The subject of properly charging the air conditioning system has been covered before with detailed procedures for charging by weight of refrigerant, monitoring the sight glass, and etc., but this thread is not about those techniques. The A/C system of my Audi A8 Quattro is charged in a different manner and I wonder if this same process could be used on a 928.

There is only a high pressure port on the Audi. There is no low pressure port. The manual clearly states that this is the case and details the following method of charging the refrigerant: engine off (not running at any time during this process); the entire system must be at a stable, precisely measured temperature; and refrigerant is added (or removed...) through the high pressure port until an exact pressure (specified according to temperature by a chart in the manual) is reached.

Thoughts?

 

I have no idea what it says in the Audi document but what you have presented as written is technical jibberish I would politely suggest. A basic knowledge of thermodynamics is required to understand why but in simple terms a given pressure has a given equiilibrium temperature - the problem is the system has a certain amount of liquid in equilibrium with the gas and based on static pressure alone it is not possible to know how much gas is in there. Whether there is some magic ingredient on the Audi I have no idea but on the 928 most certainly there is not.

On the 928 you can either fill directly by weight or you can fill dynamically as I tend to do. My procedure evacuates the system, I connect the bottle to the LP port via the gauge set and I sit the bottle on the ground with the discharg port highest. I purge the line between the bottle and the gauge manifold and then with gas passing slightly I tighten the connection to the manifold. I then weigh the cylinder and with the engine not running I fill the system until it reaches equilibrium pressure. I know from experience this takes about 200gms of gas. Once equikibrium has been attained I start the car, switch on the ac with the blower at mid range, and then open the gas again. I hold the engine at about 2000 rpms and watch the discharge pressure. When the system is getting close to the right amount of gas the discharge pressure starts to rise and when it hits 250 psig I close the gas and reweigh the cylinder. This invariably shows that I have put 1kg of gas into the system and only takes a few minutes.

Dr Bob has just highlighted another approach in another current thread using a static fill through the HP port.

 

No low side port eh? Unusual. Fred R gives some accurate methods as a weighed charge should be ok unless there have been mods to the system. Another way would be to put in a new suction filter with a valve on it.
A good easy way to charge a system, using the low side while running, is to give it a partial liquid fill (can upside down) and then flip it to upright for a vapor finish. Have a buddy sit inside with the fan on high and
check for cold air coming out. When it just starts making cold on a 70 degree day you have reached minimum charge. This is called a starved evaporator and is actually the most efficient charge. Unfortunately it is only good at that OAT.
The big factor now is whether your system has a receiver. This is the volume cushion that compensates for outside air temp. If it has a receiver you can go ahead and dump a bit more in.
A beginner can charge car and window air conditioners this way.
Remember to purge lines before charging. you can purge the whole system with no pump if you do a couple vapor charges and releases to atmosphere.

 

I agree. Something is missing in the description of Audi charging. Temp and pressure are not enough. They will not determine quantity. Must use weight delivered to an evacuated system.
Good luck,
Dave

 

I hope that no one thinks that I have made this up because I have nothing better to do; and, I was not asking for a recap of the usual methods. BTDT.

The car referenced is a 1998 Audi A8 Quattro. It has no low pressure port. The manual describes the refrigerant charging process as detailed above. I do not know if this method is used in earlier, later, or other models.

I don't really believe that this method violates any law of thermodynamics or is causing Boyle to spin in his grave, and I don't really care if it does. Audi specifies this method, so it is practical even if strange. Perhaps the values were empirically determined: Audi specifies some odd troubleshooting and test procedures. Being able to monitor both high and low side pressures is a better approach, but despite any amount of technical gibberish that I may read (or write), this is what was done.

 

Hi RET,
My point is that something is missing from your description. Maybe if you gave a sample pressure and temperature, it might shed light. The method does not describe gas or liquid charging or the state of fill of the existing system.
In general, with the engine not running, the high and low pressure ports (where two ports are present) are at the same pressure, and that pressure is determined by temperature. That pressure will be the same as long as liquid refrigerant is present in the system, regardless of the amount present. Once there is no liquid present, then temperature and pressure do indeed follow Boyle's Law.
There might be details of the procedure and these may be specified in the workshop manual in a footnote or similar. It might be possible to charge from the high pressure side, in the gas form, by temp and pressure alone, but you might need a special high pressure charging canister. Some details are missing, or I simply don't understand their thinking, which is possible.
Remember most factory systems are charged strictly by weight into an evacuated system, so Audi's method just sounds a little different. I suspect there is more in the method, hidden somewhere in the manual.
Good luck,
Dave

 

You posted some information, you asked whether it could be applied to the 928 and you asked for opinions. I advised you very clearly that I could not comment on the Audi model but in the context of the 928 it is jibberish and offered an explanation as to why- do you understand/agree with this concept ?-if not would you like some further guidance?

I then outlined the procedure I prefer to use, I then referenced the procedure Dr Bob mentioned in another thread because that can be done using the high pressure port only which is about as close to what you wrote as can be implemented on the 928 but you still have to measure the weight of gas added and you have to let static head push the liquid into the system but personally I do not go that way.

 

FredR - I might have overreacted to what I interpreted as condescension in your earlier reply. I apologize if none was intended or present.

Despite previous comments, there still might be some sense in the described technique.

First: I must admit that I might have missed and/or forgotten some important details, and that I only had copies of the supposedly important pages from the Bentley(?) service manual.

Second: The information and original manual link came via one of the Audi forums I belonged to, but the link history is long lost. However a search today yielded the following that looked vaguely familiar, and although for a '97 A6, may have been excerpted from the same manual. (We all know that anything found on the Internet must be true….)

<snip>
Checking

• Switch ignition OFF.
  Connect high pressure hose of refrigerant recovery/recycling/recharging unit Kent-Moore ACR4 or equivalent, to high pressure service valve.
  Leave low pressure hose of refrigerant recovery/recycling/re- charging unit disconnected.
  

NOTE : Only a high pressure service valve is used for A/C system servicing and checking pressures. A low pressure service valve is not installed in the A/C system. Perform all A/C system pressure checks through the high pressure service valve.

Outside (ambient temperature) Base pressure in refrigerant system (engine OFF)

+15 °C ( 59 °F) 3.9 bar ( 56.5 psi)
+20 °C ( 68 °F) 4.7 bar ( 68.2 psi)
+25 °C ( 77 °F) 5.6 bar ( 81.2 psi)
+30 °C ( 86 °F) 6.7 bar ( 97.2 psi)
+35 °C ( 95 °F) 7.8 bar (113.1 psi)
+40 °C (104 °F) 9.1 bar (132.0 psi)
+45 °C (113 °F) 10.5 bar (152.3 psi)

1) All components of A/C system must be at the specified temperature for an accurate pressure measurement.

If A/C refrigerant system base pressure is less than specified on chart, system is low on refrigerant.
<end snip>

The assumption of the "Audi" method is that a static system has a fixed empty volume (whether in the form of hoses, compressors, condensers, etc., or the inside of a pressure cooker); and different volumes of refrigerant that partially fill the system (so that some refrigerant remains liquid and some boils off to form gas) will result in different pressures when at the same stabilized temperatures. FWIW, on the occasions that I have added refrigerant, I have observed an increase in the static pressure (engine off, and high and low pressures allowed to equalize).

 

Hi RET,
I guess it's immaterial, since we wouldn't really use that technique, but pure material, including liquid and gas in a sealed container, will equilibrate to give a vapor pressure which is solely dependent on temperature. Those numbers you listed indeed look pretty much like those pressures, and therefore indicate that there is liquid present in the system, but do not tell how much. It's pretty much the same when you have a cylinder of CO2. It stays at about 1000PSI (depending on temp) until all the liquid is gone, then the pressure starts to fall. I'm not arguing. I just think something is missing.
Good luck,
Dave

 

I'd need to look harder at the Audi system, but you can answer some basic questions here. 1) Does the system use an expansion valve, or is it a "more modern" system that uses a fixed orifice tube for metering refrigerant to the evaporator? 2) Does it have a liquid receiver after the condenser but before the evaporator?

Mfrs have figured out that they can avoid the costs and complexities of the expansion valve by using a fixed orifice tube instead for metering refrigerant flow through the system. If you've worked on a home or smaller commercial refrigerator, you've seen the system in action although the flow orifice is a not nearly as elegant as an orifice tube. The orifice tube resembles a tapered plastic screen, so it can catch any possible system debris without plugging completely. Refrigerant flow is related to compressor speed, and the system relies on having just the right mass of refrigerant in the system to work correctly. In a static state at typical ambient conditions there is no liquid present in the system, so you can use the indicated pressure and a known system-wide stable temperature to determine state of charge. The test/charge port can be anywhere in the system really, since everything is at the same pressure throughout during test and fill. These systems use a very simple dessicant drier with no liquid reservoir at all. These are typically just a small drawn cylinder with an axial port in each end, placed somewhere in the higher-pressure piping.


To answer your original question, you can not use this method to reliably determine the state of charge in your 928 AC system.

 

If folks are a little over sensitive at the moment it is not too surprising given many of us do not know whether we will be intubated this time next month [or worse]. I smile at these sort of things but then I am British and we believe we have the best sense of humour in the world [with the British climate and the doube taxation system you need an exceptional sense of humour just to survive!].

The thermodynamics of refrigeration systems are the same no matter how the process is achieved. When I fill the system [as stated earlier] initially I bring the system to equillibrium with the bottle of R134 which for the most part is full of liquid when new and this happens when approx 200 gms have been transferred. At this point the ac will start. That the system fill capacity is 1000 gms tells us that some 800gms must be present in the liquid phase with the system static.

The chart you attached is simply the equilibrium pressure of the refrigerant used at the prevaiing ambient temperature. Now as you wil probably have noticed, the 928 ac system in line with other systems has a low pressure start permissive switch- not enough pressure and the system will not engage. For that to work in all circumstances the setpoint of that switch has to be compatible with the lowest temperature the ac system might be used at- the aim being to protect the compressor from damage.

Now, if you have a 928 system that is leaking slowly the driver can sense that something is amiss because the system rapidly loses performance just before the point at which the switch locks the system out. I perceive that point to be when the static system has no liquid component but I am not 100% sure of that. Between max fill and the point at which the performance tips, the performance seems pretty much the same and this I put down to the system having enough gas to be at the saturation point. Once the gas inventory drops below this point performance collapses and then the low pressure switch prevents operation.

The achilles heels of the 928 system are the compressor and the expansion valve. The compressor shaft seal technology is very old hat and fugitive emissions are more or less the normal modus operandi. The expansion valve commonly lets go with age. The Audi system invariably will not use an expansion valve, rather a fixed orifice system [nothing to leak]. I have no idea how Audi ac compressors are sealed but if they have implemented a more robust seal desgn that eliminates leaks they may well have decided that an equilibrium fill is all that is needed and have designed the system elements [the evaporator and the condensor] such that they can deliver the required duty. This being the case the system would only need a HP connection and a simple static fill [no engine running] would be all that was needed, no weighing scales needed.

Now, if you do the 200gm static fill on the 928 my guess is it will work to some extent or other but will likely not last too long before the low pressure switch locks it out due to a small loss of gas.