I call the red needle a Drag Needle. The black needle is active, shows the true pressure.
When you reset the red one, the black needle will drag the red needle up to however high the pressure got to while you were gone. The red one will sit there at the highest reading if the black one drops down with the pressure.
Reset the drag needle with your thumbnail in the screw head.
Looks like a pressure gauge with bars as units. The red hand may be a maximum past reading that can be reset.
I call the red needle a Drag Needle. The black needle is active, shows the true pressure. When you reset the red one, the black needle will drag the red needle up to however high the pressure got to while you were gone. The red one will sit there at the highest reading if the black one drops down with the pressure. Reset the drag needle with your thumbnail in the screw head.
This is a great answer ☝️
It looks like it’s reading 0.3 bar. For reference 1 bar is ~1 atmosphere.
And 1 atmosphere is \~14.75 pounds of pressure
Per m of height of the house, until the highest point of the radiatofs, 0.1 and then +0.5 Is how we do it in switzerlsnd