During the CitationFieldprojectMPACE (Fall 2004) field project the CitationInstrumentLaserhygrometer was measuring 4-5 degrees low. Comparison with dewpoint generator on October 3, 2004 shown the CitationInstrumentLaserhygrometer was measuring low.

Randy May looked over the October 3, 2004 Calibration check and concluded that the reason for the poor agreement between the dewpoint generator and the TDL was that the Beer-Law corrections were not being applied to the data since the test was done at a pressure above 1000 mbars. During aircraft flights this problem will not occur since the pump will be running which results in a reduced pressure in the TDL cell compared to ambient static pressure.

The disagreement during aircraft flights seem to be related to an incorrect calculatation of the cell pressure based on the Citation measured static pressure. See Plot below.

Based on the September 30, 2004 first flight I calculated new TDL cell Chamber calibration constants to calculate the TDL cell pressure based on the measured static pressure.

Using these new calibration constants results in better agreement between the TDL frost point and air temperature in sampling in clouds (time index ~5000)

Hence the problem was caused by the addition of the CSI flow to the TDL outlet which resulted in a different relationship between the measured static pressure and the TDL cell pressure.