Since water is hydraulic, it does not compress and cannot store energy. Water alone will not propel the filter lid with any force or for any distance.

Air, on the other hand, compresses easily. What we measure as “zero” on the pressure gauge is relative pressure and is actually close to 15 psi of absolute pressure. Absolute pressure is measured in relation to a vacuum. The difference is atmospheric pressure, or the weight of the air above us. Zero psi at absolute pressure is a vacuum, as found in space. So, when air is under 15 psi of relative pressure on our gauge, it is actually at 30 psi absolute pressure. Under this pressure, it occupies only half the space it took up when it was at 0 psi on the gauge. Doubling the absolute pressure of a gas (in this case air) compresses it to half the volume. At 30 psi on the gauge or 45 psi absolute pressure, the air occupies only one-third the original volume. Tripling the pressure compresses the air to one-third its former volume. See the chart below.

Absolute Pressure (Atmospheric plus Relative Pressure) Relative Pressure (what you read on the filter pressure gauge) Stored Energy Volume
15 psi 0 psi 1 1
30 psi 15 psi 2 1/2
45 psi 30 psi 3 1/3

Changes in volume and pressure affect stored energy.

Why is this important? If the filter you’re servicing is half full of air, about a foot in depth, and under 30 psi relative pressure on the gauge, that air is a coiled spring poised to expand to occupy three times its current volume. Let the locking mechanism fail now and the filter lid is likely to be found in the neighbor’s yard, along with anything that stood in its way.

When the filter tank fails, pressure is released quickly, but not before the top of the filter reaches an extremely high speed. The force is so great that it accelerates the lid to high velocity in the first inch of travel. After quickly reaching this velocity, the lid is only coasting. But, Einstein’s theory tells us that velocity is the most important factor in determining the amount of energy involved in any action. Do you remember E = mc2 or energy equals mass times velocity squared? Velocity is most important because it’s squared in the equation. That means that doubling the velocity quadruples the energy. Any object in the way will be seriously damaged, including your face or head. When a fiberglass filter lid recently blew off in an equipment pit, the only metal parts involved were the small pieces contained in an otherwise plastic pressure gauge. But the force from the release of pressure was sufficient that after traveling just over three feet, the metal parts still left a small pit in the concrete ceiling, along with a large mark from the rest of the lid.


Hazard Country

Service techs must be aware of safety concerns lurking around the pool.