DO 4 WRONGS MAKE A RIGHT?: Understanding the implications of anemometer selection for Continuous Monitoring

2024 was the warmest year on record. In fact, the 10 warmest years of NOAA’s 175-year record have all taken place within the last decade (NOAA, 2024). As we feel the consequences of a warming world (changes in global weather patterns, increases in forest fires, more frequent floods and natural disasters), both private companies and governments are taking action to reduce the world’s greenhouse gas emissions.   

Because methane has a greenhouse gas potential that is 28 times that of carbon dioxide, curbing methane emissions is critically important. As a result, many companies—especially in the oil and gas industry—have turned to continuous monitoring systems as alert and quantification mechanisms for effectively detecting fugitive or unplanned methane emissions.  

The Importance of Wind for Methane Monitoring  

As part of a continuous monitoring system, a set of methane sensors are placed within and/or at the perimeter of a site. Although these sensors measure methane concentrations, these data are only useful with respect to wind data. Without an understanding of how the wind moves methane particles from a leak source to the sensor, we cannot accurately detect a leak nor calculate site emissions. As a result, high-quality wind data is the cornerstone to high-quality methane monitoring.  

In a series of four blog posts, we will explore wind data. First (Chapter 1) we will evaluate the similarities and differences of the two main types of wind measurement devices used to measure wind. Then (Chapter 2) we will turn to a field study to compare data quality across an area of uniform wind characteristics. Next (Chapter 3), we venture to an oil and gas site in the Permian basin and analyze wind data collected in an on-site application. Lastly (Chapter 4), we compare low- and high-cost sonic anemometers to determine, through data, whether quality differences in the same technology can impact results. These analyses will allow us to use empirical data to justify the hardware that we use, and how that choice of instrumentation can impact the resulting data quality.  

 When it comes to measuring both wind speed and wind direction, the two instruments that are typically used for this task are the sonic and cup-and-vane anemometer.   

The cup-and-vane anemometer consists of a series of horizontal cups that are attached to a rotating shaft, whose rotation frequency is proportional to the wind speed. A second mechanical arm (the ‘vane’) with a wing is additionally used to determine wind direction. Whereas this instrument is typically found at a very affordable price point, there are several drawbacks to this instrument in terms of accuracy:  

1. For determining a wind speed, the cups need to be pushed by the wind. At low wind speeds, the power of the wind is often not enough to move the cups. As a result, wind speed measurements are often inaccurate in low-wind conditions. Because this is a mechanical measurement device, there is often a time lag between a wind gust and the ensuing rotation and recording of the wind. 
2. Like the ‘cups’ of the anemometer, the ‘vane’ also requires force to push it in a certain direction. At low windspeeds, the wind may not have a sufficient force to push the ‘vane’ in the correct direction, leading to inaccurate results on wind direction. 
3. Both the ‘cup’ and ‘vane’ components are devices that freely spin. As a result, to get accurate wind readings it is essential for the instrument to be mounted absolutely vertical, as even the smallest tilt can bias wind data to the direction of tilt. 
4. Lastly, there are many moving pieces associated with a cup-and-vane anemometer. Because of its reliance and sensitivity to the movement of these pieces, the instrument is very vulnerable to impacts of weather (e.g. freezing conditions causing parts to bind up) and the environment of installation (e.g. dust causing parts to jam).   

A sonic anemometer utilizes ultrasonic sound waves to detect how fast these waves travel across from one transducer to another. Unlike the mechanical cup-and-vane anemometer, the sonic anemometer has no moving parts, allowing it to be physically more robust and require less maintenance. Although it is important to mount a sonic anemometer vertically, a slight tilt in the sensor will not produce a significant bias to the data as would a mechanical cup-and-vane anemometer. This is significant, as sensors may be in remote locations not regularly visited and subject to environmental conditions that can tilt the sensors. Due to the use of ultrasonic sound without mechanical parts, this instrument has a fast time response to wind changes and gusts, a higher sensitivity, and the ability to measure wind speed and wind direction accurately at low wind speeds.   

In addition, the sonic anemometer outputs a status code, so that you can always remotely confirm its operation, thereby preventing erroneous wind readings. One main drawback of a sonic anemometer is its high cost—which can be >$1,000 (or 10 times the cost of a cup-and-vane anemometer). Despite this drawback, considering the importance of wind measurements for methane detection, the high cost of the instrument is justified.  

At ChampionX we support our decisions with scientific evidence. To thoroughly investigate and compare the quality of data produced from a cup-and-vane anemometer to a sonic anemometer, we set out to Red Feather Lakes, Colorado. In a wide and open field devoid of tall vegetation (Figure 1a), we mounted both anemometers on a pole (Figure 1b). Using one logger—to ensure simultaneous measurement—we recorded readings from both devices at 1-second intervals across 12 days. Below, we evaluate the data, comparing across four sonic and cup-and-vane anemometers. 

Assessing Data Reliability: 

One aspect of using equipment of different quality is not just the accuracy and precision of the data output, but also in the robustness of the instrument itself. Of the four cup and vane anemometers that were deployed in the field, one systematically malfunctioned around 120 degrees, providing incorrect wind data (Figure 2). Unless the user performs in-depth analysis of each sensor, this malfunctioning sensor would likely be undetected and remain in use within a continuous monitoring system. The impacts of a malfunctioning sensor are significant—as it would point to the wrong source, leading to incorrect conclusions of emission sources and methane emission quantification. The Gill anemometer that ChampionX uses, on the other hand, has a specific error code that notifies the system when a sensor is malfunctioning, limiting the risk of this occurring at your site. 

Comparing Cup-and-Vane and Sonic Anemometer Data 

Next, we compare the cup-and-vane and sonic anemometers mounted on the same pole (Figure 3). Although the two anemometers produce wind data that aligns closely with one another at wind speeds above 2 m/s, once the wind speed drops below 2 m/s the wind direction data diverges significantly across the instruments. In fact, the data points to wind directions consistently being over 10°different, and as much as 45°off, across a period of over 90 minutes. The implications of this error are substantial, potentially leading to incorrect source identification and emission quantification (Figure 4). To evaluate how often this discrepancy would occur at a site, we evaluated one year’s worth of wind data at a site in the Permian Basin. There, we found that wind below 2m/s occurred over 36% of the time. It follows that there may be incorrect source attribution and quantification of site emissions >1/3 of the year, just because of the choice of and investment in quality wind measurement devices.  

Comparing Sonic Anemometer Data Across a ‘Typical’ Site Distance 

To evaluate the quality and synchronization of wind data from sonic anemometers at distances of 150-200 meters (generally the distances between sensors at a typical site), we compared the readings across all poles (Figure 5). Results show that there is a high correlation between the anemometers (Figure 6), with minimal wind direction differences—even at wind speeds below 2 m/s. These data show not only the homogeneity of wind across a site of this size, but most importantly demonstrates the consistency and quality of the sensors across different devices.