Limitations of Gas Detection Tubes and Pumps

Gas Detection Instruments

• There are four categories of hazardous atmospheres: toxic, oxygen deficiency/asphyxiating gases, explosive/flammable gases or vapours, and smoke, aerosols, fumes (particulate contaminants).

Intrinsic Safety

• Intrinsic safety is a design applied to electrical equipment and wiring for hazardous locations.
• It limits energy, both electrical and thermal, to a level below that required to ignite a specific hazardous atmospheric mixture.

Selecting Gas Detection Equipment

• Gas detection equipment must be selected based on the specific needs of the incident.
• Mine rescue gas detection involves the use of direct-reading instruments (DRI).
• All equipment must meet relevant health and safety legislation, standards, and regulations.

Performance Criteria/Specifications

• Response time: ability to react to its specific gas in the specified time (e.g., 90% of reading in 30 seconds).
• Noise/drift: how much readings fluctuate when the quantity or concentration of a substance stays the same.
• Limit of detection (LOD): the lowest quantity or concentration of a substance that the instrument can register within a margin of confidence.
• Accuracy: the degree to which the measurement of a quantity of a substance matches up with that quantity's actual value.
• Precision: the degree to which repeated measurements under unchanged conditions show the same results.
• Dynamic range: the ratio between the largest and smallest possible signals.

Limitations of Tubes and Pumps

• Tubes and pumps are manufacturer-specific.
• No alarm system, therefore not recommended for continuous monitoring.
• Dependent upon operator's interpretation of results.
• Application can require longer periods of time in atmospheres for results to appear (up to several minutes).
• Limited shelf-life.
• Only accurate within +/- 20%.
• Many cross-sensitivities.

Types of Detectors

• Drager, Gastec, and CMS are common types of detectors.
• Electronic gas detectors are complex systems that include a sensor(s), alarms, a battery, and a display.
• They are available as single, multi-gas, stationary (non-portable), and specialized units.

Gas Testing

• During underground exploration, gas testing should be performed at intersections or where conditions change.
• Sampling at the top, middle, and bottom of a space must be done in accordance with manufacturer specifications.
• Sample every 1-2 meters (3-7 ft) due to the weights of gases.

Bump Testing and Calibration

• Bump testing and calibration are essential elements of gas detector maintenance.
• Functional ("bump") testing involves a brief exposure of the monitor to a known concentration of gas(es) to verify sensor and alarm operation.
• Manufacturers recommend that a bump test be performed on every instrument prior to each day's use.
• Calibration ensures that detectors perform properly and provides insight into the condition of the sensors in the instrument.
• Calibration gas cylinders are certified and traceable mixtures of gas concentration.

Steps for Gas Detector Operation

• Visually inspect for damage and contamination.
• Turn on the instrument in good atmosphere and check battery level.
• Zero the instrument.
• Bump test (functionally check) the instrument.
• Clear the peaks if applicable.