It is built different, so think different.
Not the small needle shape stem that make ths valve better than other. It is non rising stem design, can't judge its ability on flow control from the stem size.
Actually, it looks like 1-3 series model, same as alan's, check the pdf you will see the official stem diameter(orifice)
Can you please let me know how you adjust the bubble rate, and did you check leak and make sure it is leak free before you test the valve?
Attention to detail when you build, think outside of the box when looking for a solution.
Everything checked out leak free before I test it. I though that the burkert solenoid makes the different with different voltage input, so I test it with 12VAC and 24VAC. Both the same result.
When I adjust the bubble, the bubble increase from 1bps to 5-10bps with just a tiny bit adjust of the needle. feel like it's out of control. I unplug the power of the solenoid at the point of about 5bps and leave it there for a while and then plug the power back, the bps decrease to 1bps or almost nothing out. Don't know what happen.