The Weight of Gases: Important Considerations in Gas Detection

Types of Gas Detectors

There are two primary kinds of gas detectors: fixed gas detectors, which are intended for permanent continuous monitoring in work environments, and portable gas detectors, which are designed to be carried or worn by workers as they move about. In this article, we will discuss the importance of considering the weight of gases to be detected when deciding where to install a fixed gas detector.

First things first, we would like to clarify what we mean by fixed gas detector. By fixed gas detector, we mean any gas detection equipment that is installed permanently at a specific location in the work environment for the purpose of continuously monitoring gas levels in the air. For example, in a plant which uses propane gas as a fuel, it is vital to monitor the presence of any gas leaks from the distribution pipes. The fixed gas detector detects the presence of a gas via a built-in sensor, which is used in conjunction with an indicator/alarm unit and external buzzer to ensure worker safety. Gas comes into contact with the sensors in one of two ways, either by diffusion or by suction. Diffusion-type gas detectors detect gas that comes into contact with the sensor as it spreads throughout the air. Suction-type gas detectors, on the other hand, use a built-in pump to draw in the surrounding air, allowing them to detect the presence of gas further away from the detector.

What is the Specific Gravity of a gas?

When choosing the right gas detector for your needs, it is important to consider factors such as the properties and location of the gas(es) you want to detect, as well as their concentration and the location where the gas detector will be installed. It is essential to select the right gas detector and the appropriate location, and this cannot be done without considering the weight of the gas(es) to be detected. In the same way that ice placed in a glass of water will float rather than sink, some gases will float to the top of a room when released into the air. This occurs because each type of gas has a different weight.

To better understand the weight of gases, a concept known as specific gravity is used. Specific gravity indicates whether a gas is heavier or lighter than air. Air is given a specific gravity of one, and other gases are given a numeric specific gravity relative to air. In other words, if the specific gravity of a gas is greater than one it is heavier than air, and if it is less than one it is lighter than air. Gases with a specific gravity greater than one tend to settle and accumulate near the bottom of a space, whereas those with a specific gravity less than one tend to float and accumulate toward the top of a space.

Atomic Weight and Molecular Weight

So how do we determine the specific gravity of a gas? First, we must calculate the weight of a gas. The weight of a gas is determined by the molecular weight of the molecules from which it is formed. A gas is made up of particles known as molecules, which are groups of two or more atoms bonded together. Each type of atom has its own weight, and the weight of a molecule is in turn determined by the type and number of each atom it contains. For example, the atomic weight of oxygen is 16, and one molecule of oxygen is made up of 2 oxygen atoms, making the molecular weight of oxygen 16×2＝32. In the same way, the atomic weight of nitrogen is 14 and there are two nitrogen atoms in one molecule of nitrogen. Therefore, the molecular weight of nitrogen is 14×2＝28.

What, then, is the molecular weight of air? Air is composed primarily of nitrogen and oxygen at a ratio of approximately 8:2. Therefore, the molecular weight of air can be calculated as follows: 28×0.8＋32×0.2＝28.8.

We will now discuss how to calculate the specific gravity of a gas. The specific gravity of a gas is calculated by comparing its molecular weight to that of a reference gas (typically air). For example, to calculate the specific gravity of hydrogen gas, we divide the molecular weight of hydrogen gas (2) by that of air (28.8) for a specific gravity of 0.07.

Specific gravity is not affected by changes in temperature or pressure. In an environment at 0℃ and an atmospheric pressure of 1, we know that the number of molecules in 22.4 liters of volume will always remain constant at 6.02×1023. This amount is referred to as one mole. When changes in temperature or atmospheric pressure occur, the number of molecules which occupy a given volume increase or decrease relative to the change in temperature or pressure, regardless of the type of molecule. Because the number of molecules occupying the same volume in the same environment will always remain constant no matter what kind of gas is present, we know that the specific gravity is dependent solely upon differences in molecular weight.

Detecting Lighter Gases

Some examples of gases whose specific gravity is less than one include hydrogen (0.07), ammonia (0.59), and methane (0.55). Because these gases tend to accumulate near the ceiling, they typically require placement of a gas detector closer to the ceiling. This does not mean, however, that placement near the ceiling is always ideal for detecting such gases. For example, if a ceiling is not within reach, installing a gas detector on the ceiling would be inconvenient from a maintenance standpoint. Diffusion- and suction-type detectors should be selected based on the gas being detected and the location where it will be installed. Below is an image which illustrates gas detection by suction using a pump or aspirator.

Detecting Heavier Gases

Some gases whose specific gravity is greater than one include propane (1.6), benzene (2.69), hydrogen sulfide (1.19), and carbon dioxide (1.52). Because these gases tend to accumulate near the ground, they are typically best detected when the gas detector is placed closer to the floor. As mentioned above regarding lighter gases, diffusion- and suction-type detectors should be selected based on the gas being detected and the location where it will be installed. Below is an image which illustrates detection by diffusion near the ground.

As you can see, gases, like liquids and solids, also have weight, and this property must be taken into consideration when deciding where to install a fixed gas detector.

Our Solutions

Below you will find some of our leading fixed gas detectors, organized by suction- or diffusion-type.

Suction Type

・With built-in pump

➡D58 Series, GD-70D

D58 Series: These fixed gas detectors feature an explosion-proof construction and large flow rate pump built in.

GD-70D: This lineup of smart fixed gas detectors can detect over 260 different gases. Ideal for non-explosion proof areas such as research laboratories or cleanrooms.

SD-D58, click here GD-D58,click here GD-70D,click here

・For use with an external pump or aspirator unit

➡SD-3 DRI/RP-D58

SD-3 Diffusion-type gas detectors meet various international standards (including proposed standards) and can be used together with the RP-D58 explosion-proof pump unit or an aspirator unit as a sampling system. Perfect for use in a wide range of applications, even for monitoring under strict conditions (desired standards, environmental conditions, etc.) or in elevated spaces.

SD-3 DRI, click here RP-D58, click here

Diffusion Type

・SD-3 Series

SD-3 series fixed gas detectors feature rugged housing and explosion-proof construction suited for even the harshest environments and meet various international standards (including proposed standards) for use worldwide.

SD-3 series, click here

・600 Series (EC-600、RI-600、OX-600)

This lineup of fixed single-gas detectors offers three power supply options: AC, DC and dry battery. The dry battery type in particular does not require installation work, translating into reduced initial costs. Can be used together with a remote sensor (sold separately) which allows for placing the sensor at a distance of up to 20 meters.

EC-600, click here RI-600, click here OX-600, click here

Here at Riken Keiki, we offer a wide range of other products adapted to meet your specific needs and placement location. When choosing which fixed gas detector to install and where to install it, each situation is uniquely different, so please do not hesitate to contact us if you have any questions or concerns.

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