Y-Axis Scaling
This operation sets the upper and lower Y-axis limits
for the screen display of a particular channel. The default values
are -1 to 1. If you pick one of the pre-defined voltage
conversions, the Y-axis scaling will be preset automatically.
Similarly, if you use the 'Direct calibration' function, the Y-axis
scaling will be based on the range of values used during the calibration
procedure. If you wish to set a different scaling from the default
or the pre-defined values, enter your choices in the two edit fields and
click the 'Selection OK' button.
Note that the scaling you select applies only to the SCREEN display. Even if values are offscale on the screen, they will still be recorded correctly in the data file -- provided they do not exceed the voltage limits of the A to D hardware. Also, note that the gain function is fixed for oscilloscope channels (this is set from the SAMPLE RATE window).
When Continuous
chart scanning option is selected (A-D
menu; available in Chart mode only), all channels are scanned
sequentially and continually throughout the sample interval. The Averaging
window is not available, and differences in the number of averaged readings
between channels are not possible. This mode provides a more continuous
sampling of each channel than in the 'normal' averaging mode, wherein each
sample is sampled for N averages before proceeding to the next channel.
It differs from the 'continuous averaging' option described below
in that all channels are scanned continuously, not just the last
one.
| As for the normal averaging mode, in continuous scanning mode, the number of averaged readings per recorded sample depends on the number of channels, the sample interval, and the speed of the computer and the A-D converter. |
The number of readings that can be averaged depends on the
number of channels, the sample rate, the speed of the computer, the absolute
value of input voltages (low voltages take slightly longer to read than
high voltages), and the conversion type used (power functions take longer
to calculate than linear or polynomial conversions). LabHelper
offers a default maximal averaging value, based on the sample interval and
the number of channels used (it assumes the same number of samples are averaged
for all channels). You can pick a different number by changing the
values in the edit fields. Remember that if you select a larger number
you may slow LabHelper down to the extent that it cannot keep up
with the requested sampling rate. If that happens, it will record
data correctly and, when data are saved, will calculate the actual sampling
rate.
In Chart mode, all of the channels can be set to
can be set to make repeated readings until the time of the next sample by
switching the "Continuous averaging (all channels)" button
to 'on'. The program cycles through all the channels in an endless
loop until it is time for the next sample (at that point all of the readings
are averaged and stored). In this mode at least one reading is always
made.
In Chart mode, the final channel can
be set to continue making readings until the time of the next sample by
switching the "Continuous averaging (last channel only)"
button to 'on'. In this mode at least one reading is always made.
The "Absolute value" button converts
all voltage readings on the final channel to their absolute value (i.e.,
negative values become positive; this is available only if continuous averaging
is on).
NOTE: sample averaging is automatic if you have selected Oscilloscope plus Chart mode. In this mode a voltage reading is made from ALL channels when the oscilloscope reading is made. When the sample interval for chart channels has elapsed, the stored readings for the chart channels are averaged, plotted, and saved. Thus the number of samples averaged for chart channels is roughly equal to the value:
chart sample interval / oscilloscope sample interval
Use of this method simplifies calculations and allows the highest possible oscilloscope sampling rate.
When the oscilloscope sampling rate is discontinuous, LabHelper
automatically averages 10 samples for all chart samples between each sampling
of the oscilloscope channel. This allows some averaging despite the
delay induced by discontinuous scope sampling (you get at least 10 samples
averaged for each chart channel reading, more if the chart sample interval
is long).
When you are satisfied, click the 'Selection OK' button.