Many industrial worksites produce atmospheric hazards that could put their employees and the environment at risk. Toxic environments such as carbon monoxide, hydrogen sulphide, and corrosive atmospheres like ammonia, chlorine and sulphur dioxide are common atmospheres that need monitoring to ensure that the legal limits are not exceeded. Other dangerous atmospheres in the petroleum, construction and manufacturing industries include oxygen deficiency and flammable or explosive environments.
In the Alberta Labour Code, employers are required to implement a WHMIS program to train employees about chemical hazards. Workplace Hazardous Materials Information System provides the exposure limits that employees must not exceed without engineering , administrative controls or personal protective gear.
Electronic gas monitors are a popular option to determine if the atmosphere workers are entering might be immediately dangerous to life and health. They can be set to provide audible and visual alarms at preset exposure limits to alert employees to changes in air quality.
There are a multitude of choices when considering gas detection equipment.
Personal monitors are a less expensive option but are limited to one gas. They are met to be worn on a persons body, exposed to the atmosphere. They can provide continuous monitoring but may be limited to ranges below 500 ppm. Unfortunately, if the alarm rings, the monitor is picking up a rise in gas at close range to the workers nose. They may be inhaling the toxic chemical while the alarm is ringing.
Portable continuous monitors have sensors for a variety of gases, such as oxygen deficiency and enrichment, H2S and CO and the lower explosive limits for hydrocarbons. They can be set to issue a warning at a low and high alarm level and often have a wider range of detection capability. These portable placed monitors can be used for a team of personnel or set up between a worker and the potential source of the gas release. They are more expensive than personal portable monitors and must be positioned correctly to be effective warning systems.
Fixed continuous monitors issue a site wide warning and provide automatic response capabilities. Sensors can be placed at the most predictable sources for gas releases and may be tied in to a control room or central display area. The area that is exposed can be shut down, ventilated and alerted to rescue crews. Instrumentation trained personnel can monitor the area and evacuate personnel from the gas environments. They are significantly more expensive and require personnel on shift continuously.
Although electronic devices can be life savers, they have distinct problems. Sensors tend to be ruined by corrosive or wet environments, or over ranging them with high levels of gas. Dust can clog sensors and silicone will ruin the sensor heads. In cold environments, the batteries tend to die.
They require significant care, such as bump testing , and calibration. Maintenance problems could include replacing the lcd displays and batteries. It is wise to consider
whether a calibration service is easily accessible with quick turnaround if repairs are needed. The cost of calibration can quickly exceed the original cost of purchasing the detector.
When considering whether to purchase an electronic monitor, compare the costs and maintenance requirements to keep them accurate versus the convenience of renting equipment on a per needs basis.
Arliss T E Levine, CRSP, CHSC
Certified Health and Safety Auditor
Expanded Safety Guide
What Electronic Gas Monitors Detect and Why They Matter
Electronic gas monitors are portable or fixed instruments that detect and alarm on hazardous atmospheres. On any Alberta oil & gas, industrial, or confined-space site, a personal 4-gas monitor is standard PPE for every worker. The four gases in a standard 4-gas monitor are oxygen (O2), lower explosive limit (LEL) combustible gas, hydrogen sulphide (H2S), and carbon monoxide (CO). Additional single-gas or multi-gas units add ammonia, chlorine, sulphur dioxide, or volatile organic compounds depending on the process hazard.
Alberta OHS Atmospheric Testing Requirements
Alberta OHS Code Part 5 (Confined Spaces) and Part 18 (Personal Protective Equipment) require atmospheric testing before entry to any confined space and continuous monitoring during entry. The acceptable atmosphere is:
- Oxygen: 19.5% to 23.0% by volume. Below 19.5% is oxygen-deficient (impaired judgement, unconsciousness); above 23% is oxygen-enriched (accelerated combustion, fire risk).
- Flammable gas or vapour: less than 10% of the LEL. Above this the atmosphere is considered explosive-hazard and requires evacuation.
- Hydrogen sulphide: Alberta occupational exposure limit (OEL) is 10 ppm 8-hour time-weighted average, 15 ppm 15-minute short-term exposure limit.
- Carbon monoxide: 25 ppm 8-hour TWA in Alberta.
Sensor Technologies Inside a 4-Gas Monitor
- Oxygen: electrochemical (galvanic or polarographic) sensor with a consumable electrolyte. Typical lifespan 12–24 months.
- H2S and CO: electrochemical sensors, specific to the target gas. Typical lifespan 24–36 months. Cross-sensitivity to other gases exists (e.g. H2 can trigger CO sensor).
- LEL: catalytic bead (pellistor) sensors respond to a broad range of combustibles calibrated typically to methane or pentane. Non-dispersive infrared (NDIR) sensors are used in oxygen-poor environments where catalytic sensors cannot function.
Alarm Setpoints — Standard Practice
Personal monitors are typically configured with:
- O2 low alarm: 19.5% · O2 high alarm: 23.0%
- LEL low alarm: 10% LEL · LEL high alarm: 20% LEL
- H2S low alarm: 10 ppm · H2S high alarm: 15 ppm
- CO low alarm: 25 ppm · CO high alarm: 50 ppm
An audible, visible, and vibrating alarm must activate immediately at low-alarm threshold. Workers must evacuate on any alarm and re-enter only after re-testing shows atmospheric acceptability.
Bump Testing vs. Calibration — Both Are Required
A bump test is a brief exposure of the monitor to a known gas concentration above the alarm setpoint to verify that all sensors respond and alarms activate. Bump testing is required before every shift where the monitor is used. Bump testing takes about 30 seconds and does not require the monitor to be adjusted.
Calibration is a full adjustment of the sensor response using certified span gas. Manufacturer recommendations typically specify calibration every 30 days minimum, or immediately after a monitor is exposed to alarm-level gas, dropped, or reads suspect values. Full calibration takes 5–10 minutes and adjusts the sensor output curve.
Common Monitor Errors and Failures
- Sensor poisoning: catalytic LEL sensors are poisoned by silicones, lead compounds, sulphur, and hydrogen sulphide over time. Poisoned sensors read low or fail entirely.
- Oxygen sensor depletion: galvanic oxygen sensors have a finite chemical lifespan; readings drift low as the sensor ages.
- Water saturation: monitors used in high-humidity or wet environments can experience condensation across sensor faces, causing erratic readings.
- Temperature extremes: most sensors have specified operating temperature ranges. Alberta winter operation requires monitors rated for -20°C or lower.
Fixed Detection Systems
Facilities with continuous H2S, combustible gas, or oxygen-deficiency risk (sour gas plants, treatment lagoons, refineries) require fixed detection with area alarms and, in critical zones, automatic shutdown systems. Fixed systems must be inspected and calibrated per manufacturer schedules and a documented maintenance record maintained. Facility emergency response plans must define worker response to fixed-system alarms (evacuation routes, muster points, roll call).
Training Requirements
Every worker who wears or uses a gas monitor must be trained on the specific model in use. Training must cover the hazards being detected, alarm setpoints and response, pre-use inspection and bump testing, wearing position, and the site emergency procedures on alarm. In sour-gas operations, this training is delivered as part of H2S Alive certification. Confined-space entry teams complete this training as part of their Confined Space or Confined Space Monitor courses.
Content reviewed by Arliss Levine, CRSP, CHSC — lead safety instructor at Safety Mom · Allstar Enviro Safety Calgary, an Energy Safety Canada Authorized Training Provider.
