View Full Version : How to use a PT chart?
Lets say my gauges (for r12 r22 r502) say that the pressure is 40psi and 40psi on the pt for r502 matches 45.5psi for r507. Does that mean the pressure in my system is 45.5psi. O btw this is a system charged with r507 and this is theoretical
runmc
07-27-2004, 02:51 AM
The psi doesn't change no matter which gauges you use.
It's the temperature relation that changes. If your gauges are for r-22, they will tell you what temperature r-22 is at a certain psi.
By using a t/p chart, you can use any gauge to get the psi - then take the psi and match it with the temperature on the corresponding chart.
Did that help?
k i think i got it.... ao basicaly if the pressure on the low side is 5.5psi with r507 that means that it boiling point it -40?
runmc
07-27-2004, 01:55 PM
You are reading the chart correctly.
If you take the same set of gauges and hook them up to a unit with r-12 and the psi was 5 then the comparison temperature would be -9f - (r22 = -30f) - (r134a -3f) and so on. The gauges all read the psi the same no matter which gas you use. The only thing different about the gauges is the scale (temperature - pressure scale) The chart is telling you what the temperature of the refrigerant is at that pressure and visa-versa
I've never used or seen them, but I think you can change the face plates out on the gauges to correspond with the gas u are using. Actually you are looking at a t/p chart on the gauges for the specified gases
Make sence??
I hope I explained that right:shrug:
Yeah you did explain it right, thanks! On my guages I can take offt he platic cover so it would only make sense that you can change the pt chart thingy. Btw is there any other reason to know the pressure/temperature for the gas you are using besides knowing how cold the evap will get?
runmc
07-27-2004, 02:48 PM
Btw is there any other reason to know the pressure/temperature for the gas you are using besides knowing how cold the evap will get?
OH YEAH!!!;)
Just ask Gary Lloyd that question..
Hobocrow
07-27-2004, 07:54 PM
Originally posted by JSU
Btw is there any other reason to know the pressure/temperature for the gas you are using besides knowing how cold the evap will get?
Caaalllliiiiing, ah, Gaaaaaarrryyy Lllllyyyyoooooddd .... ;)
i mean besides being able to say that the ambient is 80f and the pressure of the gas...
Gary Lloyd
07-28-2004, 12:14 AM
Refrigeration is a series of heat transfers, starting with the transfer of heat from the CPU to the refrigerant in the evaporator, and ending with the transfer of that heat from the refrigerant in the condenser to the room air, with several transfer points in between.
If we look at the temps on each side of a transfer point, we can see how efficient that transfer is, thus we can pinpoint problem areas.
Saturated suction temperature aka SST (low side pressure converted to temperature) and saturated condensing temperature aka SCT (high side pressure converted to temperature) are two very important indicators when combined with other temperature readings. They tell us everything we need to know.
To use your example, an excessive difference between the ambient temp and SCT tells us that the condenser to air heat exchange is inefficient, indicating a need for a larger condenser and/or more airflow.
This is just one small example. Optimizing the heat transfer across each of the transfer points is the difference between a system that gets cold and a system that kicks butt.