The calibration of formaldehyde online monitoring equipment is a key step in ensuring the accuracy and reliability of monitoring data. The calibration process usually includes preparation before calibration, calibration operation, analysis and processing of calibration results, etc. The specific methods and precautions are as follows:
1、 Preparation before calibration
1. Equipment inspection
Confirm that the monitoring equipment is operating normally and there are no fault alarms (such as sensor contamination, circuit abnormalities, etc.).
Check whether the sampling pipeline is unobstructed and leak free, and avoid external interference (such as dust, water vapor) affecting the calibration results.
Ensure stable power supply to the equipment and that environmental parameters such as temperature and humidity meet calibration requirements (usually conducted in a constant temperature and humidity environment).
2. Preparation of standard materials
Standard gas: Use nationally recognized formaldehyde standard gas (such as concentration of 1ppm, 2ppm, etc.), and pay attention to the validity period and uncertainty of the standard gas (usually relative expanded uncertainty ≤ 5%).
Liquid standard solution: If the equipment supports liquid calibration (such as phenol reagent method), a known concentration of formaldehyde standard solution (such as 1mg/L, 10mg/L) needs to be prepared accurately using a volumetric flask and prepared for immediate use.
3. Preparation of calibration tools
Calibration instruments: such as gas dilution devices (for diluting standard gases), flow meters (for calibrating sampling flow rates), and temperature and humidity meters (for monitoring environmental parameters).
Protective equipment: Wear protective gloves and masks during the calibration process to avoid contact with the skin or inhalation of formaldehyde.
2、 Calibration operation steps
1. Zero point calibration (blank calibration)
Purpose: To eliminate equipment baseline drift and environmental background interference (such as trace amounts of formaldehyde in the air).
Method:
Introduce zero gas (high-purity nitrogen or clean air filtered by activated carbon) for 510 minutes. After the equipment reading stabilizes, set the zero point (the reading should be close to 0mg/m ³).
If the zero drift exceeds the allowable range of the equipment (such as ± 0.01mg/m ³), it is necessary to check whether the pipeline is contaminated or whether the sensor is aging.
2. Span calibration (range calibration)
Purpose: To verify the measurement accuracy of the device within the target range.
Method:
Gas calibration method:
1. Select the appropriate concentration of standard gas based on the device range (such as 0.5mg/m ³ standard gas when the range is 01mg/m ³).
2. Use a gas dilution device to introduce standard gas into the sampling port of the monitoring equipment, and control the flow rate within the recommended range of the equipment (such as 0.5L/min).
3. After the reading stabilizes (usually taking 35 minutes), record the deviation between the device display value and the standard gas concentration.
If the deviation exceeds the allowable error (such as ± 5%), the span calibration coefficient of the equipment needs to be adjusted (through the built-in software or button operation of the equipment) until the reading is consistent with the standard value.
Liquid calibration method (applicable to equipment such as spectrophotometry):
1. Inject a known concentration of formaldehyde standard solution into the reaction tank of the equipment and start the calibration procedure.
2. The device automatically performs color reaction and measures absorbance, and calculates the concentration value based on the standard curve.
If the deviation between the measured value and the standard value exceeds the allowable range, it is necessary to redraw the standard curve or adjust the equipment parameters.
3. Multi point calibration (optional)
For high-precision monitoring requirements (such as laboratory or scientific research scenarios), multi-point calibration (such as 0.1mg/m ³, 0.5mg/m ³, 1.0mg/m ³) is required to ensure accuracy over the full range.
The calibration steps are similar to span calibration, in which standard gas is introduced in order of concentration from low to high, and the calibration coefficients of each point are adjusted separately.
3、 Analysis and processing of calibration results
1. Recording and archiving
During the calibration process, the following information needs to be recorded:
Calibration date, calibration personnel, equipment model and number;
The concentration, uncertainty, and expiration date of standard substances;
Zero and span readings before and after calibration, adjustment factors;
Parameters such as environmental temperature and humidity, atmospheric pressure, etc.
Calibration records need to be archived and saved as the basis for equipment metrology certification and data traceability.
2. Calibration cycle
The regular calibration cycle is generally 36 months, but it needs to be adjusted according to the frequency of equipment use and environmental conditions (such as high humidity and high pollution environments, which need to be shortened to 13 months).
Special circumstances: Calibration should be performed immediately after equipment maintenance, sensor replacement, or abnormal fluctuations in monitoring data.