Operator Manual, Dual-Column Cloud Condensation Nuclei Counter (CCN-200)
DOC-0128 Rev F
6 7
© 2017 DROPLET MEASUREMENT TECHNOLOGIES
12.2.2.1
CCN Temperature Adjustment
The program will readjust temperature set values if any of these four following conditions are met:
1)
Delta T(C) changes
2)
T 2*(T1 – T Sample) < T1 Set
3)
T Sample > T1 Set
4)
The unit has looped back to SS #1
Conditions 2 and 3 ensure that the temperature at the top of the column stays slightly above the
temperature of the sample. If the top of the column temperature is not above the sample
temperature (
T Sample
), the air comes into the top of the column from the Nafion humidifier, which
is at 100% RH, can condense and cause fog to form at the top of the column. Thus, if the sample
temperature rises by more than the (
T1 Set – T Sample
) value, the system will reset all of the
temperatures up to maintain this temperature difference. If the sample temperature drops by twice
the difference of the (
T1 – T Sample
) value, the T1 value will be reset lower.
It is also important that the operating temperature of the CCN be higher than the dew point of the
air being sampled. If the CCN is operated in a room with substantial air conditioning, and the
temperature of the air in the room is less than the dew point of the sampled air, aerosol changes can
occur.
When the CCN adjusts set temperatures, the new values are calculated using the equations below.
•
T1 Set
new
= Sample Temp Setpoint
+ (T1 – T Sample)
•
T2 Set
new
= T1 Set
new
+ ½ Delta T(C)
•
T3 Set
new
= T1 Set
new
+ Delta T(C) * (1 – (%TG
)/100)
•
Nafion Set
new
= T1 Set
new
+ (T Nafion – T1)
•
Inlet Set
new
= T1 Set
new
+ (T Inlet – T1)
•
OPC Set
new
= T3 Set
new
+ (T OPC – T3)
In order to protect the laser diode in the OPC, if
OPC Set
new
> 55 °C, then
Delta T(C)
is reduced using
the following algorithm:
1)
Delta T(C)
new
=
50 +
Delta T(C)
-
OPC Set
new
2)
All temperatures are readjusted using
Delta T(C)
new
3)
Steps 1 and 2 are repeated until
OPC Set
< 55°
It is important that the OPC operate at a higher temperature than the bottom of the column, T3. This
temperature differential ensures that the OPC optics will not fog. Studies at DMT have shown that
the OPC temperature can be as high as 5 °C above T3 with no loss of concentration. At an OPC
temperature of 7 °C above T3, there is a 20% loss of droplets.
6
Sample Temp Setpoint
is a temporary variable used by the software. It is equal to whatever
TSample
was when conditions 2 or 3 were last true.
7
%TG
is a temperature-gradient adjustment factor. For details, see section 14.0.
