I don't think there is one. My understanding is that the water vapour content is combined with the pressure and temperature range of the system to determine whether there is a risk of water vapour condensing in the system.

In a well designed and maintained compressed air system there should be no problem with water vapour in the compressed air system. Put simply, when air is compressed the energy applied results in the air temperature increasing. Thus the smaller volume of air can hold the water in suspension. Were we to allow the air simply to flow into the system in this state as it cools in the system the ability to hold the water will reduce resulting in condensation in the system. Then when the air expands as it is used it will cool rapidly resulting in a reduced ability to hold water, hence the ‘condensation’.

A well designed system will have a cooling system or other means of removing the excess water (in the trade it used to be called an aftercooler, but it is a long time since I designed and installed compressed air systems) from the compressed placed immediately after the compressor. The minimises the potential for condensation in air receiver, pipework and when breathed. It prevents corrosion in pipework, etc.
So if there is a problem with water vapour in the breathing air you might want to check out your system.

Chris

For reference:

www.hse.gov.uk/research/rrpdf/rr427.pdf