VacTran 3 Manual (779748), страница 24
Текст из файла (страница 24)
time data. Both dialogs look and work inmuch the same way.Alternatively, use the speed button as shown below:© 2011 Professional Engineering Computations22811.6VacTran 3Raw data dialog descriptionTwo input fields are given at the top of the dialog, one for gas load and one for elapsed time. Initially, the data list isempty, and the input fields are filled with default values. Pull down menus changes time units and gas load units(pressure, volume and time). These can be used at any time to convert all data and update the graph window.The data list in the center of the dialog contains a scrolling window of gas load data.
In a new gas load model, thislist will be initially empty. Use the scroll bar to move up and down the list. Clicking on one row of the list willupdate the input fields at the top of the dialog. To change the value of these numbers, click in each input field andedit the number as you would with a word processor. Add it to the list using the Add command, or replace theselected list value with the Replace command.Two pull-down menus are provided to change the number format of the displayed data. Buttons on the right side ofthe dialog are provided for the basic editing functions. These Add, Replace, and Delete commands are alsoavailable under the Model menu.© 2011 Professional Engineering ComputationsRaw data gas load models11.7229Raw data dialog commandsAdd (Ctrl+A): Adds the data in the input fields to the list.
If the global graph update option is selected, and at leasttwo data points are in the list, the graph window is updated. Pressing the Return key has the same effect asclicking on Add. VacTran will sort the data according to the sort option currently selected. It is not necessary toenter an even spacing of data. Since VacTran uses interpolation to determine gas load between data points, astraight-line section of a curve may be represented by two points. Other areas might be represented by a largenumber of points to obtain a smoother curve. Model size is limited only by available memory and disk space, butusually no more than 10 to 20 points are needed.The Replace, Delete, and Multiply commands are available only if there is data in the list.Replace (Ctrl+R): Replace the currently highlighted list selection with the data in the input fields.
The list isresorted according to the current sort option, available under the Defaults menu. This option will be dimmed if themodel is empty.Delete (Ctrl+D): Delete the currently highlighted list selection. This option will be dimmed if the model is empty.Multiply:Multiply the entire list of gas load data by a specified value. The dialog shown below is used to enter the multiplier.A multiplier of 2 or more can be used if several of these gas sources are to be added together in parallel. Themultiplier only affects the gas load data, while the pressure data remains unchanged. The multiplier value must begreater than 0.
This option will be dimmed if model is empty.Data from file: It is useful to combine raw gas load files under certain circumstances. This command allowsanother external file to be added to the current one. Where data overlaps in time, it is added together.The resulting gas load model will contain the number of entries equal to the larger of the two files, and ranging fromthe highest to the lowest elapsed time for the two combined gas loads.
Resulting data is interpolated from the datafrom each gas load model.© 2011 Professional Engineering Computations23011.8VacTran 3Menu commands for gas load modelsIf there is more one entry in the raw gas load model, the following options are available in the pull down menus:command:Add, Replace, and Deletemenu: ModelFunctions perform exactly as described in the dialog commands section.© 2011 Professional Engineering ComputationsRaw data gas load models231command:Gas load vs timemenu: GraphGas load vs time for the gas load model is plotted on the currently selected axes in the Primary Graph Window.Gas load is in current pressure-volume/time units.
Gas loads are usually plotted either in Y log - X linear or log-log.© 2011 Professional Engineering Computations23211.9VacTran 3Right-click optionsClicking the right mouse button will show this menu:Sort time up and Sort time down determine the order that gas load data is displayed. The data is not affected.Graph controls… activates the dialog for graphing optionsUpdate open system models searches for open system models and conductance studies that are using this gasmodel, and forces an update.
This function happens automatically when you save the model after changes.© 2011 Professional Engineering ComputationsRaw data gas load models23311.10 Opening existing raw data modelsTo open an existing Raw Data Gas Load: Under the File menu, select the Open... command, or click on the iconas shown:In the Open Document dialog that appears, select Raw Data Gas Load and click on OK.A file selection dialog then allows you choose the directory and name of the raw data file.© 2011 Professional Engineering Computations234VacTran 311.11 Raw data gas load exampleAn outgassing curve is one in which the gas load typically decreases with time.
The relationship is often linear on alog-log scale, with gas load decreasing a decade with every decade of time.An example model is included in the examples directory, called Example1.raw, which is a simple representation ofa typical gas load.Example1.raw is shown on a log-log axis.common units are atm-cm3/second.Although torr-liters/second is the gas load units used here, other11.12 Raw data gas load file formatThe raw data model is stored as an ASCII text file, and can be edited with most word processors. If you prefer touse a word processor to enter all the data, save the file as text. Change the text file extension from "TXT" to"VTRAW", and then open the file. When editing files outside VacTran, follow the formatting guide below or VacTranwill report an error reading the file.The following format is required for raw data gas loads. Time is in seconds and gas load is in torr-liters/second.[ Gener al ]Tot al =3[ 0]Ti me=1Load=1e- 6[ 1]Ti me=100Load=1e- 7[ 2]Ti me=1000Load=1e- 8© 2011 Professional Engineering ComputationsOut gas libraries12235Out gas librariesEvaporation of liquids and desorption of gases in a vacuum system are principal contributors to the phenomenoncalled outgassing.
In VacTran, outgassing does not include leaks, which are addressed separately.Even under controlled conditions, a vacuum system might not out gas at a consistent rate each time it is operated.Prediction of the performance of such a moving target can be challenging. Out gassing can be the most difficultcomponent of total gas load to quantify consistently and accurately, because a number of factors can contribute towide variations.The longer a vacuum system operates, the lower the rate of outgassing.
For this reason, some systems aredesigned to stay at low pressures for long periods, so that once outgassing has decreased to an acceptable level,the process can operate indefinitely. These vacuum vessels are usually designed with at least one "load lock", anadjacent vacuum vessel used as an interface to the outside world. The effects of outgassing on the process vesselare then minimized.The principal constituent of initial outgassing for many materials is water vapor. When a material is exposed to theatmosphere, water vapor is adsorbed into its surface.
At low pressures, water contributes significantly to the totalinitial gas load. Over time, depending on surface conditions, temperature, and the previous amount of atmosphericexposure, the rate of water vapor outgassing diminishes, and bulk outgassing dominates. Outgassing curves oftenhave a change of slope, usually after 1 to 10 hours, which reflects the transition from water vapor outgassing to bulkoutgassing from the interior of the material.The outgassing process can be sped up by a technique called baking. Heat is applied to the vessel walls or thepart in question, resulting in dramatically faster evaporation and desorption rates.
VacTran does not have thecapability to calculate the effect of baking on a vacuum system directly. If the increased outgassing rates formaterials are known from experimentation, they certainly can be modeled.See also:Why use an out gas library?Units of measure for out gassingCaveats - out gas librariesDefinition of out gas dataGeneral formula for out gassingExample CalculationGas load start and stop timeCreating out gas librariesOut gas library dialog DescriptionOut gas library dialog commandsOpening out gas librariesOut gas library format12.1Why use an out gas library?Out gas libraries are a convenient way to store data on a variety of candidate materials which would have potentialuse in a vacuum system.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
