This 2.5" glycerin filled pressure gauge has a bronze bourdon tube and a 1/4" MPT connection. These are great for keeping track of pressure differential in your large filters or venturi style injectors. Can also be installed in a pump or backflow prevention set-up for monitoring or testing. Available in 200 PSI.Please Note: An air bubble is normal in all liquid filled pressure gauges to allow for expansion and contraction of the oil. Over time or transportation some oil may leak out or evaporate due to environmental conditions. This will not harm the gauge. The liquid is used as a buffer or cushion for the needle (reduces vibration) which provides for a more accurate reading.