The gas detection industry has used specialty gas mixtures to calibrate or function test their equipment over the past 40 years. While the gas detection manufacturers have looked to alternatives such as self-calibration, gas generators, and capillary tubes, the truth remains that calibration gas is a key stakeholder in the gas detection market and it is likely to stay that way for many years to come.
Calgaz, part of the Air Liquide Group, has been a maj or player in this global market since its very inception 40 years ago. Working hand in hand with the major gas detection OEMs and their partners, Calgaz ensures that quality gas is readily available in order to guarantee the safety of all of those in this very vital sector which serves workers across industry, keeping them safe from the hazards of toxic gases, flammable environments, and oxygen depleted confined spaces. Calgaz has been the pioneer in supplying gases in non-refillable cylinders (NRCs) since the 1980s and in developing equipment such as the award winning patented Demand Flow Regulator to support industry across the world.
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 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.”
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.
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.
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.
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.
Chris Street is Director of Global Business Development and Strategy, Air Liquide Calgaz.