ating ▁b iod ies el ▁has ▁been ▁difficult ▁to ▁measure ▁by ▁taking ▁samples , ▁since ▁water ▁and ▁oil ▁separate . ▁However , ▁it ▁is ▁now ▁possible ▁to ▁measure ▁the ▁water ▁content ▁using ▁water - in - o il ▁sens ors . ▁ ▁Water ▁cont am ination ▁is ▁also ▁a ▁potential ▁problem ▁when ▁using ▁certain ▁chemical ▁catal yst s ▁involved ▁in ▁the ▁production ▁process , ▁substantial ly ▁reducing ▁catal yt ic ▁efficiency ▁of ▁base ▁( high ▁p H ) ▁catal yst s ▁such ▁as ▁pot ass ium ▁hydro x ide . ▁However , ▁the ▁super - crit ical ▁m eth an ol ▁production ▁method ology , ▁where by ▁the ▁trans ester ification ▁process ▁of ▁oil ▁feed stock ▁and ▁m eth an ol ▁is ▁effect u ated ▁under ▁high ▁temperature ▁and ▁pressure , ▁has ▁been ▁shown ▁to ▁be ▁largely ▁una ffect ed ▁by ▁the ▁presence ▁of ▁water ▁cont am ination ▁during ▁the ▁production ▁phase . ▁ ▁Av ail ability ▁and ▁prices ▁ ▁Global ▁b iod ies el ▁production ▁reached ▁ 3 . 8   mill ion ▁tons ▁in ▁ 2 0 0 5 . ▁Appro xim ately ▁ 8 5 % ▁of ▁b iod ies el ▁production ▁came ▁from ▁the ▁European ▁Union . ▁ ▁In ▁ 2 0 0 7 , ▁in ▁the ▁United ▁States , ▁average ▁ret ail ▁( at ▁the ▁p ump ) ▁prices , ▁including ▁federal ▁and ▁state ▁fuel ▁tax es , ▁of ▁B 2 / B 5 ▁were ▁lower ▁than ▁pet role um ▁dies el ▁by ▁about ▁ 1 2  