Psychrometric correlation with ASHRAE 55

Hi @chris,
Sorry for bugging you so much with so many psychrometric stuff.
Here is a new one.
When comparing the plotted chart in LBT to the ASHRAE 55, under the same conditions, the resulting comfort polygons show some differences that i like to understand.
The conditions are:
Met: 1 - 1.3
Clo: 0.5 - 1.0
Air speed: 0.1 m/s
This is ASHRAE’s chart:

And this is the LBT’s:

To summarize the differences:

  1. The summer polygon: Thinner, it lower left corner is about 1C shifted to the right. The upper left corner should have some more RH
  2. The winter polygon: The lower left corner is shifted about 2C to the left. The upper left corner should be at about 82% RH and it gets more than 95% in LBT.
  3. Both polygons have some overlapping in ASHRAE’s and are disconnected in LBT’s.
    Tried to show in one image the differences, like so (hope it is clear enough).In red is LBT and ASHRAE is on the background:

All settings in LBT, besides those mentioned are default.


Hey @AbrahamYezioro ,

No worries and good questions. Those static comfort polygons leave a lot of the PMV design space undescribed but here’s what I think the author of those graphics was thinking. Given that these polygons are supposed to be “between 1.0 and 1.3 met,” they decided to just depict them in the middle of this range, using a met of 1.15 for both polygons. When I do this, I get pretty good alignment with the graphic published in the standard: (33.1 KB)

Depending on what version of ASHRAE-55 you took the graphics from, the air speed they assumed may also be slightly different. In the most recent version of the standard, they changed the “still air threshold” from 0.2 m/s to 0.1 m/s. Given that your graphic still says “0.2 m/s”, I would assume it’s from an older version of the standard and so they might have drawn those polygons using 0.2 instead of 0.1. But this should be a comparatively smaller difference than the met.

In any event the “modern” version of that graphic that the ASHRAE-55 committee members point everyone to these days is the CBE Tool here:

You should find very good agreement between the Ladybug psych chart and that interactive graphic, particularly because the ladybug code was derived from that tool’s javascript code.

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Hi @chris,
You are right, as always.
The 1.15 met did the trick. I indeed gave two met values as ASHRAE sets.
I used also 0.1 m/s for still air. If you see inside the comfort polygons there it says 0.1. To the right they say 0.2 for increasing Operative temp …