Static A/C pressure.
 

Anyone know what it should be...?

Low side pressure with the compressor/system having not been operated for a day or so, refrigerant evenly distributed throughout the system?

'88 Carrera.

Mine reads 65PSI.

Without the pump running, there will be no differential between high side and low side pressures. Testing has to be done while the system is operating.

Really? I swear, I saw the high side gauge peg higher vs low side when I connected it right before the test. Will need to check the next top off time.

I also recall my low side was about 70ish before turning the engine on for the recharge.

Layperson here... There may be a "normal" resting system pressure but you would have to include your temp, humidity and elevation from sea level to compare to anyone else. Once turned off, the system will fight to reach equilibrium hence my statement above. I'm not sure the numbers are diagnostically relevant without the pump doing it's thing. Maybe you can snag an HVAC guy to flame me and set us straight.

Gee West, we thought you knew this stuff, but we will help you out, lol

http://forums.pelicanparts.com/uploa...1360377577.jpg

Draco,

If you were just operating your system and then connected the gauges you might see the high side is higher than the low, however once you turn off the system the two side equalize shortly after a given time, say 15 to 30 minutes tops. Because the high side pressure pushes through the TEV, then through the evaporator and back to compressor low side. This is normal.

Post dates from June 2011, lots of water under the bridge, lots to have learned.

....

This is a better thread wild man.

http://forums.pelicanparts.com/porsche-911-technical-forum/557512-porsche-early-c-tutorial-not.html#post7261831

NOT

Like Kuehl said...

But I could elaborate some.


How fast the system equalizes is primarily a function of the state of the expansion valve, TXV. Provided high side pressure (vs low side) is present and sustained the TXV will automatically adjust its opening apperture as a function of the temperature sensed at the evaporator exhaust line.

The TXV will be fully open to allow liquid refrigerant to flow freely into the evaporator core until/unless the temperature at the evaporator outlet/exhaust line declines to ~33F.

Once the "target" exhaust temperature is reached the level of apperture opening will be continously modulated in order to sustain just enough liquid refrigerant flow to keep the evaporator core almost at the point of freezing up, but not quite.

When you shut down the engine the A/C blower will stop providing warm airflow to, through the evaporator. Should the evaporator core happen to be at the point of freezing up at the instant you shut down the engine the result will be an extended period for the pressures tom equalize.

of the A/C blower stoppage the cooling/heat transfer also stops. Now, with no "heating" of the refrigerant remaining wihin the evaporator core the TXV will not remain mostly closed, or maybe even fully closed until the evaporator core temperture begins to rise.

More, edits, later..off to dinner

Oh... let's elaborate even more!

WHEN THE VERY HOT 911 ENGINE AND AC SYSTEM IS TURNED OFF ......
THE AC HIGH SIDE PRESSURE DOES NOT RISE!
IT DROPS ! as the system pressures equalize.

AND, WHEN THE ENGINE IS TURNED OFF THE VERY HOT ENGINE DOES NOT RAISE THE PRESSURE IN THE DECK LID CONDENSER CAUSING THE AC SYSTEM PRESSURES TO RISE.

AND, WHEN THE ENGINE IS TURNED OFF THE VERY HOT ENGINE DOES NOT CAUSE YOUR AC SYSTEM TO LEAK MORE REFRIGERANT!

REPEAT AFTER ME WWEST....

"THE VERY HOT ENGINE DOES NOT CAUSE YOUR AC SYSTEM TO LEAK MORE REFRIGERANT!"

AND, REPEAT AFTER ME WWEST

"ADDING FANS TO THE DECK LID CONDENSER TO PREVENT SYSTEM REFRIGERANT LEAKS IS A WASTE OF TIME AND MONEY BECAUSE THAT'S NOT WHY THE SYSTEM LEAKS REFRIGERANT"

Stop being such an idiot, of course the leakage rate will increase as a direct function of the pressure differential, regardless of the location/source of the leak.

And of course, just as the pressure of a liquid or gas in a sealed container will decline as the container is cooled, the inverse is also TRUE.

So, remove the cooling effects of the engine driven fan on the HOT contents of the rear lid condensor and what happens? NOTHING...as long as there is no source of additional HEAT.

But what about a HOT engine that has just been shut down, where does all that engine HEAT disapate with no fan to wick it away into the open space under the engine.

Up, UP, and AWAY!

Radient and convection effects, combined, will create an outward and upward flow of HEAT, HEATED air, up where? The engine lid condensor, of course.

So, we have just taken a volume of HOT liquid refrigerant at 350PSI that was being actively cooled but is not be passively HEATED

What might your average 7th grade science teacher say would be the result on that volume of liquid refrigerant..?


lThen what happens to the air pressure in a tire as you travel more and more distance and the tire heats up more and more.

Kueehl..I do not take you for an idiot, far from it, moreso otherwise than anything.

So, once again...

The air cooled engine 911 is setting there idling with the A/C in operation and each time the A/C clutch is released the high side pressure is quite possibly at 350 degrees. Only things can happen that will result in the high side pressure declining below that peak.

1.) Liquid refriigerant flow through the TXV and thereby eventually into the low side as a gas.

2.) Additional cooling of the rear lid condensor due to a rise in engine RPM.

Agreed, so far..?

More...

The rate at which the TXV alllows the liquid refrigerant to flow into the evaporator and thus into the low side is a direct function of the temperature at the evaporator outflow line/tube.

That outflow line flow volume is a direct function of how much HEAT is being absorbed into the evaporator core due, primarily, to the heat content of volume of airflow produced by the A/C evaporator blower motor.

Agreed? Still with me?

Wild man West,

Let's cut to the chase.

How many automobile conditioning systems have evacuated, charged and tested over the past 42 years?

How many 911 air conditioning systems have evacuated, charged and tested over the past 29 years?

How many hours have you spent working on Porsche automotive air conditioning systems?

How many different 911's have you driven over the past 42 years?

How many automotive refrigerant lines have you built over the past 29 years?

How many Porsche owner's have you positively helped, other than posting opinions here in the forums, over the past 29 years?

How many automotive air conditioning systems have you designed over the past 29 years?

Describe your collection of automotive air conditioning tools and equipment.

This is really the crux of the entire discussion. Actual, real-world experience, vs. hypotheticals and vague allegations of profit-seeking. In the world of science, if I have a hypothesis (derived usually from a moment where I look at something and say "hmmm, well, that's strange - I wonder what happened there?"), I test it by doing experiments. First, I try to replicate the strange results. Then, I isolate the cause of the strange results. In the world of automobiles, this process is often called "troubleshooting". Those who are good at troubleshooting usually have a scientific mindset. Which is why I often get along very well with good mechanics. Analytical thinking is more of a function of how a person is, rather than how a person is trained, I have found. But that is neither here nor there. The bottom line is that one party in this discussion has more crediblity that the other. And the years of experience are the deciding factor over who has credibility and who doesn't.

Interesting chart, thanks Charlie. I believe I understand the R134 and R12 columns, which I assume show the static pressure temps for the corresponding ambient temps. What I don't get are the R401A and B columns. Is that a different refrigerant or it it referring to something else? If they are refrigerants why does it not just list pressures like it does for R134 and R12?
On a related note, I was showing about a 100 psi last night with ambient temps about 90 but what's odd is its STILL in the same ballpark and its a relatively chilly 72 at 6:30 am today. Maybe my gauges are getting buggy?
Thanks,

The difference between 401 A and B is the percentage of the constituents of the blend.
400 series refrigerants are Zeotropic Blends of two or more refrigerants that will fractionate and boil and condense at slightly different temperatures. This is known as glide.

Some refrigerants have a common molecular makeup but a different way of assembling the molecule. The difference is indicated by a capitol or lower case letter after the number.

Buck,

Gee, this thread is like an old yard sale.

But, I'll try to answer your questions as best I can on 1/2 cup of coffee.

The chart tossed up there is a Pressures and Temperatures Chart (P&T) which happens to list 4 types of refrigerant. You can find P&T charts all over the place which have a mixed bag of various refrigerants. What's on the chart is usually based upon the market the chart was designed for (auto, residential/commercial HVAC, etc).
It really does not matter what various types of refrigerant are on the chart, the numbers never change; you just need one for the refrigerant you are using.

A P&T chart is just one tool you can use to help you diagnose what is happening at 3 points in an AC system, typically condenser, evaporator and drier; its all relative to the 'saturation' of the refrigerant (you can google this), a state of refrigerant liquid and vapor.

To give you one idea on how you can use the chart (context is R134a) you could measure the temperatures at the evaporator inlet just below the TEV and at the outlet where the suction hose is connected, look up those temperatures on the R134a chart and you could surmise the expected pressures at those give temperatures; or visa versa.

Hence the P&T chart is used to help you 'think' along the way.