Marshall shock-proof pressure gauges have rugged construction and are liquid filled for vibration dampening. Instructions: Use wrench on HEX to tighten/loosen. DO NOT tighten/loosen by turning gauge case. Damage to the gauge may occur and warranty will be VOID. Use thread sealant and test connection for leaks. DO NOT operate if a leak is detected. Install gauge so that the nipple plug is facing up. Once installed, cut the nipple on the plug. This allows the case to be vented at all times ensuring an accurate reading without additional venting/cooling. DO NOT cut the nipple if it is not facing up. DO NOT USE in applications where the pressure exceeds the top scale of the gauge or in applications with excessive vibration, pressure pulsation, excessive temperature, or corrosive media.