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If it is called "Relative" Humidity, what is it "Relative" to?

by S. Jamison, NJ Industrial Controls   Opinions are mine alone

It is called Relative Humidity because it is the humidity relative to the Dry Bulb or more correctly, the Chamber Air Temperature.  If we move the Chamber Temperature Sensor from the ductwork next to the RH Probe and put it behind the samples, what is our Relative Humidity now relative to?  The Vaisala Probes measure air temperature at the thin film sensor to create the "relative" part of the measurement.  Therefore the humidity is still relative to the temperature local to the probe and not the specified and controlled chamber temperature.

If the probe is moved to a new location, and that location is 4C different in temperature at 42C nominal chamber temperature, that will equate to a difference in RH at the probe still located in the duct of 4% RH won't it?

If there is a 3 to 4C difference between the probe locations, doesn't that change the test as the samples see it?

In addition, having the Chamber Temperature measured behind the Specimens / Rack means the temperature at that point is influenced by the spaces between the samples or how much light is hitting the wall.  A Ci5000 running ISO-4892 was having difficulty maintaining the RH.  This particular instrument was filled with tensile bars having a fair amount of space between the samples in addition to the normal spaces between the holders.  Running at 0.51 W/m2, 65C BST, 36C CHT, and 60% RH, the RH would only make it to 38% no matter how the air pressure / water pressure was adjusted to the Humidity Nozzle.  Shielding the probe helped, but pushing the probe all the way in so there was no thermal contact of the wall / mounting hardware to the probe helped the most indicating there is some conduction of heat from the wall to the probe that is more pronounced the more space between holders.  Note the relationship of the CHT local to the RH Probe (34C) and the CHT behind the rack (38C.) 

Shielding the CHT probe and inserting

temperature Probe in with it.

Probe reading

Screen Shows

Conditions local to RH probe:

59% RH, 34C CHT

So . . . local to the RH probe there are 21 g of water / kg of air.  Figuring backwards to the area of the CHT probe, at a probe temperature of 38, the RH in the area of the CHT probe is 46%?