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Ian,
My duty cycle measures 80% high/20% low. A low corresponds to a magnet positioned in front of the sensor. From a surface area standpoint, the 5mm magnets represent 8% of the collar’s circumference so the additional 12% must be due to field strength tripping the switch’s thresholds on approach.
I’ve attached a picture of my setup. The collar is plastic with a single set screw (aligned with shaft keyway) purchased from McMaster Carr.
My apologies on the sensor. Mine is made by Hamlin, model 55140-3H-02-A although Cherry does make a similar model MP102101.
A function generator with adjustable duty cycle would be a helpful tool but odds are you don’t have one. As a test you could attach more magnets around the circumference so that the space/mark ratio improves to the point the tach responds. With that “minimum” ratio known, you could then find larger diameter magnets that can satisfies this criteria with only 2.
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