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Chapter 11 - Software Operation for Charge Decay Analysis.

This chapter describes the Analysis Software which may be accessed:

It is prescribed reading for a new user of the software, so that he may grasp the range of its capability, but need not be learnt, as it is referenced in the index.

Analysis Preparation Menu

This menu is shown in figure 11.1 (also in figure 2.6). It offers a status panel, three numbered actions and Quit.

Figure 11.1 Analysis Preparation Menu Screen

Status Panel

Normally with the data from the charge decay run successfully filed, the status panel will indicate as follows:

If there is a problem ..

If the file fails to load, the first two selections will be unavailable, and the status panel will indicate:

Firstly ensure that the disc was not changed or removed while a File / Directory Picker is showing. Otherwise, see Chapter 13 ("Trouble-shooting", § Disc Drive Problems).

Selection 1. Analyse with SELECTABLE parameters

Use this selection when the default parameters, explained in the section on Analysis Parameter Menu below, have proved unsuitable for the analysis required. This selection causes the data file to be reloaded in analysable form, and if the reload is successful, the Analysis Parameter Menu is presented. In the unlikely event that the data cannot for some reason be reloaded at this point, a screen message will appear, then a rerun (selection 3 below) will be offered, with selections 1 and 2 unobtainable.

Selection 2. Analyse with DEFAULT parameters

Use this selection to start the analysis at once, when there is no reason to suppose that the default analysis parameters, explained in the section on Analysis Parameter Menu below, are unsuitable. This selection causes the data file to be reloaded in analysable form, and if the reload is successful, on-screen analysis begins at once, using default parameters. In the unlikely event that the data cannot for some reason be reloaded at this point, a screen message will appear, then a rerun (selection 3 below) will be offered with selections 1 and 2 unobtainable.

Selection 3. Repeat Run

Use this selection to repeat a charge decay run using unchanged Run Parameters. Note that the data file for this repeat run will NOT replace the data file for the original run. This selection takes you back to the the Sample Identity Entry Screen if the JCI155v4 is in manual control mode, or to the Instrument Parameters Menu if the JCI155v4 is in computer control mode. This is the appropriate place to quickly start a new charge decay run with identical Run Parameters to the previous one.

Selection 4.
Quit to Main Menu

Use this selection to start the next charge decay run with different run parameters, or if you require another main menu selection.

Analysis Parameters Menu

This menu is shown in the figure 11.2 below and is encountered when proceeding from selection 1 of the Analysis Preparation menu above. It offers a Proceed action, six editable analysis parameters, a Default / Cancel line, and a Quit action.

Figure 11.2 Analysis Parameters Menu

Proceed Action

Leads to the on-screen Analysis. Use this action to proceed with analysis when the analysis parameters are satisfactory.

Averaging Range

The averaging range is the number of surface voltage readings in the data file that are averaged to produce a single reading for analysis, and is by default 1, i.e. there is effectively no averaging by default.

For various reasons, e.g. sample movement, electrical interference, fluctuations may be observed on the graphical display that are judged to degrade the charge decay curve, and may be averaged out. The number of readings over which the averaging is performed may be set up to 255 here. Note that the time over which the averaging is performed will depend on the phase (see table in figure 10.2 above).

Note also that the number of readings used for the analysis is not significantly affected by increasing the averaging range N. Each individual reading on the data file is replaced by the average of the N readings about it. There is a slight displacement if averaging range is an even number. At the start of the decay, before N readings are available, the averaging is performed by dividing the sum of the available readings by N.


Figure 11.3 Effects of Averaging Range.



No averaging, the effect of averaging shown by the shaded background.








Averaging by 6.

As shown in figure 11.3 above, though the apparent rise time is lengthened, the part of the curve from which decay rates are calculated is not much affected by averaging.

Spike Suppression

To effect spike suppression, where the spikes may be arbitrarily large, due to interference on the serial link, each reading in the data file is replaced with the median value of a group of an odd number of readings centred on the replaced reading. To suppress N consecutive spikes there must be N readings on either side in the group. The spike suppression is applied in a similar fashion to the averaging range.

The default spike suppression value of 1 suppresses all single spikes. If spikes are still obvious on the graphical display where they might have affected the analysis, the analysis can be repeated with a higher value.

Time Constant Range

This parameter is used to control the generation of local Time Constant values shown during on-screen analysis and on the graphical display. The local time constant is calculated whenever the absolute magnitude of the surface voltage is reduced by the stated percentage since the previous determination / start of charge decay. If inspection of the analysis or graph shows the local Time Constant to fluctuate rather than change gradually, increasing the Time Constant Range % may well cure this more satisfactorily than increasing the average range.

This parameter is subject to the following:-

Local time constants are calculated by computing the reciprocal of the slope of the best straight line approximation to the graph of (surface voltage) against (time) for the period since the previous determination / start of charge decay.

The default and minimum for this parameter is 5%, and values up to 25% may be set.

Zero base line shift

Despite the good zero stability of remote surface voltage sensing in the JCI155v4, it is possible on long decay time constants to find the local Time Constant going to zero or infinity eventually, due to zero drift exceeding the ever declining available signal. This may be observed during on-screen Analysis and / or the Graphical display. When the Time Constant at low surface voltages is important, the Zero drift that has occurred may be estimated and corrected using this parameter. The effect on readings at the start of the decay is normally negligible.

Analysis Period Start

Analysis Period End

These values may be edited so that the time for surface potential to decay from a desired fixed value (Analysis Period Start) to a fixed percentage of that value (Analysis Period End) will be displayed on the Analysis Screen (figure 2.7) and the Graph Screen (figure 2.9) as Vs and Ve start and end voltages. The settings automatically assume the correct polarity for the discharge. If the surface potential at the start of the charge decay is less than Vs, then Vs is taken as the surface potential at the start of the charge decay. The range of the Analysis Period Start is ± 15 to ±10,000 Volts, while the range of Analysis Period End is ±5 Volts (as a percentage) to 100%

Default / Cancel

Use this selection for cancelling all changes and / or restoring all defaults. See the Chapter 9 ("Menus" §§Restoring Defaults ... ). above, and / or read the instruction line before you press anything!

Note also that the default and current values shown on this menu at program start up are initially the same, and may be set using a setup file. When a charge decay run completes, or a charge decay file is loaded for display, the default values are set to those appropriate for the charge decay, while the current values are left unchanged.

This feature allows you to

Quit

This selection returns to the main menu.

On-Screen Analysis

An example of the On-Screen Analysis is shown in figure 11.4 below (also in figure 2.7). This screen actually has four possible stages of presentation, indicated by different messages on the bottom line of the screen. The stages and messages are:

  1. Peak finding state (initial state):
    Message: "Please wait while finding peak ..".
    During this message the initial (peak) data phase is scanned to locate the point at which charge decay analysis should start, which is just after the largest reading (with Averaging Range and Spike Suppression applied). Only the data items superscripted 1 in the data item review below show at this stage. When the peak is located, the screen automatically transfers to:
  2. Analysis Run state:
    Message: "Terminate, losing data: <ESC>; Pause <RET>".
    This instruction appears while the analysis is progressing, and indicates the functions of the and keys. During this state the items superscripted 2 in the data item review below appear gradually. If the key is pressed, the analysis is abandoned, and any subsequent graphical display will only be complete up to that point. If the key is pressed, analysis is paused, and the instruction line changes, as shown below:
  3. Analysis Pause state:
    Message:
    "Continue:<ESC>; Step:<Ret>; Local TC's:­¯, PgUp/Dn"
    This message appears while analysis is paused. Pressing the key returns to the Analysis run state; pressing the key causes one step of the analysis to be performed. In this state the display of time / surface voltage / local Time Constant may be examined, and if it is bigger than the available window may be scrolled through using the indicated keys as follows:
  4. Analysis complete state:
    Message:
    "Quit: <ESC>; Proceed <Ret>; Local TC's:­¯, PgUp/Dn"
    This message appears when the analysis is complete. In this state the display of time / surface voltage / local Time Constant may be examined, using keys as described for the Analysis Pause State above. Pressing the key or the key causes the program to leave the Analysis Screen, in order to advance to the Graph Preparation Menu, Proceed or Quit Selections respectively.

Analysis recording

If the Analysis Screen is quit using the return key, an attempt is made to append the analysis information to the report file produced at the end of the Charge Decay Run.

Review of on-screen Analysis Data

Figure 11.4 On Screen Analysis (completed).






NOTE! Analysis and Run figures may differ significantly when Averaging Range and Spike Suppression are set differently!

Graph Preparation Menu

This menu is shown in figure 11.5 below (also in figure 2.8). It offers four numbered actions and Quit.

Figure 11.5 - Graph Preparation Menu Screen

Selection 1. Graph with SELECTABLE parameters

Use this selection when the default parameters, explained in the section on Graph Parameter Menu below, have proved unsuitable for the Graph required. This selection causes the Graph Parameter Menu to be presented.

Selection 2. Graph with DEFAULT parameters

Use this selection to start the graph at once, when there is no reason to suppose that the default graph parameters, explained in the section on Graph Parameter Menu below, are unsuitable. This selection causes the graph to begin at once, using defau lt parameters.

Selection 3. Repeat Analysis

Use this selection to repeat an Analysis, e.g. with parameters revised in the light of the graph. This selection takes you back to the the Analysis Preparation Menu.

Selection 4. Repeat Run

Use this selection to repeat a charge decay run using unchanged Run Parameters. Note that the data file for this repeat run will NOT replace the data file for the original run. This selection takes you back to the the Sample Identity Entry Screen if the JCI155v4 is in manual control mode, or to the Instrument Parameters Menu if the JCI155v4 is in computer control mode. This is the appropriate place to start a repeat of the previous charge decay run.

Selection 5.
Quit to Main Menu

Use this selection to start the next charge decay run with different run parameters, or if you require another main menu selection.

Graph Parameter Menu

This menu is shown in figure 11.6 below and is encountered when proceeding from selection 1 of the Graph Preparation menu above. It offers a Proceed action, four editable Graph parameters, a Default / Cancel line, and a Quit action. These parameters can usefully be set so that a number of Charge Decay Graphs whose default parameters are slightly different may be given uniform scales for compariso n purposes.

Figure 11.6 - Graph Parameter Menu

Proceed Action

Leads to the on-screen Graph. Use this action to proceed with the graph when the graph parameters are satisfactory.

Y-axis max. Volts

This sets the desired surface voltage at the upper end of the Y-axis. Note that the actual value may be rounded out to the nearest displayable Y-axis interval. As indicated values in the range ± 3,000 Volts may be set.

Y-axis min Volts

This sets the desired surface voltage at the lower end of the Y-axis. Note that the actual value may be rounded out to the nearest displayable Y-axis interval. As indicated values in the range - 3,000 Volts to the previous Y-axis max. setting may be set. (If the Y-axis max. has been previously set to a lower value than indicated here, and this parameter is not edited, then the actual values on the graph will be reversed.)

Time axis max.

This controls the scale of the X-axis. It is particularly useful when it turns out that a long decay has an interesting start.

TC display

The inevitable uncertainties in the calculation of local time constant can be displayed on the graph, by selecting Line or Block displays of the standard deviation (s) from the following selections:-

Local time constants, TC, are calculated by computing the reciprocal of the slope S of the best straight line approximation to the graph of (surface voltage) against (time) for the period since the previous determination / start of charge decay. The standard deviation, s S, is calculated for the slope, and the the local TC ± sTC is calculated as the reciprocal of S ± sS. If sS exceeds S, then TC + sTC is taken as infinite. Because of the reciprocal relationship, and the non linear nature of the Local Time Constant display Range, the display of standard deviation is not symmetrical about the local time constant.

TC display range centre

The centre of the Local Time Constant display Range will be set to the nearest 1,2, or 5 × whole power of 10 seconds. The TC display is non linear to cope with the very wide range of local time constants observed on many samples.

Note that the scale will always run from 0 (zero) up to ¥ (infinity). The range from 0.2 to 5 × centre value is approximately logarithmic. The precise value of any point on this scale is given by the formula:

Default / Cancel

Use this selection for cancelling all changes and / or restoring all defaults. See the Chapter 9 ("Menus" §§Restoring Defaults .. ) above, and / or read the instruction line before you press anything!

Note also that the default and current values shown on this menu at program start up are initially the same, and may be set using a setup file. When a charge decay analysis completes, the default values are set to those appropriate for the charge decay, while the current values are left unchanged.

This feature allows you to

Quit

This selection returns to the graph preparation menu quit line.

Graphical Display Screen

The following figures show a graphical display with the three different types of TC display.

Fig 11.7a - Plain graphical display of Charge Decay and Local Time Constant

Fig 11.7b - Graphical display with line display of local TC ± s

Fig. 11.7c - Graphical display with block display of local TC ± s


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