Blog – Calibration Gas

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Sorry, this entry is only available in Vietnamese.

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What take effect to shelf life of a calibration gas cylinder        

Gas cylinders are made by many different manufacturers, are available in a variety of sizes, can be disposable or refillable and filled with a low, high or pressurized concentration. Generally speaking, the shelf life of calibration gas, (also known as span gas), is dependent on three factors:

  1. Gas Type
  2. Gas Concentration
  3. Gas Cylinder Quality and Size

1. Gas Type

Calibration gases can be divided into two types: reactive and non-reactive. “Reactive” is a broadly used term for chemicals that have some instability under certain conditions and may react with certain materials, moisture, oxygen or other chemicals. Reactive gas mixtures include gases such as ammonia (NH3), chlorine (Cl2), hydrogen sulfide (H2S), sulfur dioxide (SO2), hydrogen chloride (HCl), hydrogen cyanide (HCN), etc. Reactive gas mixtures are normally filled in aluminum cylinders with stainless steel valves that have been treated to minimize reactivity with the reactive gas. These mixtures have a shorter shelf life, typically 6 months to one year, because the concentration of the reactive gas is likely to dissipate over time.

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“Non-reactive” is a broadly used terms for chemicals that are stable under most conditions and are not affected by moisture, oxygen or other chemical interactions. Non-reactive gas mixtures include alkane or alkene hydrocarbons (methane (CH4), propane (C3H8), hexane (C6H14), isobutylene (C4H8), etc.), nitrogen (N2), hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), etc. Non-reactive gas mixtures are normally filled in steel cylinders and typically have a shelf life of about 3 years.

2. Gas Concentration of Reactive Gases

In some cases, a higher concentration of a reactive gas can have a longer shelf life than a lower concentration. In low concentrations, a few reactions can have a much larger effect on the overall composition of the mixture than the same reactions in a highly concentrated mixture.

3. Gas Cylinder Quality and Size

A well-made gas cylinder will have, on a microscopic level, the smoothest interior walls as possible. If the walls are rough, it allows the gas to come in contact with a larger surface area which increases the likelihood of a reaction with contaminants or the cylinder material itself. The quality of the internal walls and the material of the valves are both factors that affect the shelf life of reactive gases. In addition to the quality of the materials, larger, high pressure cylinders allow for longer shelf life because the ratio of the internal wall surface to gas volume is substantially less and thus there is less potential for a reaction.

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Regardless of the type of gas mixture, cylinders that do not bear a legible written, stamped or stenciled identification of the contents should not be used. It is also important to note the expiry date and not to use the gas past that date. If an inappropriate amount of calibration gas is used or if expired gas is used during calibration or bump testing, the result could be improper calibration and may result in a potentially dangerous situation.

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[i] http://www.critical-environment.com

cylinders

Non-Refillable Cylinder

Why We Need Calibration Gases
All gas detectors are comparative devices and susceptible to drift, which may lead to an un­der or overestimation of the true concentra­tion of the gas being detected. These effects can be minimized, but not entirely eliminat­ed, through the application of a documented calibration and maintenance procedure in line with the manufacturer’s instructions and recommendations.
For gas detection equipment, calibration is typically a relatively simple process where the zero point and a single up-scale value are assessed, which either can be around 50 per­cent of the measurement range or at a specif­ic safety related level. Typically for methane
Calgaz has been the pioneer in supplying gases in non-refillable cylinders (NRCs) since the 1980s.
50 percent of the lower explosive limit (LEL) is used.
The most common method of calibrating of gas detectors in the field is through the use of a pressurized cylinder of a calibration gas mixture available from specialty gas suppliers.
Depending upon the nature of the instru­ments to be calibrated, the cylinder may contain a single calibration gas in an inert balance gas, or a number of components. Calibration gases are available in a wide range of low and high pressure cylinders in many different sizes. These can range from containing a few liters of gas in an NRC up to several thousand liters of gas in a large high pressure refillable cylinder.
Reactive gas mixtures are calibration gases that contain at least one component which is classified as “reactive” because it may react with certain materials, moisture, oxygen, or other chemicals. Non-reactive gas mixtures containing alkane or alkene hydrocarbons (e.g. methane, ethane, or propane), or other stable gases such as nitrogen, hydrogen and carbon dioxide, which do not have any re­active components, are classified as “non­reactive.”
Why Non-Refillable Cylinders
Typically for the gas detection instruments the move has been away from the larger high pressure refillable calibration gas cylinders (HPC) towards the smaller NRCs since the mid-1980s. Unlike refillable HPC cylinders, NRCs are referred to by a nominal gas capac­ity and not by water capacity and are usually supplied in a range from 11 to 116 liters of gas, with working pressures between 11 and 70 bar.
NRC products have historically been sup­plied in both aluminum and steel cylinders, but there is a move in the industry to harmo­nize all NRC products to aluminum. There are however, some specific applications glob­ally that will still require steel cylinders, such as mining.
The main advantage of the NRCs is their portability. They are easy to use and enable calibration to be carried out in the field as well as in the workshop or laboratory. The NRC products tend to use a standard C10 valve which allows the regulators and other equipment to be interchanged between gas mixtures, again helping the engineer on site to minimize the amount of excess equipment used with them.
As NRCs are lightweight and portable, they are easily and safely transported by air freight allowing the cylinders to be supplied to loca­tions globally. In addition, there is no monthly rental cost and transportation costs are lower. Specialty gas companies also generally offer a recycling service for the cylinder material.
Innovation
One recent development in the industry has been the hybrid solution of offering small lightweight cylinders to industry giving all the above advantages but developing cylin­ders which can now be refilled by specialty gas companies. This improves the environ­mental footprint of the cylinders and reduces waste. However, it should be noted that due to differing regulations with regards to dan­gerous goods, these “refillable” NRC products must be shipped back “as full” in Europe even if fully evacuated, while in North America it is only necessary for the cylinders to be filled to under 2 bar (29 psi) to classify them as empty.
It also should be noted that rules and reg­ulations change periodically and it is up to the customer to determine whether they can legally ship back such products to the manu­facturer.
Advantages of NRC Products
NRC products have real advantages when only a minimal quantity of a gas mixture is required for calibration and they also are the better option for the single point calibration of gas detectors in the field. Multi-point cali­brations may be carried out in the laboratory using several NRCs, which occupy minimal floor space and have advantages in handling and storage.
Stability
The reactive components in the NRC cylin­ders ensure that the gas mixtures have a de­fined warranty for their shelf-life; however, due to improvements in cylinder quality and treatment processes, leading companies can achieve up to two years of shelf-life for hydro­gen sulfide and sulfur dioxide mixtures and three years for non-reactive gases. This is like­ly to increase over time as further research is done to understand even more the processes which cause the breakdown of gas stability over the long term.
One major trend over the past few years has been for the leading companies to be able to produce complex gas mixtures in NRCs with amazing stability. Indeed, the levels of stabil­ity are now approaching those of the same mixtures in the more traditional HPC.
Author: Chris Street
Director of Global Business Development and Strategy, Air Liquide Calgaz
From www.specialtygasreport.com
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Eastern Sea Co,. Ltd – The Official Distributor of Calgaz in Vietnam

As the industry evolves there is an increasing need for off the shelf mix­tures, short lead times, and global support combined with local distribu­tion hubs. Leading companies such as Calgaz now have invested in manu­facturing plants outside their home markets with US and Eu­ropean manufacturing as well as offices and hubs strategically located throughout the world.
 Taking that oportunity, Eastern Sea has reached out and become The Official Distributor of Calgaz in Vietnam. We commit to you the world class standard quality from Calgaz Product along with our enthusiatic staves, will support you with all we have.
Sincerely!