Which instrument is used to measure sag in transmission lines?

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Sag is the vertical drop of a transmission line conductor between supports, which changes with temperature, wind, and tension. The sag scope is the instrument designed specifically to quantify that drop across a span, giving a direct and calibrated reading of how far the conductor sags. This makes it the best tool for assessing clearance and meeting design or regulatory requirements, because you get a precise measurement that can be tied to the actual span geometry and operating conditions.

A stop watch measures time, not geometry, so it can’t provide a sag value. A dynamometer gauges tension rather than vertical sag, so it doesn’t directly tell you how far the conductor hangs. The Unisag watch isn’t the standard instrument used for direct sag measurements, and isn’t as reliable for obtaining the precise sag reading needed for safe clearances.