Originally posted by mdman Unless you are deriving compression ratio or setting pressure controls accurately then the only useful information on the gauge is the sat temp (even more true when you work with several refrigerants).
...I do get pissed every time someone makes reference to head or suction pressure because they are missing this major point. The top techs out there will not flame this thread and they are all nodding right now.
Actually there is more to the pressures than just that. Looking at the suction pressure gives a quick 'heads-up' on if the coil may be icing up or already iced up. Poor airflow also results in lower pressure so it helps in diagnosing if there is restricted airflow. Looking at the liquid pressure gives a 'heads up' on the condenser performance. The pressures are needed if the tech is going to plot out the PE diagram so that the whole performance of the system can be viewed at one time. Don't underestimate the value of what the pressures will tell.
Everything you've mentioned is a "head-up" on refrigerant heat exchange. If the evaporator coil has reduced airflow the EST (you say suction pressure) will follow down, the amount depending on metering type. If you've already thinking in terms of EST then your one suction line temperature from determining SH. Then knowing what kind of metering control you have will take you straight to the problem or prompt the next logical question.
We are both referring to the same measurements here, but the terms that foster better understanding here are saturation temperature and not pressure.
How many different nominal pressure ranges do you have to keep in your head when you go from R22 high temp, to 410A, to a 502 box?
Also, many P-E charts are scaled in psia so your going have to add 15 anyway. I have one that I scaled in degrees F because I also think in the Queen's English. LOL
Perhaps it depends on which way a person best comprehends the AC system. Saturation temperatures instead of pressure may work for you but not for me. The temperatures must be estimated on the gauge at my altitude, thus it is more accurate to just go with the pressure and keep a PT chart that is altitude adjusted handy for when the charge is in the ball park. When I drop 3,000' I still use the same method to get the same results. Then when I do an R-134a system I use the same method. When using R-410a the same method still works. When I come across R-502 I am still good to go without trying to mull over temperature conversions. It is a method that covers all refrigerants rather than just one or two and works better for me at these high altitudes. I can see where you are coming from with your method and see how it can serve you best. Whereas others may do it differently than you does not mean that they are ignorant or any less of a technician - it just means the perspective is at a different angle.
Personally I prefer the Sporlan PT chart. The pressures are PSIG and the charts come adjusted for different elevations.