California Code, Business and Professions Code - BPC § 12724

(a) Except as provided in this section, a weighmaster weighing a vehicle for certification shall determine both gross and tare weights with all persons off the scale and vehicle, unless both the gross and tare weights are determined without leaving the weighing location.

(b) Predetermined tares for vehicles moving earth, stone, rock, sand, gravel, and asphalt paving material may be determined with the driver in the vehicle, if the gross weight is determined in the same manner and the weighmaster indicates on the weighmaster certificate that the driver was on the vehicle for both gross and tare weight.