Does a regulator outside of the fridge accurately reflect the pressure of a cold keg?

[TAK]

If the CO2 tank and regulator are outside the fridge, does the regulator push the appropriate pressure to maintain the stated PSI when the keg is inside the fridge?

In other words, say the regulator is set to 10 PSI. That is regulating the equilibrium of pressure between the room temp gas in the tube and in the tank. But, as the gas moves further down the line, into the fridge, and into the keg, it gets colder. So, is it safe to assume that the keg is 10 PSI if the regulator is at a different temp?



Obviously lots of people use this setup. I'm just curious if you need to adjust pressure to compensate for temp differential.

Also, any suggestions on running lines into a fridge? I've ran across some stories of people who get the fridge specs but still find a hidden coil in the fridge with their drill bit.

 

[Dan]

That's a question I've never thought of, just always assumed the pressures were the same.

As far as drilling a hole in the sides of a fridge, that's always a bit scary but not hard to do safely. If you can find a diagram of the lines running inside that's great but I've never had much luck finding them. I've drilled holes in two refrigerators successfuly. Try drilling the tiniest hole you can just through the skin of the fridge and then probe around with a the drill bit, a paper clip searching for wires, cooling lines. It's not foolproof but does work. Just proceed cautiously.

I followed Donosborn's lead with succcess.

 

[jbaysurfer]

If the system is closed (if it's leak proof it's closed) the pressure in the keg is what the regulator reads. Co2 expands when it's warm so when it hits the cool keg it might not yield the pressure you intended instantly, but if the valve remains open, the regulator will continue to push c02 until it's equalized at the pressure on your dial.

Hope that makes sense.

 

[TallDan]

Yes.

Pressure and temperature are independant. 12 psi is 12 psi, regardless of temperature. The volume of co2 in the beer is a function of both temperature and pressure. As long as the beer is at the temperature and pressure that you want, you will have the right volume of co2.

 

[TrickyDick]

Pressure * Volume = n(a constant) * R (forgot what this one stands for) * Temperature.

Thus, when temperature decreases, pressure usually also decreases unless there is a change in volume, which wouldn't happen in a sealed system.

 

[TallDan]

Pressure * Volume = n(a constant) * R (forgot what this one stands for) * Temperature.

Thus, when temperature decreases, pressure usually also decreases unless there is a change in volume, which wouldn't happen in a sealed system.

http://en.wikipedia.org/wiki/Ideal_gas_law

n is the amount of gas, R is the constant.

you have a regulator ensuring that the pressure in the gas line stays where you have it set. That pressure is the same at both ends of the gas line. There are more co2 molecules in the cold end than the warm end, but the pressure is the same. It is true that when temperature decreases the pressure also does assuming that the volume stays the same, but that is for a given number of molecules of co2.

 

[TrickyDick]

http://en.wikipedia.org/wiki/Ideal_gas_law n is the amount of gas, R is the constant. you have a regulator ensuring that the pressure in the gas line stays where you have it set. That pressure is the same at both ends of the gas line. There are more co2 molecules in the cold end than the warm end, but the pressure is the same. It is true that when temperature decreases the pressure also does assuming that the volume stays the same, but that is for a given number of molecules of co2.

Makes sense.

 

[Yuri_Rage]

The ideal gas law assumes a closed system with no regulation. However, the regulator opens the system, making those equations moot (or, more accurately, more difficult to solve, given that gas amount is regulated and will vary with temperature and volume). The reading on the outlet (keg) side of the regulator is indeed the pressure inside the keg, regardless of temperature or keg location.

Perhaps I will confuse things by saying that the inlet (tank) side of the regulator will show marked differences in tank pressure as temperature changes. That's because the tank itself behaves more or less like a closed system - there is no regulator to affect its pressure (by supplying or venting gas amount), only gas amount (n) and temperature.

 

[zachattack]

It's a great question, but it's not a concern.

In other words, say the regulator is set to 10 PSI. That is regulating the equilibrium of pressure between the room temp gas in the tube and in the tank. But, as the gas moves further down the line, into the fridge, and into the keg, it gets colder. So, is it safe to assume that the keg is 10 PSI if the regulator is at a different temp?

In order for there to be a pressure drop along the gas line, there has to be an active flow of CO2. When you're actively and continuously flowing a significant amount of CO2 to the keg (unless there's a leak this is pretty much only while you have the faucet open), the pressure inside the keg might be slightly less than the regulator gauge reads, but this is due more to the resistance (drag) inside the gas line than temperature. A big change in the temperature of gas in the line might cause the pressure to change a tiny bit, but again only while gas is actively flowing. Chances are it's nothing you'd ever notice, and as soon as you close the faucet it's going to equilibrate almost immediately. If you wanted to do an experiment, you could place a pressure gauge on the cold keg and compare it to the gauge on the warm regulator. But you'd have to make sure the two gauges give identical readings first .

Again, it's not possible for there to be a pressure drop if there's no flow, and it's not possible for there to be flow unless there's a pressure drop to drive it. Normally you open the faucet, the pressure in the keg drops, and CO2 flows through the regulator to increase the pressure back to your setpoint.

An extreme temperature change might cause enough of a pressure drop to create flow, but in a tiny system like this it's not really going to happen. If you connected two huge tanks full of compressed CO2 and threw one of them in a freezer, the pressure would drop in the cold tank, CO2 would flow from the warm tank to the cold tank to equalize the pressure, and then there would be no flow. Everything would be at the same pressure.

There's also resistance in the regulator's diaphragm, which is why when you're flowing a good bit of gas the pressure at the regulator will drop, but then it comes back up as the flow stops. For more critical applications you buy regulators with different Cv ratings depending on your desired flow. But in a kegerator, there's a trickle of CO2 going in as you carbonate but otherwise the only significant flow of CO2 is when you purge the keg or pour a beer. And this isn't the same scenario you're worried about.

I've mentioned this before and it bothered a few people, but Ohm's Law (V = I*R) is universally taught to engineers as an excellent analogy for, among other things, fluid flow. Pressure and voltage are very analogous, as are flow rate and current. The CO2 line can be thought of like an electrical circuit: in order for there to be a voltage drop through a resistor, there needs to be current flowing. And in order for there to be a pressure drop through the tube, there needs to be CO2 flowing. The electrical analogy is often a lot easier to visualize for someone that's relatively new to fluid mechanics.

Yuri makes a good point about the tank side pressure changing with temperature, but it's important to note since highly compressed CO2 is generally stored as a liquid, not a gas, the tank pressure is a function of the CO2 vapor/liquid equilibrium behavior, not gas laws.

 

[TAK]

Thanks for all the quick replies. This is as I figured, but it was definitely helpful to hear some others articulate it.

Dan - Thanks for posting that video too. I might try to move my tank outside of my fridge now.