The Puzzle
Analysis
The measurement is
dependent on the change of capacitance because the pressure in the chamber has
changed the distance between capacitor plates of the sensor. The capacitor sensor shifts the current
signal out of phase with the voltage signal so that the current leads the
voltage. The puzzle provides an
opportunity to discuss phase shift from a visual perspective. It also allows a discussion of the leading vs
lagging options. Students don't easily appreciate these out of
phase concepts so explaining the diagram will be valuable. Setting the visual with a 90 degree phase
shift should help them see smaller phase shifts. This specific graph emphasizes that at a time
just a bit before 10:01:05 AM, the red curve current value is approaching its
maximum positive while the voltage value is lagging behind. (it has a negative value now and will have a
zero value at 10:01:05 AM.) The voltage
signal become positive as it crosses the zero volt value on the blue voltage
plot. The blue curve will reach its
maximum (allowing that this is just a cartoon with no precision) at 10:01:06
AM. Thus, the voltage curve lags the
current curve. (Some of us learned about ELI the ICE man, so if you are that vintage,
now is a nice time to bring that mnemonic up with your students. If not, well it's just another pearl that has
slipped out of phase with current times!)
Question
Do the two plots below represent what the Technician expects
to see in this constant pressure situation? YES or NO
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