BY CHRISTOPHER WRENN AND MIKE SEGARS
A gas detector is a “flashlight” that lets people “see” gases they would not normally be able to see.
In recent years, the fire service has increasingly relied on engine companies to respond to carbon monoxide (CO) detection calls. The number of CO calls has outstripped the ability of haz-mat teams (the traditional responders) to respond to such calls. Because of the increased emphasis on energy-efficient building construction, homes and offices are now more airtight, which has increased the potential for the buildup of CO, a by-product of incomplete combustion. To handle the increased workload, engine companies now often respond to CO calls.
However, unlike haz-mat teams, engine company firefighters typically lack extensive gas detection training and experience, including gas detector maintenance. If gas detectors are not properly maintained and calibrated, their readings may be unreliable. But by using two verification methods, calibration and CO detector tubes, engine companies can quickly and easily increase and maintain CO detector reliability.
LIKE A FLASHLIGHT
A gas detector is a “flashlight” that lets people “see” gases they would not normally be able to see. If using a flashlight to see in a dark basement, it is advisable to confirm that it works before going down the basement steps. The same is true for CO detectors, which are designed to help users “see” a toxic gas that is normally not visible. Like a flashlight, gas detectors need to be checked (i.e., calibrated) to confirm proper operation before use.
When a CO sensor fails, it may register zero and thus provide a false safe reading. The only way to verify that a CO sensor is working is to calibrate it using a known concentration of CO. Often, CO detectors may sit on an apparatus for a long time without ever being used or calibrated. During the heating season, when CO calls increase, the neglected CO detector may not provide reliable results.
CALIBRATING A CO DETECTOR
Some firefighters will put a CO detector in the exhaust pipe of a car or truck to see if it works. This causes problems for two reasons. First, the catalytic converters on new cars have drastically reduced the amount of CO present. Second, other products of combustion (e.g., water vapor, hydrocarbons, and particulates) can clog sensor filters and membranes, thus permanently disabling the CO sensor. Never use vehicle exhaust to test a CO sensor!
For the best service and reliable readings, CO sensors should be regularly calibrated with a known calibration gas throughout the year, especially at the beginning of the heating season, and once a month thereafter. Calibration gas is “confidence in a can.” If the CO detector is giving erroneous readings, using the calibration gas can help to verify that it is performing properly. The calibration gas should be carried on the apparatus along with the detector. If a user gets an unusual reading at a CO call, the detector’s accuracy can be quickly field-verified by exposing the detector to a known concentration of CO. This is easy to do because CO calibration gas is cheap and stable.
FOUR CALIBRATION METHODS
Bump: The detector is exposed to CO gas; and the user confirms that the sensor responds.
Field verification: The detector is exposed to the CO gas; the user verifies that the detector’s display is within ±10 percent of the value indicated on the calibration gas cylinder after the display has stabilized. If the cylinder contains 50 ppm of CO, the detector should display between 45 and 55 ppm. If the detector is outside of this range, the user should perform a field calibration.
Field calibration: This takes field verification to the next step. The detector is exposed to CO gas. If the reading is not within 10 percent of the value on the calibration gas cylinder, the detector is adjusted accordingly. In older detectors, this was accomplished with dials or a screwdriver. Newer detectors use a microprocessor to perform this function automatically, and field calibration is typically accessed using a special series of keystrokes.
Factory calibration: The user must return the detector to the factory for calibration. Typically, this is required only on older detectors. When purchasing CO detectors, make sure that factory calibration is not required. Also, determine that all calibration and maintenance are simple and can be performed by a qualified person in the department. Otherwise, you may need extra detectors so there is still one available to use while others are at the factory calibration center.
USING A CO DETECTOR
Using gas detectors requires special skills and calibration to provide the best results. Users must have some knowledge of gas behavior. For example, if an engine company gets a CO call and opens doors and windows on arrival to ventilate the structure, it will be difficult or impossible to track the source of the CO. Also, some common gases and vapors can provide false readings on a CO detector. Calibration, training, and verification techniques will virtually eliminate these false alarms.
CO Tubes
The CO tube is one of the oldest and most reliable measurement techniques. A known volume of gas is drawn through a glass tube filled with silica substrate that has been treated with a chemical that changes color when exposed to CO. The CO concentration is read on the tube in the same way as you read the temperature on a glass thermometer. Some dismiss the use of tubes because they are perceived as “old-fashioned” and “low-tech.” But “low-tech” is an advantage for engine companies. CO tubes are factory-calibrated and, if properly stored, will last two years and will need no user calibration. Tube pumps sell for as little as $165; a box of 10 tubes sells for as little as $25. This means that an engine company can easily be outfitted for less than $200 for two years.
CO tubes have some disadvantages. The tubes are not continuous monitors; they provide only “snapshots,” so a new tube is needed to sample every room or area suspected of having CO in it. Tubes are used once and then thrown away.
Unlike CO detectors, CO tubes do not provide instaneous measurements; it can take from one to five minutes for a reading. Bellows-style tube pumps are slower and can introduce significant user error if the bellows are not squeezed properly. It is best for engine companies to consider the simpler and more rugged syringe-style pumps. Metal syringe-style pumps are virtually indestructible.
Continuous-Measurement CO Monitors
Continuous-measurement CO detectors use an electrochemical CO sensor and provide highly accurate continuous measurements and a quick response in approximately 20 seconds. The quick response allows a user to move from room to room using the CO detector like a Geiger counter to locate the CO source.
CO detectors may include various features such as peak hold, short-term exposure limit (STEL), and time-weighted average (TWA). However, the STEL and TWA values are Occupational Safety and Health Administration average alarms industrial hygienists use and typically are not useful to engine companies, which are usually interested in the instantaneous reading and the peak reading only.
When purchasing CO detectors, choose one with an easy-to-read display. When outfitting an engine company with CO detectors, put CO calibration gas on the apparatus also. This allows users to calibrate the CO detector monthly and verify the detector’s performance on-scene.
CO DETECTION SCENARIOS
Printed circuit board plant. A portable CO detector registered 60 ppm in the cafeteria of a printed circuit board plant with no obvious cause. When the detector was taken outside the plant, the CO reading dropped to zero, and a fresh air calibration was performed. On reentering the plant, the CO detector again showed a reading of 60 ppm.
The detector was taken outside again and exposed to 50 ppm of CO calibration gas. The meter read 47 ppm (a field verification), which showed that the device was working properly. For good measure, it was recalibrated (a field calibration). However, there was no obvious source of CO, so personnel employed a CO tube, which read approximately 50 ppm CO. With both the electrochemical CO sensor and the CO tube reading positively for approximately the same concentration of CO, it was safe to assume that CO was present. Using the CO detector like a Geiger counter, personnel discovered the source was a heat-shrink packaging machine that was producing 150 ppm of CO in the operator’s breathing zone. The company immediately had the machine tuned up.
Food warehouse. In a food warehouse, a portable CO detector showed a CO concentration of 80 ppm. The facility, however, used battery-powered forklifts. Propane-fueled forklifts are a common source of CO in warehouses, which ruled them out as a possible source. Warehouse personnel pointed out that the CO measurement registered in an office in the battery charging room. Without an immediate answer to the CO levels, a CO tube was used to verify the CO concentration, but it did not register any CO. Lead-acid batteries generate hydrogen gas while charging. The CO detector manufacturer’s cross-sensitivity table showed that 80 ppm on the CO sensor means that there was approximately 200 ppm of hydrogen present, well below its lower explosive limit (LEL) of four percent (40,000 ppm).
Household CO alarm. A home CO alarm activated, and a haz-mat squad responded. The haz mat squad’s CO detector indicated no CO was present. The CO tube used didn’t indicate the presence of CO. However, the house smelled strongly of lacquer paint fumes. Home CO detectors have a number of cross-sensitivities and are commonly cross-sensitive to paint fumes. The haz-mat squad used a photo ionization detector (PID) to confirm the presence of volatile organic compounds (VOCs), and the house was ventilated.
CHOOSING A DETECTION METHOD
Determining which method may be more appropriate depends on the number of CO calls your department makes, the total cost of the detection method used, and the cost per measurement. Additionally, consider the department’s technical needs compared with the advantages and disadvantages of each individual method.
CO detectors provide very quick response, but they tend to be more costly and require more maintenance and calibration than CO tubes. Tubes are inexpensive and easy to use but have a higher cost per use and don’t provide continuous readings. However, CO detectors and CO tubes represent two drastically different techniques to reach the same goal of measuring CO. Used together, they complement each other’s effect and can provide greater confidence in CO measurement and prevent false alarms. For example, if a homeowner suspects that CO is present but the detector’s digital display reads zero, then a CO detector tube can be used to verify the reading. If both the detector and tube read zero, it is extremely unlikely that CO is present.
References
- RAE Systems: Sensor Specifications and Cross-Sensitivities (Technical Note TN-114)
- RAE Systems: Handbook of Gas Detection Tubes & Sampling Pumps (Second Edition 2001)
CHRISTOPHER WRENN is the product applications manager for RAE Systems.
MIKE SEGARS is the tube products manager for RAE Systems and an instructor at Texas A&M Fire School.
