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Instructions for charging by pressure & temp?

Leggie on Sat August 18, 2012 11:57 PM User is offline

The simple answer is "weigh in as specified on label or factory manual" but that does not work when the information is not available.

The guideline differs depending on where I look.
Here is what Master Cool shows http://www.mastercool.com/media/89660-INST_web.pdf
The silly gauge on charge kit shows 25-45 psi

Since temperature to pressure is not a linear relationship across the entire range, the rule "number of psi = "number of *F x nn + yy " does not seem like the correct way.
Perhaps the better way is saturated pressure at ambient + xx degrees.

What are some good ways to charge very old systems that do not have the specs available or systems with aftermarket components that make factory specs useless?

I'd like instructions for both R-12 and R-134a systems with & without sight glass.

Edited: Sun August 19, 2012 at 12:06 AM by Leggie

buickwagon on Sun August 19, 2012 3:13 AM User is offline

Compare the temperature of the evaporator inlet to the temperature of the evaporator outlet. If the inlet is warmer than the outlet, the system is overcharged. If the outlet is warmer than the inlet, the system is undercharged.

Heat is absorbed primarily through the vapourization of the liquid. If the system is just right, the last vestige of refrigerant is vapourizing as it hits the outlet. Once vapourized, it can "superheat" quickly ("superheated" just means the temperature is above the vapour point. In an undercharged system, there is not enough liquid in the evaporator so it contains some superheated vapour -- ie: part of the evaporator near the outlet is warmer than the rest. In an overcharged system, there is more liquid than can be vapourized within the evaporator alone, so vapourization is delayed in the evaporator (keeping part of the evaporator near the inlet warmer than the outlet) and continues in the discharge line.

In a CCOT system, some additional liquid is required to accommodate inefficiencies, so it is common for a bit of liquid to make it into the accumulator. Only vapour should exit the accumulator though. Also, some may have the orifice at or near the condenser to minimize the hissing noise transmitted to the passenger compartment, so the entire line from the condenser to the firewall may be cold too.

Pressure alone is not an adequate guide, as there isn't a second low-side test port to compare inlet and outlet pressures. IR heat guns are convenient, but not necessarily accurate as different surfaces can influence the reading. Best is an accurate (or at least matched) pair of temperature probes.

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I've saved hundreds on service by spending thousands on tools.

mk378 on Sun August 19, 2012 11:39 AM User is offline

CCOT charge until the evaporator outlet line gets cold, the same or slightly colder than the inlet. This indicates there is enough to have liquid spilling out of the evaporator. Accurate temperature measurement is essential as the difference is slight.

In a TXV system, the TXV acts to prevent the evaporator from ever completely filling with liquid, so the above method will not work. On those, watch the high side primarily. When it rises dramatically it's overcharged (the condenser is filling up with liquid, which impairs condensation) and you may need to take some out. There is some leeway in charging a TXV car as the receiver will buffer some refrigerant.

Add in small amounts and allow operation to stabilize for several minutes before deciding to add more. All this assumes the system is otherwise in working order and just needs to be charged. Always keep track of the actual weight put in and write it down so you have a spec for next time.

Leggie on Sun August 19, 2012 5:35 PM User is offline

Quote
Originally posted by: buickwagon
Compare the temperature of the evaporator inlet to the temperature of the evaporator outlet. If the inlet is warmer than the outlet, the system is overcharged. If the outlet is warmer than the inlet, the system is undercharged.

Some cars have TXV on the engine side of firewall, but some don't. What about when you don't have access to both? My car is such example. High side goes inside the evaporator box which houses TXV. This line can be 130F on a hot day.

Also, for cars with sight glass, how do you use it? I remember seeing a PDF somewhere that you charge it until its clear, then add 50g.

buickwagon on Mon August 20, 2012 7:14 PM User is offline

I guess, when in doubt, the ultimate arbiter is the performance. Undercharge it, then slowly add additional refrigerant so long as the performance improves. Stop adding when the performance levels off.

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I've saved hundreds on service by spending thousands on tools.

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