Tests outgassing

In the standard outgassing test, the oven was operated at 150°C for a period of 225 hours. This is not an unrealistic test temperature since thermal solar collectors can reach 125-200°C,... 

Figure 4.2 Scheme of the stainless steel high vacuum bakeable bench equipped with mass spectrometer for outgassing tests on vacuum components. IG ionisation gauge. 

Before being used in a vacuum panel, the open cell PU foam needs a preliminary heat treatment in air, generally carried ont at 120-150 °C for 10-60 minutes to remove water and other volatile species which otherwise would desorb and rapidly cause the vacuum to deteriorate. The result of a typical outgassing test carried out at 23 °C on a foam sample baked at 120 °C for 30 minutes is shown in Figure 4.4 for all desorbed gases but water. Water is difficult to quantify since it sticks to the walls of the system and only partially reaches the mass spectrometer. Water can be estimated as the difference between the total absolute pressure and the sum of the partial pressures of the other gas species, which can be accurately quantified with the mass spectrometer. 

Product Name/ Supplier Description Viscosity (cps) Shelf life (-40°C), Pot Life (25 C) Cure Schedule hr °C Hardness (Shore A, unless indicated) NASA Outgassing Test Results (ASTM E 595) Lap- Shear Strength (psi) CTE (ppm/°C) A fC) Volume Resistivity (ohm-cm) Thermal Conductivity (W/m-K)... 

To assure safety and reliability, NASA and other government agencies require all pol5mieric materials to be qualified and, as a minimum, to pass outgassing tests defined in ASTM-E-595 This specification defines two... 

We would like to thank Shannon Novotny for molding the samples, Chris Lewis for performing the outgassing testing, and Entegris management for support of this project. 

Figure 1. Schematic diagram of apparatus used for outgassing testing.

Fig. 10 shows that the flow stress of the hydrogen-alloyed compacts is essentially less than that of the outgassed ones at all test temperatuics. The flow stress relation between the hydrogen-alloyed and outgassed compacts depended on the strain. At equal strains at test temperatures, this ratio could achieve 2 or more. Thus, the effect of hydrogen on the properties of compacted powders is much similar to that observed on bulk titanium. 

Porosity characteristics were determined by N2 adsorption-desorption at 77 K (Dubinin method) with a Thermoquest Sorptomatic 1990. Powder samples were outgassed (10 4 Torr) and heated to 450°C before each test. 

The specific surface area of the fibers was determined using inert gas adsorption in a commercial volumetric adsorption system (Micromeritics Instrument Corp.). Krypton gas was used because of its sensitivity to the small specific surface areas of the glass fibers ( 0.2 mz/g). The fibers were degassed at 100°C to a pressure of 80mTorr before introducing the adsorbate gas into the sample chamber. Several samples were also outgassed at 80 and 200°C (to 80 mTorr) to confirm that outgassing was sufficiently complete under the standard test conditions. A standard five-point surface area determination was made for each inert gas adsorption experiment. 

When a vacuum system is suspected of having a leak, one of the first tests is to determine whether the leak may be caused by outgassing. An easy way to determine this is to chart the rate of pressure loss versus time. To chart this rate, obtain the lowest vacuum you can in a reasonable amount of time, then close the section in question from the pumping section by a stopcock or valve. Next, periodically over a few minutes, or an hour or two (or three), note the pressure and elapsed time. As seen in Fig. 7.55, a real leak will indicate a constant rate of pressure rise over time while an outgassing problem will indicate a decreasing rate of pressure rise over time. 

Apply the respective system factors. Studies and experience show that the calculated pump-down time for a vacuum system must be corrected by an appropriate system factor. This factor makes allowance for the normal outgassing of surfaces exposed to atmospheric air. It also provides a basis forjudging whether a system is pumping down normally or whether some problem exists that must be corrected. Table 6.33 lists typical system factors that have proven reliable in many tests. To use the system factor for any pump, apply it this way ta = tS. where ta is actual pump-down time, in min t is computed pump-down time from step 3, in min S is system factor for the type of pump being considered. 

It should be kept in mind that any change in surface area, surface chemistry, or microporosity will result in a change in the energy of immersion. Because immersion calorimetry is quantitative and sensitive, and because the technique is not too difficult to apply in its simplest form, it can be used for quality testing. The preliminary outgassing requires the same care as for a BET measurement, but, from an operational standpoint, energy of immersion measurements are probably less demanding than gas adsorption measurements. 

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