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Frequency Counter Cylinders (RPM)
The Cylinders (Cyl) control is at the far right end of the Frequency Counter window. It works in conjunction with RPM mode, and should be set to twice the number of trigger events per revolution. The default value is 2, for one event per revolution.
Note: The Cylinders setting also affects the frequency axis display in CPM (RPM) Spectrum mode.
If you are using the RPM mode as a tachometer to measure engine RPM based upon spark or fuel injection pulses from a single cylinder on a 4-stroke engine, set this control to 1. (Set it to 2 for a 2-stroke engine.)
If you are monitoring all spark or fuel pulses on a 4-stroke engine, set this control to the actual number of cylinders. For example, use this when monitoring the coil wire or breaker points on a conventional distributor engine. For a 2-stroke engine, set it to twice the number of cylinders.
If you are monitoring pulses from the crankshaft position sensor, set Cylinders to twice the number of actual teeth present on the gear... do not count any missing teeth used to provide a reference. (They don't produce trigger pulses, so the Frequency Counter won't count them either.)
However, if you are monitoring a simulated sensor that allows you to trigger on just the first pulse of each revolution, set Cylinders to 2 for one event per revolution. If you set the Trigger Level to trigger on every pulse, as you would with a real engine sensor, then use twice the actual teeth.
The Engine_Sim macro mini-app sets Cylinders to 1, since it produces one trigger pulse per two revolutions (one cam revolution). It does this even when monitoring the Crank channel, which actually consists of two complete crank revolutions; only one of them has a reference tooth high enough to exceed the trigger level setting of 97%.
For all other motors and rotating machinery, set Cylinders to twice the number of pulses per revolution, which may be quite large if you are sensing teeth on a large gear. The Cylinders control accepts values up to 2000, which will support a 1000-tooth gear.
If you want to measure the RPM of a machine shaft that is geared to a shaft with a more accessible gear for sensor placement, you can set Cylinders to reflect the ratio.
where SensorTeeth is the number of teeth on the gear with the sensor, and GearRatio is the overall ratio between that and the target shaft. If the target shaft is directly driven by a gear driven by the sensor gear, then
GearRatio = TargetTeeth / SensorTeeth.
But there may be any number of intermediate shafts and gears. The basic idea is that Cylinders must be set to twice the number of teeth sensed per target revolution, wherever that target may be.
This will also work with chain drives, but will only give a rough estimate of RPM with belt drives due to slippage.
Note that Cylinders only accepts integer values, so the ratio method can't be used directly in every case. However, if you know the actual number of teeth on all relevant gears, a custom macro meter can be created that will give full accuracy. Use the Contact Us link in the Help menu or on the Daqarta site for no-obligation help creating the macro for your particular situation.
See 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.
See also Frequency Counter
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