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====== Problem ======
During the Sikorsky field project in January 2005 the FSSP liquid water measurements were about half of the King Probe liquid water measurements. This is apparent on January flights. The FSSP was cleaned, aligned and calibrated on January 5, 2005 by DMT. During the L3Com project (November/December 2004) the FSSP liquid water measurements agreed very well with the King liquid water measuremetns.

The FSSP was cleaned on January 28, 2005 which did not improve the King/FSSP liquid water content agreement on the January 29, 2005 flight.

===== Cause =====
During the January 5, 2005 calibration by DMT ..."The gain resister was increased to give a laser reference voltage of 8.5V up from 7.5V." This change in FSSP laser reference voltage is evident on the FSSP reference voltage measured during Citation flights. See examples plots below.

{{dept:atmos:citation:instruments:fssp:04_12_06_16_04_33.fssp_refvoltage.png}}
{{dept:atmos:citation:instruments:fssp:05_01_29_16_43_51.fssp_refvoltage.png}}

The FSSP reference voltage was getting low during the end of the L3Com project. During the beginning of the MPACE project the reference voltage was more typical of the values during the beginning of the Sikorsky project (See for example the 05_01_29_16_43_51 flight). Hence, it is important to watch the FSSP reference voltage as an indications of possible problems with the FSSP.

During the ground calibration on January 31, 2005 the reference power was more stable than during a typical aircraft flights (See plot below). From the January 31, 2005 FSSP calibration data it is evident that the reference power starts low and climbs to higher value as the FSSP warms up. Therefore, it is important to wait at least 10 minutes before doing a FSSP bead calibration check to allow the FSSP reference voltage to stabilize. 

{{dept:atmos:citation:instruments:fssp:05_01_31_21_22_14.fssp_refvoltage.png}}

The FSSP reference voltage noise level jumps right atfer takeoff (See plot below). This increase in reference voltage noise could be the result of the FSSP heaters coming on or an increase in virbration. The FSSP probe heaters were turn on twice near the end of the January 31, 2005 calibration run for testing with only a small increase in the FSSP reference voltage noise level (See plot above).

{{dept:atmos:citation:instruments:fssp:05_01_29_20_46_44.fssp_refvoltage.png}}

On June 4, 2004 the FSSP Laser was changed. Plots below show the FSSP Reference voltage on a flight with the new laser and one with the old laser.

{{dept:atmos:citation:instruments:fssp:04_06_15_19_29_40.fssp_refvoltage.png}}
{{dept:atmos:citation:instruments:fssp:04_04_23_19_29_24.fssp_refvoltage.png}}

===== Solution =====

Send probe into DMT to see what the problem is. Bill Dawson looked at the FSSP on February 2, 2005 and determined that their was a problem with the laser. Since a new laser would take at least 5 days to get, Bill fixed the laser reference voltage as a short term fix. The FSSP was return and flew on February 3 and February 4. During both days there was excellent agreement between the FSSP liquid water content measurement and the King Probe liquid water content measurement.

===== Bill Dawson Comments (Feb. 19, 2005 Emali) =====

I essentially disabled the laser reference detector because the value was unstable, while the laser power was stable. I put a trim pot in place of the reference system so the value fed to the pulse height analyzers could be adjusted. So, the 0.1 or 0.2 value is expected. The PHA's are receiving a good value of -8.75V.

===== Post February 2, 2005 Problem =====

After Bill Dawson February 2, 2005 work on the FSSP we had a problem with the FSSP powering up correctly on some flights (e.g. March 7, 2005 flight). After cycling the power several times the FSSP starts to work correctly and gives good agreement with the King Probe liquid water content measurement.

===== Cause February 2, 2005 Problem =====

The High Voltage Power Supply was failing

===== Solution February 2, 2005 Problem =====

The FSSP was returned to DMT for repairs once the Citation return to GrandForks? on March 19, 2005. The Hight Voltage Power Supply was replaced to fix problem with FSSP not powering up. The FSSP Prism was replace due to deterioration of the aluminum backing on the hypotenuse. The FSSP laser was replaced do to February 2, 2005 problem with poor laser reference stability.