Fundamentals of Vacuum Technology (1248463), страница 52
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5.13 Partial flow principle3Signal riseQ =pS100%1,095%Leak rate150 `B) Response time: t95% = 3 · SV = 3 · 24.66` = 18.25 smThrottle100%95%Partial flow ratio = Fraction of the overall flow to the leak detectorQQLD1γ = LD ==QQHe QLD + QPFP 1 + nnnQQLD = γ · QHeQVSMSSignalamplitude3QV}γ ... Partial flow ratio0,5100%95%021Compensation period, e.g. t95% = 3 · τ = 3 V·S(τ = V ... Time constant)Sto3 · V = 1 (3 · V)Dead time S + S’ 2S3· VSTime3 · SV = 2 (3 · V )S/2Fig. 5.14 Signal responses and pumping speed121HomeLeak detection1.
Center: The specimen with volume of V is joined directly with the leakdetector LD (effective pumping speed of S).either due to one large leak or several smaller leaks Ð more gas flows intothe unit than the maximum permissible throughput rate for the leak detector.2. Left: In addition to 1, a partial flow pump with the same effectivepumping speed, Sl = S, is attached to the test specimen.5.73. Right: As at 1, but S is throttled down to 0.5×S.The signals can be interpreted as follow:1: Following a Òdead periodÓ (or Òdelay timeÓ) up to a discernible signallevel, the signal, which is proportional to the partial pressure for helium, willrise to its full value of pHe = Q/Seff in accordance with equation 5.9:Q ⋅ 1 − e τ Seff –tpHe =(5.9 )The signal will attain a prortion of its ultimate value aftert = 1 τ .
. 63.3 %t = 3 τ . . 95.0 %t = 5 τ . . 99.3 %t = 2 τ . . 86.5 %t = 4 τ . . 98.2 %t = 6 τ . . 99.8 %The period required to reach 95 % of the ultimate value is normally referredto as the response time.2: With the installation of the partial flow pump both the time constant andthe signal amplitude will be reduced by a factor of 2; that means a quickerrise but a signal which is only half as great. A small time constant meansquick changes and thus quick display and, in turn, short leak detectiontimes.3: The throttling of the pumping speed to 0.5 S, increases both the timeconstant and the signal amplitude by a factor of 2. A large value for t thusincreases the time required appropriately. Great sensitivity, achieved byreducing the pumping speed, is always associated with greater timerequirements and thus by no means is always of advantage.An estimate of the overall time constants for several volumes connectedone behind to another and to the associated pumps can be made in aninitial approximation by adding the individual time constants.5.6Leak detection techniquesusing helium leak detectors5.7.1 Spray technique (local leak test)The test specimen, connected to the helium leak detector, is slowly tracedwith a very fine stream of helium from the spray pistol, aimed at likelyleakage points (welding seams, flange connectors, fused joints), bearing inmind the time constant of the system as per Equation 5.8 (see Fig.
5.14).The volume sprayed must be adjusted to suit the leak rate to be detectedand the size and accessibility of the object being tested. Although helium islighter than air and therefore will collect beneath the ceiling of the room, itwill be so well distributed by drafts and turbulence induced by movementswithin the room that one need not assume that helium will be foundprimarily (or only) at the top of the room during search for leaks. In spite ofthis, it is advisable, particularly when dealing with larger components, tostart the search for leaks at the top.In order to avoid a surge of helium when the spray valve is opened (as thiswould ÒcontaminateÓ the entire environment) it is advisable to install achoke valve to adjust the helium quantity, directly before or after the spraypistol (see Fig. 5.15). The correct quantity can be determined easiest bysubmerging the outlet opening in a container of water and setting the valveon the basis of the rising bubbles.
Variable-area flowmeters are indeedavailable for the required small flow quantities but are actually tooexpensive. In addition, it is easy to use the water-filled container at any timeto determine whether helium is still flowing.The helium content of the air can also be detected with helium leakdetectors where large leaks allow so much air to enter the vessel that the 5Limit values / Specifications forthe leak detector1. The smallest detectable leak rate.2. The effective pumping speed at the test connection.3. The maximum permissible pressure inside the test specimen (alsothe maximum permissible inlet pressure).
This pressure pmax will beabout 10-1 for LDs with classical PFPs and about 2 to 10 mbar for LDswith compound PFPs. The product of this maximum permissibleoperating pressure and the pumping speed S of the pump system at thedetectorÕs test connection is the maximum permissible throughput:Qmax = pmax á Seff, connector(5.10)This equation shows that it is by no means advantageous to attain highsensitivity by throttling down the pumping speed. The maximum permissiblethroughput would otherwise be too small.
The unit is not functional when ÐAvoiding the Òhelium surgeÓ when the pistol valve is openeda) Throttle hose orb) Adjustable throttle valve ahead of the spay pistolMinimum helium quantity for correct display: Changing the setting for the throttle shall notaffect indication.The minimum quantity is always much smaller than one would set without a flowmeter (e.g.by listening for flow or letting the helium flow across moistened lips). The simplest checkwithout a flowmeter: Letting gas bubble through water.Fig. 5.15 Helium spray equipment122HomeLeak detectionppm share of helium in the air is sufficient for detection purposes. The leakrate is then:Display (pure He) Display (atmosph. He)ÑÑÑÑÑÑÑÑÐ = ÑÑÑÑÑÑÑÑÑÑÐ15 á 10-6QL = Display (pure He)= 2 á 10+5 á Display (atmospheric He)(5.11)5.7.2 Sniffer technology(local leak test using the positivepressure method)Here the points suspected of leaking at the pressurized test specimen (seeFig.
5.4, d) are carefully traced with a test gas probe which is connectedwith the leak detector by way of a hose. Either helium or hydrogen can bedetected with the INFICON helium leak detectors. The sensitivity of themethod and the accuracy of locating leaky points will depend on the natureof the sniffer used and the response time for the leak detector to which it isconnected.
In addition, it will depend on the speed at which the probe ispassed by the leak points and the distance between the tip of the probeand the surface of the test specimen. The many parameters which play apart here make it more difficult to determine the leak rates quantitatively.Using sniffer processes it is possible, virtually independent of the type ofgas, to detect leak rates of about 10-7 mbar á l/s.
The limitation of sensitivityin the detection of helium is due primarily to the helium in the atmosphere(see Chapter 9, Table VIII). In regard to quantitative measurements, theleak detector and sniffer unit will have to be calibrated together. Here thedistance from the specimen and the tracing speed will have to be includedin calibration, too.5.7.3 Vacuum envelope test(integral leak test)Vacuum envelope tests are integral leak tests using helium as the test gas,in which the test specimen is enclosed either in a rigid (usually metal)enclosure or in a light plastic envelope. The helium which enters or leaves(depending on the nature of the test) the test specimen is passed to ahelium leak detector, where it is measured. Envelope tests are made eitherwith the test specimen pressurized with helium (Fig. 5.4c) or with the testspecimen evacuated (Fig.
5.4a). In both cases it may be necessary toconvert the helium enrichment figure (accumulation) to the helium standardleak rate.5.7.3.1Envelope test – test specimen pressurizedwith heliuma) Envelope test with concentration measurement and subsequentleak rate calculationTo determine overall leakiness of a test object pressurized with helium theobject shall be enclosed in an envelope which is either rigid or deformable(plastic). The test gas leaving the leaks accumulates so that the heliumconcentration in the envelope rises. Following an enrichment period to bedetermined (operating period) the change in concentration inside theenvelope will be measured with a sniffer connected to the helium detectionunit.
The overall leak rate (integral leak rate) can be calculated followingcalibration of the test configuration with a reference concentration, e.g.atmospheric air. This method makes it possible to detect even the smallestoverall leakiness and is suitable in particular for automated industrial leaktesting. Due to gas accumulation, the limits for normal sniffer techniquesare shifted toward lower leak rates and the ambient conditions such astemperature, air flow and sniffer tracing speed lose influence. When usingplastic envelopes it is necessary to take into account helium permeationthrough the plastic envelope during long enrichment periods.b) Direct measurement of the leak rate with the leak detector(rigid envelope)When the test specimen, pressurized with helium, is placed in a rigidvacuum chamber, connected to a helium leak detector, the integral leak ratecan be read directly at the leak detector.5.7.3.2Envelope test with test specimenevacuateda) Envelope = Òplastic tentÓThe evacuated test specimen is surrounded by a light-weight (plastic)enclosure and this is then filled with helium once the atmospheric air hasbeen removed.
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