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Question on "can charging"

wgabriel10 on Sun September 19, 2010 8:53 PM User is offline

Year: 2004
Make: chev
Model: S10 Blazer
Engine Size: 4.3L
Refrigerant Type: R134
Ambient Temp: na
Pressure Low: na
Pressure High: na

After reading some posts/answers back earlier in the year for education, I have a question about charging from the typical 12 oz cans. In one post a statement was made that it took 3-4 oz of refrigerant (R134 in this discussion) to fill up the manifold gauge set. I knew that it took some amount, but did not realize it could be this much.

When I was recharging my S10 last Friday, I just put in 2 full cans (24 oz) and on the last can, did the weighing to put in another 5 oz for 29 oz total, which was just a bit more than the 28 to 28.8 oz called for (depending on whether you use the 1.75 lb (hood sticker) or 1.8 lb (vehicle manual) spec, thinking that this would offset the "loss" in the gauge set and charging line purge and whatever gas was left in the "empty" cans (there was no detectable liquid left). If the info from the post I read is true, I could be a bit undercharged.

So, my question is: Does "filling" a manifold gauge set typically cause a loss of this amount of refrigerant?

Thanks for your advice/education in advance -- Watson

Edited: Sun September 19, 2010 at 9:29 PM by wgabriel10

TRB on Mon September 20, 2010 12:35 AM User is offlineView users profile

I've never measured it! But personally think you may retain an once at the most in the lines of a guage set. High side is closed when charging unless using a professional changing station. When using a can method you can shut the can tap and the compressor will pull out most of what's in the line. Shut the low side gauge and now there should be little refrigerant in the gauge set itself.


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emsvitil on Mon September 20, 2010 1:21 AM User is offlineView users profile

I've measured it..........

I had a partial can, and a triple beam balance.....

3 48" hoses connected to service port adapters. Did a slight 2 second vent to purge the lines.

Weighed can before and after to get:

To charge yellow line and manifold, 3.4 grams

Then 2 grams each for the high and low lines.

So the hoses and manifold hold roughly 8 grams or 1/4 oz (28.4 grams/oz)

Also weighed cans after charging and then after venting can, and found 3 grams were left in 1st can 5 grams in second can (only needed 2 cans).


NickD on Mon September 20, 2010 8:07 AM User is offline

Always a guess as to how long to purge a can, and has to be done every time the can is changed. Too short, and you risk getting air in the system and in particular that first can where you have negative pressure in your system that can suck in a lot of air in the process of purging.

Kicking myself in the butt for sleeping and not buying 30# containers of R-12 when I could at 27 bucks each, could have been super rich today.

I just connect one set of manifold gauges, the blue to the pump, the red to the tank with valve closed, and the yellow to a second set of gauges connected to the vehicle as normal. Can draw a sharp vacuum clear up to the tank, no need to purge, close off the pump and open the tank for charging. Can leave both the red and blue valves open to the vehicle to draw a complete vacuum from both ports, and even initially charge that way. Then close off the red for low side charging. Close off the tank and draw most of the refrigerant out of the lines when done.

So simple, so intelligent, so reliable, screw cans.

iceman2555 on Mon September 20, 2010 2:11 PM User is offlineView users profile

Some time, long ago in history, we 'tested' this issue with different gauge sets and different hose. The test was the manifold hose set was evacuated and then connected 30lb that was placed on a scale. The center hose was purged, the scale was 'tare' once more and then the manifolds were opened. Test were performed with 48 in, 72 in and 96 in hoses. The average amount necessary to 'charge' the hoses was 2 to 2.5 oz per hose.
In the earlier days of recovery and recharge machines, several of the machine manufactures actually stated in their instruction manual to add 2 oz per hose for a base recharge.
This amount may sound insignificant and would be with a large capacity system, say a early model Chevy Surb with rear air, however, when compared to a late model vehicle with a service recharge of 14 oz, then this amount of possible undercharge can be a major contributor to possible performance issues and premature compressor failures.

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

TRB on Mon September 20, 2010 2:34 PM User is offlineView users profile

Did you try and reclaim the refrigerant in the lines or just shut off the gauges and measure?

If you start with the high side closed you not going to get any refrigerant it that line. If you continue to use the compressor to suck in refrigerant after the can tap or cylinder valve is closed. I would think there would be a small amount left in the lines.

I do agree with low volume capacities. A proper charge is a must!


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Cussboy on Mon September 20, 2010 3:02 PM User is offline

Originally posted by: NickD
Kicking myself in the butt for sleeping and not buying 30# containers of R-12 when I could at 27 bucks each, could have been super rich today.

Maybe if you had done that and sold them when the demand for R-12 was still high, like in the late 1990s. Now there are relatively few vehicles which still run on R-12 (like my 1988 B2200 truck, and I do have my own R-12), so not a ton of demand. Yes, I know asked-for prices for R-12 are still prettyhigh, but doubt that there's that many buyers.

I think a lot of shops don't even service R-12 any longer because of the need for separate equipment, espeically for recovery.

NickD on Mon September 20, 2010 7:08 PM User is offline

Originally posted by: Cussboy
Originally posted by: NickD
Kicking myself in the butt for sleeping and not buying 30# containers of R-12 when I could at 27 bucks each, could have been super rich today.

Maybe if you had done that and sold them when the demand for R-12 was still high, like in the late 1990s. Now there are relatively few vehicles which still run on R-12 (like my 1988 B2200 truck, and I do have my own R-12), so not a ton of demand. Yes, I know asked-for prices for R-12 are still prettyhigh, but doubt that there's that many buyers.

I think a lot of shops don't even service R-12 any longer because of the need for separate equipment, espeically for recovery.

R-134a won't go to waste, can always use it to clean my keyboard.

Positively impressed with my motorhome refrigerator, has a large separate freezer compartment that is -10*F and only consumes an average of 25 watts! It uses ammonia as a refrigerant, they should go back to that.

Yeah, I know all about buying low and selling even lower, but you can make that up in volume.

wgabriel10 on Mon September 20, 2010 8:25 PM User is offline

LOL NickD -- I know about making up losses in volume!!

Speaking of volume -- volume of gauge sets, hoses, etc -- I surely do appreciate all the answers.

It does look that for smaller charge volume systems that use of manifold gauge sets can have some significance in measuring the amount of refrigerant added to a system during charging. It also is apparent that every time one hooks up a set of manifold gauges, take readings, and then remove them, the total system charge left in the system could be affected.

When I was charging my system, yea, I had the HP line hooked up as well as the LP, so I got the whole effect. I always do it that way in order to also see the HP reading at the same time. The compressor probably pulled out most refrigerant in the LP line until I shut the LP valve, but the HP line volume is a total loss as far as I can see. Am going to do some thinking and will probably add a bit of R134 to make up the difference.

I also want to look into another related item in seeing if a system is "charged enough". Somewhere I read in the recent archives about measuring the tube temperatures of the inlet and outlet of the evaporator during operation to make determinations if the evaporator was "flooded", indicating that the system is properly charged. I need to pull out my refrigeration book and try to better understand the "flooding" reference; however, I think I already know what that is. Tell me if I am wrong -- It means that the system has enuff refrigerant such that the evaporator will continually have enough expanded (cold) gas in it such that with the air flow across the evaporator being cooled, there is no degradation of evaporator temperature.

In other words -- the creation of cold gas in the evaporator "keeps up" with the BTUs lost to the flow of outside inlet air moving across the evap coil. If it cannot keep up, the outlet gas temperature of the evaporator will "degrade" in temperature, by getting warmer. As such, the system is not meeting its optimum cooling capacity.

I think the post I was reading related more to R12 to R134 conversions; however, from a thermodynamics standpoint, what was said should apply to whatever refrigerant was used and also should apply to any system.

I really want to say, but can't be positive with data, that no vehicles that I have ever owned have had situations where the evap inlet and outlet temps were "real close" to being equal. By touch, I always think I remember the outlet being warmer than the inlet. I know for sure that I can say that about either of the two heat pumps I have had in my house when in a/c mode.

Thanks again for the answers, guys! Personally, I get a kick out of talking over this stuff. Problem with me being an engineer, I guess, even if it is electrical.


1stbscout on Tue September 21, 2010 12:59 AM User is offlineView users profile

ok so here is the problem. It is nearly impossible to have your "cake and eat it too". LOL
A flooded evap will cool only as well as the different system temps and pressures allow it to.
Hard to grasp but here it is....... where to start.......
Depending on who you want to believe.... the process is simple yet complex ..... LOL
We will start with the refrigerant....... each one has a different pressure/temp relationship. Depending on it's intended usage it will work better or worse than the next one. If you want you can even make up your own.
So... Pick one..... makes no real difference just for the sake of this we will pick R12.
R12 has a specific Temp/pressure relationship. For our purposes we are most concerned about where it "boils" where it condenses and the latent heat required to do each "job".
Looking at your gauge set you have temp scales inside (or out, depending on where they come from) that tell you the temp the refrigerant will boil at on the low side (suction if you will) and the condensing temp on the high side. It is up to you to understand the "critical" temp and pressure of the refrigerant you are using. As such I will ignore that for this discussion).
So... If I "release R12 into a "room" (the evaporator) that is at atmospheric pressure it will boil at -29.8 C. If we do not make arrangements to remove the "vapor" that is produce the pressures will equalize and the refrigerant will cease to boil. As such the temp will change.
Next we need to look at a strange occurrence. If the refrigerant is not at the same temp as it's boiling point some part of the expansion is used to drop the liquid to -29.8 C before the rest can boil. This is known as it's latent heat... I think... Have been drinking and LOL... Oh hell I give for tonight.

Simply put if the evap it too full... the refrigerant cannot boil and as such it will not cool to it's max capacity. the cooling will occur outside of the evap. Obviously this is not what we want. So we then turn to "superheat". This is the amount of heat the refrigerant absorbs over and above it's theoretic "boiling point", at the given conditions, before it is "released" to the compressor to start the process over.

Clear enough ???? LMAO need sleep ... hope y'all understand.

Edited: Tue September 21, 2010 at 1:01 AM by 1stbscout

NickD on Tue September 21, 2010 3:01 PM User is offline

I have heard of a flooded evaporator, personally, never ran into one. Visualizing one, could be one where the orifice lost its seal or someone forgot to put one in. If normal, sure would have to take a great deal of overcharging to flood one, before that happens, the high end would blow up or the compressor would seize. Or lets just chalk this up to my inexperience.

If I put my hand on the evaporator outlet with engine running at rated speed and its warmer than the inlet, there are problems in the system. Now trying to think back where that is normal, so far a blank.

But charging with cans is not completely foreign to me, very easy to charge with air that gives you artificial pressures and very poor cooling. And how many seconds, days, or weeks do you let that can purge to make sure it pure refrigerant? So much nicer to draw a vacuum clear up to that can or tank valve.

WaterGuy on Tue September 21, 2010 8:53 PM User is offline

I'm still working to improve my brain's comfort level on what makes this marvelous chemical act as it does. The release of heat from oxidized fuel into a space with subsequent rise in temp is relatively simpler to grasp, but the idea of a chemical (refrigerant) of and by itself lowering the temperature of a space is just strange. R-12 or 134a has to evaporate and absorb heat to do this, and I guess the spray of all those little molecules at minus 21 F or thereabouts is just grabbing and holding onto any heat it can find. When the heat is gone from the space, the temp drops, and how far it drops depends on the amount of spray atomized into the evaporator. Is that right? But isn't there a mix of liquid and gas entering, with all of it turning to gas as it rises to the outlet and absorbs cabin air heat(?) In other words, a temperature gradient, cool coming in, hot going out-- as it's pulled toward the compressor(?). There is interaction going on here I would think which causes the change; however if I pick up a can of 134a at WalMart (sorry!) I'm pretty dead certain that everything in that can- vapor, liquid, metal can and all, is at ambient store temperature. I don't have a chart handy, but seems like psi and temp match up around 70 degrees F. To me that means a saturated vapor situation, where molecules are moving back and forth inside the can from gas to liquid. A lot of the general troubleshooting guides seem to say something like a static pressure on a vehicle's AC system is in neighborhood of 50 psi. But how can that be, if ambient is 80 or 90 degrees F, for which the chart would indicate a much higher psi exists, for what I assume would be the saturated vapor condition. So... maybe I'm just way off base but that seems to indicate that a vehicle's system is not really charged to a saturated vapor condition, with for instance a flooded evaporator.... liquid does exist perhaps at points in the system but that's a dynamic, gradient kind of thing which changes from low to high side and perhaps changes to some "degree within each side of the system as well and the chart doesn't exactly apply..... I'm just trying to understand by making outlandish assumptions I guess.

Blessed are those who have not seen and believed.

NickD on Wed September 22, 2010 6:04 AM User is offline

Change of state is the key, from a liquid to a gas and you want pure liquid feeding the orifice for best performance. When you compress and condense this gas, it becomes a liquid and gives off heat in the process. That heat is recovered when that liquid returns to a gas. What causes the liquid to change to a gas is differences in pressures, the liquid before the orifice is in relatively high pressure state, in the evaporator, at a low pressure state, permitting the refrigerant to expand.

Liquid is in a low energy state, gas is a much greater energy state, to gain that much higher energy state, it needs heat, so absorbs it from the evaporator. Can delve into quantum mechanics on this subject and blow your brain trying to comprehend it, but this is a natural event that occurs in nature, easier just to accept it. Trick is to find elements on this earth that can convert pressures into temperatures or vice-versa at reasonable pressures and temperatures and preferably with such a compound that doesn't kill people. Last time I checked, over 4,000 refrigerants have been concocted over the last 120 years, but the government makes it simple with only a minute handful to select from.

V = PT, the relationship between pressures and temperatures discovered by Robert Boyle in 1662, but it took humanity over 230 years to get any practical use out of this basic relationship. With volume constant, if pressure goes up, temperature must come down. Called Boyle's law, but he didn't make this law, he just discovered it.

Always a theoretical and practical side to science, been working with electrons most of my life, but never saw one, sure felt them at times. Some delve in strictly the theoretical side with no practical value, other make practical use out of these sciences and accept they exist in nature.

Practical side of a can of R-134 is that its internal pressure is about 70 psi at 70*F, but can rise to over 300 psi if left in a vehicle in the sun where the temperature can hit 180*F. Practical side of this, is the can will explode.

Another practical side is taking a 110*F shower and stepping into a 70*F room, not only the cooler temperature, but the water is evaporating from your body that is removing heat from your body. You find yourself freezing to death. Drying off quickly with a towel helps.

1stbscout on Wed September 22, 2010 9:48 AM User is offlineView users profile

actually NickD is correct. It is the change in state from a liquid to a vapor that does most of the cooling.
Using water/ice for an easier to use example.
If we start with ice at 32* (there may or may not be any water present). As the ice melts the temp stays at 32* until the entire amount of ice has melted and all turned to water. After which the entire mass of water will gain temp until it approaches it's boiling point. The same thing happens at boiling point. The temp (which changes with atmospheric pressure) remains stable at 212* until the last of the water is changed to steam.

So why is this process so important?
The amount of heat that is absorbed to change ice to water is within 36 BTUs of the the amount needed to heat water to it's boiling point and yet the temp stays at 32* the whole time. At its boiling point there is once again a "pause" where the amount of heat absorbed without temp change is fairly great. (simplistic explanation at best )

Refrigerants do the same thing. As they are not a pure gas they do not follow directly the laws which govern pure gases. Technically they evaporate to a vapor instead of a gas. And like water the change of state determines how much heat they can absorb. Each one is designed to work within a given temp/pressure range such that their boiling point and condensing point works within the system they are being used in. Just a few ounces in a large system and less than 0.01 ounce in a small system can effect how well they cool.

One other often forgotten aspect it what happens when the liquid leaves the expansion device. As the liquid enters the evaporator the first thing it does is cools itself to the temp that the core/refrigerant needs to be at in order to meet the current pressures. So if you have 79* liquid entering the evap and the evap is at 32* first the expanding liquid cools all the liquid to 32* then the rest evaporates from this point. This is why higher liquid temps do not do as much cooling and the efficiency of the condenser is important.

If the amount of liquid in the evap is proper. It will all turn to a vapor before it leaves the evap. This is where superheat comes in. Most systems work best when there is a certain amount of heat absorbed over the "boiling" point. The difference in the theoretic temp the refrigerant/core (shown by/on your gauges on the inside scales) is at and the actual temp if the refrigerant is superheat.

Lastly, if there is not enough "load" on the system the evap can completely "flood" and liquid can be returned to the compressor. We all understand how this is not a good thing.

NickD on Thu September 23, 2010 6:03 AM User is offline

Water was considered as a refrigerant in the sense of pulling all the heat out of it in aiding a geothermal system by making ice cubes. Taking three pounds of water, can only pull 3 BTUs out of it for each drop in *F, but in changing state from a liquid to a solid, can get 442 BTUs out of it.

Sounds good on paper but does have a minor problem in mechanically moving those ice cubes and storing it so it can be used to cool in the summer.

Whereas just 3 pounds of AC refrigerant that is constantly recycled can produce 36,000 BTUs per hour without a storing problem. Problem with existing heat pump/AC systems, trying to use just one refrigerant over a huge temperature range, it can't be efficient over that entire range, perhaps 2 or even 3 different refrigerants should be used. In AC, the key disadvantage, is that the hotter it gets, the less efficient is the system.

AC and heat pumps are more of an energy transformation system like an electrical transformer that can input any voltage and transform it to a higher or lower voltage. Even the the ambient temperature is say, -40°F outside, really extremely hot when compared to the absolute zero temperature of ?459.67°F. And it takes a lot less energy to transform this heat rather than burn a pound of fossil fuel. Can be as high as 18 times less with current models. But again over a very narrow temperature range.

One severe restriction imposed by our EPA and other such emission governmental agencies throughout the world is to develop a safe refrigerant that can be dumped into the atmosphere besides have good heat transfer characteristics. They rather do this so manufacturers can use cheap crap in the production of refrigerant type equipment and pick on the repair industry themselves for recovery with huge fines. Recover what? If all the refrigerant didn't already leak out, there wouldn't be a problem.

Any kind of confined energy is extremely dangerous, you may feel very secure with a full tank of gasoline on the freeway. But if some idiot is going to smash into you where that gasoline can burn you to death, you would wish your tank was empty. Same problem exists with nuclear, years of energy in that little box.

If you really want to meet with a group of dumb engineers, deal with the EPA, one reason why they are there, can't hack it in private industry. But if you are a good engineer, the pay is excellent, only have to work for a short 30 years, get full medical benefits, and retire a 55 with 75% pay. But to do this, have to be able to lose your self-respect and put up with 30 years of pure BS. This is what our country has become.

tomw on Fri September 24, 2010 11:55 AM User is offlineView users profile

I remember from physics the formula PV=nRT ... don't remember what they all stood for, though, as it was long ago... That said, it seems to me that the amount by weight of refrigerant that would purge the hoses and manifold would totally be based upon ambient temperature. Given that a fraction of an ounce of liquid will pressurize a container to the formula stated pressure, as the refrigerant evaporates, it will expand as much as it can on the way through the hoses and manifold. It will keep expanding until it reaches the pressure for the stated temperature curve, but, in atmosphere, the pressure is ~13.x psi, so will expand seemingly forever.
What's that mean? Trying to explain in simpler terms, a fraction of an ounce of refrigerant, at room temperature, will boil to gas, increase its volume until it reaches its temperature/pressure equilibrium, and stop. There is not much pressure from the atmosphere at sea level, the temperature for humans to be comfortable takes the pressure of most refrigerants into the 70+psi boiling point range, so it will boil, and expand enough to fill the hoses and manifold with a very expanded gas. How much? Depends on temperature would be my answer. My guess is that ounces of R12 would expand enough to fill a bunch of hoses and manifolds. Consider the gas that you use for cooking. How many ounces of LPG do you think it would take to fill the hose from the regulator to the burner? Not much, as it expands significantly, and displaces the air that previously filled the hose.
Interesting question.

simplificate and add lightness

NickD on Fri September 24, 2010 12:03 PM User is offline

In a practical sense, measuring static pressures only tells you that you may have only ounce or two of refrigerant left. Has nothing to do with the actual quantity of the refrigerant in the system.

Also in the practical sense, purging a can is nothing short of a wild ass guess, better not to have to purge, period. And means available so you don't have too.

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