‘Deleting’ data points using the Windower NOVA Technical Note 3

NOVA Technical Note 3
‘Deleting’ data points using the Windower
Case study: how to delete invalid data points in FRA data?
In NOVA, data points cannot be deleted. This is because NOVA is developed to be
GLP compliant (GLP stands for Good Laboratory Practice). The GLP guidelines
dictate that the original raw data must always be visible. This means that no data
points can be removed from a NOVA data set.
NOVA does however provide a number of data handling tools that can be used to
extract data points from a measurement. Technical note #1 illustrated how the
Windower tool can be used to extract a small selection of data points for data
analysis purposes.
In this case study, the Windower will be used again to illustrate how invalid data
points can be ignored in order to use the data analysis tools more efficiently. More
information on the Windower can be found in the NOVA user manual.
1 – FRA frequency scan
To illustrate this case study, a .nox file is provided. Import the Fuel cell test data
[NOVA 1.10].nox file as data into your database and load the data into the
explorer frame.
This data was originally acquired using the FRA software. It was obtained in
combination with a TDI RBL488 – 300 A electronic load. The data was recorded
on a single large area fuel cell, at 120 A DC current, using a 9 A AC amplitude.
The fuel cell was fed with pure oxygen during the measurement.
Note
More information about the PGSTAT/FRA + electronic load combination can be
found in application note FC03, FC04 and FC05.
Figure 1 shows the original Nyquist plot for this measurement.
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Figure 1 – Original Nyquist plot for the fuel cell data
The Nyquist plot clearly shows the typical shape for a PEM fuel cell impedance
measurement. Inductive behaviour is observed at high frequency and an O-type
diffusion contribution is measured at low frequency. However, one data point
(highlighted in Figure 1) seems to be invalid. In order to obtain reliable data from
the fitting algorithm, it might be required to ignore this data point.
Figure 2 shows the Bode plot for the same measurement. The invalid data point is
located in the sub-hertz range and could be due to instabilities of the fuel cell
during the measurement.
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Figure 2 – The original Bode plot for the fuel cell data
This technical note will illustrate how the Windower tool can be used to ignore
the invalid data point.
2 – Identifying the invalid data point
Select the Bode plot modulus and right-click on the plot. Choose the Step through
data option from the context menu (see Figure 3).
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Figure 3 – Choosing the Step through data option
The mouse pointer will change shape from an arrow pointer to a ‘finger’ pointer.
Click the invalid data point on the Bode plot to display its coordinates (see Figure
4).
Figure 4 – Locating the invalid data point in the Bode plot
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The label displayed below the invalid point in the Bode plot (see Figure 4) indicates
that this point was recorded at 0.63095 Hz and that it is data point #43 out of 51.
Press the insert key on the keyboard to add the label to the plot.
3 – Adding a Windower to the data
Since the invalid data point cannot be deleted, the Windower can be added to the
original data. The Windower tool will be used to select only the valid data points.
To add a Windower to a data set, right-click the Import FRA data item in the data
explorer frame and choose the Add Windower option from the context menu
(see Figure 5).
Figure 5 – Adding the Windower to the Import FRA data item
The Windower can also be added using the quick access toolbar. Click the Import
FRA data item in the explorer frame and click the
button in the quick access
toolbar to add a windower to the data (see Figure 6).
Figure 6 – Adding a Windower using the quick access toolbar
When the Windower is added, a popup menu will appear (see Figure 7).
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Figure 7 – The Windower dialog window will appear when the Windower command is added
to the procedure
The Windower tool needs a source. The source is a signal that can be used to
identify the data points that need to be selected by the tool. In this case, we want
to keep all the data points except the data point # 43, recorded at 0.63095 Hz.
This means that we have at least two possible sources for the Windower:
•
•
Index: the invalid point can be ignored by windowing out the data point
with index equal to 43.
Frequency: the invalid point can also be ignored by selecting all the
frequencies except 0.63095 Hz.
In this technical note, we will illustrate the Index-based solution but the
Frequency-based solution can be used exactly the same way.
In the dialog window shown in Figure 7, set the source signal to Index using the
dropdown list. When this is selected, the dialog window updates and displays a
list of checkboxes corresponding to the individual values for the index signal (see
Figure 8).
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Figure 8 – Setting the source to the Index signal
Using the checkboxes, it is possible to select the data points to include in the
windower tool. To quickly select the points, it is possible to right click the list of
checkboxes and select the select all option (see Figure 9).
Figure 9 – Selecting all the available checkboxes using the right-click menu
With all the checkboxes selected, scroll down the list of box and uncheck the box
corresponding to data point #43 (see Figure 10).
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Figure 10 – Unchecking the box for data point #43
Click OK to validate the setting of the windower.
A new item, called Windower, will be added to the data explorer frame, below the
Import FRA data item (see Figure 11).
Figure 11 – The Windower is added to the Import FRA data item
The Windower can now be used to select only the valid data points. Before we
can do this, we need to identify the invalid data point that needs to be ignored.
4 – Plotting the selected data points
Having completed the data selection process, it is now possible to create a new
Nyquist plot, which will only contain the selected data points. Right click the
Windower item in the explorer frame and choose the Nyquist plot from the Add
plot option in the context menu (see Figure 12).
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Figure 12 – Adding a new Nyquist plot to the Windower
It is also possible to add a plot to the windower item using the quick access
toolbar (see Figure 13).
Figure 13 – Adding a Nyquist plot to the windower
The Nyquist plot should look like the one displayed in Figure 14. The invalid data
point has not been selected by the windower data handling tool and it will
therefore not be plotted.
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Figure 14 – The new Nyquist plot
Note
Right-clicking the Windower in the explorer frame also allows us to add a Fit
and Simulation analysis tool to the windower. Since the data provided by the
windower does not contain the invalid data points, only the valid points will be
fitted (see Figure 15).
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Figure 15 – Adding a Fit and Simulation analysis item to the Windowed data
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