Understand gas concentration units and sensor types

Instrument knowledge: Understand gas concentration units and sensor types

Instruments and sensors play a key role in various fields, especially in gas concentration measurement. Understanding the units of gas concentration and the different types of sensors is an important step in ensuring accurate measurements.

Gas concentration unit:

PPM (parts per million)

PPM is the volume concentration unit of gas, expressed in parts per million. It is commonly used in low concentration gas detection instruments.

mg/m³ (milligrams per cubic meter)

mg/m³ is the unit of mass concentration of gas, which indicates the mass of a certain gas in milligrams per cubic meter of space. Usually, there is a conversion formula between PPM and mg/m³, but the relative molecular weight of different gases is different.

%VOL (volume content occupancy)

%VOL also represents the volume content occupancy of gas, which is usually used to represent the gas content at high concentrations. 1%VOL is equal to 10,000 PPM.

%LEL (lower explosion limit)

%LEL represents the lower explosion limit unit of the gas. When the combustible gas concentration reaches 100% LEL, an explosion will occur when encountering an electric spark. Different flammable gases have different lower explosion limits.

Conversion relationship between PPM and mg/m³:

The conversion relationship between PPM and mg/m³ depends on the relative molecular weight of the gas. The general conversion formula is: PPM = mg/m³ × 22.4 ÷ gas relative molecular weight. NBchao Tips: Different gases have different relative molecular weights.

Three-wire and four-wire systems:

In instruments, there are usually two different signal output methods, namely three-wire system and four-wire system.

• Three-wire (3-wire): Usually used for instruments with 4-20mA signal output. It includes three wires: positive power supply, negative power supply (negative 4-20mA signal) and positive 4-20mA signal.

• Four-wire (4-wire): an instrument usually used for RS485 signal output. It includes four wires: positive power supply, negative power supply, positive RS485 signal and negative RS485 signal.

Both systems are equipped with a set of first-level alarm relay outputs as standard, and customers can also choose to equip a second set of second-level alarm relay outputs. These relay outputs are usually used to control field equipment such as fans and solenoid valves.

Types of sensors and their advantages and disadvantages:

Different types of gas sensors have different advantages and disadvantages in various applications. Here are some common sensor types:

Electrochemical sensors (liquid electrolyte)

The service life is generally 2-3 years. Fast response, high sensitivity, and good linear output. However, it needs to be used in an oxygen-containing environment and is easily disturbed by temperature, humidity, pressure and similar gases. Often used to detect low concentration gases.

Catalytic combustion sensor

The service life is generally 2-3 years. Only used to detect flammable gases and requires oxygen to function properly. Commonly used to detect the lower explosion limit (%LEL) of flammable gases.

Infrared sensor

The service life is generally 3-5 years. High precision, good stability and good repeatability. It is not affected by temperature and other gases, but needs to work in specific temperature, pressure and humidity environments.

Semiconductor sensor

The service life is generally 2-3 years. Fast response and sensitive