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I came across physicsforum.com while trying to answer a puzzling question I'm facing. I'm building a portable nitro coffee dispenser and need to know how much PSI I need to put in a small nitrogen tank to adequately dispense the beverage.

Here's the facts;

I've played around with the numbers but I really don't know how to calculate the PSI in the keg as the volume of liquid decreases.

- I have a 64oz keg that needs to maintain a pressure of 35 psi throughout the duration of serving. The average size of a pour will be 8 oz. Temperature of the fluid and keg will be 10 degrees Celsius.
- The 64oz keg can be pressurized to 35 PSI prior to hooking up the 4 oz tank (via a 5 LBS tank.)
- I have a 4oz tank (co2 tank that will instead run pure nitrogen. Limited to 1800 psi.) I have a regulator that will run at the desired 35 PSI. Length of tubing from tank to keg is 7". Temperature should be around 18-19 degrees Celsius.
- From what I've read the faucet provides about 2-3 PSI resistance. I'm unsure how much PSI is lost during the pour but based on what I've seen testing it I'd say there's a 5 PSI loss per 8 oz.
- The 4oz tank will be filled from a 5 LBS tank that runs at 2200 PSI but the regulator can only handle 500 PSI. After running the numbers I believe I can only ever achieve a PSI of 450-470 PSI in the 4 oz tank.
- Do I need to buy a new regulator that can output a higher PSI and how much more pressure do I need? (I've found an 800 PSI output regulator that I might be able to squeeze 900 PSI out of. I don't want to buy it if it's not enough PSI to serve a 64oz keg.)

I do know from my *flawed* math and experimentation that 400 PSI in a 4 oz tank isn't enough to serve the entire 64oz while maintaining 35 PSI. Can anybody show me what I'm doing wrong? I had thought 350-400 PSI would have been enough to dispense the 64 oz at atleast 30 PSI but my tests aren't confirming my hypothesis thus far (to be fair I did have a leaky fitting I've fixed. I ran out of N2 so I haven't had a chance to test it again.)

I appreciate any insight sharper minds can provide!

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# How much pressure do I need to power a keg full of coffee

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