▁filled ▁with ▁a ▁compatible ▁low ▁pressure ▁fluid ▁called ▁the ▁buffer ▁fluid . ▁ ▁This ▁buffer ▁fluid / space ▁may ▁be ▁monitor ed ▁to ▁detect ▁performance ▁of ▁the ▁assembly . ▁ ▁Unfortunately , ▁the ▁definition ▁of ▁“ tan dem ▁se al ” ▁was ▁often ▁stated ▁in ▁a ▁confusing ▁manner . ▁ ▁In ▁particular , ▁a ▁tan dem ▁se al ▁was ▁usually ▁described ▁as ▁two ▁se als ▁pointing ▁in ▁the ▁same ▁direction ; ▁that ▁is , ▁in ▁a ▁face - to - back ▁orientation . ▁ ▁This ▁orientation ▁is ▁not ▁necessary ▁to ▁the ▁function ▁of ▁the ▁configuration ▁and ▁the ▁API ▁chose ▁to ▁use ▁the ▁term ▁Arr ang ement ▁ 2 ▁instead ▁of ▁tan dem ▁in ▁the ▁API ▁ 6 8 2 ▁standard . ▁ ▁Double ▁se als ▁ ▁A ▁double ▁se al ▁consists ▁of ▁two ▁sets ▁primary ▁se aling ▁surfaces ▁with ▁the ▁space ▁in - between ▁the ▁two ▁se als ▁filled ▁with ▁a ▁compatible ▁press ur ized ▁fluid ▁called ▁the ▁bar rier ▁fluid . ▁ ▁This ▁bar rier ▁fluid / space ▁may ▁be ▁monitor ed ▁to ▁detect ▁performance ▁of ▁the ▁assembly . ▁ ▁Unfortunately , ▁the ▁definition ▁of ▁“ double ▁se al ” ▁was ▁often ▁stated ▁in ▁a ▁confusing ▁manner . ▁ ▁In ▁particular , ▁a ▁double ▁se al ▁was ▁usually ▁described ▁as ▁two ▁se als ▁pointing ▁in ▁the ▁opposite ▁direction ; ▁that ▁is , ▁in ▁a ▁back - to - back ▁orientation . ▁ ▁This ▁orientation ▁is ▁not ▁necessary ▁to ▁the ▁function ▁of ▁the ▁configuration ▁and ▁the ▁API ▁chose ▁to ▁use ▁the ▁term ▁Arr