Calibrating a hydrogen analyzer is the core step in ensuring the accuracy of its measurement results, and the appropriate method should be selected based on the instrument type (such as thermal conductivity, electrochemical, gas chromatography, etc.) and application scenario (such as industrial processes, laboratory analysis, etc.). The following are common and critical calibration steps, common problems, and solutions to help the system complete the calibration work:
1、 Preparation before calibration
The preparation of specifications before calibration is the basis for avoiding errors, and the following aspects should be focused on:
Instrument and environmental inspection
Confirm that the instrument is in normal working condition: power on and preheat (according to the instructions, usually 30 minutes to 2 hours), check whether the power and gas connections are firm and leak free (you can apply soapy water to the interface and observe if there is any bubbling).
Environmental condition control: Temperature (generally 15-35 ℃, avoid severe fluctuations) and humidity (≤ 85% RH) should meet the requirements of the instrument, and be kept away from strong electromagnetic interference (such as large motors and frequency converters) and corrosive gases.
Selection of standard gases for calibration
Standard gases must meet the requirements of "quantity traceability": issued by qualified institutions, with valid certificates and clear concentration markings (such as hydrogen purity of 99.999%, H ₂ concentration in mixed gases of 5.00% ± 0.02%).
Concentration range matching: Calibration points should cover the commonly used measurement range of the instrument (such as instruments measuring 0-100% H ₂, standard gases of 0%, 20%, 50%, 80%, and 100% can be selected); A trace hydrogen analyzer requires the selection of low concentration standard gases, such as 10ppm and 50ppm.
Storage and use: The standard gas should be stored upright, avoiding direct sunlight. Before use, slowly open the valve (to prevent component adsorption caused by sudden pressure changes), discharge the residual gas in the pipeline (about 3-5 minutes), and ensure that the gas introduced into the instrument is the true concentration of the standard gas.
Auxiliary tools and record preparation
Prepare a high-precision flow controller (controlling the flow rate of standard gas, usually 0.5-2L/min, consistent with the working flow rate of the instrument), pressure gauge, timer, and calibration record sheet (including calibration date, instrument model, standard gas information, measured values, deviations, etc.).
2、 The core steps of calibration (taking the common "thermal conductivity hydrogen analyzer" as an example)
1. Zero point calibration (for "trace hydrogen analyzers" or scenarios where background errors need to be eliminated)
Applicable situation: When measuring low concentration hydrogen gas (such as ppm level), it is necessary to calibrate the zero point with "zero gas" (gases without hydrogen gas, such as high-purity nitrogen and argon, with a purity of ≥ 99.999%) to avoid background interference.
Operation steps:
Connect the zero gas to the instrument inlet through the gas path, adjust the flow rate to the specified value of the instrument (such as 1L/min), and steadily introduce it for 10-15 minutes (ensuring stable instrument readings).
Observe the displayed value of the instrument. If it does not show "0", enter the "Calibration Menu" of the instrument, select the "Zero Calibration" function, and set the current displayed value to "0" (some instruments require manual recording and calculation of deviation for manual correction).
2. Range calibration (key step to ensure accuracy of concentration measurement)
Operation steps (taking "multi-point calibration" as an example, which is better than single point calibration):
Introduce the selected standard gases in descending order of concentration (to avoid residual high concentration gases affecting low concentration measurements).
After each standard gas is introduced, maintain a stable flow rate and wait for the instrument reading to stabilize (usually 5-10 minutes, until the reading fluctuation within 30 seconds is ≤ the instrument accuracy requirement, such as ± 0.1% FS). Record the instrument display value (set as "measurement value A") and the actual concentration of the standard gas (set as "standard value B").
After completing all calibration point measurements, calculate the "deviation": deviation=(measured value A - standard value B)/standard value B × 100%. If the deviation is ≤ the allowable error of the instrument (such as ± 1% FS), the calibration is qualified; If it exceeds the tolerance, a "range correction" is required.
Range correction: Enter the instrument calibration menu, select "Range Calibration", enter the standard value B of each standard gas, and the instrument will automatically fit the calibration curve (some old-fashioned instruments require manual adjustment of the potentiometer to make the displayed value consistent with the standard value); After correction, it is necessary to re introduce the standard gas for verification to ensure that the deviation meets the requirements.
3. Repeatability and stability verification
After calibration, it is necessary to verify the repeatability of the instrument: repeat measurements of the same standard gas (such as intermediate concentration points) 3-5 times, calculate the relative standard deviation (RSD), and if RSD ≤ 2% (adjusted according to instrument accuracy), it indicates good stability of the instrument; If the repeatability is poor, it is necessary to check whether the gas path is leaking and whether the sensor is aging (such as contamination of the thermal conductivity cell filament and failure of the electrochemical sensor).
3、 Data processing and recording after calibration
Calibration result determination: If the deviation of all calibration points is within the technical specifications of the instrument (if the instrument accuracy is ± 0.2% FS, the deviation needs to be ≤± 0.2%), the calibration is judged to be qualified; Otherwise, it is necessary to recheck the standard gas and instrument status, and calibrate again.
Record and identification: Complete the calibration record form, including the calibrator, reviewer, deviation data, and whether it is qualified, etc; Paste the "calibrated qualified" label on the instrument, indicating the calibration date and the next calibration date (usually recommended to calibrate every 3-6 months, and can be calibrated monthly for frequent use or critical scenarios).