A Guide to Instrumentation
Battery-powered, direct reading instruments are classified as two groups - single-gas instruments or multiple-gas instruments - typically monitoring one or a combination of the following atmospheric conditions:
- Oxygen deficiency or enrichment
- The presence of combustible gas
- The presence of certain toxic gases
Depending upon the capability of the instrument, monitoring can be conducted simultaneously for oxygen and combustible gas, or for oxygen, combustible gas, and toxic gases.
No matter which type of instrument is used to check environmental gas concentrations, regular monitoring should be performed because a contaminant’s level of combustibility or toxicity might increase even if it initially appears to be low or non-existent. In addition, oxygen deficiency can occur unexpectedly.
To determine the composition of an atmosphere, reliable instruments should be used to draw air samples. If possible, do not open the entry portal to the confined space before this step has been completed. Sudden changes in atmospheric composition within the confined space could cause violent reactions, or dilute the contaminants in the confined space, giving a false low initial gas concentration.
When testing permit-required spaces for acceptable entry conditions, always test in the following order:
- Oxygen content
- Flammable gases and vapors
- Potential toxic air contaminants
In order for combustion to occur, there must be three elements:
- Oxygen to support combustion
- Heat or a source for ignition
This is known as the fire triangle, but if you remove any of the legs, combustion will not occur.
How Combustible Gas Monitors Work
To understand how portable combustible gas detection instruments work, it is first important to understand what is meant by the Lower Explosive Limit (LEL), and Upper Explosive Limit (UEL). When certain proportions of combustible vapors are mixed with air and a source of ignition is present, an explosion can occur. The range of concentrations over which this reaction can occur is called the explosive range. This range includes all concentrations in which a flash will occur or a flame will travel if the mixture is ignited. The lowest percentage at which this can happen is the LEL; the highest percentage is the UEL.
Single-Gas Monitors for Oxygen Deficiency
Oxygen indicators measure atmospheric concentrations of oxygen. Concentrations are generally measured over a range of 0 to 25 percent oxygen in air, with readings being displayed on either digital readout or an analog meter.
Single-Gas Monitoring for Combustible Gas
Single-gas instruments for monitoring combustible gases and vapors are generally calibrated on pentane and are designed for general-purpose monitoring of hydrocarbon vapors. Such instruments operate by the catalytic action of a heated platinum filament in contact with combustible gases. The filament is heated to operating temperature by an electric current. When the gas sample contacts the heated filament, combustion on its surface raises the temperature in proportion to the quantity of combustibles in the sample.
Multiple-Gas Monitors for Oxygen, Combustible and Toxic Gases
Toxic gases and vapors, which can be inhaled or absorbed through the skin, are frequently found in confined spaces. Sometimes, these atmospheric hazards can also displace oxygen and may incapacitate the body’s ability to maintain respiration. Some toxic gases and vapors can also cause long-term physical damage to the body in cases of repeated exposure.
Diffusion-type instruments are available for simultaneous measuring the LEL of combustible gases, oxygen levels, and toxic levels (in parts per million) of H2S, CO and other toxic gases. Alarms also alert the user to low and high oxygen levels. Remote sampling pump adapters are available to convert these diffusion-type into pump-style instruments.
Photoionization Devices for Toxic Gases and Vapors
A photoionization detector, featuring micro-processing technology, uses ultraviolet light to ionize molecules of chemical substances in a gaseous or vaporous state. A real-time digital readout allows the user to make an immediate determination of gas and vapor concentrations.
To ensure the accuracy of all monitoring and detection equipment, calibration should be performed regularly. If the instrument reading differs significantly from the values of the known standard, the instrument should not be used until it has been adjusted or, if necessary, repaired.
Questions about Instrumentation?
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