FAAM CORE General Eastern GE 1011B Chilled Mirror Hygrometer

Instrument

N.B. This instrument is no longer routinely calibrated. Dewpoint data from the Buck CR2 chilled mirror hygrometer should be used instead where possible. Where Buck data is unavailable, data from the General Eastern may be used, but only after careful comparison between the Buck and General Eastern for previous flights. 

 

The 1011B thermoelectric hygrometer measures the dewpoint of ambient air using the chilled mirror technique. Air is sampled using a passive inlet, nominally free of any particles, and passed over a mirror. The mirror is thermoelectrically cooled until it reaches a temperature at which condensation begins to form (the dew or frost point), and is then held at that temperature. The presence of condensation is sensed optically. By definition this is an absolute measurement of dewpoint temperature.

System Components

The GE system consists of the following elements:
  • An inlet is mounted on the aircraft skin permitting sampling of the ambient airstream. A pressure gradient is created in flight using the difference in flow across two ports, and the ambient air is thus passed through the internal portion of the inlet containing the mirror and sample optics.
  • A control module is mounted on the Aft Core Console providing a visual display of the humidity as well as a user interface.
  • Power for the above components is supplied by a further electronics unit on the aft core console, which also outputs the measurements to the aircraft data system.
 
GE Inlet

Documentation

The original GE manual is available here:
icon GE Manual (10.32 MB)
A report on the Humidity measurements made by the FAAM aircraft features information on the GE here:
icon Humidity pdf WGNov2007 (58 KB) pdf (58.65 kB November 2007) (58 KB)

Data

GE "dewpoint" data are recorded at 4hz by the aircraft data recording system. The instrument also outputs status information based on the GE mirror stability. "Dewpoint" measurements output by the GE are in fact readings of the mirror temperature. Under many circumstances this is almost equivalent to dew/frost point with the following caveats:

      1. The GE is nominally capable of measuring data in the dew point range -75 to +50°C. Its actual capability is strongly dependent on the ambient temperature and the instrument data are typically subject to increasing positive bias below around -25°C, possibly due to temperature effects in flight. The instrument time response is a function of the phase of the condensate layer and the mirror temperature. At dewpoints around 0°C the response time might be expected to be of the order of a few seconds, becoming many minutes at -40°C and below. Data acquired in these conditions should therefore be used with caution.
      2. As is typical of chilled mirror instruments, the GE data mirror data become ambiguous between 0°C and -30°C in which range it is often difficult to distinguish the phase of the condensate layer. This is important since ice and water have different vapour pressures at a given temperature. To put it another way, for a given ambient humidity level, the dew and frost points will be different by a known, non-trivial amount. Since it is not obvious whether the mirror temperature reported is a frost or a dew point, the true ambient humidity cannot be determined from the mirror temperature alone. Supercooled liquid water (at below 0°C) typically only has lifetimes of tens of minutes on the GE mirror, and below -40°C theory predicts that homogenous freezing will occur. FAAM have not seen instances of a GE liquid water layer below -30°C. Careful examination of the data by the user may therefore be required with this instrument to distinguish the mirror phase and therefore the true ambient humidity.
      3. No correction is made for pressure. There is no pressure measurement within the GE inlet or sample chamber. For there to be flow through the inlet there must be a small reduction in the pressure in the inlet below ambient static pressure. Although small, this is unknown.
      4. GE dewpoint measurements can be positively biased in cloud, as the sampling method appears to allow cloud water into the sample chamber which then floods the mirror.

Further Details

Contact Alan Woolley at FAAM