Direct-reading atmospheric monitoring equipment may be divided into two broad categories: electronic and color indicating. Electronic instruments may further be divided by display type, either digital or analog, and by the type of sensor used.
Electronic instruments
Electronic instruments are available with a wide variety of sensors. This discussion will focus on several of the more common types used in confined space entry operations.Oxygen sensors are a special type of electrochemical sensor that provide direct percentage readings. Flammable sensors are most commonly catalytic bead type, or a less common flammable sensor is the infrared variety. Both provide percent of lower explosive limit (LEL) readings. The advantage of the infrared sensor is that it is not oxygen dependent. Toxic sensors in multi-gas instruments are typically electrochemical sensors. There are about a dozen commonly available toxic sensors. Two of the most commonly used in confined space work are carbon monoxide and hydrogen sulfide.
Instruments for confined space work typically have a four-sensor capability, with five-gas instruments becoming more common.
Dealing with VOCs
In work environments where volatile organic compounds (VOCs) are likely to be present in confined spaces, a photo ionization detector (PID) is another type of useful instrument. These units provide ppm readings of materials such as solvent vapors that will be hazardous to personnel from an exposure perspective below the level at which they would pose a flammable hazard. The instrument uses an ultraviolet lamp to ionize the gas or vapor within the device. These instruments are very sensitive and quite accurate, even at low concentrations. The disadvantage is that they are not selective. Your judgment will be critical in interpreting the readings as the instrument “sees†all VOCs with an ionization potential (IP) below the level of the lamp energy. 10.6eV is the most commonly used lamp. Lamps are available in several energies ranging from 9.2 to 11.7.If you need to monitor VOCs with higher IPs such as chlorinated solvents then a flame ionization detector (FID) may be needed. A hydrogen flame is the ionization method in these instruments. This hydrogen flame operates at approximately 15.4eV, so it will ionize materials that the PID would not.
A detailed discussion of colorimetric tubes is beyond the scope of this article. Although used for substances that cannot be detected with electronic instruments, they are a tool that may be required for evaluating the atmosphere within the confined space.
Testing the tools
Once you have selected the appropriate instruments you need to confirm that they are functioning properly. Each day the instrument will be used it should be zeroed and bump tested. Zeroing must be done in fresh air to ensure that the values reset to normal will in fact be accurate. Bump or function testing confirms that the sensors, display and alarms on the unit will operate when exposed to materials they are designed to detect. Bump test gas or calibration gas may be used for this test. This is not calibration, just a test of the basic operation of the instrument.Calibration must also be done regularly. Calibration confirms or resets the accuracy of the instrument by matching the instrument reading to a known quantity. At a minimum, the interval recommended by the manufacturer must be used. Anytime you cannot explain the readings you get or become uncomfortable with the readings, calibration is the first step in troubleshooting the instrument.
Calibration gas expires, so make certain that you have current gas available. For the common confined space configuration of oxygen, flammable, carbon monoxide and hydrogen sulfide, a single cylinder that contains all of these gases is available.
Don’t be fooled
Effective atmospheric monitoring is critical to confined space safety. Careful attention must be paid to selecting an appropriate strategy, using the correct equipment, making certain that equipment is functioning correctly and interpreting the results of testing.
Modern instruments have made the mechanics of monitoring very easy. Do not be lulled into a false sense of security by this ease of use. We must still exercise judgment to ensure that the work environment is safe for the crew.
SIDEBAR: Testing: Is the space vertical or horizontal?
The process of testing a confined space will be affected by the configuration of the space. For both vertical and horizontal spaces, if any opening into the space exists other than the entry hatch, make your initial readings through this opening before the hatch is removed. This will provide early warning if an extreme atmospheric condition exists in the space. If this is not possible, then conduct your first test immediately after the entry point opening has been opened slightly and before it is completely removed.For vertical entry spaces, the remote sampling tube needs to be lowered slowly through the entire depth of the space. Conditions may vary within the space, and it is critical that the entire depth be checked. Pay particular attention to low spots such as sump pits.
For horizontal entry spaces, extending your reach into the space without entering is the important issue. Most manufacturers offer probes of various lengths as an accessory. These are often relatively short. Duct taping the tubing to the end of a telescoping painter’s pole can allow extended reach into the space.
For atmospheric monitoring during the entry, two options are available. The end of the remote sampling tube may be placed near the work area and the instrument left with the attendant, or the instrument may be taken into the space by the entry crew.
Anytime the space is left unattended the atmosphere should be checked as it was for the initial entry.
