Frequency Counter - RPM Measurement

Controls: Options Menu >> Frequency Counter >> RPM
Macros: FcountMode=RPM, Freq, Total

When this button is active the large Frequency Counter readout shows the equivalent RPM of the Trigger Source, assuming it is detecting events from an engine, motor, or other rotating machinery.

For use as an engine tachometer, this mode works in conjunction with the Cylinders control to compensate for the number of cylinders on an engine, where you are triggering on spark or fuel injection pulses. Be sure to read the Engine RPM Measurement topic under Automotive Applications in the Applications section. It contains information about Trigger settings, input voltage limiters to prevent damage to your sound card, and tips on how to obtain trigger signals from spark plug wires or fuel injectors without direct electrical contact.

For electric motors or other rotating equipment, set Cylinders to twice the number of pulses per revolution. The default setting is 2, which assumes one pulse per revolution.

Observe the same considerations as for Hertz Measurement with respect to triggering.

Note that these RPM measurements have high resolution (fractional RPM, if desired) even at 10 updates per second (Fast). Conventional benchtop frequency counters count the number of pulses in a one-second or ten-second interval, and require you to multiply the reading by 60 yourself to convert to RPM. Even after waiting ten seconds, you would still only get 6 Hz effective resolution.

You can use the Min/Max buttons at any time to see the minimum and maximum RPM values since the last Reset.

Macro Notes:

FcountMode=RPM or FcountMode=1 sets the Frequency Counter to RPM mode. See Macro Notes under Frequency Counter - Hertz Measurement for the complete list of mode names and numbers.

Freq is an internal variable that holds the current Frequency Counter readout value, dependent on the chosen mode (Hz, RPM, msec, Total, or SpecTot). You can assign it to a variable such as F via F=Freq, or you can display it via Msg=Freq, or use it directly in an expression.

Total holds the count of total cycles counted since the last Reset, valid during Hz, RPM, or msec modes. This allows macros to get the Total value even when these other modes are active. Note that this does not work in SpecTot mode.

See also Frequency Counter