The AC831 is a gas spring equipped monitor mount and is configured for a certain weight of monitor and can be adjusted to meet the weight of your monitor.
Find the Allen screw displayed below
Keep the monitor positioned in the middle by lifting it or pushing it down.
Now use the allen key to adjust the tension of the gas spring.
If the monitor lowers when released, turn the Allen key to the + and repeat until the monitor stops lowering.
If the monitor rises when released, turn the Allen key to the – repeat until the monitor stops rising.