Standard leak

Disposable paper bags (standard, leak-resistant, and L 75-210 120 mm 380Wx300dx 1100 H... 

STANDARD LEAK TEST PRESSURE setting 3 PERCENT... 

Leak <—> Hole Q... Leak rate, In short Leak Substance quantity trhough hole per unit of Ume Definition 0 = At Heliim standard leak rate p, = 1 bar. P2 < 1 mbar (Ap = 1 bar) Test gas s Helium... 

Fig. 53 Examples for conversion into helium standard leak rates...

Test leaks (also known as standard leaks or reference teaks) normally comprise a gas supply, a choke with a defined conductance value, and a valve. The configuration will be in accordance with the test teak rate required. Figure 5.9 shows various test leaks. Permeation teaks are usually used for leak rates of 10 ° < < 10, capillaries, between 10 and 10 ... 

N cuum 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 a helium leak detector, where it is measured. Envelope tests are made either with the test specimen pressurized w/ith helium (Fig. 5.4c) or with the test specimen evacuated (Fig. 5.4a). In both cases it may be necessary to convert the helium enrichment figure (accumulation) to the helium standard leak rate. 

Although on occasion it may be useful to know the relative size of one leak to another, the specific size of a leak is irrelevant. If the leak is large in relation to the standard leak, it should be repaired. On the other hand, if the leak is close to the size of the standard leak (and your system does not get below 10 7 torr), there is little reason to repair the leak. 

Even if there is no reason to quantify any given leak, it is still important to establish a leak limit for the helium leak detector before leak hunting begins. This limit is set with a standard leak. 

A standard leak is a small container that leaks helium at a very slow and specific amount. As mentioned before, helium diffuses through borosilicate glass (or through ceramic or capillary tube). This diffusion is very consistent at any given pressure and temperature. Within the standard leak is a thimble of borosilicate glass (see Fig. 7.62) around which is the stored helium. The helium leak rate var-... 

Standard leaks come from the factory with their calibrated leak rate imprinted on their sides. The approximate annual attenuation is also indicated. After a certain amount of time (every 5 to 10 years) they can be returned to the factory for either recharging and/or recalibration. Recalibration is not critical if the helium leak detector is being used only for leak location and not leak calibration. 

Standard leaks should be handled carefully and not dropped because sharp hits can break the glass inserts. They should not be stored with their valves closed. Otherwise, an artificially high accumulation of helium would be created in the area between the valve and glass tube. The valve provides temporary stoppage of helium calibrations between its use during leak readings. 

The response time for a helium leak detector is the time required for a known leak rate to be indicated on the leak detector from zero to 63% of its maximum equilibrium level. 87 Of course the helium leak detector must be calibrated with a standard leak so that the maximum reading can be properly set on the leak detector. The response time is dependent on the quality of vacuum, the size of the leak, physical barriers (such as constrictions, bends, or traps) between the leak and the leak detector, and the proximity of the leak to the leak detector. Because of so many factors, there can be no set specific response time. 

In spite of these lesser annoyances, I moved in some equipment and began hooking it up. To supply helium to a gas chromatograph, I connected a cylinder via copper tubing to the supply line, turned on the cylinder, set the pressure at fifty psi, went in to the lab to get the air out of the lines, turned the valve — and took a shower without soap. The standard leak test is — or was — with water. No one had suggested to the contractor that the stuff ought to be dumped. 

The apparatus used in this study consisted of a vacuum chamber containing a spherical cryosurface and an independent gas addition system. The gas addition system, shown in Fig, 1, comprised a surge tank, a barometer for measuring the upstream gas pressure, and a series of five calibrated standard leaks, with individual valves, arranged in parallel, permitting individual use of each leak or any combination of leaks. The entire gas addition system has a separate pumping station. 

The vacuum chamber (Fig. 2) is a 20 x 39-in. stainless steel cylinder of all heliarc-welded construction. The chamber is suspended in an insulated aluminum tank. The chamber volume after correction for the pump lines and cryosphere was calculated to be 202 liters. This chamber size makes possible the use of commercially available standard leaks, without introducing significant errors in the time measurement when calibrating the gas analyzer by the rate of pressure rise method. The 7-in.-diameter cryosphere has a calculated surface area of 995 cm . The coolant transfer lines to the cryosurface are vacuum-jacketed to prevent possible condensation. 

C = capture coefficient k = Boltzmann constant, ergs/°K M = molecular weight, g/mole n/m = fraction of molecules with energy greater than j Pe = pressure in the chamber, mm Hg Po = pressure in front of standard leaks, mm Hg Pi = vapor pressure of condensate at mm Hg P = universal gas constant, ergs/mole-°K S =s pumping speed, liters/cm -sec Te= temperature of chamber, °K Tff= temperature of the gas, °K To = temperature of the standard leaks, °K... 

The number of defective fuel elements can be calculated by dividing the corrected iodine activity concentration ( cotr,i-i3i) by the concentration of the same nuclide observed with a standard leak A st). 

Leak rates down to 10 Torr-liters/sec of nitrogen can be detected using helium leak detection methods. For accurate measurement the leak detector must be calibrated with a standard leak. 

---