PERMEATION TUBES
The permeation tube is a small container made of inert polymeric material, filled with a pure chemical compound in equilibrium between its gas and liquid or solid phase. At a constant temperature, the device releases the compound through its permeable wall at a stable and controlled rate.
Inserted into a carrier gas flow with a regulated flow rate, the permeation tube enables the generation of reference mixtures with precise concentrations, used for calibrating gas analyzers, testing hazardous gas alarms, and studying the effects of specific substances on materials or biological systems. Thanks to its reliability and stability, it is ideal for any application requiring a constant concentration of a trace chemical compound.
Available permeation rates typically range from 20 ng/min to 500 µg/min, allowing for concentrations from ppb up to high ppm levels.
Applications of Permeation Tubes
- Calibration of gas analyzers – Used to generate reference gas mixtures with precise concentrations to ensure the proper functionality and accuracy of measurement instruments.
- Testing of hazardous gas alarms – Enable the verification of toxic and flammable gas detectors, enhancing safety in industrial and laboratory environments.
- Studies on the effects of chemical substances – Applied to analyze the impact of specific chemical compounds on materials (e.g., corrosion, degradation) and biological systems (e.g., toxicology, environmental effects).
- Generation of reference gas mixtures – Used in laboratories to produce gases with known concentrations, essential for scientific testing, research, and development.
- Environmental monitoring – Employed to assess the presence of atmospheric pollutants and calibrate measurement instruments for air quality control.
- Chemical and pharmaceutical industries – Utilized for quality control and the verification of chemical compound stability in various industrial processes.
- Workplace safety – Applied to ensure compliance with air quality regulations in enclosed environments and to reduce exposure risks to hazardous gases.
- Simulation of gas exposure scenarios – Used in scientific studies to replicate real-world exposure conditions to specific chemical compounds under controlled conditions.
- Cleanroom monitoring and compliance – Used for the calibration of gas analyzers in cleanroom environments to maintain air purity standards and ensure accurate detection of trace contaminants.
- Electronic nose calibration and testing – Enable the precise delivery of volatile organic compounds (VOCs) and trace gases to evaluate and standardize the response of electronic nose (e-nose) sensor arrays in laboratory and industrial applications.
Thanks to their reliability, stability, and broad range of available concentrations, permeation tubes are highly versatile tools for various scientific, industrial, and environmental applications.
SF₆ Permeation Tubes in Methane Measurement from Ruminants
Methane emissions from ruminant livestock represent the largest global source of methane, significantly contributing to the increasing concentration of greenhouse gases in the atmosphere. The SF₆ tracer technique is widely used to determine methane (CH₄) emission rates in ruminant animals. This method allows for precise and accurate measurement of CH₄ emissions from individual grazing animals.
A permeation tube releasing SF₆ is placed inside the rumen of the cow, while the animal is equipped with a sampling system designed to collect air from around its mouth and nostrils over an extended period of time.
The permeation tube is a small stainless-steel container with a polymeric window, filled with pure SF₆ (Sulfur Hexafluoride) in two-phase equilibrium between its gas and liquid states. At a constant temperature, the device releases the compound through its permeable wall at a stable and controlled rate. In this case, the permeation tubes are certified at 39°C, corresponding to the internal temperature of the rumen.
The SF₆ is exhaled along with the methane produced from ruminal fermentation. The gases are collected in a PVC canister, designed to fit around the cow’s neck. Once the sampling period is complete, the canister is removed from the animal and connected to a dilution system, where the final pressure is recorded. Nitrogen is then gradually added until the canister pressure reaches approximately 1.2 atm.
The pressurized gas in the canister is then transferred to a gas chromatograph, where methane emission rates are calculated using the following equation:
where:
Methane emission rate
Sulfur hexafluoride release rate from the permeation tube
Measured methane concentration in the sample
Measured sulfur hexafluoride concentration in the sample
This formula allows for an accurate estimation of methane emissions by relating the known SF₆ release rate to the measured concentrations of both gases in the sample.
Advantages of Permeation Tubes
- High precision and stability – Provide a consistent and predictable emission rate, with values traceable to NIST standards, ensuring reliable and repeatable results.
- Adjustable concentration – The gas concentration can be easily modified by regulating the flow of the carrier gas, offering great flexibility for various applications.
- Wide concentration range – With permeation rates ranging from 20 ng/min to 500 µg/min, they allow for gas concentrations from ppb to high ppm levels, adapting to different calibration needs.
- Safer and more practical alternative to gas cylinders – Permeation tubes are compact, lightweight, and easy to transport, eliminating the need for bulky and hazardous high-pressure gas cylinders.
- Extended operational lifespan – Certain models, such as Extended Life Tubes with impermeable reservoirs, offer significantly longer service life compared to standard tubes, reducing replacement frequency.
- Lower operational costs – A single tube can generate a wide range of concentrations, reducing the need for multiple gas mixtures and lowering procurement and storage costs.
- Reliable for long-term applications – Ideal for scientific studies, environmental monitoring, and industrial testing requiring a continuous and stable supply of low-concentration gases.
- Minimal storage space and improved efficiency – Unlike gas cylinders, permeation tubes require little storage space and can be conveniently kept in a small refrigerator, rather than in large storage areas for gas tanks.
Thanks to these advantages, permeation tubes represent an efficient, safe, and flexible solution for calibrating analytical instruments, environmental monitoring, and various industrial applications.
How to use permeation tubes in Lab
Permeation tubes release the compound at a constant emission rate while maintaining a controlled temperature. However, the permeation rate exhibits an exponential dependence on temperature, increasing by approximately 7% for every 1°C variation. This phenomenon makes precise thermal regulation essential to ensure the stability and reliability of the generated gas mixtures.
To achieve highly accurate gas standards, it is crucial to use an advanced thermostatic system with a tolerance of ±0.1°C, minimizing temperature fluctuations and ensuring a stable and reproducible emission rate.
The permeation tube, or a combination of multiple tubes, is placed in a U-shaped glass holder and immersed in a thermostatic bath, where the temperature is rigorously maintained at a constant level. The diluent gas, typically air or nitrogen, is preheated through a bed of glass spheres, optimizing heat exchange and ensuring uniform and controlled flow conditions.
Chemical List
Gas Concentration Conversion between ppm and ng/sccm
Gas concentration can be expressed in parts per million (ppm), a relative unit that represents the ratio of gas molecules to the total number of air molecules, or in nanograms per standard cubic centimeter per minute (ng/sccm), an absolute unit that indicates the gas mass per unit volume and time.
The conversion between these units can be performed using the following equations:
where:
conversion factor from
to
conversion factor from
to
The conversion between these units can be performed using the following equations:
| Chemical Name | CAS Number | Molecular Weight | K1 | K2 |
|---|---|---|---|---|
| (+)-Limonene | 5989-27-5 | 136.24 | 0.180 | 5.570 |
| 1- Butanethiol | 109-79-5 | 90.19 | 0.271 | 3.687 |
| 1,1,1,3,3,3-Hexamethyldisilazane | 999-97-3 | 161.38 | 0.152 | 6.598 |
| 1,1,1,3,3-pentafluorobutane | 406-58-6 | 120.05 | 0.204 | 4.908 |
| 1,1,2-Trichloroethane | 79-00-5 | 133.4 | 0.183 | 5.454 |
| 1,2,3-Trimethylbenzene | 526-73-8 | 120.19 | 0.204 | 4.914 |
| 1,2,4-Trimethylbenzene | 95-63-6 | 120.19 | 0.204 | 4.914 |
| 1,2-Dichloro-1,1-difluoroethane | 1649-08-7 | 116.94 | 0.209 | 4.781 |
| 1,2-Dichloroethane | 107-06-2 | 98.96 | 0.247 | 4.046 |
| 1,2-Diethylbenzene | 135-01-3 | 134.22 | 0.182 | 5.487 |
| 1,4-Butanediamine | 110-60-1 | 88.15 | 0.277 | 3.604 |
| 1,4-Diethylbenzene | 105-05-5 | 134.22 | 0.182 | 5.487 |
| 1-Butanol | 71-36-3 | 74.12 | 0.330 | 3.030 |
| 1-Ethyl-4-methylbenzene | 622-96-8 | 120.19 | 0.204 | 4.914 |
| 1-Hexanethiol | 111-31-9 | 118.24 | 0.207 | 4.834 |
| 1-Hexanol | 111-27-3 | 102.18 | 0.239 | 4.177 |
| 1-Methoxy-2-propanol | 107-98-2 | 90.12 | 0.271 | 3.684 |
| 1-Methoxy-2-propyl acetate | 108-65-6 | 132.16 | 0.185 | 5.403 |
| 1-Methyl-2-pyrrolidone | 872-50-4 | 99.13 | 0.247 | 4.053 |
| 1-Methylnaphthalene | 90-12-0 | 142.2 | 0.172 | 5.814 |
| 1-Pentene | 109-67-1 | 70.13 | 0.349 | 2.867 |
| 1-Propanethiol | 107-03-9 | 76.15 | 0.321 | 3.113 |
| 2,2-Thiodiethanol | 111-48-8 | 122.18 | 0.200 | 4.995 |
| 2,3,3,3-Tetrafluoropropene | 754-12-1 | 114.04 | 0.214 | 4.662 |
| 2,3-Dimethylpentane | 565-59-3 | 100.2 | 0.244 | 4.096 |
| 2,4,6-Trichloroanisole | 87-40-1 | 197.45 | 0.124 | 8.072 |
| 2,4-Dimethylheptane | 2213-23-2 | 128.26 | 0.191 | 5.244 |
| 2-Aminoethanol | 141-43-5 | 61.08 | 0.400 | 2.497 |
| 2-Chloroethyl ethyl sulfide | 693-07-2 | 124.64 | 0.196 | 5.096 |
| 2-Hexanone | 591-78-6 | 100.16 | 0.244 | 4.095 |
| 2-Methoxy-4-methylphenol | 2785-89-9 | 152.2 | 0.161 | 6.222 |
| 2-Methyl-2-propanethiol | 75-66-1 | 90.19 | 0.271 | 3.687 |
| 2-Methylbuta-1,3-diene | 78-79-5 | 68.12 | 0.359 | 2.785 |
| 2-Methylnaphthalene | 91-57-6 | 142.2 | 0.172 | 5.814 |
| 2-Methylphenol | 95-48-7 | 108.14 | 0.226 | 4.421 |
| 2-Methylthiophene | 554-14-3 | 98.16 | 0.249 | 4.013 |
| 2-Nitrotoluene | 88-72-2 | 137.15 | 0.178 | 5.607 |
| 2-Propanethiol | 75-33-2 | 76.15 | 0.321 | 3.113 |
| 3-(Propenylthio)-1-propene | 592-88-1 | 100.19 | 0.244 | 4.096 |
| 3-Methyl-1-butanol | 123-51-3 | 88.15 | 0.277 | 3.604 |
| 3-Methylindole | 83-34-1 | 131.17 | 0.186 | 5.363 |
| 3-Methylphenol | 108-39-4 | 108.14 | 0.226 | 4.421 |
| 4-Hydroxy-2,5-dimethylfuran-3-one | 3658-77-3 | 128.13 | 0.191 | 5.238 |
| 4-Methoxy-4-methylphenol | 93-51-6 | 138.18 | 0.177 | 5.649 |
| 4-Methyl-1-(1-methylethyl)-bicyclo[3.1.0]hex-2-ene | 3387-41-5 | 136.24 | 0.180 | 5.570 |
| 4-Methyloctane | 2216-34-4 | 128.26 | 0.191 | 5.244 |
| Acenaphthene | 83-32-9 | 154.21 | 0.159 | 6.305 |
| Acenaphthylene | 208-98-8 | 152.19 | 0.161 | 6.222 |
| Acetonitrile | 75-05-8 | 41.05 | 0.596 | 1.678 |
| Acrylonitrile | 107-13-1 | 53.06 | 0.461 | 2.169 |
| Ammonia | 7664-41-7 | 17.03 | 1.436 | 0.696 |
| Anthracene | 120-12-7 | 178.23 | 0.137 | 7.287 |
| Benzaldehyde | 100-52-7 | 106.12 | 0.230 | 4.339 |
| Benzene | 71-43-2 | 78.11 | 0.313 | 3.193 |
| Benzo[a]anthracene | 56-55-3 | 228.29 | 0.107 | 9.333 |
| Benzo[a]pyrene | 50-32-8 | 252.31 | 0.097 | 10.315 |
| Benzonitrile | 100-47-0 | 103.12 | 0.237 | 4.216 |
| Benzothiazole | 95-16-9 | 135.19 | 0.181 | 5.527 |
| But-3-en-2-one | 78-94-4 | 70.09 | 0.349 | 2.865 |
| Butanal | 123-72-8 | 72.11 | 0.339 | 2.948 |
| Butane | 106-97-8 | 58.12 | 0.421 | 2.376 |
| Butyl acetate anhydrous | 123-86-4 | 116.16 | 0.211 | 4.749 |
| Cadaverine | 462-94-2 | 102.18 | 0.239 | 4.177 |
| Carbon disulfide | 75-15-0 | 76.14 | 0.321 | 3.113 |
| Carbonyl sulfide | 463-58-1 | 60.1 | 0.407 | 2.457 |
| Chlorobenzene | 108-90-7 | 112.56 | 0.217 | 4.602 |
| Chloroform | 67-66-3 | 119.38 | 0.205 | 4.881 |
| cis-Penta-2,3-diene | 1574-41-0 | 68.08 | 0.359 | 2.783 |
| Cyclododecane | 294-62-2 | 168.34 | 0.145 | 6.882 |
| Cyclohexane | 110-82-7 | 84.16 | 0.291 | 3.441 |
| Cyclohexanone | 108-94-1 | 98.15 | 0.249 | 4.013 |
| Cyclohexylmethane | 4292-75-5 | 98.19 | 0.249 | 4.014 |
| Cyclopentanone | 120-92-3 | 84.12 | 0.291 | 3.439 |
| Decamethylcyclopentasiloxane | 541-02-6 | 370.77 | 0.066 | 15.158 |
| Decane | 124-18-5 | 142.29 | 0.172 | 5.817 |
| Dibutyl sulfide | 544-40-1 | 146.29 | 0.167 | 5.981 |
| Dichloromethane | 75-09-2 | 84.93 | 0.288 | 3.472 |
| Diethyl cyanophosphonate | 2942-58-7 | 151.11 | 0.162 | 6.178 |
| Diethyl disulfide | 110-81-6 | 122.24 | 0.200 | 4.998 |
| Diethyl ether | 60-29-7 | 74.12 | 0.330 | 3.030 |
| Diethyl phosphoramidate | 1068-21-9 | 153.13 | 0.160 | 6.260 |
| Diethylamine | 109-89-7 | 73.14 | 0.334 | 2.990 |
| Diisopropyl methylphosphonate | 1445-75-6 | 180.16 | 0.136 | 7.365 |
| Dimethyl disulfide | 624-92-0 | 94.2 | 0.260 | 3.851 |
| Dimethyl methylphosphonate | 756-79-6 | 124.08 | 0.197 | 5.073 |
| Dimethyl sulfide | 75-18-3 | 62.13 | 0.394 | 2.540 |
| Dimethyl sulfoxide | 67-68-5 | 78.13 | 0.313 | 3.194 |
| Dimethyl Trisulfide | 3658-80-8 | 126.24 | 0.194 | 5.161 |
| Dipropylene glycol monomethyl ether | 34590-94-8 | 148.2 | 0.165 | 6.059 |
| Dodecamethylcyclohexasiloxane | 540-97-6 | 444.96 | 0.055 | 18.191 |
| Dodecane | 112-40-3 | 170.34 | 0.144 | 6.964 |
| Epichlorohydrin | 106-89-8 | 92.53 | 0.264 | 3.783 |
| Ethanethiol | 75-08-1 | 62.13 | 0.394 | 2.540 |
| Ethenylbenzene | 100-42-5 | 104.15 | 0.235 | 4.258 |
| Ethyl 3-methylbutanoate | 108-64-5 | 130.18 | 0.188 | 5.322 |
| Ethyl acetate | 123-66-0 | 88.11 | 0.278 | 3.602 |
| Ethyl methyl sulfide | 624-89-5 | 76.16 | 0.321 | 3.114 |
| Ethyl sulfide | 352-93-2 | 90.18 | 0.271 | 3.687 |
| Ethylbenzene | 100-41-4 | 106.17 | 0.230 | 4.341 |
| Ethylene glycol monoethyl ether | 110-80-5 | 90.12 | 0.271 | 3.684 |
| Fluoranthene | 206-44-0 | 202.26 | 0.121 | 8.269 |
| Fluorene | 86-73-7 | 166.22 | 0.147 | 6.796 |
| Fluorobenzene | 462-06-6 | 96.1 | 0.255 | 3.929 |
| Formic acid | 64-18-6 | 46.03 | 0.531 | 1.882 |
| Guaiacol | 90-05-1 | 124.14 | 0.197 | 5.075 |
| Heptane | 142-82-5 | 100.2 | 0.244 | 4.096 |
| Hex-1-ene | 592-41-6 | 84.16 | 0.291 | 3.441 |
| Hexadecane | 544-76-3 | 226.44 | 0.108 | 9.258 |
| Hexamethyldisiloxane | 107-46-0 | 162.38 | 0.151 | 6.639 |
| Hexanal | 66-25-1 | 100.16 | 0.244 | 4.095 |
| Hexane | 110-54-3 | 86.18 | 0.284 | 3.523 |
| Hexyl acetate | 142-92-7 | 144.24 | 0.170 | 5.897 |
| Hexylcyclohexane | 4292-75-5 | 168.34 | 0.145 | 6.882 |
| Hydrochloric acid | 7647-01-0 | 36.46 | 0.671 | 1.491 |
| Hydrogen sulfide | 7783-06-4 | 34.08 | 0.718 | 1.393 |
| Indole | 120-72-9 | 117.15 | 0.209 | 4.789 |
| Iodomethane | 74-88-4 | 141.94 | 0.172 | 5.803 |
| Manganese dioxide | 1313-13-9 | 86.94 | 0.281 | 3.554 |
| Mesitylene | 108-67-8 | 120.19 | 0.204 | 4.914 |
| Methanol | 67-56-1 | 32.04 | 0.763 | 1.310 |
| Methyl acetate | 79-20-9 | 74.08 | 0.330 | 3.029 |
| Methyl mercaptan | 74-93-1 | 48.11 | 0.508 | 1.967 |
| Methylamine | 74-89-5 | 31.06 | 0.788 | 1.270 |
| Methylcyclohexan | 108-87-2 | 98.19 | 0.249 | 4.014 |
| Methylcyclopentane | 96-37-3 | 84.16 | 0.291 | 3.441 |
| m-Xylene | 108-38-3 | 106.17 | 0.230 | 4.341 |
| Naphthalene | 91-20-3 | 128.17 | 0.191 | 5.240 |
| Nitrogen dioxide | 10102-44-0 | 46.01 | 0.532 | 1.881 |
| Octamethylcyclotetrasiloxane | 556-67-2 | 296.62 | 0.082 | 12.127 |
| Octanal | 124-13-0 | 128.21 | 0.191 | 5.242 |
| Palladium | 7440 - 05 -3 | 106.42 | 0.230 | 4.351 |
| Pendimethalin | 40487-42-1 | 281.31 | 0.087 | 11.501 |
| Pentadecane | 629-62-9 | 212.41 | 0.115 | 8.684 |
| Phenanthrene | 85-01-8 | 178.23 | 0.137 | 7.287 |
| Phenol | 108-95-2 | 94.11 | 0.260 | 3.848 |
| Phosforic acid | 7664-38-2 | 98 | 0.250 | 4.007 |
| Pinacolyl methylphosphonate | 616-52-4 | 180.16 | 0.136 | 7.365 |
| Propane | 74-98-6 | 44.1 | 0.555 | 1.803 |
| Pyrene | 129-00-0 | 202.25 | 0.121 | 8.269 |
| sec-Butyl mercaptan | 513-53-1 | 90.19 | 0.271 | 3.687 |
| Sodium hydroxide | 1310-73-2 | 40 | 0.612 | 1.635 |
| Sulfur dioxide | 7446-09-5 | 64.07 | 0.382 | 2.619 |
| Sulfur hexafluoride | 2551-62-4 | 146.06 | 0.167 | 5.971 |
| Tertineolene | 586-52-9 | 136.24 | 0.180 | 5.570 |
| Tetrachloroethene | 127-18-4 | 165.83 | 0.148 | 6.780 |
| Tetrachloromethane | 56-23-5 | 153.82 | 0.159 | 6.289 |
| Tetradecane | 529-59-4 | 198.39 | 0.123 | 8.111 |
| Tetrahydrothiophene / THT | 110-01-0 | 88.15 | 0.277 | 3.604 |
| Thiophene | 110-02-1 | 84.14 | 0.291 | 3.440 |
| Toluene | 108-88-3 | 92.14 | 0.265 | 3.767 |
| Tolylene-2,4-diisocyanate | 584-84-9 | 174.16 | 0.140 | 7.120 |
| trans-Penta-1,3-diene | 2004-70-8 | 68.12 | 0.359 | 2.785 |
| Trichloroethylene | 79-01-6 | 131.39 | 0.186 | 5.372 |
| Tridecane | 629-50-5 | 184.37 | 0.133 | 7.538 |
| Triethylamine | 121-44-8 | 101.19 | 0.242 | 4.137 |
| Triethylene glycol | 112-27-6 | 150.17 | 0.163 | 6.139 |
| Trimethyl phosphate | 512-56-1 | 140.1 | 0.175 | 5.728 |
| Trimethylsilanol | 1066-40-6 | 90.2 | 0.271 | 3.688 |
| Valeraldehyde | 110-62-3 | 86.13 | 0.284 | 3.521 |
| Vinyl chloride | 75-01-4 | 62.5 | 0.391 | 2.555 |
| α-Pinene | 7785-70-8 | 136.24 | 0.180 | 5.570 |
| β-Pinene | 18172-67-3 | 136.24 | 0.180 | 5.570 |
Application of the Equations
These formulas enable quick conversion between ppm, a commonly used unit for air quality assessments and trace gas analysis, and ng/sccm, which is widely applied in flow measurements and mass-based analytical systems.
Accurate conversion between these units is essential in fields such as environmental monitoring, industrial emissions analysis, quality control in manufacturing processes, and scientific research.