| Vacuum | Most widely used in the world for gas detector tube systems. Sample air is aspirated (drawn) into the detector tube manually by pulling the handle of the sampling pump (that the detector tube is inserted in). It can be quickly and easily used by anyone anywhere. |
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| Injection | Used for some gas detector tube systems. The air sample is first drawn into a syringe before being injected into the detector tube. There are numerous high-concentration measuring applications of this type available for carbon dioxide, propane, etc. |
| Sampling pump | Used for both gas detector tube systems and sensor systems. Sample air is automatically aspirated by the motor driven pump at a prescribed rate for a prescribed time. The GASTEC GSP series automatic gas samplers use this method. |
| Compressed air | This system is used to measure compressed air/gas from a compressor or other pressurised vessel. The compressed air is sampled by attaching a pressure reducing valve with flowmetre (between the vessel and the detector tube) in order to enable the pressurised air/gas inside the vessel to flow via the flowmetre into the detector tube at a fixed rate of flow for a predetermined time. It can be used to check for impurities in scuba-diving tanks or pressurised gas containers. |
| Diffusion | This method is used for some gas detector tube systems as well as sensor systems. Air is not aspirated by a sampling pump but allowed to diffuse into a sensor or detector tube placed in a factory or laboratory environment . A diffuser accelerates the gas diffusion rate to produce a longer colour change layer and the mean value for a 1-10 hour period is obtained. GASTEC passive dosimetre tubes utilise this principle. |
| Vapour-liquid equilibrium |
When a solution is put in a sealed container, any substances dissolved in it vapourise and diffuse until the concentration remaining in solution and the vapour equilibrate at a specific ratio that is unique to that substance: its specific vapour-liquid equilibrium. To obtain the true concentration of the target substance in solution, simply draw the air into an appropriate detector tube with the GV-100 Gas Sampling Pump and read, and then either: a.) multiply the reading by the correction factor unique to the specific target substance or b.) correct the reading by using the correction graph for the target substance. |
| Reagent-adding | A specific reagent is added to the sample to change the target substance into a gas that is easy to measure. A sample of generated gas is aspirated into an appropriate detector tube with the Model GV-100 Gas Sampling Pump. Tube readings should be corrected by multiplying by prescribed factors that are in a known proportionate ratio to one gram of said reagent. |
| Capillary tube |
This method utilises capillary action, which causes the sample liquid to rise up the narrow detector tube. With this method, measurements are quite simple. The detector tube end is merely immersed vertically in the liquid sample (directional arrow pointing upward) and a reading is taken after the prescribed sampling time has elapsed. The GASTEC lineup includes a dissolved sulphide detector tube which can be used to detect sulphur ion (S2-) in water. |
| Liquid aspiration | This method entails the forced sampling of a liquid using a strong aspiration (suction) via a syringe with a filter and then extracting it with a dropper pipette through a detector tube. Using the indicated value (that is read) and a prescribed calculation factor the volume of the target substance contained (in the liquid sample) can then be extrapolated. This method is useful, for instance, when measuring the salinity of raw concrete or bulk aggregate. |
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About gas detector tubes
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General information on gas measurement
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Reference
