Calibration of ammonia gas detectors is a core step in ensuring their measurement accuracy and alarm reliability. However, ammonia's strong adsorption properties, "memory effect" on sensors, and corrosiveness to pipelines make its calibration process far more complex than that of conventional gas detection. For EHS managers, metrology engineers, and third-party calibration organizations in the international market, establishing a rigorous and traceable calibration standard is fundamental to ensuring the safety of on-site personnel and meeting compliance audit requirements.

　　Calibration Cycle and Applicable Standards

　　The calibration cycle for ammonia gas detectors needs to be set differently depending on the equipment type and usage scenario. The periodic functional testing cycle for stationary ammonia gas detectors is typically no more than 6 months, and for portable devices, no more than 3 months. Formal metrological verification or calibration, whether portable or stationary, is once a year. Recalibration must be performed immediately in the following situations: after instrument repair or sensor replacement, after a change in installation location, after a periodic functional test fails, and when there is doubt about the measurement data.

　　Preparation Before Calibration

　　Adequate preparation is a prerequisite for successful calibration. First, the compliance of the standard gas must be confirmed: a certified ammonia standard gas with air as the base gas should be used, because electrochemical ammonia sensors require oxygen to participate in the reaction, and a nitrogen base gas will cause the sensor response to be low and the calibration results to be distorted. The standard gas concentration should cover the commonly used detection range, with a common reference concentration of 50 ppm. Standard gases at concentrations of 20%, 50%, and 80% of full scale should also be prepared for indication error testing. The standard gas must be within its validity period and have complete traceability information.

　　Second, the choice of tubing material is crucial. Because ammonia reacts with rubber and ordinary plastics, calibration tubing must use PTFE or stainless steel tubing; rubber tubing is strictly prohibited. Furthermore, the instrument must be fully warmed up before calibration—portable devices should run for at least 1 hour, and stationary devices for even longer—to ensure the sensor reaches a stable operating state.

　　Standard Calibration Procedure: Five Steps

　　A proper ammonia gas detectors calibration should include the following five core steps:

　　Step 1: Zero-point calibration. In a clean air environment, introduce zero-point gas and confirm the zero-point indication after the reading stabilizes. It is particularly important to note that ammonia sensors exhibit a "memory effect" after a high-concentration shock; even with clean air, the reading may not return to zero for an extended period. Therefore, the sensor must be thoroughly purged with zero-point gas before calibration until the reading is completely stable; otherwise, a "false zero" will cause a shift across the entire measurement range.

　　Step Two: Indication Error Test. Introduce ammonia standard gas at three concentrations: 20%, 50%, and 80% of full scale. Record the readings after the indications stabilize at the specified flow rate. Repeat the measurement three times at each concentration point, taking the average as the instrument reading for that point, and then compare it with the standard gas value to calculate the indication error.

　　Step Three: Repeatability Test. Introduce standard gas at approximately 50% of full scale. Record the readings after the indications stabilize. Repeat the measurement six times and calculate the relative standard deviation of each measurement to evaluate the instrument's repeatability.

　　Step Four: Response Time Test. First, zero the instrument using zero-point gas. Then, introduce standard gas at 50% of full scale while simultaneously starting a timer. Record the time required for the indicated value to rise to 90% of its stable value from the moment the standard gas is introduced. Repeat this process three times and take the average as the instrument's response time.

　　Fifth Step: Alarm Function Verification. Introduce standard gas at a concentration higher than the alarm setpoint to confirm that the instrument's audible and visual alarm functions trigger normally, and record the alarm action value. Repeat the measurement three times and take the arithmetic mean as the final alarm action value.

　　Calibration Records and Compliance Documents

　　Complete calibration records are crucial evidence for compliance audits. Calibration reports must include the following elements: the model and serial number of the instrument being calibrated, the concentration and batch number of the standard gas used for calibration, the temperature and humidity of the calibration environment, the indicated values at each calibration point (comparison before and after calibration), the response time, repeatability results, alarm function verification results, and the final pass/fail conclusion. All data must be traceable to the standard substance, and both the calibrator and the verifier must sign for confirmation. Calibrations that only issue a "certificate of conformity" without any data records will be considered invalid during an audit.

　　Special Considerations for Fixed Equipment

　　For fixed ammonia gas detectors installed in workshops, cold storage facilities, or outdoors, on-site calibration is strongly recommended over disassembly and reassembly. Disassembly and reassembly may damage the equipment or alter the installation angle, and differences between laboratory and on-site conditions can lead to decreased accuracy after calibration. Furthermore, process operators must be notified before calibration, and bypass interlocks or DCS settings should be adjusted if necessary to prevent accidental triggering of interlocks during calibration. When the site is a hazardous area, calibration personnel must wear appropriate personal protective equipment, and exhaust gases must be properly treated; direct discharge in enclosed spaces is strictly prohibited.

　　In summary, ammonia gas detector calibration is a highly specialized task. Its core elements can be summarized as follows: using air as a baseline gas, employing PTFE or stainless steel piping, thorough purging to eliminate memory effects, performing a complete multi-point calibration procedure, and issuing traceable and compliant documentation. Only by strictly adhering to these standards can we ensure that every ammonia gas detector provides accurate and reliable data at critical moments, providing a solid guarantee for personnel safety and compliant operation.