Atmospheric analysis

Technical Note No. 99/1 - Measuring Waves in Lakes and Estuaries is Quite Tricky, data can be Improved by Adjusting Sensor Noise Level
It sounds paradoxical, but the most difficult task when measuring waves (Task 4/0 or Task 4/1) is knowing when there aren't any! The danger is that sensor noise - not real signal - gets transformed into estimates of wave height, which will be wrong. The "reality checks" applied by DOBIE have been described in a previous Technical Note, and these are meant to catch those bursts where sensor noise has been (incorrectly) transformed.

Technical Note No. 2000/1 - Conversion of Hydrostatic-Depth Spectrum to Sea-Surface-Elevation Spectrum Since the release of version 19.5, PEDP reports beside each spectral estimate Sh(f) when processing Task 4/1 spectra. is defined as:
where z* is the depth of DOBIE below mean water level, is the mean water depth and k is the wavenumber corresponding to f, the frequency of the spectral estimate.

NIWA's mobile air quality monitoring trailers contain equipment capable of measuring a range of key air contaminants and meteorological parameters.

Modern marine geoscience requires accurate and detailed subsurface information for the mapping of geologic structures and sedimentary sequences beneath the continental margin. In order to acquire this information NIWA purchased a Multi Channel Seismic system (MCS) in December 1996, consisting of a 24 channel streamer, seismic source and acquisition hardware. 

NIWA has been engaged in an active UV research programme since the 1980s. The cornerstone of this work is the routine accurate measurement of the spectrum of surface UV irradiance to quantify UV climatologies, to understand the causes of UV variability (temporally and spatially), and to monitor long-term changes.

Setup
The following setup was used for testing the cosine response and transmission of the diffuser.
In this setup the lamp distance is as large as possible. The reason for this is that the lamp should resemble a point light source. If the lamp is placed at a close distance this would lead to large errors especially at bigger zenith angles. The diam eter of the windings inside the lamp are about 5 mm. A high accuracy power supply is used to keep the lamp current constant (Optronics OL 83A). The voltage across the lamp filament is monitored directly.

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] ]

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.

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.

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.