How to use a vacuum pressure temperature table

By using a vacuum pressure temperature table it is possible to determine the correct vacuum level you need to achieve to remove moisture and  confirm that there is no moisture left in the system.

Temp °C Pressure mBarA Pressure Torr Microns
100 1013.25 762 762,000
90 701.10 534 534,000
80 473.60 361 361,000
70 311.60 237 237,000
60 198.92 149 149,000
50 123.30 94 94,000
40 73.75 56 56,000
30 42.42 32 32,000
20 23.37 18 18,00
10 12.27 9.3 9,300
0 6.11 4.6 4,600
-10 2.30 1.73 1,730
-20 1.10 0.83 830
-30 0.36    0.29 290



Determine the temperature of the environment the pipework is in and use the lowest temperature to determine the vacuum level from the vacuum pressure temperature chart.

Connect your vacuum pump and a good vacuum gauge as per normal.

As the pressure drops the moisture in the pipework will begin to boil off.

The pressure achieved must be lower than the boiling point of water based on your temperatures taken, for example, if the lowest temperature you read was 0C then you must achieve a vacuum below 4.6 Torr. The moisture will not begin to boil off until you are below this temperature.


Checking for moisture removal

To determine if all of the moisture has been removed isolate the system from the vacuum pump, leaving the vacuum gauge connected. If the pressure rises and continues to rise this indicates that moisture is still boiling off (assuming you have no leaks).  Continue evacuating the system until all of the moisture is removed if this is the case.

If this is still not successful and you have no leaks continue on to a triple evacuation and if possible, replace the system oil. Remember, you cannot vacuum for too long or to too deep a pressure and you should regularly change your vacuum pump oil.

Do not charge any refrigerant into the system until you are sure you have cleared all of the moisture.

For more info see

System evacuation part 2

System evacuation part 3