What Type of Signal Would You Recommend for Wide Range Measurements
In contrast, if you are using
a pulse generating system, you can extend the observation period to increase the
certainty. For example, if you are collecting a 100Hz signal and wait for 1
second, you can calculate your flow rate to 100 pulses +/-1. This is based on
the fact that you may have just missed the next pulse, or caught the lead edge
of the next pulse. This +/-1 count of the right hand digit cannot be overlooked.
Now, if you wait for 10 seconds and collect 1000 pulses, your +/-1 count is a
0.1% uncertainty. If you would like to further reduce the uncertainty, you could
wait for 100 seconds and be able to calculate the rate with a 0.01% uncertainty.
Returning to the fixed
uncertainty that exists in an analog signal. When you set 10.00 volts equal to
100% of the flow rate, the uncertainty in the right hand digit is equal to 0.1%
at the full flow rate ( 1 part in 1000). When you drop to 10% of full flow, the
output is 1.00 volts and your uncertainty is now 1%. At 1% of full flow, the
uncertainty rises to 10%. The meter may be accurately parsing up the flow and
spinning at exactly the right speed, but the output resolution cannot report the
flow beyond the right hand digit.
Valve testing and real time
data gathering may require that you pass rapidly through a range of flow rates.
Stopping at each flow rate and collecting a statistically valid sample may not
be practical. So there is a place for analog signals in flow testing, but their
answers can only be as good as the resolution of the data. Please keep this
uncertainty in mind when setting pass/fail criteria.
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