Common knowledge terms for combustible gas detectors
Parts Per Million (PPM) is a concentration measurement unit commonly used in the field of gas detection. For example, a mixture of air containing 1 PPM of hydrogen sulfide means that there is one unit volume of hydrogen sulfide per million units of gas volume.
The explosive limits are divided into lower explosive level and upper explosive level. The units of LEL and UEL are usually percentages, referring to the percentage of a certain gas contained in air (or oxidizer). In an environment below LEL, it cannot burn due to too little combustible gas. When the concentration of combustible gas in the environment is higher than UEL, it cannot burn due to too much gas. The LEL and UEL values of various combustible gases can be obtained from relevant data.
The Threshold Limit Values (TLV) indicate that when the concentration of a certain gas in the air is below this threshold, the health of workers who are fully and continuously exposed to that environment will not be harmed. When referring to this value, it must be based on the standards issued by the country, and the latest revised value should be used. TVL includes the following two parts:
The average threshold (TLV-TWA) represents the time weighted average concentration value in the environment. The vast majority of workers working in this environment on an 8-hour daily and 40 hour weekly schedule will not have any health issues.
The instantaneous threshold (TLV-STEL) is defined as a weighted average of 15 minutes, where the concentration of a harmful gas in the workplace must not exceed its specified threshold at any time during a working day, even if the total weighted average reaches the average threshold. The number of times in a day that the average threshold is exceeded but below the instantaneous threshold must not exceed 4 times, and the duration of each time must be less than 15 minutes.
Dangerous concentration (IDLH): If workers do not wear gas masks or lack evacuation experience, and the gas concentration in the work environment reaches a dangerous concentration, a 30 minute stay can cause permanent damage to the human body or weaken the health level of the human body (such as reduced vision).
The RS485 serial bus specifies a dual ended electrical interface form, and its standard is to transmit signals through dual ended wires. If one of the lines is in logic 1 state, the other is in logic 0 state. Due to the bidirectional differential nature of the voltage circuit, common mode interference in the transmission circuit can be suppressed, greatly improving communication performance.
The concentration range between the lower and upper explosive limits of combustible gases in a mixture of combustible gases and air is called the explosion range.
