Marshall shock-proof pressure gauges have rugged construction and are liquid filled for vibration dampening. This liquid filled pressure gauge can be used for air, oil, fuel, and any other medium that is not corrosive to the copper alloy internals. 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. 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.