# Thread: Relative Humidity - ASHRAE v ACCA

1. Regular Guest
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Many thanks Carnak.
I'll give that a try.

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Carnak, after a 2 days of trying, I'm sorry to say I'm struggling to follow your instructions.

Are you able to dumb it down to the point where I can identify specific columns in the ASHRAE table, and perform a formula to generate the ACCA Grains data? Using Birmingham AL as a control, I need to be able to generate somewhere close to: 34Grains@55%RH / 41Grains@50%RH / 47Grains@45%RH.

Something like: Cooling Dry Bulb @ 1% minus Mean Coincident Wet-Bulb = etc

Does the ASHREA table contain enough data to generate these numbers?

3. Indoor temp 75F

at 55% RH holds 71.5 grains

at 50% RH holds 64.9 grains

at 45% holds 58.3 grains

Dry bulb 92 and coincidental wet bulb 75 air holds 103.9 grains

So probably ACCA would say to maintain 55% use (103.9-71.5) = 32.4 grains of mositure in every cubic foot of outdoor air that infiltrates in, used for ventilation in the manual j calc.

to maintain 50% RH (103.9-64.9) = 39 grains in every cubic foot of infiltration

to maintain 45% (103.9-58.3) = 45.6 grains

you can try it for the 0.4% levels, you can also try it keeping it 74F inside.

Noiw what I get anal about, worst case humidity design

its 75 degrees outside and the air contains 135 grains of mositure

now to be able to maintain 55% RH inside, for every cubic foot of air that infiltrates in, you have to remove at least 135-64.9 grains = 70.1 grains.

how often does the AC run when it is 75 and cloudy? Need a tight structure to keep infiltration out and some internal gains to get the ac cycle on/off

4. if you know two things about the air, the pysch chart will give you all the other air properties, like Grains of moisture, the dew point, specific volume, the enthalpy etc

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Carnak
Do I understand that these RH figures are constant at an indoor 75f? i.e.
55% RH = 71.5 grains
50% RH = 64.9 grains
45% RH = 58.3 grains

If so, I need only be concerned with the outside conditions which vary to calculate the grains differance?

How did you resolve 103.9 outside grains by inputting "Dry bulb 92 and coincidental wet bulb 75 air". Is there a table or formula that you use?

6. its called the psychrometric chart

you need to search for it here, online

I gave a link to some free demo programs, see post 17 & 18

75 is a common indoor design temperature, maybe ACCA uses 74, maybe they take the outdoor condition a little different on the dry bulb wet bulb

For an indooor temperature of 75F and RH of 55,50 & 45 percents, you use the chart to find 71.5, 64.9 and 58.3 grains of mositure

then you use the chart to find the grains of moisture in the outdoor air.

ACCA grains are the difference between the mositure indoor and outdoor.

7. trane sells a laminated 11x17 chart, has good examples on the back

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Carnak
This is nice little utility but it doesn't state the grains figure.
http://www.sugartech.co.za/psychro/index.php

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Have found a better online calc that does the job.
http://www.envirochex.com/Psychro.htm

Thank you very much for your ongoing advice Carnak, I'm pleased I got there in the end!
Much appreciated.

10. Originally Posted by unclebingo
Carnak
This is nice little utility but it doesn't state the grains figure.
http://www.sugartech.co.za/psychro/index.php
use IP units, multiply humidity ratio by 7000 to get grains

11. Originally Posted by unclebingo
Have found a better online calc that does the job.
http://www.envirochex.com/Psychro.htm

Thank you very much for your ongoing advice Carnak, I'm pleased I got there in the end!
Much appreciated.
you're welcome, don't fall victim to GIGO read up on the topic

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Garbage In Garbage Out, right.
I've noticed there are disparities between ACCA and ASHRAE data when calculating design grains difference between inside / outside grains.
Birmingham AL - ACCA
45% - 47
50% - 41
55% - 34

Birmingham AL - ASHRAE
45% - 45.53
50% - 38.93
55% - 32.33

Indoor assumes 75F dry bulb on both. ACCA seems to round up 2 grains?

If only the outside Dry Bulb and Coincident Wet Bulb figure could easily be calculated without the need of an online tool. Otherwise I could enter the formula in the Excel sheet and let it generate the numbers.

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