Marshall shock-proof pressure gauges have rugged construction and are liquid filled for vibration dampening. Getting an Accurate Reading When Gauge Heats Up: You should vent or cool a liquid filled 0-15 psi gauge when it gets hot to ensure an accurate reading. To cool: wrap the gauge case with a wet cold towel. Once the gauge temp comes down to ambient your reading will be accurate. To vent: push the vent plug to the side a little to equalize the gauge case pressure with the atmospheric pressure. Once the gauge case pressure is equalized your reading will be accurate.