Relative vs. Absolute Humidity

[ATodd@UH.EDU]

   As chance would have it, the harpsichord list is also having a discussion
about humidity, etc. Below is a forwarded message from Bud Fine with
Hubbard Harpsichords that I though some of you might be interested in:

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First off, Hubbard recommends keeping its instruments at approximately 40%
relative humidity (RH) at "normal" temperatures, meaning from, say, 65-72
deg.  That's ideal. Obviously, an ideal is something someone strives to
attain, but I think the point is clear.

Now, for a little complication. This came up in a very different context many
years ago.  My memory was jogged by Mr. Feeser's comment above, that when the
temp. dropped drastically in his home due to a power outage, the RH dropped
ONLY 3%.

Normally when considering the conditions under which an instrument should be
kept, we speak of  RELATIVE humidity and  temperature as the two critical
factors. We strive to keep our humidity gauge at, say,  40%. That's fine as
long as we stay within certain temperature parameters.  Mr. Feeser reported
a low temperature of 45 deg  but was reassured that all was reasonably well
moisture-wise because  the RH had "only" dropped from 35% to 32%. Let's
examine that more closely.

First, two definitions:

RELATIVE humidity: ratio of the quanitity of vapor actually present in a
given space to the greatest amount possible in that space at a given
temperature. [The "greatest amount possible" changes substantially as
temperature increases or decreases].

ABSOLUTE humidity: amount of vapor actually present in a given space
expressed as weight/given volume, usually as grains/cubic foot.

Let's see what this means with regard to the conditions Mr. Feeser's
instrument experienced.

His "normal" conditions were 35%RH at 68 deg.  Under those conditions,
according to the Table  before me, a cubic foot of aqueous vapor weighs 2.618
grains (a grain is.065 grams).

Then the conditions changed to  32%RH at 45 degrees.  When that happened,
 ABSOLUTE humidity, that  is, the weight of a cubic foot of aqueous vapor,
dropped to approx. 1.092 grains.

We then have to ask, what would be the RELATIVE humidity equivalent of an
ABSOLUTE humidity of 1.092 grains at 68 degrees, the temperature at which Mr.
Feeser usually kept his instrument?  Interpolating from my table, I get
approximately 15% RH!

I hope that's clear.  What I'm saying is that  Mr. Feeser's instrument at
45deg/32%RH was in a very dry environment equivalent  to 68deg, 15%RH.

In other words, DRY and DANGEROUS.  I would recommend that if anyone
experiences the kind of temperature drop that Mr. Feeser did, put all plants
in the house around the instrument, water them and in addition place some
pans of water under and around the instrument.

We all have become very accustomed to thinking in terms of RH.  But relative
humidity is just that, relative, and a very amorphous concept.

Now, I'm certainly not a physicist and make no claims to expertise in this
area.  I'd welcome discussion of the topic and certainly would like to be
proven wrong, for it would make things a great deal easier if, in fact, Mr.
Feeser 's instrument was "safe" (or relatively so) at 45 deg., 32%RH.  [All
of the above changes somewhat with atmospheric pressure, but let's leave that
aside for now.]

If I am correct, however, than we have to start thinking in terms of  RH AND
temperature in combination, because as far as I know, the humidistats that we
use to read or control moisture in our rooms all read in terms of relative
humidity, not absolute humidity.  Please correct me if I am wrong.

One can easily prepare a chart which would, say, give RH values necessary to
maintain on your humidistat which would be equivalent of 70deg/40%RH, as
temperature changes.

Bud Fine
Hubbard Harpsichords
BJFine@AOL.COM
Date: Tue, 06 Feb 1996 10:25:52 -0500

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   Bud, and I'm sure some others who have been participating in the
discussion, would be interested in any observations any of you might be
able to make.
   I would like to assume permission to forward to that list any comments
made here, unless requested not to.
   A lot of the CAUT's here, including me, have to maintain harpsichords, so
I feel that any help the specialists can give us would be welcome. ]
   Thanks for your indulgence.

Avery Todd, RPT
University of Houston
atodd@uh.edu