equation 5.5a:
Seff = effective pumping speed at the rotary vane pump at the branching point
K = Helium compression factor at the turbomolecular pump
The counter-flow leak detector is a particular benefit for automatic vacuum units since there is a clearly measurable pressure at which the valve can be opened, namely the roughing vacuum pressure at the turbomolecular pump. Since the turbomolecular pump has a very large compression capacity for high masses, heavy molecules in comparison to the light test gas, helium (M = 4), can in practice not reach the mass spectrometer. The turbomolecular pump thus provides ideal protection for the mass spectrometer and thus eliminates the need for an LN2 cold tap, which is certainly the greatest advantage for the user. Historically, counter-flow leak detectors were developed later. This was due in part to inadequate pumping speed stability, which for a long time was not sufficient with the rotary vane pumps used here. For both types of leak detector, stationary units use a built-in auxiliary pump to assist in the evacuation of the test port. With portable leak detectors, it may be necessary to provide a separate, external pump, this being for weight reasons.
Partial flow operation
Where the size of the vacuum vessel or the leak makes it impossible to evacuate the test specimen to the necessary inlet pressure, or where this would simply take too long, then supplementary pumps will have to be used. In this case the helium leak detector is operated in accordance with the so-called “partial flow” concept. This means that usually the larger part of the gas extracted from the test object will be removed by an additional, suitably dimensioned pump system, so that only a part of the gas stream reaches the helium leak detector (see Fig. 5.13). The splitting of the gas flow is effected in accordance with the pumping speed prevailing at the branching point. The following then applies:
where g γ (instead g!) is characterized as the partial flow ratio, i.e. that fraction of the overall leak current which is displayed at the detector. Where the partial flow ratio is unknown, g γ (instead g!) can be determined with a reference leak attached at the vacuum vessel:
Connection to vacuum systems
The partial flow concept is usually used in making the connection of a helium leak detector to vacuum systems with multi-stage vacuum pump sets. When considering where to best make the connection, it must be kept in mind that these are usually small, portable units which have only a low pumping speed at the connection flange (often less than 1 l/s). This makes it all the more important to estimate – based on the partial flow ratio to be expected vis à vis a diffusion pump with pumping speed of 12000 l/s, for example – which leak rates can be detected at all. In systems with high vacuum and Roots pumps, the surest option is to connect the leak detector between the rotary vane pump and the roots pump or between the roots pump and the high vacuum pump. If the pressure there is greater than the permissible inlet pressure for the leak detector, then the leak detector will have to be connected by way of a metering (variable leak) valve. Naturally one will have to have a suitable connector flange available. It is also advisable to install a valve at this point from the outset so that, when needed, the leak detector can quickly be coupled (with the system running) and leak detection can commence immediately after opening the valve. In order to avoid this valve being opened inadvertently, it should be sealed off with a blank flange during normal vacuum system operation.
A second method for coupling to larger systems, for example, those used for removing the air from the turbines in power generating stations, is to couple at the discharge. A sniffer unit is inserted in the system where it discharges to atmosphere. One then sniffs the increase in the helium concentration in the exhaust. Without a tight coupling to the exhaust, however, the detection limit for this application will be limited to 5 ppm the natural helium content in the air. Many leak detectors have Zero-functions, where the natural background can be subtracted and hence lower leak rates can be found. In power plants it is sufficient to insert the tip of the probe at an angle of about 45 ° from the top into the discharge line (usually pointing upward) of the (water ring) pump.
The time constant for a vacuum system is set by
τ = Time constant
V = Volume of the container
Seff = Effective pumping speed, at the test object
Figure 5.14 shows the course of the signal after spraying a leak in a test specimen attached to a leak detector, for three different configurations:
- Center: The specimen with volume of V is joined directly with the leak detector LD (effective pumping speed of S).
- Left: In addition to 1, a partial flow pump with the same effective pumping speed, Sl = S, is attached to the test specimen.
- Right: As at 1, but S is throttled down to 0.5◊S.
The signals can be interpreted as follow:
1: Following a “dead period” (or “delay time”) up to a discernible signal level, the signal, which is proportional to the partial pressure for helium, will rise to its full value of pHe = Q/Seff in accordance with equation 5.9:
The period required to reach 95 % of the ultimate value is normally referred to as the response time.
2: With the installation of the partial flow pump both the time constant and the signal amplitude will be reduced by a factor of 2; that means a quicker rise but a signal which is only half as great. A small time constant means quick changes and thus quick display and, in turn, short leak detection times.
3: The throttling of the pumping speed to 0.5 S, increases both the time constant and the signal amplitude by a factor of 2. A large value for t thus increases the time required appropriately. Great sensitivity, achieved by reducing the pumping speed, is always associated with greater time requirements and thus by no means is always of advantage.
An estimate of the overall time constants for several volumes connected one behind to another and to the associated pumps can be made in an initial approximation by adding the individual time constants.