How do direct-flow and counter-flow leak detectors work

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Depending on the way in which the gas from the test object is supplied to the mass spectrometer, one can differentiate between two types of helium leak detectors:

The direct-flow leak detector
The counter-flow leak detector

Fig. 14: Comparison between main flow leak detector and counterflow leak detector

Fig. 14: Comparison between main flow leak and counterflow leak detector

Figure above shows the vacuum diagrams for the two leak detector types. In both cases, the mass spectrometer is evacuated by a high-vacuum pump system.

Direct flow leak detector

In case of the direct flow leak detector, the gas to be inspected is supplied to the mass spectrometer via a cold trap. The cold trap is cooled with liquid nitrogen (LN₂) and is basically a cryopump in which all the vapors and other contaminants condense. In case of the diffusion pump, which was usually used in the past, the LN₂-cooled cold trap was therefore an effective protection for the mass spectrometer against the oil vapors discharged from the diffusion pump.

The auxiliary pump serves for pre-evacuation of the test object and the required connection lines. In order to be able to connect the high-vacuum side of the running high-vacuum pump with the test object, the auxiliary pump must evacuate the test object to a pressure below 5·10^–2 mbar. Only then it is possible to open the valve between auxiliary pump and cold trap. The high-vacuum pump must not evacuate the test object, the required connection lines and the mass spectrometer to a pressure below 2·10^–4 mbar. Afterwards, the mass spectrometer may start operating in order to detect leaks.

Depending on the size of the leak in the test object and the pumping performance of the vacuum pumps used, pumping times may be very long. In case of a very large leak, the abovementioned pressure values may not even be reached at all.

Counter-flow leak detector

The right part of Fig. 14 shows the diagram for the counter-flow leak detector. One immediately recognizes the substantial difference to the diagram of the direct-flow leak detector: Here, the high-vacuum pump only evacuates the mass spectrometer (smaller volume, very small leak rate) and not the test object (large volume, large leak rate in general).

It should be noted that, in case of the counter-flow leak detector, the supply of the gas to be inspected is carried out between the roughing pump and the high-vacuum pump. This means that the roughing pump and the auxiliary pump must bring the test object to a pressure at which the roughing side of the running high-vacuum pump can be connected to the test object. In case of today‘s counter-flow leak detectors, this so-called start pressure amounts to several millibars. If the start pressure at the inlet of the leak detector is reached, it immediately switches over to the measuring mode.

The partial test gas pressure p_{FV, TG} between roughing pump and high-vacuum pump is increased by the test gas TG (TG = helium or hydrogen) which flows into the leak detector.

When the high-vacuum pump is running, the partial test gas pressure (p_{HV, TG}) on the high-vacuum side of the pump is significantly smaller than on the fore-vacuum side of the pump (p_{FV, TG}). Therefore, a certain amount of the test gas flows - against the delivery direction of the high-vacuum pump - from the fore-vacuum side to the high-vacuum side of the high-vacuum pump. This phenomenon is the reason why this kind of leak detector is referred to as counter-flow leak detector“.

In equilibrium, the following partial test gas pressure will be present on the high-vacuum side, i.e. between high-vacuum pump and mass spectrometer:

p_{HV, TG} = p_{FV, TG}/C_{0, TG}

In this case, C_{0, TG} refers to the compression of the high-vacuum pump for the test gas TG at a test gas flow of zero (the net gas flow of the test gas through the high-vacuum pump is zero).

Nowadays, the high-vacuum pump in counter-flow leak detectors is always a turbomolecular pump with compound stage. This high-vacuum pump type features a high fore-vacuum consistency (few millibars) and therefore allows for the abovementioned high start pressures in the millibar range. Therefore, the leak detection process can be carried out much faster than with a leak detector with oil diffusion pump (fore-vacuum consistency of an oil diffusion pump ⋍ 5 ·10^–1 mbar).

Turbomolecular pumps feature a very high compression for heavy gases (hydrocarbon, oil vapors). Therefore, the following applies: In contrast to light test gases such as helium and hydrogen, heavy gas particles basically cannot reach the mass spectrometer. The turbomolecular pump is thus an optimal protection for the mass spectrometer and renders a liquid nitrogen-cooled cold trap obsolete.

Counter-flow leak detector in partial-flow operation

If evacuating the test object to the required start pressure is impossible or takes too long due to the size of the test object or the leak, an auxiliary pump (auxiliary pump system) must be used in addition to the leak detector.

The leak detector will then be operated in a so-called partial-flow configuration. Since the auxiliary pump usually has a higher performance than the roughing pump integrated into the leak detector, the larger amount of the test gas will flow through the auxiliary pump and only a small amount of the test gas will flow through the roughing pump.

However, the partial test gas pressure at the inlet of the roughing pump and at the inlet of the auxiliary pump p_{FV, TG} will be identical. Therefore, the total test gas flow from the test object amounts to

q_L = p_{FV, TG} · (S_{RP, TG} + S_{AP, TG})

with

S_{RP, TG} = pumping speed of the roughing pump integrated into the leak detector for the test gas in l/s
S_{AP, TG} = pumping speed of the auxiliary pump for the test gas in l/s

This is the true leak rate which the leak detector is supposed to display. The electronic system of the leak detector, however generates the following display

q_{L, display} = p_{FV, TG} · S_{RP, TG}

The following results from:

The leak rate q_{L, display} which is displayed by the leak detector equals the product of the true leak rate q_L and the partial flow ratio γ:

q_{L, display} = q_L · γ

γ = S_{RP, TG}/(S_{RP, TG} + S_{AP, TG}) (partial-flow ratio)

The partial-flow ratio is calculated by means of relation above stated.

In practice, it often makes sense to determine the partial-flow ratio experimentally. To do this, one installs a calibration leak with the leak rate q_L directly at the leak detector (operation without auxiliary pump). The leak detector will then indicate the true leak rate qL of the leak detector on the display. The value q_L must be recorded. Now, one installs the same calibration leak at the test object, puts the auxiliary pump in operation and records the indication on the display of the leak detector. The leak detector now indicates q_L, display. The partial-flow ratio γ being sought will then result from the quotient of q_{L, display} and q_L:

γ = q_{L, display} / q_L(partial flow ratio)

Fig. 15: Example for usage of a leak detector with partial flow principle

Connection to vacuum systems

The connection of a leak detector to vacuum systems with multi-stage vacuum pump sets is usually carried out by means of the partial-flow method. When considering where to best make the connection, it must be kept in mind that the leak detector is usually a small, portable unit which has only a low pumping speed at the connection flange (typically with S_{RP, TG} ⋍ 2 m³/h). 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 S_{AP, TG} = 10,000 l/s = 36,000 m³/h 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 valve (variable leak). 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 connected (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.

Another method for connecting a leak detector to larger vacuum systems is to insert a sniffer into the atmosphere-side outlet of the system. One then sniffs the increase in the test gas concentration in the exhaust.

S_LD = S_{R, He}pumping speed of the roughing pump built into the leak detector for helium in l/s at the branching point
S_AP = S_{AP, He}pumping speed of the auxiliary pump for helium in l/s at the branching point

Time constants

The time constant for a vacuum system is provided by:

t = V_ch / S_eff

V_ch = Volume of the vessels in l
S_eff = Effective pumping speed for the test gas at the vessel in l/s

Fig. 16: Signal responses and pumping speed

Fig. 16 above shows the course of the signal after spraying a leak in a test object attached to a leak detector, for 2 different configurations:

The test object (volume V_ch) is directly connected to the leak detector LD (effective pumping speed for the test gas = S_LD).
In addition to 1, an auxiliary pump ( = partial-flow pump) with the same effective pumping speed S_AP = S_LD is connected to the test object.

The two corresponding signal curves are shown on the fig. 16:

Curve 1: After a "dead time“ t₀ the signal proportional to the partial test gas pressure p_TG increases over time t according to the relation

p_TG = (q_L/S_eff) · { 1 − exp[ − (t − t₀)/τ ] }

After a certain period of time, the signal reaches a portion of its ultimate value

t − t₀ = 1 τ 63.3 % of ultimate value
t − t₀ = 3 τ 95.0 % of ultimate value
t − t₀ = 6 τ 99.8 % of ultimate value

The ultimate value of the signal is proportional to p_TG = q_L/S_eff since the exponential term will disappear for t - t₀ >> τ.

The time span t - t₀ which is required to reach 95% of the ultimate value is referred to as response time. This is given by 3 τ.

This provides the following result for curve 1: The ultimate value of the signal is proportional to p_TG = q_L/S_eff = q_L/S_LD = p₁

Response time = 3 τ = 3 V_ch/S_eff = 3 V_ch/S_LD = τ₁

The following applies for curve 2 ( = partial-flow operation): The ultimate value of the signal is proportional to p_TG = q_L/S_eff = q_L /(S_LD + S_AP) = 0.5 · p₁

Response time = 3 τ = 3 V_ch/S_eff = 3 V_ch/(S_LD + S_AP) = 0.5 · τ₁

Due to the installation of an auxiliary pump ( = partial-flow pump), the response time will always be shortened and the ultimate value of the signal will always be decreased. In the above example, the response time is halved but the ultimate value of the signal is halved as well.

A short response time means a quick change and display of the signal. This provides that advantage that the expenditure of time required for leak detection can be significantly reduced. The consequential downside that the ultimate value of the signal is smaller does, in most cases, not result in any severe problems due to the very high sensitivity of today‘s leak detectors.

Conclusion: Partial-flow operation reduces the expenditure of time for leak detection!

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.

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