Helium measurement units

We consider a thin, horizontal, fused-silica plate of area. I and thickness H. Initially, both horizontal surfaces of the plate are in contact with air (Figure 2.3c). We assume that the air is completely soluble in silica. At time t 0. the air below the plate is replaced by pure helium, which is appreciably soluble in silica. The helium slowly penetrates into the plate by molecular motion, and eventually appears in the air above the plate. This molecular transport of one substance relative to another is known as diffusion. The air above the plate is removed rapidly, so that there is no measurable helium concentration there. In this system, the index / shows helium and j shows silica, and the concentrations are given by the mass fractions w, and i. respectively. Eventually, the concentration profile tends toward a straight line with increasing t, and we have w, = r(U at the bottom surface, and w, = 0 at the top surface of the plate. At steady-state diffusion, the molar flow vector that is the flow rate of helium per unit area j, is proportional to the concentration gradient in terms of the mass fraction V r,... 

The principal source of helium is certain natural gas fields. The helium contents of more than 10,000 natural gases in various parts of the world have been measured (9). Helium concentrations of a few are Hsted in Table 2. In the United States, recovery of helium is economical only for helium-rich gases containing more than about 0.3 vol % belium. Most of the United States helium resources are located in the midcontinent and Rocky Mountain regions, and about 89% of the known United States supply is in the Hugoton field in Kansas, Oklahoma, and Texas the Keyes field in Oklahoma the Panhandle and Cliffside fields in Texas and the Riley Ridge area in Wyoming (11). 

The true density of a solid is the average mass per unit volume, exclusive of all voids that are not a fundamental part of the molecular packing arrangement [55]. This density parameter is normally measured by helium pycnometiy, where the volume occupied by a known mass of powder is determined by measuring the volume of gas displaced by the powder. The true density of a solid is an intrinsic property characteristic of the analyte, and it is determined by the composition of the unit cell. 

These units are the most sensitive and also provide the greatest degree of certainty. Flere certain is intended to mean that there is no other method with which one can, with greater reliability and better stability, locate leaks and measure them quantitatively. For this reason helium leak detectors, even though the purchase price is relatively high, are often far more economical in the long run since much less time is required for the leak detection procedure itself. 

Here the points suspected of leaking at the pressurized test specimen (see Fig. 5.4, d) are carefully traced with a test gas probe which is connected with the leak detector by way of a hose. Either helium or hydrogen can be detected with the INFICON helium leak detectors. The sensitivity of the method and the accuracy of locating leaky points will depend on the nature of the sniffer used and the response time for the leak detector to which it is connected. In addition, it will depend on the speed at which the probe is passed by the leak points and the distance between the tip of the probe and the surface of the test specimen. The many parameters which play a part here make it more difficult to determine the leak rates quantitatively. Using sniffer processes it is possible, virtually independent of the type of gas, to detect leak rates of about 10 mbar l/s. The limitation of sensitivity in the detection of helium is due primarily to the helium in the atmosphere (see Chapter 9, Table VIII). In regard to quantitative measurements, the leak detector and sniffer unit will have to be calibrated together. Here the distance from the specimen and the tracing speed will have to be included in calibration, too. 

Tritium 0-10 tritium units 0.01 tritium units Measure ashore Helium-3 ingrowth a>... 

Helium-3 is a decay product of radioactive tritium (3H, half-life = 12.44 years) that has been produced by nuclear bombs as well as naturally by cosmic rays in the upper atmosphere. Because virtually all 3He atoms escape from the surface ocean to the atmosphere, the 3He/tritium ratio in subsurface seawater samples indicates the time since the water s last exposure to the atmosphere. Both 3He and tritium are measured by gas mass spectrometry. Alternatively, tritium may be measured by gas counting with a detection limit of 0.05 to 0.08 tritium unit, where 1 tritium unit represents a 3H/H ratio of lxl0 18. A degassed water sample is sealed and stored for several months to allow the decay product 3He to accumulate in the container. The amount of 3He is then measured by mass spectrometry, yielding a detection limit of 0.001 to 0.003 tritium unit when 400-gram water samples are used. With this technique, the time since a water mass left the surface can be determined within a range from several months to 30 years. 

During the last decade a different track has been followed for the tritium dating method, namely, measurement of the decay product 3He (section 10.1) as a substitute for the initial tritium concentration. Tritium-derived 3He (tritiogenic helium) is built up in the groundwater as the contained tritium disintegrates hence, the 3H 3He ratio decreases with age. To facilitate the calculations both nuclides are expressed in TR (tritium ratio) units, that is, 3H H and 3He H ratios of 10-18. In principle, the age of groundwater can be determined by measuring the tritium concentration left... 

Before being used for flow experiments, each disk was tested with helium for acceptability. The rate of helium flow at 10-cm. pressure gradient across the disk was measured at several mean pressures between 5 and 50 cm. Unless the mass of gas passing per unit time was constant—i.e., the flow through the disk followed Knudsen s law—the disk was rejected. About one disk in ten passed this test the others showed increased flow at high pressures. 

The porosity of a catalyst or support can be determined simply by measuring the particle density and solid (skeletal) density or the particle and pore volumes. Particle density pp is defined as the mass of catalyst per unit volume of particle, whereas the solid density p, as the mass per unit volume of solid catalyst. The particle volume Vp is determined by the use of a liquid that does not penetrate in the interior pores of the particle. The measurement involves the determination by picnometry of the volume of liquid displaced by the porous sample. Mercury is usually used as the liquid it does not penetrate in pores smaller than 1.2/m at atmospheric pressure. The particle weight and volume give its density pp. The solid density can usually be found from tables in handbooks only in rare cases is an experimental determination required. The same devices as for the determination of the particle density can be used to measure the pore volume V, but instead of mercury a different liquid that more readily penetrates the pores is used, such as benzene. More accurate results are obtained if helium is used as a filling medium [10]. The porosity of the particle can be calculated as ... 

The size of the kelvin, the SI temperature unit with symbol K, is defined by the statement that the triple point of pure water is exactly 273.16 K. The practical usefulness of the thermodynamic scale suffers from the lack of convenient instruments with which to measure absolute temperatures routinely to high precision. Absolute temperatures can be measured over a wide range with the helium-gas thermometer (appropriate corrections being made for gas imperfections), but the apparatus is much too complex and the procedure much too cumbersome to be practical for routine use. 

Thirds I want you to take a look at the units of the quantities shown in the control har. The pressure is measured in the unit atm. This is not a reference to quick cash hut rather an ahhreviation for atmospheres. One atmosphere is a pressure roughly equal to the air pressure at sea level. Volume is measured in liters a unit with which you should he familiar. The third and fourth control bars indicate the number of atoms of helium and neon that are present. The unit is mol which stands for the word mole. For now just think of this number as an indicator—not an exact count—of the number of atoms in either the simulation or the real gas the simulation represents. For example the default value of the number of moles of helium is 1.0. Clearly, there s more than one atom of helium in the simulation. Later on, you 11 find out how many atoms of a real gas this 1.0 represents (a lot ). The temperature is measured in degrees Kelvin, or K. Water freezes at 273.16 degrees Kelvin, which is 0 degrees Celsius or about 32 degrees Fahrenheit. 

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