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.

Gas Cylinders

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)

Permeation Tubes

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

Injection Methods

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

Diffusion Methods

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

Evaporation Methods

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.

Accuracy Remarks
Injection 5-9%
  • The accuracy strongly depends on the accuracy of the injection devices.
  • When multicomponents test gas is generated, the accuracy is not as precise as for pure components.
Permeation 2-5%
  • The main source of the uncertainty comes from the permeation rate, which is influenced by thermodynamic and physical state variables, such as temperature, pressure and gas flow.
  • Permeation methods can be used to generate very low concentrations (ppb level), the uncertainty of the calibration itself also has a strong influence on the overall accuracy.
Diffusion 3-5%
  • The most important uncertainty is the diffusion rate. The variation of it is related to the compound itself, the design of the diffusion device, and thermodynamic state variables.
  • In analogy to the permeation method, the calibration itself contributes to the overall uncertainty.
Evaporation 5-15%
  • The marked deviation results from the dilution system and temperature variation.
  • The vapour pressure of the compound is also one considerable factor. The error can be minimized for compounds with a low vapour pressure.

For information regarding pricing and availability get in touch with our team

Request pricing