To select an appropriate calibration gas cylinder for gas detector, you must determine the type of gas, its concentration, and the cylinder material.
1.Select the cylinder body material based on the chemical properties of the gas.
The objective is to prevent reactions (such as adsorption or oxidation) between the inner wall of the cylinder and the gas.
For inert or non-adsorptive gases-such as O2, N2, CH4, etc.-carbon steel cylinders are used, as they are chemically stable and cost-effective.
For reactive or corrosive gases-such as CO, CO2, SO2, etc.-aluminum alloy cylinders are used; their smooth inner walls minimize adsorption.
For highly reactive or strongly corrosive gases-such as Cl2, H2S, NH3, NO2, etc.-carbon steel cylinders featuring an internal coating (e.g., Teflon or silicon) are used. For corrosive gases, the valve must also be constructed from stainless steel.
2.Determine the cylinder specifications (size) based on usage volume and application scenario.
Common cylinder volumes include 1L, 2L, 4L, 8L, 40L, etc.
For portable applications or temporary calibration tasks-where usage frequency is low-1L to 4L cylinders are recommended.
For stationary systems or applications requiring frequent calibration, cylinders with a capacity of 8L or larger are recommended.
3. Shelf Life of Common Gas Cylinders for gas detector
A. Stable gases: 1 year.
B. Low-concentration corrosive gases: Less than 6 months (e.g., Cl2, H2S, NH3, etc.).
C. Highly reactive gases (e.g., NO2, Cl2, etc.) have an even shorter shelf life.
4. How to Select the Appropriate Gas Concentration?
As a general rule, it is recommended to select a concentration between 50% and 80% of the instrument's full scale. For example, if the instrument's detection range is 0–100 ppm, select a calibration gas with a concentration of 50–80 ppm.
Avoid the Non-linear Zone: For most sensors, linearity tends to be poor within the 0–10% and 90–100% ranges of the full scale, which can lead to inaccurate calibration.
Ensure Practical Accuracy: Routine measurement values typically fall within the middle range of the scale; calibrating at this concentration ensures maximum accuracy for the values most frequently encountered during operation.
Maintain a Safety Margin: This ensures that the sensor will not be damaged by occasional measurements that exceed the full scale.
A.For environmental monitoring or occupational safety applications, it is recommended to select a concentration that is 2 to 5 times the alarm threshold or regulatory limit. This ensures accurate measurement specifically around the critical alarm threshold.
B.Combustible Gas Detection (LEL): Select a concentration of 50% LEL (approximately 2.5% vol methane) for calibrating sensors designed to detect gases such as methane, propane, or hydrogen.
C.Oxygen Detection: Use an oxygen concentration of 18% or 21% vol to calibrate sensors for detecting oxygen-enriched or oxygen-deficient environments.
5. Single-Point or Multi-Point Calibration (Considered for Specific Circumstances)
Typically, only a single calibration gas cylinder is required, with a selected concentration between 50% and 80% of the full scale. Multi-point calibration utilizes 2 to 3 cylinders containing calibration gases of different concentrations:
Low Concentration Point: 10–20% of the full scale (used to calibrate for zero-point drift).
Medium Concentration Point: 40–60% of the full scale.
High Concentration Point: 80–90% of the full scale.
Avoid a Common Misconception: Using a calibration gas with an excessively high concentration is not necessarily better; concentrations exceeding 90% may cause the sensor to enter its saturation or non-linear range, thereby compromising calibration accuracy.