Dew Point Sensors Standard
Instrument Description
This humidity sensor sensor provides a direct measurement of ambient ambient dew point temperature using an optically monitored chilled / heated mirror surface. The temperature of the mirror is monitored and controlled to maintain a uniform level of condensation on the surface.
Measurements Made and Methods Used
Operational Status: Part of Standard State Parameter Package
- requestable, ready for deployment
General Description:
- A mirror attached to a thermoelectric cooler (or heater) is mounted inside a small chamber that is exposed to the airstream. If the mirror temperature is colder than the dewpoint inside that chamber, dew or frost accumulates on the mirror surface. A small light source illuminates the mirror, and a sensor detects the amount of light reflected from the mirror. A feedback control circuit maintains the temperature of the mirror just at the threshold for condensate, and a temperature sensor measures the temperature of the mirror, which should then correspond to the dewpoint in the chamber. Because airflow effects can change the pressure (and hence the dewpoint) in the chamber from that present outside the aircraft, the pressure in the chamber is also measured and used, with measurement of the ambient pressure, to correct the measured dewpoint to the ambient value. Other measurements of humidity including mixing ratio and specific humidity are then calculated from the measured dewpoint. For details, see the discussion of processing algorithms and the tutorial on state parameters under "Documentation" at the end of this section.
- The following is a photograph of the exposed housing used for these sensors:
Measurement Characteristics:Range:-80 to 50C; Accuracy: 0.5C > 0.0 or 1.0C < 0.0; Response Time: 0.2 to 10 s, response time varies as as function of dew point depression
- Overall estimate of uncertainty:
- Response time:
- Precision:
- Other measurement characteristics (comments on signal/noise, bias limits, etc):
Calibration Methods: Annual recertification by manufacturer; pre and post deployment calibrations using CARI Calibration Cart
History of Significant Changes to the Instrument: cavity pressure measurements added to GV sensors
Description of Software, Data Handling, and Data Flow:
Software Components (data acquisition, display, processing, and analysis): Standard "nimbus" processing
Data Processing Chain (from flight to archival): See the discussion of processing algorithms referenced in the "Documentation" section at the bottom of this page. The standard processing converts to temperature measured by the sensor in contact with the mirror into an ambient dewpoint and other derived humidity variables, but there are some subtleties involved that are documented in those notes. The primary output variables are dew point DPXC and vapor pressure EDPC. DPXC represents the ambient dew point, even below 0ºC, as related to temperature by the Murphy and Koop (2006) equilibrium vapor-pressure relationship (without enhancement of the equilibrium vapor pressure by the presence of air). It is corrected for the difference in pressure between the sensor housing and the ambient air and for the enhancement factor that arises from the presence of air in addition to water vapor. DPXC is selected by the project manager on the basis of which sensor (right or left) appears to provide the best measurement in a particular project and flight. These become part of the standard netCDF data archives produced by the RAF. See the StandardDataPath page for more information.
QA Procedures and Needs: comparison against redundant measurements and against temperature during cloud passes
Internal users: See http://wiki.eol.ucar.edu/rafscience/DewPointSensorsStandard (last edited 2012-12-08)
