Critical Assessment of Data using MTDATA
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
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).
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
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.
||Should the default optimisation algorithm in MTDATA
be changed to UPOLYT ?|
- 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.
||Can phase names be supplied as subscripts to
keywords in macros?|
- Not at present. The keyword plus subscript needs to be
supplied, for example
- where the macro class.mac contains the command,
classify absent ********
||Can I plot experimental property against calculated
value in the ASSESSMENT module?|
- 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.
||I would like to be able to identify experimental
points by property or by reference|
||What is the recommended syntax for labelling
- 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
- Further details are given in the HINTS
AND TIPS section.
||Do you plan to allow component amounts to be set
from user variables? |
- 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) !
||Do you plan to provide a graphical user interface
for entering experimental information? |
- 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
||I would like "site fraction" to be available as an
||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. |
||I would like "heat capacity" and "entropy" to be
available as experimental properties.|
- 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.
||Can I specify my own value for the "penalty
- 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 !
||Do you plan to write a general function evaluator to
supplement (or replace) the RATIO, PRODUCT, DIFFERENCE, SUM,
LOG10 and LN operators now available? |
- 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.
||Can you arrange for the list parameter command to
list parameters in numeric order rather than in the order they
were selected for change?|
- This will be done in version 4.73 of MTDATA. It should
make individual parameters easier to find in long lists.
||Why do I get an error message when trying to LINK
the values of two parameters?|
- 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 !
||Can corresponding parameters valid over different
temperature ranges be LINKed together automatically?
- The LINKing of such parameters will be automatic in
version 4.73 of MTDATA.
||Is it possible to express experimental uncertainties
as percentages? |
- 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
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
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.
When a diagram is produced using the PLOT command of the
ASSESSMENT module, comparing calculated and target data for selected
- 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
log p(S2)-BAL:0.50 and
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.
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) /
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.
For a current list of forthcoming events please see the MTDATA News
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:
NPL Materials Centre
TW11 0LW, UK
(0)20 8943 7098
Fax: +44 (0)20 8614 0425