3-2
The "SetPnt" column lists control setpoints used as a target value in order to control these
parameters. Only those parameters which the generator directly controls have values listed in
this column. For instance, since the 2500 has no control over the chamber pressure, that
parameter has no value listed in the "SetPnt" column.
The "Actual" column lists all of the measured data and calculated parameters of the
generator.
A description of each of the Control/Display parameters follows:
%RH @Pc -
The %RH calculated at the chamber pressure Pc. This calculation ignores
the chamber temperature and Effective Degree of Saturation (section 1.2.5),
and assumes that the chamber is at saturation temperature. The humidity
%RH @ Pc is calculated from the Pressure Ratio (section 1.2.4) and
Enhancement Factor Ratio (section 1.2.6), resulting in
%RH @ Pc =
Pc
Ps • ƒ
Pc,Ts
ƒ
Ps,Ts
•100
Note that the enhancement factor for the chamber (the numerator of the
Enhancement Factor Ratio) is calculated using saturation temperature rather
than chamber temperature.
%RH @ PcTc - The %RH calculated at the chamber pressure Pc and chamber temperature
Tc relative to saturation temperature Ts. This is the most accurate
calculation of %RH at the point in the immediate vicinity of the chamber
temperature probe. Placing the chamber temperature probe at the humidity
sensing point of devices under test gives the actual value of the relative
humidity being imposed on the devices, as it is dependent on both pressure
and temperature.
Viewing the difference between %RH @Pc and %RH @PcTc gives a good
indication of the gradient which exists between the humidity at the walls of
the chamber (since they are at saturation temperature) and the humidity at
the chamber temperature probe. The humidity %RH @PcTc is calculated
from the formula
%RH @PcTc =
Pc
Ps • ƒ
Pc,Tc
ƒ
Ps,Ts
•
es
ec •100
Note that this formulation utilizes the Pressure Ratio (section 1.2.4), the
Effective Degree of Saturation (section 1.2.5), and the Enhancement Factor
Ratio (section 1.2.6) found in equation 3 of section 1.2.3. Note also that the
enhancement factor for the chamber (the numerator of the Enhancement
Factor Ratio) is calculated using chamber temperature.
SATUR xxx - The saturation pressure measurement, Ps, may be displayed in units of bar,
mb, Tor, kPa, "Hg, or psi. Various humidity values are generated by
controlling the saturation pressure.
CHMBR xxx - The chamber pressure measurement, Pc, may be displayed in units of bar,
mb, Tor, kPa, "Hg, or psi. The chamber is designed to operate at ambient
barometric pressure only.
