Slightly high LP AC reading

Discussion in 'High Tech Retrofits' started by badazz81z28, May 1, 2019.

  1. frank5s

    frank5s Veteran Member

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    Someone correct me if l am wrong . The outer ring is pressure. The inner ring in red is refrigerant temps, not pressure
    Different refrigerant have different temps at the same pressure
     
    Last edited: May 3, 2019
  2. tom3

    tom3 Veteran Member

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    Yup. Pressure is temperature, temperature is pressure in refrigeration. I'd say that's a bit overcharged. High side will go up when it gets hot outside. Temperature is pressure you know......
     
  3. badazz81z28

    badazz81z28 Veteran Member

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    I’m not convinced VAs techs are smart on the system. Their answer is to pull vacuum and charge with 28oz. I’m not purging this system to do exactly what I already did. It blows cold....it’s almost like don’t pay attention to the gauges unless the system isn’t performing. I don’t see how it’s even possible to have 6 psi on the low pressure side. As soon an you put a couple oz of R134 in the system it’s at 30-40 psi and doesn’t change all the way to 28oz
     
  4. mrfleet12

    mrfleet12 Veteran Member

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    Badazz, how much refrigerant did you use to charge your system, is everything still working fine?
     
  5. badazz81z28

    badazz81z28 Veteran Member

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    I did my best to put in 28oz. I used a kitchen scale. It seems to be working fine. I decided to disregard the the pressure #s.
     
  6. mrfleet12

    mrfleet12 Veteran Member

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    Thanks
     
  7. supwicha

    supwicha Veteran Member

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    The purpose of this long post is to help you not spin your wheels, and spend money on an improperly tuned system. If you are reading this and are a real AC tech, this will be a boring read for you. No, no, no people :) The gauges do NOT represent temperatures. While I understand the gauges in the photos seem to be representing that, high side temperatures of the refrigerant will far exceed what this particular gauge set is indicating. While temperature changes pressure, your interest is not about refrigerant temperature. Basic system overview: The compressor drafts refrigerant from the receiver drier and pressurizes it and at this point it is a hot high pressure gas. It is passes through the condenser where the gas is condensed to a high pressure liquid (if this process fails, the system will not function), This high pressure liquid now passes through either an orifice tube or expansion valve. This is where the magic happens, this process now takes the high pressure liquid and during the quick expansion and drop in pressure, and vaporizing of the refrigerant, this process is what gets very cold (this is why if you vent a can of refrigerant, it is cold. The liquid in the bottle sitting there is not cold, but the expansion and release reaction is). This low pressure vaporized liquid now passes into the evaporator. The evaporator does NOT make cold air, it absorbs and transfers heat from the warmer air. This HEAT that has now transferred from the cab of your car into the refrigerant leave the evaporator to the receiver driver, and starts the process all over again. AC systems are "heat pumps", they absorb heat in the cab, and expel the heat outside of the cab. The high side temperatures are going to be very hot, and if working properly, the low side will be cool to cold and depending on humidity wet. While the manufacture do charge based on volume, this assume that the SYSTEM IS STOCK with no modifications due to prior testing and established pressures. Once you determine the exact volume for your application THAT WORKS PROPERLY, you too can charge by volume later. If you have changed the system, converted to a different refrigerant type, change the size of the drier, swapped out the compressor, forget charging by weight. Any knowledgeable AC tech knows that you must charge based on pressure AND outside ambient temperatures assuming the system is working as designed. There are charts that easily define this. Your static pressure while the system is off will vary based on ambient temperatures. Assuming the system has no leaks, your reading on a 60 degree day will be lower than a 100 degree day. This methodology is only a guideline, but not a final answer in how to charge your system. Just google "R134a Charging Pressure Chart" and you will see some GUIDELINES as to what to expect for suction and discharge pressures based on ambient temperatures. 18PSI is too low for suction on a R134a system at ANY ambient temperature. For example, on a 70 degree day, you could expect to see 35-40 psi on the low side and about 140-160 psi on the high side. This also assumes that your system is working properly, and if you have performed a R12 to R134a conversion, and if using a stock condenser is never going to work very well. Since I don't know your system configuration, if you have converted, or what you have changed, I can't really provide more detail as to how your system is performing. If you post up good detail about how your system is built, stock, modified, converted to R134a, etc, I can better answer what is going on. No two vehicles in these forums are going to be the same as how to charge them unless the vehicles are identical in how they are equipped. Lastly, if your system has been modified or only partially converted, no chart is going to help you as the system must be matched up correctly for optimum performance. In my particular conversion, I'm using a updated R134a drier, late model R134a compressor, a R134a condenser from a '95 caprice. The orifice is located in the condenser, not at the evaporator inlet. My system running at idle on a very hot day with the electric fans running, will produce solid cool air all day long, and I'll have to turn it down. But again, this is a matched system.
     
    Last edited: Jun 20, 2019
  8. supwicha

    supwicha Veteran Member

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    I'm sorry, but this is not correct advice. While the manufacture does charge by weight, this is because they have already determined the proper pressures for a given factory ambient temperature through testing and/or engineering calculations. Once the OEM has made this determination, for the ease and efficiency of the manufacturing process, they charge by weight for simplicity and reduced manufacturing time. But again, because they have tested/calculated what volume of refrigerant is needed to get the desired PRESSURES. This assumes that any vehicle in question, that the system is stock with no modifications, original receiver drier size, and refrigerant type. If anything has changed and more likely than not it has, you must establish a new baseline using pressures and ambient outside temperatures. If you weigh your refrigerant during this process and have a properly functioning system based off of pressure, moving forward for that particular vehicle, you can then simply charge by weight because you have the correct baseline.
     

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