Dr Lisa Whalley

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Instrument Support Level 1
Instrument Support Level

Instrument Support Level 1

Manufacturer & Model

General Eastern, Optica Chilled Mirror Hygrometer

CEDA Archive Name


Data Products


Insurance Value


Physical Dimensions

34 cm x 22 cm x 18 cm, 16kg


50 cm x 25 cm x 65 cm, ~25kg

Daily Facility Charge



Calendar 2:
Chilled Mirror Hygrometer

Chilled Mirror Hygrometer

The Optica Hygrometer offers precision dew point measurement. Precisely controlled by GE’s chilled mirror sensing technology – providing NIST traceable humidity, temperature and pressure measurement anywhere via the worldwide intranet.

The Optica Chilled Mirror Hygrometer measures dew point temperature by regulating the temperature of a polished metal mirror by the use of optical feedback such that a constant mass of dew or frost is maintained. The temperature of the mirror is measured with a precise PRTD and is by definition equal to the dew or frost point.

The dewpoint hygrometer provides a measurement of:

  • dew/frost point from –80°C to +85°C (–112°F to +185°F)
  • relative humidity from 0.002% to 100%
  • moisture content from 500 ppbv to over 5.71 × 105 ppmv

Optical condensation hygrometry is a precise technique for determining the water vapor content in gases by directly measuring dew point or frost temperatures. Using this technique, a metallic mirror is cooled until it reaches a temperature at which a thin layer of condensation begins to form on it. The dew layer is detected optically, and the mirror is held at that temperature. The mirror temperature, measured with a platinum resistance thermometer, is an accurate indicator of the dew or frost point. Because these hygrometers are so accurate, they are widely used as a standard in many of the world’s metrology laboratories.

The condensate mirror is illuminated with a high-intensity, solid-state, light-emitting diode

(LED). A photodetector monitors the LED light reflected from the mirror. The photodetector is fully illuminated when the mirror is clear of dew, and it receives less light as dew forms. A separate LED and photodetector pair are used as a known reference to compensate for any thermally induced changes in the optical components. The photodetectors are arranged in an electrical bridge circuit, the output current of which is proportional to the light reflected from the mirror. The bridge output controls the electrical current to the thermoelectric cooler. A large bridge current develops when the mirror is dry, causing the mirror to cool toward the dew point. As dew begins to form on the mirror, less light is reflected, and the bridge output decreases. This, in turn, causes a decrease in cooling current. A rate feedback loop within the amplifier ensures critical response, causing the mirror to stabilize quickly at a temperature that maintains a thin dew or frost layer on the mirror surface. A precision thermometer element embedded within the mirror directly monitors this dew point temperature.

The instrument is calibrated against a primary humidity standard as and when required.

  • The user will need to supply two UK (240 V) plug sockets
  • Instrument Insurance
    • Hygrometer must be insured by the user for £50K to covers loss, theft or damage to the instrument: damage being that over and above general wear and tear. The system is insured whilst it is on the research aircraft.
    • The user is responsible for the instrument from the time it leaves AMOF to the time it is returned and signed off as in an acceptable operating condition by the IS: this will be done as soon as is possible on its return.
  • Public Liability Insurance
    • The AMOF is not liable for any damage or injury arising from the deployment or operation of this instrument when unattended by the IS.
  • Shipping Expenses
    • The user is liable for all costs arising from the shipping of the instrument both to and from a deployment.
  • IS T&S
    • The user is responsible for coving the travel and subsistence expenses of the IS while attending the instrument.
  • Footprint: 50 cm (W) X 25 cm (H) X 65 cm (D)
  • Shipping weight:  ~25 kg

The hygrometer control unit and sensor should be operated from an air-conditioned laboratory. A ¼” sampling line should be located outside the laboratory and secured to the common sampling point, ensuring that no water can enter the line.

Manual handling

  • Good lifting practices should be employed when unpacking the hygrometer, external pump, and control unit.
  • Cables and sample tubing kept away from pathways; held overhead height if necessary. Walkways kept free of clutter and access to exit kept clear.

Electric safety

  • Check hygrometer casing and cables are in good condition before switching on.
  • Attach all cables to before switching on at the mains. All equipment electrical safety tested.

Attended operation

  • There is no requirement for the system to be attended during operation from a safety standpoint.

Control unit

  • Footprint
    • 34.3 cm (L) x 33 cm (W) x 16.5 cm (H)
  • Weight (not including shipping case)
    • 5 kg
  • Power
    • 230 VAC, 2.5 amps
  • Operation temperature
    • 0 min°C to 50 max°C

Sensor unit

  • Footprint
    • 34 cm (L) x 22 cm (W) x 18 cm (H)
  • Weight (not including shipping case)
    • 16 kg
  • Power
    • 230 VAC, 300 watts
Field Data
  • Data is available to download from the logging laptop as txt files.
  • The user can download (but not delete) this data from the instrument but it should be noted that this data will not have been quality controlled.

Dew point

Archive data