Atmospheric analysis

NIWA has been using advanced scientific instruments to measure atmospheric trace gases and isotopes for over 50 years.

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    Temperature sensitivity of teflon (PTFE)

    PTFE is an excellent diffuser material for instruments exposed to the elements: It has a low coefficient of thermal expansion (0.000122mm per degree Celcius) It has a low moisture absorption (< 0.01%) It is stable against damage by UV radiation The following graph shows the measured temperature-sensitivity of teflon for the temperature range 19 degrees Celcius and 45 degrees Celcius.
    Info, questions and remarks, contact:
    Richard McKenzie [ [email protected] ]
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    Fibre relocation errors

    Sometimes it is neccessary to disconnect fibre and diffuser. Reconnection can cause a change in the diffuser-fibre distance, which could lead to errors in previous calibrations. Thermal expansion can also be a reason for a changing diffuser-fibre distance .
    The following graph shows the change in photomultiplier signal due to a changing fibre distance.
    When the fibre is relocated with care relocation errors are around 0.1mm.
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    Diffuser's Calibration Report

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    Quality of the diffuser

    Quality factors: Relative cosine error, Q and DCE
    Relative cosine error
    The quality of the diffuser is defined with the following quality factors.
    The "relative cosine error" is defined by:
    Q correction factor
    Q is a correction factor for isotropic radiation:
    This factor 'Q' is greater than 1 if the diffuser overestimates and less than 1 if the diffuser underestimates.
    Diffuse Cosine Error (DCE)
    The "Diffuse Cosine Error" shows the actual quality of the cosine response response.
    Physical meaning of the DCE: If the diffuser has a perfect cosine response this factor will be zero.
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    Cosine response at greater fibre distance

    For this experiment the diffuser is placed at a position which is 20 mm greater than the normal fibre position. In between diffuser and fibre is a teflon "light tunnel". The cosine response is shown below:
    The graph shows that the cosine response does not change dramatically, altough it improves a little.
    This suggest that the diffusers do not necessarily have to be used in combination with a fibre.
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    What does the diffuser look like?

    Drawing with dimensions of diffuser BAT 5
    The diffuser’s distance to fibre distance is matched to the fibre’s N.A. There is a slight slope in the diffuser’s surface to drain the water through a drainage hole. An 'o'-ring gives additional protection against rain.
    The diffuser can be made in almost any size.
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    Cosine response at 400 nm

    DCE,Q?
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    Alcohol

    Previous meaurements show indications that the transmission of teflon might change after cleaning the diffuser with alcohol. The next graph shows the signal before and after cleaning the diffuser with alcohol.
    This graph shows that there are no significant changes in signal after cleaning with alcohol.
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    Effective aperture

    The position of the diffuser's entrance aperture needs to be known for accurate calibration. Because the diffuser has a hemispherical shape the effective entrance aperture is not exactly located at the front surface.
    Experiments with the lamp at 1.5 m and 0.5 m show that according to the inverse square law the location of the effective entrance aperture is 1.4 mm (±1mm) behind the front surface of the diffuser.
    Info, questions and remarks, contact:
    Richard McKenzie [ [email protected] ]
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    Maintenance & Cleaning

    While in use it is best to clean the diffuser on a daily base with a piece of dry cotton. This to wipe of any dirt, dust, snow etc. If the diffuser for some reason gets really dirty it is recommendable to clean it with water and soap or alcohol. Dust inside the diffuser can be removed with a cotton bud.
    Info, questions and remarks, contact:
    Richard McKenzie [ [email protected] ]
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    Instrumentation at NIWA Lauder

    Facility
    There is a wide range of instruments at Lauder, which are listed on this page.
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    Cosine response at 300 nm

    DCE,Q?