How should the range and setpoint of the NH3 hazardous gas detector be determined appropriately?

The monitoring of ammonia (NH3) and the setting of alarm values generally need to be determined in accordance with local occupational safety and health regulations and standards, as well as the specific working environment and conditions. The following is some reference information to help you set appropriate range values and alarm thresholds: 1. Unit conversion: Gas concentration is usually expressed in two ways, mg/m³ (mass concentration) and ppm (volume concentration). The specific conversion formula is as follows: ppm = (mg/m³) × (24.45) / (molecular weight). For NH3, whose molecular weight is 17.03 g/mol, the conversion formula is: ppm = (mg/m³) × (24.45 / 17.03). For example, if a specific ammonia concentration limit specified in the GB/T 50493-2019 standard is 25 mg/m³, then the equivalent value in ppm is: 25 mg/m³ ≈ (25 × 24.45 / 17.03) ppm ≈ 35.9 ppm. 2. Setting of alarm values: Based on the screenshot you mentioned, if only the PC-TWA (Time-Weighted Average) and PC-STEL (Short-Term Exposure Limit) values are provided, you can use these values to set the alarm thresholds. Normally: - Alarm 1 (low-level alarm) can be set at a value close to or slightly below the PC-TWA value. - Alarm 2 (advanced alarm) can be set to a value close to or slightly below PC-STEL. When setting the alarm value, a certain safety margin can also be considered. For example, if the PC-TWA is 25 ppm, you might set Alarm 1 at 20 ppm to provide an early warning before the TWA is reached, giving workers time to evacuate or take action. Similarly, if PC-STEL is 50 ppm, Alarm 2 can be set at 40 ppm or 45 ppm to ensure that workers are warned before reaching the short-term exposure limit. Keep in mind that what is provided here is merely a reference; the specific alarm thresholds need to be determined based on the actual application environment, local laws and regulations, as well as the company’s internal security standards. Furthermore, regular calibration and maintenance are also important aspects in ensuring that testing instruments are accurate and reliable. In any case, the most important thing is to ensure the safety and health of employees; therefore, please be particularly careful when setting any safety-related parameters. .

Converting mass from mg/m3 to volume gives ppm; the part you highlighted in the second question explains this very clearly

Take NH3 as an example; for the alarm level 1, the value of 100% OEL is used, but there are also two other values: PC-STEL and PC-TWA. Thus, we have two values: 20 and 30. Then, for Alarm 2, there are also two values: 40 and 60. That’s why there is this difficult problem

Select values in the order given in the table; if a preceding value is available, the subsequent ones are not used.

The first threshold is easy to determine, but there’s a problem with the second value: 200% of 20 is 40, and there’s also 30 later on. Should I choose the two smaller values or just go with 40? I’m not quite sure