Selecting Proper Materials For Calibrating Gas Detection Instruments

Calibrating gas detectors seems simple enough. Just select a cylinder of gas, regulator and tubing. But each of those elements needs to be of the correct material otherwise the calibration results could fail. Having the technical understanding of the gases you are calibrating eliminates unnecessary and unwanted failures.

Too many times situations are encountered where the calibration gas was blamed for a failed calibration when either the regulator or tubing was the culprit. These three elements can be easily managed with a few basic rules of best practice.

The correct selection of materials when calibrating gas detection instruments, designing gas distribution systems and other specialty gas applications is critical to achieve accurate, repeatable results.

Choosing the Correct Gas Regulator

Regulators used for calibrating gas detectors typically fall into two categories.

1. Fixed Flow
2. On Demand

Each style of regulator is designed for specific types of instruments either diffusion or sample draw respectively. Today, each regulator type is available in brass, nickel plated brass and stainless steel.

The variety of regulator materials means we are able to select the correct regulator for our application in the same manner that specialty gas applications select their requirements.

1. Stainless steel now permits gas detection customers the opportunity to use the correct regulator for the highly corrosive (reactive) gases such as ammonia, chlorine, hydrogen chloride, hydrogen cyanide, nitric oxide, nitrogen dioxide to name a few.
2. Nickel plated brass is still fine for low concentrations of hydrogen sulphide and sulphur dioxide but if the concentrations increase in special applications then stainless steel should be used.
3. Stainless steel will not react with the corrosive gases.
4. Nickel Plated Regulators, will over time corrode and as the regulator begins to corrode it will react with the gas mixture. Often we hear, “but we have been using this regulator for years” yes and that is the problem. The nickel plating has corroded and the gas is now reacting with the bare brass.
5. Brass or nickel plated brass is suitable for non-corrosive gases such as CO2, CO, CH4, the family of hydrocarbons, refrigerants and oxygen. There are issues with CO and nickel plating at higher concentrations which should be evaluated, but most applications are suitable.

Best Practice Tips:

1. Use only stainless steel regulators for CL2, HCL, HCN, NH3, NO, NO2, PH3.
2. Use nickel plated brass for low concentration H2S and SO2 but stainless steel is still best
3. Use brass or nickel plated for non-corrosive applications

How To Pick The Correct Tubing

We often use the generic term Tygon® when referring to tubing used to calibrate gas detectors, but this brand name means many different things. There are dozens of different types of tubing with varying chemical properties, permeation rates and tolerances making it confusing and complicated.

For gas detection applications, it is best to break down the requirements for tubing by similar criteria as we have for regulators. As with regulators we must consider the corrosive nature and properties of different gases when selecting tubing.

Standard tygon® or polyurethane (C-210) tubing is fine for most non-corrosive mixtures.   But there are always exceptions.

• Styrene will adsorb onto to standard polyurethane tubing.
• Gases such as chlorine and ammonia will adsorb onto standard tygon® tubing as the gas travels from regulator to instrument. Even small lengths of tubing will be sufficient to adsorb most if not all the gas present.

So the calibration gas and regulator might be fine but the tubing is the problem creating failed calibration results.

Chemically Resistant Tubing

There are different types of tubing (such as Tygon® R-3603), which have improved chemical resistance properties. Many of these will be satisfactory for hydrogen sulphide or sulphur dioxide but again the specification need to be evaluated.

In general the “fluoropolymer” family is the best selection for corrosive gases with fluorinated polymers FEP or PTFE providing the ultimate in chemical resistance. Many of you know this tubing as “Teflon tubing”. There are many different versions of this tubing with varying permeation rates and flexibilities.

Tubing such as SE-200 is chemically resistant to most gases making it the correct choice for corrosive gas applications.   In this case it is a tube within a tube providing flexibility and chemical resistance.

Best Practice Tips

1. Use PTFE/FEP for all corrosive gases including H2S and SO2 so there is no confusion
2. Use PTFE/FEP for styrene
3. Use standard tygon® for non-corrosive gases