Sonic Temperature

Gill's range of 3-axis anemometers now has improved sonic temperature performance, providing excellent accuracy and linear response over a much wider temperature range.

Key Features:

  • Improved sensible heat fluxes
  • High performance in all conditions
  • Enhanced quality control
  • -18C to +30C linear performance around 0 degrees
  • Higher repeatability - lower variance between units
  • Improved assembly of the transducer
  • WindMaster Transducer Stalk Details
WindMaster
  • Introduction
  • Development Objectives
  • Methods & Implementation
  • Test Results
  • Outcomes & Benefits

Introduction

Sonic anemometers have been used to measure three dimensional wind vectors, temperature, and surface sensible heat and momentum fluxes since the early 1960's. They have played a pivotal role in studying the surface energy balance (SEB), which describes how the radiative energy at the Earth's surface is partitioned between latent heat flux (LE) and sensible heat flux (H).

Sonic Temperature is used for fast response covariance results as well as Ambient Temperature inputs or corrections. Improvements in Sonic Temperature allow for better closure in energy budget measurements allowing for potentially less infrastructure and cost.

Currently, Gill anemometers are excellent Wind (w') and Temperature (T') flux monitors. As a designer and manufacturer of professional 3D anemometers, Gill Instruments understands that to maintain the high levels of performance required by their customers, continuous improvements are required as new materials and manufacturing techniques become available.

Advances in manufacturing and materials have provided improvements in the performance of ambient temperature measurements. Developments in this area reduce the need and costs associated with additional temperature sensors to correct for Sonic Temperature errors and improve sensible heat flux calculations made with the Gill anemometers.

Therefore, the Gill family of 3 Axis anemometers has undergone a number of improvements to enhance sonic temperature outputs, whilst maintaining the continuity of the transducer design.

Development Objectives

Gill embarked on a programme with three objectives to improve the sonic temperature performance.

  • Reduce or remove sonic temperature delay (timing) errors by introducing improved materials and manufacturing techniques making the timing more efficient.
  • Reduce potential long term flux errors caused by path length changes in extremely high humidity environments, by continuing to innovate with the latest hydrophobic materials available.
  • Further improve the repeatability between Gill anemometers with improved automated manufacturing processes.

Methods & Implementation

Improvements to Sonic Temperature measurements are largely achieved by improving on any additional timing errors in signal analysis and/or achieving better repeatability during manufacture, in addition to long term exposure in extreme environments.

These developments, to provide improved sensible heat fluxes and higher performance in all conditions, were made in the following areas by;

Reducing error in sonic temperature from the delay time between telling the electronics to send an electronic pulse to the time a pulse is actually transmitted. Experiments with different techniques (including CFD analysis) were done to find an optimal concept to reduce these delays. By using a different bonding technique between the existing transducer crystal and the surrounding 'rubber' material, these delays were reduced significantly and with a consistent behaviour. The new design and process ensures a more rounded transducer fits the housing with less pinching which therefore resonates more freely and quickly. This amendment to the assembly of the transducer has generated the majority of the improvement to the sonic temperature performance.

Gill 3 Axis anemometers can be used in almost every environment worldwide. One of these environments can be those that are totally saturated, such as moist rain forest regions. It was found that a small contributor to errors is if the material from which the transducer arms are made is hygroscopic. Over longer time periods - 5 years or more - a change in path length can occur in a moisture absorbing material leading to performance error. By working closely with plastics manufacturers on the latest compounds to be developed that are resistant to moisture absorption, Gill has been able to identify the best material currently available. Combined with detail changes to the transducer arm that makes manufacture easier, Gill has been able to reduce to an absolute minimum any effects of moisture absorption.

Automation of manufacturing processes can lead to improvements in assembly accuracy and repeatability along with efficiencies in the manufacturing process. By investing in automation, Gill has been able to leverage improvements in the accuracy and repeatability during the mounting of the transducer arms to the upper and lower transducer alignment bosses. This enhancement provides small but measureable improvements in repeatability between anemometers and path length movement over time. The trend to more extreme environments requires long term stability of the measurement.

The three changes have all been tested with control and competitive units for comparison. Tests were done in the Gill Instruments environmental chambers (temperature and humidity controlled) as well as geographically separated outdoor environments located in the UK, USA and Italy. These took place over a period of 24 months in all-weather environments.

Test Results

To ensure impartial results, the tests undertaken in the USA and Italy were by third party academic institutes.

The results confirm that the delay timing has been substantially reduced with the enhancements, which has resulted in a significantly improved ambient temperature measurement performance.

This means that even generically calibrated 3-axis anemometers can be used in more applications to provide sensible heat fluxes and other longer term ambient temperature values without correction factors being necessary. (For best results Gill always recommends correcting these values for humidity).

As importantly, the results have also confirmed that these refinements have not had a negative impact on the already excellent w' and T' fluxes.

An additional benefit that has come from this work is that a non-linear error in T has been shifted. The results demonstrate that fluxes measured between -5°C and +5°C are now not only linear, but can be individually calibrated to have 0°C error over a much wider range of temperatures.

Outcomes & Benefits

Previously with generically calibrated 3-axis units, sonic temperature error would trend well compared to reference ambient temperatures although the offset could be as large as 4°C. With these new improvements the sonic temperature error is limited to 1.5°C in the worst case scenarios. While Gill provides individually calibrated units with a much better performance than this, in most situations the error would be substantially less.

Richard McKay, Gill Product Manager and Meteorologist reports;
"the gains we have made using CFD and assembly process improvements are small yet significant in the performance of sonic temperature throughout the 3-axis range. A key advantage is that these improvements maintain the continuity of design and we now have high performance in all conditions".

By enhancing the performance with better fluxes, the Gill 3-axis anemometer range now leads the market in performance, quality and service. This provides the customer with the reassurance that they are obtaining the most accurate results, suitable for meeting the needs of the professional and scientific users, whilst maintaining the continuity of design that ensures compatibility with historical data already obtained.

The work to maintain this position continues with ongoing work into alternative material choices, plus carrying out further investigations to provide reassurance that no detrimental effects have been introduced over longer time frames in extreme events.