Rather than using a linear ramp of frequency - which is hard to measure - can you use small steps in frequency? If you stepped 500 Hz at a time you would have 9 steps each lasting about 1-2 seconds.
Then your test looks like this:
Select frequency.
Set fan.
Set your sample rate and number of samples as suitable for the expected frequency.
Start the data acquisition.
Read the data.
If not last frequency, increment frequency and repeat.
Resetting the reat and number of samples once every 1-2 seconds should not be a major issue.
Often it is better to measure period rather than frequency when the signal is at low frequencies. Consider measuring the rotation speed with the 4th channel of the 9215. Then your data is synchronized. If you sample at 100 kHz you get almost 58 samples per cycle (not revolution) of the speed sensor signal at the highest speed.
Lynn
