Both regular and liquid-filled gauges need a vent to provide accurate readings. Otherwise pressure builds as the surrounding temperature changes, affecting accuracy as much as 3 psi. Vents would cause liquid gauges to leak so Longacre developed a vent valve to equalize pressure and maintain accuracy. To use, loosen the knob on top of the gauge a 1/4 turn, then retighten by hand. Any temperature-caused pressure difference will be purged for the most accurate reading possible.