# How to balance flow and pressure in nitrogen purged chambers?

I have an instrument that is technically a humidity controlled chamber. It measures how much moisture is picked up by the sample through a microbalance(Zone 1). Nitrogen gas flows from zone 3 and split into 2 chambers (by a union Tee ) in Zone 2. Finally the gas is vented in Zone 1, each chamber with its own exhaust vent (tubings that vent to the back).

The Problem is that the flow is uneven. or let me rephrase, the flow is even as long as the resistance against the flow is equal for both chamber. with the slightest change in resistance at one of the vents, the flow is completely diverted to the other chamber. This happens if one of the vent tubings got slightly bent, one is slightly longer than the other, etc.. It is ridiculous sometimes

My question is: if I joined the exhaust tubings together at the end (by a Y-connection), would that equalize the pressures and gas flow in the chambers? I'm no engineer but I can't think of a better place to ask

• Don't think so, once a fluid establishes a flow through one channel it needs some input to switch it to the other channel - this was investigated for a type of memory left being high iirc ... – Solar Mike Jan 22 '19 at 15:01
• Flow balances to an equilibrium based on total pressure drop through a circuit. If you join the vents together but don't address any upstream/unique restrictions, you won't change the balance of flow. If the tee ensures that both paths see the same bent tube resistance (after the tee), then yes it will work. Another way would be to intentionally create resistance you can vary on both paths. You can keep them balanced by adjusting the valve. This may or may not working depending on acceptable back pressure on the system. A back pressure regulator is a device that does this automatically. – Byron Wall Jan 24 '19 at 21:57
• @ByronWall, I was thinking about balancing the flow with flow meters. Would a back pressure regulator work better? and Do I need a regulator for each vent? – Zaid assaf Jan 26 '19 at 20:31
• If flow meter = flow controller (e.g. mass flow controller), then it will work fine. If it's a needle valve and a rotameter/floating ball, then the back pressure regulator should work better. The regulator will control pressure whereas the needle valve will just provide an obstruction. This matters if the issue changes over time. If it's just a steady state difference between the two paths (and doesn't change over time), then the needle valves will likely work. 2 back pressure regulators are likely better than 1 but you can start with 1 on the "better" path now. – Byron Wall Jan 27 '19 at 1:58