## 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 the measurement of gas concentrations. Knowing the units of gas concentration as well as the different types of sensors is an important step in ensuring accurate measurements.

Gas concentration unit:

Parts per million (PPM)

PPM is a unit of volumetric concentration of a gas, representing parts per million by volume. It is often used in low concentration gas detection instruments.

mg/m³ (mg per cubic meter)

mg/m³ is a unit of mass concentration of a gas, indicating how many milligrams of a gas are contained per cubic meter of space. In general, there is a conversion formula between PPM and mg/m³, but the relative molecular weight of different gases varies.

Understand gas concentration units and sensor types

%VOL (Volume share)

%VOL also indicates the volume content share of the gas, and is usually used to indicate the gas content at high concentrations. 1%VOL is equal to 10,000 PPM.

%LEL (Lower explosive Limit)

%LEL indicates the lower explosive limit unit of a gas. When the concentration of combustible gas reaches 100%LEL, it will explode when it encounters an electric spark. The lower explosion limits of different combustible gases are different.

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 ÷ relative molecular weight of gas. It is important to note that different gases have different relative molecular weights.

Understand gas concentration units and sensor types

Three wire system and four wire system:

In the instrument, there are usually two different signal output modes, namely three wire system and four wire system.

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

Four-wire system (4-wire) : Usually used for RS485 signal output instruments. It consists of four wires: positive power supply, negative power supply, RS485 signal and RS485 signal negative.

Both systems come standard with a set of primary alarm relay outputs, and customers can optionally have a second set of secondary alarm relay outputs, which are typically used to control field equipment such as fans and solenoid valves.

Understand gas concentration units and sensor types

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

Electrochemical sensor (Liquid electrolyte)

The service life is generally 2-3 years. Fast response speed, high sensitivity, good linear output. However, it needs to be used in an environment containing oxygen and is susceptible to interference from temperature, humidity, pressure and similar gases. It is often used to detect low concentration gases.

Catalytic combustion sensor

The service life is generally 2-3 years. It is only used to detect flammable gases and requires oxygen to work properly. It is commonly used to detect the lower explosive limit (%LEL) of combustible gases.

Infrared sensor

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

Understand gas concentration units and sensor types

Semiconductor sensor

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