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134A Pressure Chart Differences

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  • 134A Pressure Chart Differences

    Can someone explain why low side/high side gauge pressure charts vary so much? For example I see charts that show, at 90 degrees, a low side of approx. 30 psi and high side of 225. Another chart will show 50 PSI and 260 PSI? One chart even allows for differences in humidity.

    I've also seen the formulas for High Side Pressures: Ambient temp times 2 + 50. And 2.3, 2.4, or 2.5 times ambient temp.

    I know engine RPM, airflow, spraying the condenser with water all have an effect on pressures. Bottom line I never know quite what to expect when I hook up a set of gauges to do a test or charge a system.

    Last edited by 69-er; 06-29-2019, 12:02 PM.

  • #2
    Charts are the best "guesses" once the only thing you tried to achieve. It can't be that simple. You need to calculate in every condition at the time you observe them. Yes humidity totally matter. A dripping wet evaporator is less capable of exchanging heat. Factors they don't mention are so many it makes a trade out of this on it's own. Sun, altitude (YES,) engine configuration - geez YOU try to list them all out!!, fans are how strong and well suited.
    Stop with that: Look at the shape of vehicles now vs other are sloped down fronts not a billboard of a grille some type have to put condensers to one side at an angle. Firewalls and dash parts when it's just ductwork now there's lots of heat invasion in many.

    The end game is know the system and capacity by weight. There's no dipstick or perfect pressures to say it's proper just within a wide range of what is known as OK for the situation.

    Check performance inside vs the real air temp in front of a vehicle not any forecasts it's only knowing right there. Blacktop surfaces vs lighter your tree thermometer or wall says one thing yet you can't touch the surface heat is rising up fast unless windy enough.

    That's why is a zillion words - sorry. Expect it to read differently with the list of different situations when you read pressures. Any vehicle maker goes thru unreal testing to know what is the best close amount for all assorted places the mobile vehicle might be in. I don't want that job! :-)
    MetroWest, Boston


    • #3
      As far as I'm concerned the only "chart" that has any use is this one:
      R-134a vs R12 Temp Pressure Chart (http://www.acsource.net/acforum/viewtopic.php?f=1&t=8490#p30284) Postby ACProf (http://www.acsource.net/acforum/memberli

      The same information can be found right on the face of the A/C gauges.
      Charts in manuals are an engineers expectations of how the system will preform under one set of conditions. Fairly useless information. What the mechanic/technician needs to deal with it what the system is doing in the real world.
      Humidity effect performance just like a refrigerant. It takes way more transfer of heat to drop the air temp 1 deg below the "dew point" from one deg above dew point than it does to drop from 3 deg above to 2 deg above. In other words, dry air takes less energy transfer to cool than "wet" air. Just like the phase change in the condenser, more heat is release when the refrigerant changes from gas to liquid, than when the gas drops temp but remains gas.
      In a properly changed and functioning system the high pressure is the condensing temp of the refrigerant, and the low side is the boiling temp, Nothing more or nothing less. ( there are some slight exceptions having to do with compressor capacity (speed) If you rev the engine the low side may dip, showing a lower pressure, without a drop in boiling temp, but in general pressure equals temp, The pressures are telling you what is happening inside the tubing.
      An engineer might say a 40deg temp difference between the refrigerant condensing temp and the air passing thru the condenser is "acceptable" but that doesn't make the system work as well as it could, it just mean that the bean counters or other concerns mean the engineer is willing to accept the compromises made.
      Understanding how a refrigeration systems works, the physics behind how phase change work, you will not need "charts". If I can get the condensing temp of the refrigerant down to 10-15 deg above ambient temp, the whole system will work better than if the condensing temp that is 40-50 deg over ambient.
      On the low pressure side, the amount of refrigerant that can be boiled off in the evaporator is a limited by the surface area of the evaporator. Once enough refrigerant is in the evaporator such that it all boils just by the time it exits the evaporator, you have reached the capacity of the system, if the air entering is hot enough, the exit air may be above the desired temp, in short the heat load exceeds the capacity of the system and the air temp and low side pressure will be higher than ideal.
      Most charts are misleading, there is a difference between observed conditions and some ones idea of how the system "should" preform. Pressures in a properly changed system tell temp, but you can't adjust pressures to meet a graph and expect a system to work. One is telling you what is happening in the system, the other is what some engineer expects the system temps to look like under specific conditions.