# Presentation: 045440

## Technical Audit Methods and Results for Anemometers Used in the Missouri Ambient Air Monitoring Networks

- Stephen M. Hall
- Missouri Department of Natural Resources
- Air Quality Assurance Unit

## Introduction

- Wind speed and wind direction meteorological data are monitored at many ambient air monitoring sites in Missouri. Of the two quantities, wind direction has shown to be the most inaccurate.

- This presentation identifies an interpretation of the EPA wind direction data accuracy criteria and presents a method that was used to collect independent audit data.  This data was analyzed to determine anemometer orientation accuracy.

## Outline

- EPA Wind Direction Accuracy Criteria (an interpretation)

- Audit Method Development

- Audit Equipment Utilized

- Audit Results

- Closing Points

- Recommendations

- Audit Equipment Precision & Accuracy

## EPA Wind Direction Criteria

- Wind Direction Accuracy:

-  3 degrees relative to the Anemometer’s (sensor) mount or index ( 5 Degrees absolute error for installed systems)

- 5 Degrees total “System error” (orientation error + sensor index error)

## Interpretation of EPA Wind Direction Criteria for Technical Auditing

- The trick is to determine what is required to measure and verify that alignments are within 2 degrees of True North.

- Absolute Error of  Anemometer’s (sensor) alignment notch (index) from True North
-  2 degrees acceptable for orientation error.

- Allows vane angle response errors to be  3 degrees and the total system error will remain within 5 degrees.

**Notes:**
(interpretation of Volume IV Section 4.2.2.2.2.1 (Reference 1).  We defined this relationship between orientation error and vane angle response error as “total system error”
Reference 1 Section 4.2.2.2.2.1 uses the terminology “system errors” to describe the same concept.

(although if the anemometer’s bench vane angle response error is low (e.g. zero) up to 5 degrees absolute orientation error is acceptable).

## Combined Index and Orientation Error - Graphical Description

**Orientation Error (+/-)**

- True North

- Anemometer
-  Alignment Notch (index)
- (its physical orientation)

- Vane Angle Response error +/-
- (relative to Notch)

## Audit Method for Orientation

- Measure location of alignment notch with a precision compass (resolution +/- 0.5 degrees, Accuracy 0.25 degrees).

- Determine True North by compensating for the site’s magnetic declination.

- Find the difference between True North and the anemometer’s orientation.  This difference is the absolute error from True North which is independent of the Anemometer’s response relative to its alignment notch.

**Notes:**
(Various source have been utilized by agencies to determine the declination for the True North calculation. For the audits we used an aeronautical chart).

## Variety of Anemometer Installations

- 5- Mesa Systems Co. (Ultrasonic)

- Due to the overwhelming majority of the RM-young Anemometers, we developed the audit protocol specifically for these units.

- 21- RM Young Model 05103 or 05305-(AQ model) (integral Vane & Prop) (there were only 2 of the 21 that were the AQ model)

- 1- Climatronics (separate cup and Vane)

## Variety of Tower Installations

- 10 meter towers (fold down or telescoping)

- 6 meter “towers” (pole installations on top of sampling shelters)

- Note: Only about 10 of the 27 sites have anemometers mounted on towers 10 meters above ground level.

## Typical 10 Meter Tower Installation

## Typical 6 meter “tower” installation

## Orientation Audit Equipment

- Due to the variety of Anemometer installations a “flexible” audit orientation apparatus was constructed

## Orientation Audit Equipment

## Orientation Equipment

## Orientation Equipment

## Compass Precision Alignment Assembly

## Orientation Audit Results

- The following orientation audit data represent audit results from all 25 of the Network’s unique metrology sites which consist of the RM-Young installation.

- (–) measured orientation “lags” True North

- (+) measured orientation “leads” True North

- The sign of the difference was arbitrarily defined as follows:

## Slide 18

## Anemometer Vane Angle Response Error Relative to Alignment Notch (index)

- Determines the error of the anemometer’s degree response relative to the alignment notch (index)

**Notes:**
Auditing this parameter is easily performed on the RM-Young models since the manufacture offers a vane angle bench which can be purchased specifically for this purpose.

## Anemometer Installed for an Audit

## Alignment Notch Interface

- Alignment Notch Interfaced
- with bench notch (index)

- Align notch here, and you have
- made a 60 degree mistake!
- -Be CAREFUL-

## Vane Angle Response Audit Results

- 25 of the network’s sites consisting of the RM-Young models were audited over a 3 year period.
- The audit data represent all paired differences (550 total differences) over the following discreet audit points:
- 0, 30, 60, 90, 150, 180, 230, 270, 330, and 350 degrees.

**Notes:**
(response - bench setting) Some sites were audited more than once over the data collection period (third quarter 2002 through third quarter 2005). Some sites were removed or installed over the same period)

## Slide 23

## Slide 24

## Total System Error “As Found”

- The following histogram shows the combined effect of the orientation and vane angle response error with the anemometer in its “as found condition”.

## Slide 26

## Total System Error “Adjusted”

- The following histogram shows the combined effect of the orientation and vane angle response error on “Total System Error” **AFTER** the anemometer’s alignment notch was aligned to True North.

## Slide 28

## Slide 29

**Notes:**

Pareto Analysis of Bench audit Failures
The following chart is a Pareto Analysis of the Anemometer bench audit failures that were discovered over 55 bench audits
Note: Many anemometers were audited more than once over the data collection period. Some anemometers failed more than one criteria per audit but each occurrence of failure was included in the Pareto analysis 
We found an interesting correlation between the occurrence of bench audit failure and the frequency of quality control checks. Can you guess what it was?

## Audit Equipment Precision & Accuracy

- Vane angle response bench
- Orientation apparatus

- The accuracy of the audit equipment is determined by the manufacture’s published specifications.

- We gathered data to check the repeatability of the following Audit equipment:

## Vane Angle Response Bench

- Blind repeatability test-  Verified the precision of our vane angle response bench with two setups and the same anemometer.

- Maximum error for any repeated setting was within 0.24 degrees.

**Notes:**
5 angle settings evenly distributed between 0 and 355 degrees were chosen for the angles.

Each setting was repeated five times, then the anemometer was dismantled from the bench and reassembled and the points repeated for another 5 trials.

## Slide 32

**Notes:**
Out of 50 observations the maximum 3 sigma deviation from the mean degree difference was 0.12 degrees.

The manufacture’s stated accuracy of the bench (+/-0.5 degrees) could be confirmed if we were able to repeat the above experiment with the same anemometer of know precision and multiple vane angle response benches.  However simultaneous availability of these benches has prevented this analysis to date.

A total of 50 observations were made.  The absolute error of the degree difference (response - Set point) was computed for each observation.

## Slide 33

## Orientation Equipment

- Anecdotal evidence suggests repeatability will be well within one degree.

**Notes:**
we have performed an orientation with the compass without the aid of the precision alignment apparatus.  A week latter the same orientation was confirmed with the same compass with the alignment apparatus. The agreement was within one degree.

Unfortunately we have not collected significant data to determine the long-term repeatability of the orientation audit equipment.

## Comparing Two Sources of Declination Data

- Aeronautical Chart , US DOT & FAA April 18, 2002

- NOAA Site: http://www.ngdc.noaa.gov/seg/geomag/jsp/IGRF.jsp
- for April 18, 2002

## Slide 36

## Closing Points

- Orientation error is the greatest contributor to the inaccuracy of wind metrology data.

- Vane angle response errors are the greatest contributors to anemometer performance audit failure.

- Infrequent quality control checks correlate with anemometer performance audit failure.

## Recommendations

- Magnetic Interference _must_ be carefully considered when developing and implementing audit protocol for the orientation accuracy.  Keep in mind that you may need to address this issue on a site specific basis.

- When developing vane angle response criteria, establish audit points that monitor anemometer performance at both limits of the of potentiometer “Dead Zone”.

## Recommendations

- Monitoring agencies should establish routine quality control checks of the anemometer’s bench performance at least once every six months.
- Agencies that have independent auditing could reduce quality control checks to once per year with audits performed once per year and, preferably, 6 months out of phase with the QC check.

## Recommendations

- Chose a particular source for the network’s declination estimate and use it consistently when performing orientations.