FAAM CORE Buck CR2 Chilled Mirror Hygrometer
The CR2 hygrometer measures atmospheric dewpoint using the chilled mirror technigue. Air is extractively pumped through an inlet into a sample chamber and passed over a mirror-like metal surface. The mirror temperature is regulated using a stirling-cycle cooler until condensation begins to form on the mirror, this being the dew or frost point. The condensation layer is maintained at a constant level using optical detection and control, and the mirror temperature is measured using an embedded platinum resistance thermometer. By these means the dewpoint measurement is directly traceable to ITS-90.

System Components

The CR2 sample inlet consists of a pumped reverse-facing inlet and outlet mounted around 1.8m back from the nose of the aircraft. The inlet pipes are made of stainless steel and are linked to the CR2 instrument by a stainless steel line with an internal hydrophobic coating. The sample line is heated to 50°C to increase the mobility of adsorbed water.
The CR2 is mounted around 60cm from the inlet, within the aircraft skin compartment. Data are transmitted using an RS232 link. Power for the CR2, pump and heater, and conversion from RS232 to ethernet are catered for in an auxiliary electronics unit. The sample mass flow is controlled to 1 litre per minute by means of a downstream flow controller and pump.
CR2   Inlet and outlet   Sample cell and mirror


The CR2 ouputs measurements of its mirror temperature data at 1-hz, along with other state parameters used to define the stability of the mirror condensation layer. Overall uncertainties are typically within ± 0.5K, depending on the variability of the conditions and the dewpoint - FAAM plan to include a dynamic uncertainty parameter alongside the data. The mirror temperature measurements made by the CR2 and reported by FAAM can under most circumstances be used as a direct assessment of the ambient dew or frost point. FAAM are currently (November 2009) developing a process to incorporate measured cell pressure, assessment of the mirror stability and other factors to allow the CR2 data to be correctly reported as ambient dew/frost point. This will be further documented on this page when complete.

2012 Update - Buck Data Flagging

The Buck CR2 data have been incorporated into the main FAAM core dataset (ie the netcdf files) for some time now. Four parameters are available, Volume Mixing Ratio, VMR Uncertainty, Dew/Frostpoint and DP/FP Uncertainty. Uncertainties are 2-sigma estimates based on the author's best understanding of the system performance under the range of operating conditions, including calibration, drift, response time and mirror state. (If you want more details, contact FAAM)

There are flags associated with each of the four parameters above, following the same convention as the other FAAM core data. All of the flags are set to the same value 0-3. These signify as follows:
0 - Good data, mirror temperature corresponds to dewpoint (VMR calculated over water)
1 - Good data, mirror temperature corresponds to dewpoint (VMR calculated over Ice)
2 - Good data, unknown water phase (Below 273K). VMR calculation assumes an ice layer but uncertainty value is increased by the difference between ice and water VMR
3 - Bad data. Mirror temperature not representative of humidity.

2016 Update - Newer instrument and changes to data processing

A second Buck CR2 instrument was acquired and swapped onto the aircraft in early 2016. Details of this upgrade, as well as information about changes to the data processing, can be found in this document:

pdf Buck CR2 Hygrometers on the FAAM aircraft 2016 (174 KB)



Manuals for the various CR2 installation components can be found here:
The Technical Specification document can be found here
A poster produced for the NCAS 2009 staff meeting is available, which discusses the initial CR2 data analysis comparing data to those from the General Eastern and Total Water Content Probes


Further details

Contact Alan Woolley at FAAM