ROTRONIC MBW 973 - Manual (Page 14)

10 Getting Started
The Balance Indicator
Although it is directly obtained from the intensity of the mirror’s reflected light signal, the Balance
Indicator is effectively the first derivative of the dew thickness. In other words, it indicates the
rate of growth or decay of the condensed layer on the mirror. While the dew or frost layer is
growing in thickness due to an increase in condensation on the mirror surface, the indicator is
above center. The faster the layer grows the higher the indication. Conversely, if the layer is
evaporating from the mirror surface thereby becoming thinner, the indicator is below center.
The faster it decays the lower the indication. When the indicator is in the center, it indicates that
the thickness of the dew or frost layer is neither growing nor decaying and that the layer on the
mirror surface is in equilibrium with the gas. In this center-balanced indication, there is no net
exchange of water vapor between the gas and the mirror surface. If the humidity of the gas
sample is homogeneous and of low enough variability for the control system to sense a steady
value, the Balance Indicator will illuminate a green ‘Stable’ message, and the system will emit a
few short audible beeps.
The Density Indicator
The Density Indicator graphically depicts the approximate relative thickness of the dew or frost
layer currently on the mirror surface. Since the 973 has the ability to differentiate between dew
and frost layers, it will also indicate which state the mirror’s layer is in. The label within the
density indicator will change from Layer Density (when it the state of the layer is uncertain) to
either Dew Density or Frost Density (when either dew or frost is assumed). For more
information regarding Dew/Frost point determination see Dew / Frost Control on page 15.
Disable Control
To disable the control system, press the Dew/Frost Control key again. The green bar on the key
disappears indicating that the control is no longer enabled. Any dew or frost point temperature
indications will begin to rise toward the temperature of the measuring head, often within several
degrees of the ambient room temperature. Also, the density indication drops off and the
balance indication moves downward but eventually returns toward center.
External Temperature Connection
The 973 is equipped with an external
temperature sensor. This sensor, along with the
dew point and pressure measurement, allows
the 973 to accurately determine and display
relative humidity. The external temperature
probe connection is made at the back panel.
Gas Inlet Connection
While the previous example did not rely on any external gas connections, the 973 is equipped
with fittings for connection of gas inlet and outlet tubing. This allows the 973 to measure the
dew or frost point of chambers and other devices that can be connected to it via tubing. If you
have a gas source you wish to measure, such as a humidity generator or chamber, connect a
tube between it and the 973 gas inlet on the back of the unit.
STABLE
Balance
Indicator
+
-
Frost
Density Hi
0
Dew/Frost
Control