Pschrometric Chart

What does the alignment point in the psychrometric chart represent? In some chart it is located at
75 FDB & 50% RH while in others it is located at 78 FDB & 50% RH, why is there this difference?

Thanx

 

[Goorah]

The "alignment point" comes from ASHRAE comfort cooling guidelines for design space temperatures in public spaces ... especially government jobs. Depending on the issue date of the original psych chart artwork, you will see the two points mentioned. Prior to the US "energy crisis" and the adoption of energy codes by various states, the common design guideline for comfort cooling was to maintain 75F and 50% RH. Later it was raised to 78F at the same RH.

There's not much more to it than that.

When designing, just be sure to check the governing codes for the project in question.