Top 5 Methods for Generating Calibration Gas Standards
Calibration gases for testing chemical detectors can be generated by a variety of means. The five main dynamic methods are outlined below.
Flow meters attached to gas cylinders determine flow rates of gases, which are then mixed together.
Gas cylinders are the most widely-used way to create gas mixtures.
- Wide range of concentrations can be produced (from 50% down to 0.01% with high-volume flow meter, even without using multiple-step dilution)
- Size and cost of cylinders may be prohibitive
- Safety issues related to storing large volumes of gas (at high pressure)
Liquefied form of gas of interest placed inside permeation tube, permeates through walls of tube at rate governed by temperature.
- Can produce (very) low concentration mixtures – ppm to ppb
- Permeation tubes are small, portable and easily-stored
- Small quantity of analyte improves safety when dealing with dangerous gases (corrosives, explosives etc)
- Not very well suited to high concentrations
Gas slowly inserted into a flow using a syringe, pump or similar apparatus.
- Concentrations from a few percent down to ppm
- If liquid is to be injected, needs to be vaporized
- Motor-driven systems to control injection rate can be expensive
Gas evaporates from a liquid reservoir and diffuses along a tube at a rate determined by temperature, pressure and tube geometry, and then enters carrier flow across the top of the tube to produce required mixture.
- Straightforward way to produce concentrations in the ppm to high ppb range
- Works for most liquids with a sufficiently high vapour pressure
- Highly affected by temperature fluctuations – change of 1°C can produce 5-10% change in diffusion rate
- Theoretical and experimental estimates of the diffusion coefficient may differ significantly, so system may need to be independently calibrated
Carrier gas is passed through liquid form of gas of interest, causing some of the liquid to evaporate and join the stream of gas being passed through it.
- Very useful for adding single (volatile) liquid to gas stream
- Composition of the resulting mixture must be determined using some independent analytical technique
Comparative Accuracy of Gas Generation Methods
Li, Täffner, Bischoff and Niemeyer (International Journal of Chemical Engineering 2012) give the following comparison of the accuracies of four of these techniques.