U of S | Mailing List Archive | alt-photo-process-l | Re: Measuring humidity

Re: Measuring humidity



Title: Re: Measuring humidity
Same here. We use air conditioning or heating all year and actually never open the windows except to clean them. This is done with a system called a heat pump. The temperature is held at about 72 F constantly, but RH inside varies from the low 30s in winter to the high 60s in summer.

Right now it is 71F and 64% inside, and 63F and 72% RH outside. RH trends more or less with outside RH. But, heating in the winter removes a lot of moisture from the air. Obviously, cold air can hold more moisture than hot air. In the summer a typical reading at 8 am would be 70F and 90% humidity, but by 1pm the reading would be 90F and 50% RH.

Temperature and humidity are major issues for carbon printing. Sensitivity is much higher at 85F and 90% RH than at 70F and 35% RH. This absolutely has to be factored in when printing.

Sandy



At 11:41 PM -0400 9/28/06, Ender100@aol.com wrote:
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Katherine,

Temperature certainly is a variable, but my home is 70 degrees year round and it's the humidity that can vary a lot.  Humidity has a LOT of influence-at least with PT/PD.  But then, as you say, I am holding my temperature constant-and wearing the same lucky undershorts.

Best Wishes,
Mark Nelson

Precision Digital Negatives - The System
PDNPrint Forum at Yahoo Groups
www.MarkINelsonPhoto.com


In a message dated 9/28/06 7:58:11 PM, kthayer@pacifier.com writes:
But apparently I'm not the only one making this mistake, because 
we've all been using RH as if it were a meaningful variable, and 
obviously it's not, unless, as you say, temperature is held 
constant.  The graphs in the literature showing the relationship 
between humidity and dark reaction, and humidity and sensitivity, 
were no doubt constructed holding temperature constant.  
Unfortunately they don't say that, or specify the temperature, 
although I suppose we could assume 25C, isn't that the usual standard 
temperature?