Weight on bit Weight on bit Weight on bit (WOB), as expressed in the oil industry, is the amount of downward force exerted on the drill bit and is normally measured in thousands of pounds. Weight on bit is provided by drill collars, which are thick-walled tubular pieces machined from solid bars of steel, usually plain carbon steel but sometimes of nonmagnetic nickel-copper alloy or other nonmagnetic premium alloys. Gravity acts on the large mass of the collars to provide the downward force needed for the bits to efficiently break rock. To accurately control the amount of force applied to the bit, the
Standard atomic weight helium: . The "(2)" indicates the uncertainty in the last digit shown, to read ±. IUPAC also publishes "abridged values", rounded to five significant figures. For helium, . For twelve elements the samples diverge on this value, because their sample sources have had a different decay history. For example, thallium (Tl) in sedimentary rocks has a different isotopic composition than in igneous rocks and volcanic gases. For these elements, the standard atomic weight is noted as an interval: . With such an interval, for less demanding situations, IUPAC also publishes an "conventional value". For thallium, . The "standard" atomic weight