LabChart for Windows 7.3.3 or later includes an arithmetic function StepSize() that is designed to facilitate this type of analysis.
To calculate the area of the peak identified, the selection
of the peak can be manually made in the Chart View, and the “Integral Relative
to Baseline” data pad calculation can be used. This calculation is somewhat
sensitive to noise fluctuations, and filtering should be used to minimize any
error that might be induced.
The goal for the StepSize() function is to facilitate the measurement of amplitude in the situation described by the image above.
In the image below, Channel 2 is used for the Stepsize Calculation. The StepSize() Arithmetic function outputs a running measurement of the step size calculated from the source waveform. In this example, the function StepSize(ch1,0.03,0.02,0.05) is used. The result can be quantified by using the Data Pad “Minimum Value” function for the same selection made to calculate the area.
The StepSize() function measures the size of any potential
steps by performing two non-overlapping linear regressions either side of every
point in the source waveform.
The format of the function is:
StepSize(source, leftWidth [s], gapWidth [s], rightWidth [s])
- The gapWidth parameter defaults to 0 s.
- The rightWidth parameter defaults to the leftWidth.
The user can specify the length (in seconds) of the left regression, the gap between the regressions, and the right regression. The function uses the result of the each regression separately to calculate the expected value at the mid-point between the two regressions. The output of the function is the expected value from the right regression minus that from the left regression.
The gap between the regressions should be set non-zero if the step in the data is not instantaneous. This means the data values in the transition will be ignored so that they do not introduce a bias, reducing the measured step magnitude. In this file, the step transition time is between 0.02 and 0.03 seconds.