Not sure by what you mean if I flip the FM upside down the pressure will still be same, please explain. If one just flips it upside down the pressure would be the same, but the float will just settle to the bottom, so I'm not sure why one would want to flip it upside down.
The flowmeters are designed to be flipped up side down(inside glass tube need to flip as well), so the inlet/outlet can be reversed.
If the valve at bottom/inlet, the pressure inside the glass tube is the same as outlet, which is standard atmosphere pressure for all flowmeters.
but if you want to measure the flow rate while the flow media under certain pressure, the valve at the bottom/inlet setting have the problem to keep the pressure in bay. Now, the flowmeter needed to be flipped, so this certain pressure will be hold steady inside the glass tube, by the valve at the top/outlet.
check my flowmeter thread, post one, for detail, and @Immortal1
also provide detail and pictures how he dissembled and reverse his flowmeter inlet/outlet in the same thread.
if you use the HR metering valve on the co2 system to be the flow control device, there is no way to hold steady the pressure inside the flowmeter.
Yes, the temps are minor, but there's also the accuracy of the pressure gauges, so if you take the two together the errors start adding up.
there is always errors, but if they are within the tolerant range, it is ok.
or the purpose of a flow meter is thrown out the door....