DOI: 10.5878/6mnk-bb39
title: Facilitating the use of low-cost methane (CH4) sensors in flux chambers : calibration, data processing, and describing an open source make-it-yourself logger
title: Facilitating the use of low-cost methane (CH4) sensors in flux chambers : calibration, data processing, and describing an open source make-it-yourself logger

description: Data on methane (CH4) emissions using low-cost CH4 sensor in flux chambers.

The sensor used in the study in which the data was collected is the Figaro NGM2611-E13, which is a factory pre-calibrated module based on the Figaro TGS 2611-E00. 

The sensor evaluation setup was designed to resemble real measurement conditions in floating flux chambers in aquatic environments. The sensors were placed in the headspace of a plastic bucket positioned upside down on a water surface in a tank. We used a 7 L plastic bucket in which we located 20 TGS 2611-E13 sensors connected to electronic circuitry and a sensor signal logging system described in detail separately. The chamber headspace was continuously pumped from the chamber, through the measurement cell of an ultraportable greenhouse gas analyzer (UGGA, Los Gatos Research), and then back to the chamber. The UGGA served as a reference instrument for CH4. The air T and RH inside the chamber were measured with 10 K33-ELG CO2 sensors (Senseair) which have an accuracy of ±0.4 ∘C and ±3 % RH (Bastviken et al., 2015). The large number of K33-ELG sensors was due to a separate test of wireless data transfer (outside the scope of this work), and one K33-ELG sensor would have been enough for this CH4 sensor study. The entire installation was placed in a climate room to allow for varying T, and thereby also absolute humidity (H) in the chamber headspace. T and H covary under field conditions in measurements near moist surfaces. Therefore although T and H were not controlled independently, their variability under this calibration setup was reflecting flux chamber headspace conditions under in situ field conditions.

The CH4 concentration in the chamber was changed by direct injections of methane into the chamber by syringe via a tube. The CH4 concentrations during the calibration experiments ranged from 2 to 719 ppm. We performed multiple separate calibration experiments at different T and RH levels ranging from 10 to 42 ∘C and 18 %–70 %. At temperatures below 20 ∘C the RH was usually 50 %–70 %, while at temperatures &gt;20ºC , RH ranged from 18 % to 60 %. The highest absolute water vapor mole fraction was 35 000 ppm H2O. Values were recorded once per minute. T and RH values from the K33-ELG sensors were averaged among all sensors (because all sensors were in the same chamber and we could not link specific K33-ELG sensors to specific CH4 sensors).

The response time to changing chamber headspace CH4 levels differed between the sensors situated in the chamber (responding rapidly) and the UGGA (delayed response time due to the residence time of the measurement cell and tubing). The reference instrument measurement cell was large enough to be influenced by CH4 from the chamber over a certain time period (the measurement cell residence time), and if the concentration change in the chamber was more rapid, the data from the reference instrument and sensors become incomparable and need to be omitted to not bias the calibration. Therefore data were filtered to remove periods of rapid changes when the different response times caused data offsets. Some sensor data were lost during parts of the experiments due to power, connection failure, or data communication issues.

The dataset was originally published in DiVA and moved to SND in 2024.