Brüel & Kjær 3638 Manuel De L'utilisateur page 33

External Tachometer
Fig.4.4 Tachometer Type 2979
In certain cases acoustically detected RPM measurements are not possible, and an external
tachometer is needed. Type 3638 A/B supports Tachometer Type 2979 as its preferred external
tachometer, though a few other tachometers on the market are also compatible (specifically,
CarTec and Maha models).
Measuring RPM and Exhaust Noise Levels
The operating principle of Tachometer Type 2979, is based on the electromagnectic emission of
the spark plug leads at the moment when the ignition sparks.
• 2-stroke engines: One combustion per cylinder and per revolution
• 4-stroke engines: One combustion per cylinder every other revolution
The internal aerial of the tachometer picks up all ignition pulses emitted. For this to function as
it should, the path between all the plug leads, or the common lead from the coil to the distribu-
tors, and the tachometer must be direct. The angle or position of the tachometer is not impor-
tant, as long the instrument faces the plug leads.
The tachometer then processes this data to give the number of RPM.
Correct Measurement Distance
The correct measurement distance between the tachometer and spark plug leads is between 10
and 40 cm. At this range, a display indicator 'R' should flash during measurement and the
measurement value is considered stable.
If the tachometer is too close, the 'R' indicator disappears.
If the tachometer is too far away, the display shows '------', or an unstable value.
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