The atmospheric gauge tank, a nonpressurized vessel, is used to measure low flow rates or to calibrate metering devices on the separator oil lines in a testing system. When the flow rate is too low to efficiently drive oil to the burners, the tanks can also be used for temporarily storing the oil.

These skid-mounted units have two compartments.As a transfer pump empties one compartment, the other is being filled. A sight-glass level built into each tank is used to calculate the change in volume based on the physical dimensions of the tank.

Safety features include flame arrestors on each vent from the tank, a grounding strap, and a shearing roof that opens at 3.45 kPa [0.5 psi] burst pressure in the event the vessel is overpressured accidentally. The grounding strap attached to the tank prevents a static charge buildup.

The atmospheric gauge tank is frequently part of the standard equipment for well testing. However, it cannot be used when H2S is present in the effluent because the gas released from the tank is vented into the atmosphere, and that would be hazardous to personnel in the area.