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Critical Assessment of Data using MTDATA

CONTENTS
MTDATA DATA ASSESSMENT COURSE

The aim of the two day course is be to introduce to those attending the techniques involved in the derivation of thermodynamic parameters for use in phase equilibrium calculations - data assessment - and so give them the capability of building their own databases or plugging gaps in existing databases. Particular emphasis will be placed on the use of the ASSESSMENT module of MTDATA designed to optimise the values of chosen thermodynamic parameters to give the best possible agreement with experimental data.

The course covers:

  • The types of experimental data required.
  • The typical steps taken during data assessment.
  • Data assessment tools available within MTDATA.
  • The use of the ASSESSMENT module of MTDATA.
  • Hands-on experience of data assessment.
  • ASSESSMENT module handbook and documented examples.
  • No previous knowledge of data assessment is necessary to attend the course but some familiarity with the basic MTDATA commands used to set up phase equilibrium calculations is essential.

    If you are interested in reserving a place (or places) on the course in September, or if you would like further information about MTDATA courses, please contact John Gisby either by e-mail (John.Gisby@npl.co.uk) or telephone (+44 (0)20 8943 7098).

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    MTDATA DATA ASSESSMENT CLUB

    At the MTDATA data assessment club meeting / workshop, attended by existing and prospective users of the ASSESSMENT module of MTDATA, NPL staff introduced new features of the ASSESSMENT module and offered guidance in its use, illustrated by examples appropriate to oxide and aqueous systems. Malcolm Rand of WintersHill Consultancy provided a user's perspective, describing data assessment work he had performed using MTDATA. A lively discussion followed, featuring questions about the ASSESSMENT module and suggestions for future enhancements. The possibility of setting up an MTDATA data assessment club was also considered.

    An informal MTDATA data assessment club, along the lines of the MTDATA users' group, was generally held to be a good idea, allowing users of the ASSESSMENT module to share ideas and experiences with each other and with NPL. Annual meetings supplemented by a newsletter or a bulletin board were recommended by those present.

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    QUESTIONS AND ANSWERS

    The following are some of the questions asked and suggestions made at the MTDATA data assessment club meeting / workshop at NPL on 30th November 2000 with responses from NPL.

    Q Should the default optimisation algorithm in MTDATA be changed to UPOLYT ?
    A
    In general UPOLYT works best when parameters are far from their final values and LSQNDN (Ellis) and LCQNDF (Elsie) work best when parameters are close to their final values. Automatic scaling of parameters within LSQNDN should eventually perform a similar job to UPOLYT so giving good performance in both regimes. LSQNDN will then become the default algorithm. It should be noted that there is a restriction on the number of experimental data items accepted by LSQNDN, currently 1000. No such limits apply with UPOLYT except when examining results graphically.
    Q Can phase names be supplied as subscripts to keywords in macros?
    A
    Not at present. The keyword plus subscript needs to be supplied, for example
    "class.mac" phase(liquid)
    where the macro class.mac contains the command,
    classify absent ********
    !
    Q Can I plot experimental property against calculated value in the ASSESSMENT module?
    A
    Yes. In version 4.72 of MTDATA using the plot residuals and plot correlations commands. See NEW FEATURE - CORRELATION AND RESIDUAL PLOTS for more details and examples.
    Q I would like to be able to identify experimental points by property or by reference
    Q What is the recommended syntax for labelling experimental points?
    A
    A facility for identifying experimental points by property or reference was introduced after the initial ASSESSMENT module handbook had been released. It is documented in the on-line help of MTDATA version 4.72. The recommended syntax for labels is
    property-reference:condition
    Further details are given in the HINTS AND TIPS section.
    Q Do you plan to allow component amounts to be set from user variables?
    A
    Yes. In version 4.73 of MTDATA users will be able to set component amounts and component masses from user variables by means of commands like
    set n(1) user_variable(amount1) !
    set w(1) us_var(mass_of_Cu) !
    Q Do you plan to provide a graphical user interface for entering experimental information?
    A
    Other parts of MTDATA have a higher priority for implementation within the graphical user interface at the moment. Setting up experiments using standard MTDATA commands typed into a text file allows a wide range of different types of experiment to be considered. This flexibility could not be reproduced easily within a simple GUI.
    Q I would like "site fraction" to be available as an experimental property.
    Q I would like phase boundary searches with composition as a variable to be available within the ASSESSMENT module in the same way as phase boundary searches with temperature as a variable.
    Q I would like "heat capacity" and "entropy" to be available as experimental properties.
    A
    All are being considered as future enhancements to the ASSESSMENT module. Meanwhile, phase boundary compositions can often be entered in experimental data files as tie-line ends and heat capacities and entropies as combinations of Gibbs energy and enthalpy values.
    Q Can I specify my own value for the "penalty function"?
    A
    The "penalty function" is the error assigned to experiments where temporary changes in parameters during the optimisation process make calculated results impossible to obtain. Its default value of 1012 is designed to ensure that such changes to parameters are quickly reversed. The value is stored in a user variable with the name "penalty_function" so a new value, for example zero, can be specified as follows
    set user_variable(penalty_function) 0 !
    Q Do you plan to write a general function evaluator to supplement (or replace) the RATIO, PRODUCT, DIFFERENCE, SUM, LOG10 and LN operators now available?
    A
    This is a long term aim and preliminary work has already started. Note that new EXP and SQUARE_ROOT operators were added in version 4.72 of MTDATA.
    Q Can you arrange for the list parameter command to list parameters in numeric order rather than in the order they were selected for change?
    A
    This will be done in version 4.73 of MTDATA. It should make individual parameters easier to find in long lists.
    Q Why do I get an error message when trying to LINK the values of two parameters?
    A
    The target parameter, that being linked to, must have already been flagged as FIXED, OPTIMISED or STEPPED. Specify the nature (or TYPE) of the target parameter first of all, then link to it, and there should be no problem. For example
    modify index 92 type optimised ! modify index 98 link 92 !
    Q Can corresponding parameters valid over different temperature ranges be LINKed together automatically?
    A
    The LINKing of such parameters will be automatic in version 4.73 of MTDATA.
    Q Is it possible to express experimental uncertainties as percentages?
    A
    This facility will also be available in version 4.73 of MTDATA. The current plan is for a percentage uncertainty to be entered as a negative number. An example is given below in which the stated uncertainty on the measured activity value is 1%.
    list property activity(Fe) value 0.456 uncertainty -1 !

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    HINTS AND TIPS - LABELS OF EXPERIMENTS

    It is convenient to assign labels to individual experiments being considered during an ASSESSMENT session, so enabling them to be identified easily in graphical and tabular output. The recommended label syntax is property-reference:condition, where property indicates the property determined experimentally, for example act(Fe), Hmix, p(O2) or w(CaO), reference indicates the source of experimental data, for example Smith or 92JON and condition indicates conditions appropriate to the experiment undertaken, for example 1473K, 10-5atm or w(Fe)=0.3.

    Figure 1

    When a diagram is produced using the PLOT command of the ASSESSMENT module, comparing calculated and target data for selected experiments:

    • the property label is used to form the ordinate and in some cases abscissa annotation
      (if the selected experiments share a common property label)
    • experiments with the same reference and condition are indicated by the same symbol
    • points corresponding to different conditions are joined up as separate lines in
      plot value experiment !  diagrams

    The diagram above compares calculated values of the partial pressure of S2 above liquid sulphides with experimental data from two references indicated by BAL and MEY. The liquid sulphides in question contain a series of different Fe to (Fe + Ni) ratios the values of which are used as conditions in the experiment labels. The actual experimental property is 1/2 log p(S2) which appears in the ordinate annotation. Note that each separate line refers to a different Fe to (Fe + Ni) ratio or condition and that each different reference and condition combination has been assigned a different symbol. Typical labels used were 1/2 log p(S2)-BAL:0.50 and 1/2 log p(S2)-MEY:0.20.

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    NEW FEATURE - CORRELATION AND RESIDUAL PLOTS

    Users of the ASSESSMENT module of MTDATA (version 4.72) can now produce plots of calculated properties against experimental values using the plot correlations experiment ! command. An example of the type of plot produced is shown below.

    Different symbols are used for different combinations of reference and property (see HINTS AND TIPS for the recommended syntax for experiment labels). In the case of a perfect fit all of the symbols plotted should lie along the line calculated value = experimental value. This line is shown in red.

    Figure 2

    Using the new plot residuals experiment ! command, diagrams can be produced in which "errors" are plotted against a chosen abscissa variable. "Errors" are defined as follows:

    error = (calculated value - experimental value) / experimental uncertainty

    An example of this type of diagram can be seen below. Different symbols are once more used to indicate different combinations of reference and property. In the case of a perfect fit all of the symbols should lie along the error = 0 line, shown in red.

    Figure 3

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    FORTHCOMING EVENTS

    For a current list of forthcoming events please see the MTDATA News page.

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    CONTACT DETAILS

    If you would like further information about any of the items featured in this newsletter, if you have comments about the newsletter or if you would like to contribute an article to a future newsletter, please contact:

    John Gisby
    NPL Materials Centre
    National Physical Laboratory
    Teddington, Middlesex
    TW11 0LW, UK

    Tel: +44 (0)20 8943 7098
    Fax: +44 (0)20 8614 0425
    Email: John.Gisby@npl.co.uk

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    Updated 23 April 2010