I agree that a 5 PSI differential is not a good thing, but that is just how most of those variable speed type systems work. The 5 PSI only gives you a fraction of a gallon of draw from the tank before the pump starts. Then the pump slow starts, which takes some time to get the line pressurized. Technically having 2 PSI less air in the tank than the start pressure of the pump is preferred. But no matter you always need less air charge in the tank than the lowest pressure you see in the system. From the video I can see the bladder in the tank bottom out at 40 PSI, which is like a basketball on a hardwood floor, and will cause water hammer.
The reason you are not seeing water hammer on the video, is because you didn't wait long enough for the mainlines to drain out. When that 1000' of main is empty, the bladder in the tank will bounce at 40 PSI several times while the pump is refilling the lines. What changed? You probably did something to let the mainline drain, and it did not drain in the past. There is probably an air leak after the zone valve that lets the line drain out. Without an air leak, the zone valve acts like putting your finger over a straw full of ice tea, and won't let the line drain. But a small air leak will let the main drain over time. And with the main drained, the bladder in the tank will bounce and cause water hammer.
If you can't find and fix the air leak, you can put check valves with 3 PSI springs on the sprinkler heads to keep the lines from draining. The 3 PSI springs in the check valves will keep 3 PSI of pressure in the main and not let it drain empty.
With the main empty, such a small tank, and the slow start from the VFD, keeping the main from draining out is probably the only way to stop the water hammer.
This is a prime example of how a "soft start" from a VFD that is suppose to eliminate water hammer, actually causes water hammer. When VFD controls cannot make the pump immediately keep up with the demand, the pressure drops below the bladder tank pre-charge and water hammer occurs like you are seeing.
The reason you are not seeing water hammer on the video, is because you didn't wait long enough for the mainlines to drain out. When that 1000' of main is empty, the bladder in the tank will bounce at 40 PSI several times while the pump is refilling the lines. What changed? You probably did something to let the mainline drain, and it did not drain in the past. There is probably an air leak after the zone valve that lets the line drain out. Without an air leak, the zone valve acts like putting your finger over a straw full of ice tea, and won't let the line drain. But a small air leak will let the main drain over time. And with the main drained, the bladder in the tank will bounce and cause water hammer.
If you can't find and fix the air leak, you can put check valves with 3 PSI springs on the sprinkler heads to keep the lines from draining. The 3 PSI springs in the check valves will keep 3 PSI of pressure in the main and not let it drain empty.
With the main empty, such a small tank, and the slow start from the VFD, keeping the main from draining out is probably the only way to stop the water hammer.
This is a prime example of how a "soft start" from a VFD that is suppose to eliminate water hammer, actually causes water hammer. When VFD controls cannot make the pump immediately keep up with the demand, the pressure drops below the bladder tank pre-charge and water hammer occurs like you are seeing.
