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With the new optical tacometer from Power Instruments, it is possible to monitor engine revolutions from up to 2ft away from the unit for the purpose of, say, calibrating a vehicle's own rev-counter. Battery powered, the instrument uses a beam and a high-speed response photo transistor. A small piece of reflective tape — made from the same type of material used for road signs — is attached to the hub of the fan belt pulley or a similar rotating component. The light beam "sees" only its own light reflected from the tape being un affected by extraneous light such as sunshine.
The tachometer features four scale ranges — 0 to 1,000, 0 to 3,000, 0 to 10,000 and 0 to 30,000 rpm and is supplied in an aluminium case with a baked enamel finish. The accuracy is claimed to be 1th per cent of full scale and the total weight is 910 g (32oz). Inclusive of two feet of reflective tape the tachometer costs £89.50.