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Thanks, Lance. As usual, I do my best proof-reading after I hit Send. Sorry that the links longer than one line will need some cutting and pasting in order to work. In my previous posting, “to easy” should be “too easy”, and “this reference” should be “these
references”. Plurals and homonyms give me more trouble with each passing year.
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<div>And while I think I read correctly the Wikipedia conversion for the SI permeance unit, an analysis of my mental back of the envelope calculations suggests that it ought to equal one 17.5 billionth of a US Perm, rather than 17.5 billion. I hope that error
of 20 orders of magnitude hasn’t caused anyone any trouble. I will try to get Wikipedia to correct their text.
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<div>Derek</div>
<div><br>
<div>
<div>On Sep 1, 2014, at 4:07 PM, Lance Kairl <<a href="mailto:sabale@bigpond.com">sabale@bigpond.com</a>> wrote:</div>
<br class="Apple-interchange-newline">
<blockquote type="cite">If I had to work with it would drive one loopy,<br>
But to read the response bought a smile to my face,<br>
You have got to love science.......at least once in a lifetime.<br>
Great work.<br>
Thanks, <br>
still smiling lance.<br>
<br>
-----Original Message-----<br>
From: <a href="mailto:GSBN-bounces@sustainablesources.com">GSBN-bounces@sustainablesources.com</a><br>
[<a href="mailto:GSBN-bounces@sustainablesources.com">mailto:GSBN-bounces@sustainablesources.com</a>] On Behalf Of Derek Stearns Roff<br>
Sent: Tuesday, 2 September 2014 7:04 AM<br>
To: Global Straw Building Network<br>
Subject: Re: [GSBN] Perms and Grains?<br>
<br>
I think they are the same units, Chis. What could be easier to calculate in<br>
your head than grains over square feet times inches of mercury times hours?<br>
It just rolls off the tongue. It's possible that Imperial Perms are<br>
different from US Perms (we wouldn't want to make things to easy).<br>
Sometimes US and Imperial measures are the same, sometimes not. Neither of<br>
the first two links below mention Imperial perms, while the third link<br>
doesn't mention US perms. However, comparing all three links leads me to<br>
conclude the US Perms and Imperial Perms are the same. <br>
<br>
According to this reference,<br>
<a href="http://www.greenbuildingadvisor.com/blogs/dept/building-science/oriented-str">http://www.greenbuildingadvisor.com/blogs/dept/building-science/oriented-str</a><br>
and-board-impermeable-they-say <br>
http://en.wikipedia.org/wiki/Perm_(unit)<br>
http://books.google.com/books?id=OMVc-nTpqwAC&pg=PA325&lpg=PA325&dq=Imperial<br>
+perm+unit&source=bl&ots=mCK-2XkiQA&sig=tGBrDl68dIqpnupwKch9ho9jH-8&hl=en&sa<br>
=X&ei=LtwEVODMPMfNggS6kILIBw&ved=0CFgQ6AEwBg#v=onepage&q=Imperial%20perm%20u<br>
nit&f=false<br>
<br>
So the sources that you cite would seem to be claiming 37 perms and 5 perms<br>
respectively, in one of the squirreliest US/Imperial units on record. Those<br>
are very different ratings, with 5 perms being in the middle of the range<br>
for a class-III vapor retarder (1-10 perms), while 37 perms is quite vapor<br>
open. <br>
<br>
Grains come 437.5 to the ounce, which once again proves the simple,<br>
human-friendly, common-sense utility of the English system. Of course, you<br>
could think of it as .04167 pennyweights or .3161 carats. Or 480 doites.<br>
The conversions to stones, long tonnes, and short hundredweights are<br>
similarly straight forward. Conversion to other useful measuring systems,<br>
such as the Chinese units in use between 1930 and 1959, are available at<br>
this link: http://www.convert-me.com/en/convert/weight/grain.html Spoiler<br>
Alert: A grain is equal to 20.74 hao, among other things. Old French,<br>
Ancient Roman, and Biblical units are available at the same link. It's a<br>
remarkable units conversion resource. <br>
<br>
In contrast, a grain equals 64.79891 milligrams, in the metric system. That<br>
fifth decimal place could be critical, don't you think? <br>
<br>
1 US perm = 0.659045 metric perms <br>
<br>
Before looking this up, I didn't realize that the metric perm is not an<br>
official SI unit. The SI unit for permeance is kilograms per second per<br>
square meter per pascal, and is roughly equal to 1.74784x10+10 US perms. If<br>
I'm calculating correctly, I think that's over 17 billion US perms. Perhaps<br>
not the most useful unit for builders. <br>
<br>
Stay dry,<br>
<br>
Derek<br>
<br>
On Sep 1, 2014, at 2:28 PM, Chris Magwood <chris@endeavourcentre.org> wrote:<br>
<br>
<blockquote type="cite">Hi all,<br>
<br>
I'm trying to compare the permeability of two different sheathing<br>
</blockquote>
products. One tells me that it is rated at 37 imperial perms and the other<br>
says it is 5 grains/h-ft2-in (Hg min).<br>
<blockquote type="cite"><br>
Can anybody tell me what the second rating means, and how it might compare<br>
</blockquote>
to imperial perms?<br>
<blockquote type="cite"><br>
Thanks!<br>
<br>
Chris<br>
<br>
-- <br>
<br>
</blockquote>
<br>
<br>
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<div apple-content-edited="true"><span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: -webkit-auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; ">Derek
Roff<br>
<a href="mailto:derek@unm.edu">derek@unm.edu</a><br>
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