Operating a gas detector

CHECKING THE ATMOSPHERE

By Tim Morrison

There’s a lot more to gas monitoring than meets the eye. Here’s a look at the most common problems encountered in the field.

Well, we have a gas detector! We’ve heard the sales pitch, watched the demonstration and read the manual. We’ve even watched the training tape a couple of times. But all too often we still hear complaints that some people are having problems with the gas detector or, in some cases, have stopped using it altogether. And that may be why over 80 per cent of all confined space fatalities are a result of hazardous atmospheres.

For gas detection to be of use, workers must know how to operate the device and believe in its value. Therefore training is required; but there is a good deal more to proper training than just knowing how to operate a given make and model of gas detection instrument. Gas detection must be completed by an individual who knows how to operate the gas detector, knows the procedures for testing confined spaces atmospheres, and is familiar with the particular hazards that may be present in the workplace.

The most common problems we see in the field relate to the following issues:

* calibration;

* relative response and conversion issues;

* sensor reaction times;

* testing protocols; and

* permit completion.

Calibration

For a gas detector to display reliable readings in the workplace, it must be calibrated regularly. Calibration is the process of adjusting a sensor’s readout so that it shows the proper response to a known concentration of gas. There are two levels of calibration -- laboratory and field. A laboratory calibration is done within a controlled setting and, depending on the manufacturer, this will include specific parameters regarding altitude, barometric pressure, temperature and humidity. The calibration gas is applied to the sensor and the electronic readouts of the unit are adjusted to the known values for the test gas. LEL and toxic sensors must be tested at least once every six months, and the results must be kept on file.

Field testing includes either a calibration test (sometimes called a "bump" test) and an "exhalation" test. In the "bump" test, a known mixture or blend of gas is applied to the sensor and the electronic readout is noted. A second type of field test is exhaling onto the oxygen sensor. Take a deep breath, hold it for five seconds, then slowly exhale across (not into) the sensor. The readout for the oxygen sensor should decrease.

Relative response

Although a sensor is calibrated to give an accurate response to a particular gas, it can sometimes respond to other gases. This type of response is called "relative response" or "interference response" or "cross sensitivity". Virtually all sensors (with the exception of oxygen meters) have some level of response to other gases. The most common "cross sensitivity" occurs with a hydrogen sulfide (H2S) chemical sensor. One manufacturer lists nitrogen dioxide, carbon monoxide, sulphur dioxide, and mercaptan as gases that provoke inaccurate readings from their H2S sensor.

It’s vital for the gas tester to be familiar with all of the possible contaminants that might be present in the workplace. For example, if a gas tester is using a meter calibrated to methane gas, but knows that the space being tested could have been contaminated with pentane gas, he can use the "relative response" of the meter to calculate the actual pentane level.

Each manufacturer supplies a chart of "conversion factors" for carrying out this procedure and calculating the actual level of combustible gas. Conversion factors play an important role when dealing with explosive or combustible gases whenever 10 per cent of the lower explosive limit (LEL ) or more is displayed on the detector.

Timing

Except in trace gas situations, most sensors begin to show an indication of the gas they are looking for almost immediately. However, each sensor needs time to adjust to the atmosphere they are placed into, and some sensors may need up to three minutes to display the correct reading. This is the time needed for the chemical reaction within the sensors to take place. Different sensors need different times.

If, in the operating instructions, the manufacturer lists a given number of readings per minute, they are usually discussing the number of electronic signals being sent to the sensor. The sensor, however, may still be stabilizing in its new environment and may not immediately provide accurate information. Therefore, it’s important to check the operating instructions for any given meter to determine the length of time required for readings.

Testing procedure

The atmosphere of a confined space cannot be determined by smell or vision, therefore the atmosphere of the entire space must be tested prior to each entry and monitored continuously while work is in progress. A generic procedure for testing the atmosphere in a confined space is as follows:

1. Select the appropriate unit to perform the atmospheric monitoring required, depending on the contaminants that may be present. This normally includes the ability to test for oxygen, combustible gases and toxic substances. Test the battery to ensure it will last through the entire testing time.2. In a good atmosphere outside of the space, turn the unit on and allow it to warm up. Test the alarm once the unit is in the ready mode. 3. Field calibrate the oxygen meter. Keep the unit in fresh air until the oxygen display shows 20.9 per cent. Take a deep breath, hold it for three to five seconds and exhale across the oxygen sensor. The reading should decrease to about 16 per cent. If the reading doesn’t drop, take the unit out of service.4. Attach the remote sampling probe and aspirator to the unit. When using a pump or drawing a sample through a line, allow three seconds per meter of line.First test the atmosphere at the opening. Then test approximately halfway into the space after removing the cover. Finally, test six inches from the bottom surface of the space.If the oxygen is between 18 and 23 per cent, LEL’s are at zero, and toxic materials at zero ppm, the space is considered "free from atmospheric hazard" and entry can proceed.

If the oxygen readout is below 18 or above 23 per cent, remove the cover and purge or ventilate the space for at least 10 minutes before any further testing is performed. If anyone has to enter the confined space, an appropriate breathing apparatus is to be worn until the readings are acceptable.If the LEL in the initial reading shows a potential flammable/explosive atmosphere (20 per cent LEL or higher) take great care in removing the cover. Do not do any further testing until the space has been ventilated or purged for at least 10 minutes. If the LEL is less than 20 per cent, or there is a toxic gas reading but the alarm does not sound, remove the cover and continue testing. Remember, the specific gravity of any gas or vapour must be considered. It may be heavier or lighter than air, making it vital to test at the top, bottom and throughout the space.If there are toxic gases present in the confined space and the alarm sounds, remove the cover and purge or ventilate the space for at least 10 minutes before any further testing is completed. If anyone has to enter the confined space, an appropriate breathing apparatus is to be worn.

Once there is no alarm from the detector or if the worker entering the space wears a breathing apparatus (and the LEL is below 50 per cent), entry is allowed. Once in, move the detector around and test the entire space. If there is still no alarm, place the sensors near the work area and monitor the space continuously while workers are inside. Record the most significant results (highest toxic and LEL etc.) found in the interior on the entry permit.5. If the alarm sounds while the entrant is inside, stop work immediately and exit the space.

Permits

Gas detection readings are part of the hazard evaluation required of the employer by law before any worker enters a confined space. There is normally a requirement for a "permit" system that requires the gas tester to record details of the confined space entry and gas testing on a special form called an entry permit. This accomplishes two objectives. First, filling out the permit ensures that all required steps -- such as field testing the monitor -- are carried out. Second, it provides a permanent record for those who review the permit. The details recorded can assist in enhancing ventilation techniques, selecting protective equipment and tracing back any problems that may occur.

However, it doesn’t stop there. Testing -- especially if there is the slightest chance the atmosphere may become hostile -- must be continuous. Gas detection can’t be performed by an individual who comes from another department and leaves right after the test is completed. Gas monitoring in the space has to be continuous. As well, the supervisor, who has overall responsibility, must plan and organize the work as well as communicate the hazards to workers. Therefore, the gas tester and supervisor should speak after the test, not just before it.

For many, gas detection is one of those things one accepts without asking questions. It’s just another piece of equipment to be used. However, simply having a gas detector and learning how to operate that particular instrument is not enough. The gas detector and its operation are just part of an overall gas testing program that has to include staff who know how to use it, who understand the broad picture of atmospheric testing, who are familiar with the hazards in the workplace and who follow a detailed set of testing procedures.

Tim Morrison is a gas detection specialist with Safetyscope, Inc. He is based in Toronto, Ont.

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