HC Gas Detector Calibration Procedure Simulator
A hydrocarbon (HC) gas detector warns of flammable gas before it can explode. It reads in % LEL (lower explosive limit): 100 % LEL is the weakest gas mixture that can burn (for methane, 5 % by volume). It sends 4-20 mA (4 mA = 0 % LEL, 20 mA = 100 % LEL). Calibration sets zero in clean air and span with certified gas, then checks the alarms. Follow: 1 Safety and permit, 2 Set up, 3 Zero, 4 Span, 5 Verify, 6 Finish. Normal working mode can be used at any time.
Procedure map (click a step)
Hint
1 Safety and permit
Bench settings and readings
| Detector | |
| Gas reaching the sensor | |
| Display reading | |
| Output 4-20 mA | |
| Alarms | |
| Reading stable |
Calibration summary
| As-found zero reading | |
| As-found span error | |
| As-left span error (verify) | |
| Response time T90 (limit 30 s) | |
| Sensor sensitivity (limit 50 % of new) | |
| Result |
Bump test or calibration?
| Test | What it does | When | Adjusts? |
|---|---|---|---|
| Bump test | Gas on the sensor to prove it responds and alarms | Often (before use, or at short intervals) | No |
| Zero + span calibration | Sets clean-air zero and span with certified gas | At the interval in your procedure, or after a failed bump test | Yes |
| Sensor replacement | New sensor, then full calibration | Sensitivity below 50 %, poisoned, or end of life | Yes |
Flammable gas sensor technologies
| Technology | How it works | Good for | Remember |
|---|---|---|---|
| Catalytic bead | Gas burns on a hot bead and changes its resistance | Low cost, most flammable gases | Needs oxygen; poisoned by silicones, H2S, lead; fails silently |
| Infrared point | Hydrocarbon absorbs infrared light | Hydrocarbons, fail-safe, no oxygen needed | Does not see hydrogen; dirty optics read low |
| Open-path infrared / laser | Light beam across a wide area | Large areas, fence lines | Fog, rain or a blocked beam |
| Ultrasonic leak detector | Listens to the sound of a gas leak | Pressurised gas leaks outdoors | Detects leaks, not gas concentration |
What to remember
HC Gas Detector Calibration Procedure – Interactive Simulator
Hydrocarbon gas detectors are widely used in oil and gas plants, refineries, chemical industries and other hazardous areas to detect flammable gas leaks before they create a fire or explosion risk.
A gas detector must provide an accurate reading and respond correctly when gas is present. Periodic bump testing and calibration help verify the detector’s performance, sensor condition, alarm operation and output signal.
This HC Gas Detector Calibration Procedure Simulator provides a step-by-step learning experience covering safety precautions, zero calibration, span calibration, alarm verification and returning the detector to normal service.
What is HC gas detector calibration?
An HC gas detector measures the concentration of flammable hydrocarbon gas in the atmosphere. Its reading is commonly displayed as % LEL (Lower Explosive Limit).
The Lower Explosive Limit is the minimum concentration of a flammable gas in air that can ignite when an ignition source is present.
For example, methane has an approximate LEL of 5% by volume in air. Therefore, 100% LEL represents approximately 5% methane by volume.
A typical 4–20 mA output is configured as:
- 4 mA – 0% LEL
- 12 mA – 50% LEL
- 20 mA – 100% LEL
The detector sends this signal to the gas detection system, PLC, DCS or safety system, where alarms and other actions may be configured.
What will you learn from this simulator?
The simulator demonstrates the main stages of an HC gas detector calibration procedure.
1. Safety precautions and work permit
Before starting calibration, the technician must follow the approved plant procedure.
Important precautions include:
- Obtain the required work permit.
- Inform the control room before starting.
- Follow the approved procedure for inhibiting alarm or trip outputs.
- Wear a personal gas monitor.
- Check the calibration gas cylinder, expiry date and regulator.
- Ensure calibration exhaust is discharged to a safe, ventilated location.
- Prevent ignition sources in the hazardous area.
The simulator demonstrates why safety checks must be completed before calibration begins.
2. Calibration setup
Select the detector technology and configure the calibration conditions.
The simulator includes:
- Catalytic bead detector.
- Infrared (IR) detector.
- Certified methane calibration gas.
- Calibration gas flow adjustment.
- Detector power and warm-up condition.
- Zero drift and weak sensor practice faults.
The correct calibration gas concentration and flow rate must be confirmed from the detector manufacturer’s instructions.
3. Zero calibration
Zero calibration establishes the detector’s baseline reading when no target gas is present.
For this procedure, the simulator uses clean air for zero calibration. Nitrogen may be suitable for some infrared detectors, depending on the manufacturer’s instructions, but catalytic bead sensors require oxygen to operate correctly.
The technician should:
- Apply the approved zero gas.
- Allow the detector reading to stabilise.
- Check that the reading is within the permitted tolerance.
- Perform the zero adjustment using the approved calibration method.
4. Span calibration
Span calibration checks and adjusts the detector response using certified calibration gas of a known concentration.
The simulator demonstrates:
- Applying span gas.
- Monitoring the detector reading.
- Waiting for a stable response.
- Adjusting the span value.
- Comparing the displayed reading with the certified gas concentration.
Always enter the concentration printed on the actual gas cylinder. Do not assume that every calibration cylinder contains 50% LEL.
5. Alarm and output verification
After calibration, the detector must be checked to confirm that it responds correctly.
The simulator demonstrates two example alarm thresholds:
- Low alarm: 20% LEL
- High alarm: 40% LEL
It also displays the corresponding 4–20 mA signal and response behaviour.
These are illustrative settings. Actual alarm setpoints, tolerances and response-time acceptance criteria must follow the approved plant configuration and manufacturer’s specifications.
6. Finish and restore normal operation
Completing the calibration adjustment does not mean the job is finished.
The technician must:
- Stop the calibration gas supply.
- Remove the calibration cup.
- Confirm that the detector returns to its normal reading.
- Remove alarm and trip inhibits in accordance with the approved procedure.
- Inform the control room that the detector is back in service.
- Complete the calibration record and close the permit.
Never leave a gas detector or its associated safety outputs inhibited after the maintenance activity.
Bump test vs. calibration
A bump test and a calibration test are not the same.
A bump test confirms that gas reaches the sensor and that the detector responds. It does not normally adjust the detector’s zero or span.
Calibration adjusts the detector’s response using known gas concentrations.
If a detector fails a bump test, investigate the cause and perform calibration or sensor replacement as required by the manufacturer’s instructions and site procedure.
Types of hydrocarbon gas detector technologies
Catalytic bead detector
Uses a heated sensing element to oxidise combustible gas and measure the resulting change in resistance. It generally requires oxygen and can be affected by sensor poisons such as silicones, lead compounds and certain other contaminants.
Infrared point detector
Uses infrared absorption to detect hydrocarbon gases. It does not require oxygen for measurement and is not suitable for detecting every combustible gas. For example, conventional hydrocarbon IR detectors do not detect hydrogen.
Open-path infrared detector
Uses an infrared beam across an open area to detect gas along the beam path. It is commonly used for monitoring larger areas and plant boundaries.
Ultrasonic gas leak detector
Detects the ultrasonic sound produced by pressurised gas escaping through a leak. It detects the sound of a leak rather than measuring gas concentration in % LEL.
How to use the HC gas detector calibration simulator
- Start with Safety and permit and complete all safety checks.
- Move to Set up and select the detector type.
- Set the calibration gas value and flow.
- Switch on the detector and allow it to warm up.
- Select clean air and perform zero calibration.
- Apply span gas and perform span calibration.
- Check the low alarm, high alarm, output signal and response time.
- Complete the final safety checks and enter normal working mode.
- Use the practice fault options to understand zero drift and weak sensor behaviour.
You can also change the gas concentration in normal working mode to observe how the detector reading, alarm status and 4–20 mA output respond.
Who should use this simulator?
This interactive learning tool is useful for:
- Instrumentation technicians and maintenance engineers.
- Instrumentation engineers working in oil and gas plants.
- Refinery and petrochemical maintenance teams.
- Gas detection system technicians.
- Engineering students and freshers.
- Engineers preparing for instrumentation maintenance interviews.
Important technical note
This simulator is an educational model. Its gas response, flow, alarm thresholds, sensitivity limits and response-time calculations are simplified for learning. Actual calibration procedures, gas concentrations, flow rates, alarm settings, acceptance tolerances and safety-system bypass requirements must be verified against the detector manufacturer’s manual and approved site procedures.
