Methods for measuring gas permeability can be divided into two main catagories: the constant-volume variable-pressure method, and the gas concentration method. We use the second method as it provides us with highly reproducible measurements, and allows us to measure the diffusion of multiple components in a gas or liquid mixture, each component individually, rather than simply the total gas or liquid diffusion. Naturalgas and LPG are primarily methane and propane respectivily but also consist of several other components, we can measure the concentrations of the gas spiese that permeate through a sample.
Example of setup: The centre of the illustration shows the sample holder and the sample to be measured. To the left, the pressure is set with a test gas, where a continuous flow over the sample is ensured using a mass flow controller (MFC), and the pressure is maintained by a pressure controller (PC). The pressure and temperature are measured using a pressure indicator (PI) and temperature indicator (TI). To the right, the backside is flushed with a carrier gas, where the flow is controlled using a MFC. The gas is then led to a measuring instrument, in this case a gas chromatograph (GC).
The figure shows one sample holder. We can make parallel measurements and perform measurements on multiple samples simultaneously. The sample holder can be heated, if desired. It is additionally possible to collect the gas over a period of time in order to measure lower concentrations, allowing measurements of lower gas permeabilities.
When the gas permeability is monitored over time, we get a process, such as the one shown in the figure below. At time t = 0, the primary side is pressurised with gas and after a short time, gas starts to diffuse through the sample. The amount increases and reaches an equilibrium, which is the gas transmission rate.
Figure: Example of a process from start until equilibrium for the measurement of gas transmission rate
The simplest approach is to measure the equilibrium gas transmission at a temperature of 40-60 °C, as equilibrium is reached fairly quickly within this temperature range. Thin samples of 1-2 mm or less are preferred as they allow a larger transmission and equilibrium is reached more quickly.
We have performed measurements on the following gases and liquids:
Nitrous oxide N2O
Carbon dioxide CO2
Propane CH3CH2CH3 = C3H8
Iso-butane and n-Butane C4H10
Dimethyl ether (DME) CH3O-CH3 = C2H6O
Diethyl ether (DEE) CH3CH2OCH2CH3
Hydrogen disulfide (H2S)
Other gasses and liquids are also possibilities: these could include refrigerants or sulfur-containing gasses. Our instruments and their areas of applicability are listed below.
Gas chromatograph with TCD, FID and FPD detector: detection of oxygen, nitrogen, carbon dioxide using a TCD, flammable gases using an FID detector, and sulfur compounds using an FPD detector.
Quadropole mass spectrometer in vacuum chamber. Measurement of molecules up to an atomic mass of 200 amu.
FTIR instrument with a gas cell with long optical path length. Requires that the infrared light is absorbed by the molecule, but has a very low detection limit. Hydrogen, oxygen and nitrogen are not visible, but, e.g., carbon dioxide (CO2), ammonia (NH3), methane (CH4), and other hydrocarbons and nitrous oxide and refrigerants show good signals.
Furthermore, we have previously used Thermal Conductivity Transmission for detection of hydrogen and an NDIR detector for carbon dioxide (CO2).
Existing sample holders:
In the sample holder, the gas permeation of a circle of material with a diameter of either 45 mm or 80 mm is measured. Outside of this circle, there must be enough space to make a closed seal. When using the 80 mm sample holder, the sample must be at least 100 mm in diameter, but preferably 130 mm. The sample holders can be heated from room temperature to at least 90 °C. We also have a sample holder, which gives us the possibility of water cooling, allowing measurements below room temperature. Measurement of straight pipes is also an option. We have a sample holder for pipes with an outer diameter of approx. 40 mm. Further, we expand our selection of sample holders continuously, adding new sizes and shapes; please contact us for further information.
We can make parallel measurements, which means that we can measure multiple samples at the same time at different temperatures and with different liquids/gases.
Photo: Illustration of a test set-up. Usually, measurements with sample holders are performed in a fume cupboard and with insulation, thereby helps to maintain a constant temperature.
Photo: The photos above show various test set-ups.