As the syringe is used to increase or decrease the volume of air in the syringe, the pressure gauge shows a decrease or increase in pressure. Down is closed. Open the valve to make the measurement.
Volume is in cubic centimeters.
Then reconnect the tubing to the gauge. Record the pressure reading. Pull the plunger out until the pressure reading is half of the value of the original reading. Record the volume reading. This volume in the equal to the volume of the gauge, tubing,and fittings.