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General Rules for Designing Custom Gas Mixtures

We are often asked for many different gases for the same applications. When possible, the required gases can be combined into the same cylinder to reduce the overall gas cost. When creating custom gas mixtures you must be aware of potential issues from mixing different compounds together.

Reactive vs. non-reactive gases.

Different gaseous elements and compounds can react differently in the same situations. We generally separate different gases into two categories: reactive and non-reactive. We use these terms and apply them specifically to our industries and applications.

Non-reactive gases can generally be mixed with any other gas. They’re not likely to change or affect other components in the mixture and should remain as they are. Typically, non-reactive gases can be filled in any cylinder type and can be used with any regulator materials (brass, plated brass, or stainless steel). An example of non-reactive gases are inert gases including all of the noble gases.

Reactive gases are likely to react with things they come into contact with – be that the inside of a cylinder, regulator, valve, tube or pipe, as well as other types of gases. They must always be treated with extra care. When they react they form new compounds, which could be dangerous to heath or equipment.

There are some exceptions to these rules. Reactive gases can occassionally be classified as non-reactives because they won’t react with the other gases they’re commonly found and used with. For example oxygen and carbon monoxide are both reactive gases, but since they do not normally react with commonly used components, and they can be filled in any cylinder type, we treat them as non-reactive.

Similarly, there are some non-reactive gases, even inert gases, that can react to form compounds. This can happen under certain conditions including variables like pressure, or if they are ionised.

Since we generally don’t see such conditions, we can use the terms reactive and non-reactive as originally defined above.

How do I know if the gases I need are reactive or non-reactive?

There is no easy way to know if a gas is reactive or non-reactive just by name. Experience and familiarity with the compounds in question is required.

Having said that, here are some ways to help determine reactivity:

  • If the gases come in a steel cylinder they are likely to be non-reactive.
  • If the cylinder has a brass valve, they are likely to be non-reactive.
  • If the cylinder has a stainless steel valve at least one of the components is likely to be reactive.
  • If the component is toxic in its pure form it is likely to be reactive.
  • Hydrocarbons (see blog on hydrocarbons), usually used for ‘LEL’ calibrations, are not reactive even though they can be flammable.
  • Noble gases are non-reactive (with minor exceptions).

Sensor compatibility

Another issue to be aware of is the impact of added gases on the sensor you are calibrating. Even if two gases don’t react with each other they may have a negative effect on the sensor or instrument.

Many sensors have cross-sensitivities to other gases meaning that you can’t calibrate to one with the other present or your results will be incorrect. For example, an ‘LEL’ or combustible gas sensor can’t be accurately calibrated to methane if there is butane present since they will both have an effect on the same sensor.

Other gases can have negative effects in other ways, such as ‘blinding’ a sensor for a different compound. For example, many hand-held gas detectors may be configured to measure VOCs via a PID sensor, calibrated with isobutylene. The same instrument also has a combustible gas sensor, calibrated to methane. Methane will ‘distort’ the reading of a PID sensor so that it is unable to correctly read the level of isobutylene leading to an incorrect calibration. These two sensors could be successfully calibrated if the gases are kept separate.

Restrictions to shelf life or fill pressure

Mixing gases together can be the wrong decision if one of the components has a shorter shelf life than others, reducing the shelf life or warranty period of the whole mixture. This may mean a smaller cylinder is a better choice and this will increase the cost per litre; the opposite of what we’re trying to achieve. Similarly, one component may have a much lower vapour pressure than all others. This becomes a restriction to the fill pressure of the mixture which has a negative effect on the amount of gas that can be put in the cylinder.

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