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Physics of gas

mozzie on Wed June 08, 2011 4:35 PM User is offline

Year: 97
Make: Honda
Model: Accord

I successfully rebuilt my 94 Toyota truck refer system. Now I am going to work on my Honda. This question crossed my mind. I was told that it really is not possible to know the quantity of refrig. gas in the system unless I empty-vacuum- recharge the system which I did. However, if the pressure equalizes between the low side and the high side when the system is off and the engine is off and the system is newly recharged and in new and good working order then the static pressure should be constant relative to ambient temp and atmospheric pressure. If so then there should be a direct relationship between that volume of refrig. gas and it's static pressure that can be used to determine the volume in any other same make -model-and year system based on the static pressure reading at the same altitude and ambient temp. Right? or wrong?

Example: Lets just say static pressure of functional system is 60psi at given atmospheres and ambient temp with filled system at 30 oz. . Now another same but less functional system is 40 psi. static. At recovery of the gas it is determined that the disfunctional system had 20 oz. of gas. So then it could be said that two thirds of normal static pressure at a given atmosphere and temp will equal two thirds of normal capacity of gas.

The properties of any gas should be predictable so why not refrig. gas? even if a table could get me within a plus or minus 2 or 3 oz. would be great.

mk378 on Wed June 08, 2011 5:07 PM User is offline

Static pressure is based on the equilibrium between liquid and gas, it is the evaporation or condensation point. It is a property of the refrigerant only, it does not matter what make/model system it is filled into.

For example consider two refrigerant cylinders, one brand new and full and one nearly empty. If they are at the same temperature, they will have the same static pressure. You would need to weigh them to find out how much refrigerant they really contain.

Refrigerant under practical conditions is not an ideal gas. The Ideal Gas Law (where pressure is proportional to quantity) does not apply once the refrigerant starts to condense.

iceman2555 on Wed June 08, 2011 11:50 PM User is offlineView users profile

Even if there were such a chart...and undercharge of 2-3 oz of refrigerant in this system will result in the failure of the compressor due to lack of proper lubricant migration.

The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

NickD on Thu June 09, 2011 6:38 AM User is offline

Atmospheric pressure really only affects your low side gauge as it is referenced to atmospheric pressure, the greater the altitude, the lower the reading. As atmospheric pressure drops approximating one inch/Hg for each thousand feet above sea level. Micron gauges use absolute zero as the reference point where the greater the reading, the less vacuum you have in the system. Or in brief, atmospheric pressure doesn't have a thing to do with charge levels in the system.

Static pressures will vary at a constant ambient temperature if the charge becomes so low, there isn't a sufficient amount of liquid in the system to evaporate to fill the system. If at say 85*F, and this is independent of RH, would expect to read 94.5 psi static pressure. But still consider reading say 40 psi on a 15 year old vehicle good news as there is still positive pressure in the system meaning that air cannot leak in. If this system was never charged before, losing just slightly over an ounce per year that is considered normal, system can be charged and checked for leaks, providing there are no oil leaks. But if its zero, air is in the system, has to be taken apart, flushed, new receiver, new oil, find the leak, and recharge. But by adding only a few ounces to the system, that static pressure would shoot back up to full static pressure and stay there if you started off with positive pressure.

But that is no indication of a properly charged system, a properly charged system can only be tested under dynamic conditions, with a Honda, you have a sight glass and want solid liquid feeding the TXV at 2,000 rpm, doors open, AC on, blower at maximum. With an undercharged system, would see lots of foam in that sight glass. Good to do this with gauges attached to verify the system is working properly.

Yeah, would be nice if you could accurately charge a system with static pressures, but doesn't make any difference if you have a few ounces of refrigerant where there is still some liquid in the system or if you say put in twice the recommended amount of refrigerant, but will have a lot more liquid in the system. But if you did that, once you start the engine, the system will blow up in your face. Big difference between static and dynamic.

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