⚠️ Disclaimer: These tasks are guidelines only. They do not include lockout/tagout (LOTO), energy isolation, or other safety requirements. Review and verify suitability for your specific equipment and application. Add all required safety procedures per your company's policies and regulatory requirements before use. You are responsible for the safe and appropriate execution of all maintenance activities.
H₂S sensors don't announce when they've stopped working. They sit there looking fine, showing a clean zero, and you find out they've drifted or failed when someone walks into a space that should have tripped an alarm and didn't. Electrochemical gas detection has a specific problem: the cell degrades slowly over time, the sensor still sends a signal, and nothing looks wrong until you test it.
This checklist covers the PM tasks for fixed hydrogen sulfide sensors using electrochemical detection technology. It is written for maintenance technicians in the field and maintenance managers building or auditing PM programs.
How to Use This Checklist
Record actual findings — not checkbox answers. "Sensor reads 0.3 ppm in clean air" is a finding. "Zero — OK" is noise. The difference between those two entries is the difference between catching a trend and missing a failing cell.
Both checklists below are standalone documents. The Field Checklist runs first, top to bottom. The Manager Reference Checklist runs second, top to bottom. Use whichever applies to your role. Do not mix them mid-task.
Field Checklist — Critical Tasks
Visual Inspection Tasks
| Task | Freq | Type |
|---|---|---|
| Inspect sensor housing for physical damage, corrosion, cracks, or signs of moisture ingress. Replace if compromised. | Monthly | MEC |
| Verify sensor LED status indicators are functioning and display the expected standby or normal-operation state. | Monthly | ALL |
| Check all conduit fittings, cable glands, and junction box covers are fully seated and water-tight. | Monthly | MEC |
| Inspect and clean sensor guard/diffusion membrane of dust, debris, oil film, or insect contamination. Do not use solvents; use dry compressed air or a soft brush only. | Quarterly | MEC |
Operational Checks
| Task | Freq | Type |
|---|---|---|
| Confirm sensor reading displays 0 ppm (or within acceptable zero baseline) in clean ambient air. Zero/bump test as required by site protocol. | Monthly | ELE |
| Perform bump test using certified H₂S calibration gas. Verify sensor responds and alarm triggers within the required response time. | Quarterly | ELE |
| Perform full span calibration using certified H₂S calibration gas at the required concentration. Adjust as needed and record results. | Semi-Annually | ELE |
| Verify alarm setpoints (low, high, and STEL if applicable) match the site safety plan. Document any discrepancies. | Semi-Annually | ELE |
Electrical Inspection
| Task | Freq | Type |
|---|---|---|
| Check sensor age against manufacturer's recommended replacement interval (typically 2–3 years for electrochemical cells). Flag for replacement if at or past end of life. | Annually | ELE |
Documentation
| Task | Freq | Type |
|---|---|---|
| Confirm all calibration records and inspection logs are current and filed per site requirements. | Every PM | ALL |
Reference Checklist — Full Task Library
Visual Inspection Tasks
| Task | Freq | Type |
|---|---|---|
| Inspect sensor housing, mounting bracket, and conduit fittings for physical damage, corrosion, UV degradation, or moisture ingress. Confirm enclosure rating (IP/NEMA) is maintained. | Monthly | MEC |
| Verify sensor status LED and any local display are functioning and show expected normal/standby state. Confirm no fault codes are active. | Monthly | ALL |
| Inspect and clean the sensor guard, diffusion cap, and membrane of dust, particulate, oil mist, or biological contamination. Use dry compressed air or a soft brush only — no solvents. Replace diffusion membrane if damaged or clogged. | Quarterly | MEC |
| Verify the sensor's installed location remains appropriate: confirm no obstructions to gas flow, sensor is positioned at correct height for H₂S detection (low, near grade), and has not been repositioned or partially blocked since last PM. | Annually | MEC |
Operational Checks
| Task | Freq | Type |
|---|---|---|
| Confirm sensor reading is at 0 ppm (or within site-defined zero tolerance) in clean ambient air with no known H₂S sources present. | Monthly | ELE |
| Perform bump test using certified H₂S calibration gas at a concentration above the low alarm setpoint. Verify sensor responds, alarm activates, and response time meets manufacturer specification (typically ≤30 seconds T90). | Monthly | ELE |
| Perform full two-point span calibration using certified calibration gas at the required concentration. Record pre-calibration reading, post-calibration reading, and gas certificate number. Adjust zero and span as needed. | Semi-Annually | ELE |
| Verify all alarm setpoints (low alarm, high alarm, STEL, TWA if applicable) match current site safety plan and regulatory requirements. Document setpoints and confirm with control system or panel display. | Semi-Annually | ELE |
| Test alarm relay outputs and any 4–20 mA analog signal output. Verify control panel or DCS receives correct signal at 0 ppm, low alarm setpoint, and high alarm setpoint. | Semi-Annually | ELE |
| Confirm spare sensor electrochemical cell(s) and calibration gas cylinders are on-hand, within expiration date, and stored per manufacturer requirements. | Annually | ALL |
Electrical Inspection
| Task | Freq | Type |
|---|---|---|
| Check all cable entries, glands, and conduit seals for integrity. Verify no moisture, condensation, or corrosion inside junction boxes or terminal compartments. | Quarterly | ELE |
| Inspect wiring terminals and connections for tightness, corrosion, or insulation damage. Re-torque loose terminals per manufacturer specification. | Semi-Annually | ELE |
| Verify power supply voltage at the sensor terminals is within the manufacturer's specified range. Low supply voltage can cause drift or false readings. | Semi-Annually | ELE |
| Review sensor response history and calibration trend data from past 12 months. Excessive drift, repeated zero offset, or declining sensitivity indicates approaching end of electrochemical cell life. | Annually | ELE |
| Check sensor age against manufacturer's recommended replacement interval (typically 2–3 years for electrochemical cells, varies by environment). Schedule replacement if at or within 6 months of end-of-life date. | Annually | ELE |
Documentation
| Task | Freq | Type |
|---|---|---|
| Verify all calibration records, bump test logs, and PM documentation are complete, signed, and filed per site and regulatory requirements (e.g., OSHA PSM, EPA RMP if applicable). | Every PM | ALL |
Failure Modes This Checklist Targets
Electrochemical cell depletion. The sensing element consumes itself during operation and has a finite service life — typically two to three years under normal conditions. A depleted cell shows reduced sensitivity or fails to respond to gas at all, while still appearing functional on a status check.
Sensor drift. Zero and span drift over time as the cell ages, as reference electrodes degrade, or as the sensor is exposed to interferents. A sensor that hasn't been calibrated recently may be reading 20–30% low without any obvious indication.
Diffusion membrane contamination. Oil mist, particulate, biological fouling, or coating buildup on the sensor guard or diffusion membrane blocks gas access to the cell. The sensor will still read zero. It just won't respond when there's actual gas present.
Moisture and condensation ingress. Water inside the housing corrodes terminals, damages the cell, and causes erratic readings or complete signal loss. Common in outdoor, wash-down, or high-humidity environments.
Alarm setpoint mismatch. Setpoints get changed, regulatory requirements get updated, or the original configuration is discovered to have been wrong. A sensor in perfect working order that's configured to the wrong alarm thresholds is functionally useless as a safety device.
Signal output failure. Relay contacts corrode. 4–20 mA loops develop ground faults. The sensor responds to gas, the alarm fires locally, and nothing reaches the control room. The relay and output circuit are separate failure points from the sensing cell itself.