ROTRONIC MBW 973 - Manual (Page 20)

16 System Configuration
However, for mirror temperatures between 0 and approximately –30°C, the state of the
condensed layer is generally indeterminate since dew can, and does, exist on the mirror in a
meta-stable state at temperatures well below 0°C. In reality, the condensed layer on the mirror
could be dew, frost, or some combination of the two. Significant errors can result if wrongful
assumptions are made and can be in excess of 2°C.
To eliminate this problem, the 973 can automatically force all sub-zero condensation to a known
state of frost using the Force Frost function. Force Frost works by rapidly cooling the mirror
below –40°C forcing the condensate to solidify to a layer of ice or frost. The mirror will then re-
stabilize at the frost point temperature. Once the condensed layer is in a state of frost, it will
remain frost for all sub-zero mirror temperatures.
You can enable or disable the Force Frost function, and also decide at which temperature it
begins to work. From the Control Setup menu, press Dew/Frost Control (the top menu key).
Change the value of the Force Frost Below field. Mirror temperatures below this value will be
forced to frost.
Now enable or disable the Force Frost function by pressing the Force Frost Below button. A
green indicator light on the button means that the function is enabled. If the light is not
illuminated, then the Force Function is disabled, and no Force Frost will be performed.
Complete your preference by pressing Ok or the keypad’s enter key.
Enable ORIS Below
If the option is installed, the Optimal Response Injection System is used to speed system
response for very low frost point measurements typically when the gas is drier than
approximately –60°C. ORIS works by momentarily injecting a small amount of water vapor into
the dry gas stream to rapidly seed the initial formation of frost on the mirror. This process allows
the required layer to build within a matter of minutes rather than hours. ORIS is typically used
for frost point measurements below about –60°C, but allows you to set it to any reasonable
value.
You can enable or disable the ORIS function, and also decide at which temperature it begins to
work. From the Control Setup menu, press Dew/Frost Control (the top menu key). Change the
value of the Enable ORIS Below field. Mirror temperatures below this value, when there is not
yet any frost formation on the mirror, will use ORIS to more rapidly form the initial frost layer.
Now enable or disable the ORIS function by pressing the Enable ORIS Below button. A green
indicator light on the button means that the function is enabled. If the light is not illuminated,
then ORIS is disabled, and no rapid layer formation will be performed. Complete your
preference by pressing Ok or the keypad’s enter key.
Mirror Cleaning Control
When the fixed Mirror Cleaning key at the bottom of the screen is pressed, the mirror
immediately warms to a pre-specified temperature, readying the measuring head for removal of
the cover and optical assembly. If the mirror and other internal measuring head components
become exposed to normal atmospheric air while cold, the possibility of undesired condensation
exists. By warming the mirror and other internal components to a safe head removal
temperature prior to accessing the mirror, this adverse condensation is avoided.
The safe temperature to which the mirror and internal components will be warmed during a
Mirror Cleaning procedure is selectable from the Mirror Cleaning key of the Control Setup menu.
Enter a value between 20 and 50°C for the Min Head Removal Temp.