This is my first post in this forum. First and foremost, big thanks to anyone who takes time out to help me with this. I work for a fire protection company; one thing i service is fire hydrants. I recently discovered this and do not understand why this is happening: A hydrant contains only air in its barrel at atmospheric pressure. When the hydrant valve is opened and the 150psi water source begins to fill the hydrant, the air in the hydrant is compressed. (Note: the hydrant is not discharging water, only filling the air occupied barrel between the valve and top section.) This is to be expected. However, when a gauge is placed on the top section to measure the pressure in the barrel I expected to find the gauge reading 150psi - the water source pressure. Instead the pressure was much higher. Anyone know why or know of a formula to calculate this? Thanks for any info in advance!