Validation of Qube Axon Ambient CH4 Concentration Readings
Testing Report: Validation of Qube Axon Ambient CH4 Concentration Readings using 100 PPM Calibration Gas
1.0 Introduction
Several organizations have expressed an interest in testing Qube’s Axon continuous methane emissions monitoring devices using compressed calibration gasses. In support of this, Qube has undertaken a project to identify the correct experimental apparatus and procedure for such tests, which are described herein.
2.0 Calibration Gasses
Calibration gasses are available in a variety of compositions. Any gas blend used for Axon concentration reading verification must satisfy several conditions:
The gas must contain a methane concentration less 2,500 ppm (100 ppm is recommended and widely available).
The gas must be a “balance air” gas. That is to say, the blend must be comprised of air and methane (rather than an inert gas and methane). Qube’s sensors require the presence of oxygen to function. Therefore, any calibration gas used must contain oxygen.
The gas must be hydrated. Calibration gasses are typically dry, meaning that the partial pressure of H2O in the air constituent of the gas is zero. This places the relative humidity of the gas outside Qube’s calibrated range. Dry, untreated calibration gas cannot be used for Axon concentration reading verification testing.
A 100 ppm CH4 air balance calibration gas was used for the test described in this report. The gas was supplied by NorLAB of Boise, Idaho in a 103 liter (1.52 liters compressed) steel cylinder at 1,000 psi, with a variable flow-rate regulator capable of delivering between 0.1 and 7.0 liters per minute. Please refer to Appendix I for Certificate of Analysis.
3.0 Testing Apparatus
A special apparatus was designed and constructed for testing with calibration gasses. Calibration gas is first supplied through a variable-rate regulator to a brass ½” NPT manifold via 1/8” ID PVC tubing.
The gas is first passed through a damp 30 micron water filter for hydration. Downstream of the filter outlet, the gas passes through a gate valve and enters the Axon inlet vent via direct threaded connection to the daughter card enclosure. A second gate valve is also included to create a purge loop for clearing the daughter card enclosure after the test is complete.
Finally, a choke is installed on the Axon outlet vent via direct threaded connection (3/4” NPT), with reduction to a 1/8” ID tubing whip. The choke is necessary due to the low calibration gas flow rate (0.1 L/min.) required to properly hydrate the gas. Without the choke, there is a risk of dilution of the calibration gas in the daughter card enclosure via the outlet vent due to turbulent flow across the outlet fan.
Figures 3.1 & 3.2 illustrate the testing apparatus.
Figure 3.1: Testing Apparatus – Top View
Figure 3.2: Testing Apparatus - Front View
4.0 Testing Procedure
The tests are conducted according to the following procedure:
Remove the hydration filter cartridge from the filter housing and run under tepid water for 2 minutes, slowly rotating the filter cartridge. Then reinstall the cartridge in the filter housing.
Flush the hydration filter with calibration gas. With the Gas Supply Gate Valve and Purge / Bypass Gate Valves open, open the variable rate regulator valve on the gas cylinder to 1 L/min. Flow for two minutes. Close the Gas Supply Gate Valve and then immediately close the variable rate regulator valve.
Leave the Purge / Bypass Gate Valve open, allowing ambient air to circulate through the daughter card enclosure until the Axon CH4 concentration returns to baseline (approximately 1.9 – 2.1 ppm).
To conduct a test, close the Purge / Bypass Gate Valve, open the variable rate regulator valve on the gas cylinder to 0.1 L/min and immediately open the Gas Supply Gate Valve. Flow for the desired test duration.
To conclude the test, simultaneously close the Gas Supply Gate Valve and open the Purge / Bypass Gate Valve, then immediately close the variable rate regulator valve.
Figure 4.1 illustrates the flow path during a test, while figure 4.2 illustrates the flow path immediately prior to and immediately following a test.
Figure 4.1: Flow Path During Test
Figure 4.2: Purge / Bypass Flow Path
5.0 Results
Using the apparatus and procedure described here, Axon 01407 produced CH4 concentration readings of 100 ppm (+/- approximately 1ppm) during a 12-minute test, with a calibration gas flow rate of 0.1 L/minute, as shown in figure 5.1.
Chart 5.1: Test Results
6.0 Verification
The CH4 concentration of the Calibration gas was verified using Qube’s Los Gatos GGA-30p Off-Axis ICOS analyzer. The GGA-30p also produced readings of 100ppm.
The verification test must be run separately from the Axon test. Concurrent testing is not feasible because the GGA-30p’s compressor produces a gas flow rate exceeding the effective flow capacity of our hydration filter. The GGA-30p does not require hydration of the calibration gas.
Plot 6.1: OA ICOS Analyzer Verification of CH4 Concentration.
7.0 Discussion
Balance air calibration gasses containing methane concentrations up to 2,500ppm may be used to verify Qube Axon performance, using the apparatus and procedure described in this document. The critical step is hydration of the air constituent of the calibration gas, which is accomplished by slowly passing the gas through a damp filter membrane. This technique maximizes surface area and contact time between the gas and the water held in the filter membrane.
Tests performed with dry calibration gas yield unpredictable results – typically underreporting of the calibration gas concentration by 50% or more.