Every gas detector is only as useful as the numbers it is told to act on. A sensor can measure perfectly, but if the alarm setpoints are wrong, the alarm will sound too late, too often, or not at all.
After years of commissioning, calibrating and troubleshooting gas detection systems in the field, I can say that bad setpoints cause as many problems as bad sensors.
This guide explains what alarm setpoints are, how low alarm, high alarm and TWA work, how they differ, and how to choose them sensibly.
What Are Alarm Setpoints?
Alarm setpoints are the gas concentration values at which a detector triggers a warning or emergency alarm. Most detectors use three kinds of setpoint:
Low alarm
The first warning. Gas is present at a level that needs attention, but conditions are not yet critical.
High alarm
The emergency level. Take immediate action, such as evacuating, shutting down or ventilating.
TWA alarm
An average-exposure alarm. It triggers when the worker’s average exposure over a working shift reaches the occupational limit, even if no single reading looked dramatic.
You will also see STEL (Short-Term Exposure Limit), a 15-minute average, on most portable multigas detectors.
Why Alarm Setpoints Matter
Gas hazards do not announce themselves. Hydrogen sulfide (H2S) can paralyze your sense of smell, carbon monoxide (CO) is odorless, and oxygen depletion gives almost no warning. The detector is the worker’s early warning system, and the setpoints define how early that warning comes.
Setpoints that are too high mean workers are exposed longer before anyone knows. Setpoints that are too low create nuisance alarms, and crews learn to ignore them. Both outcomes are dangerous, which is why setpoints deserve engineering attention rather than factory defaults left untouched.
What Is a Low Alarm?
The low alarm (often called Alarm 1 or A1) is the first level of warning. It tells the user that gas has appeared at a level that is not yet immediately dangerous but should not be ignored.
What it should trigger
- Check the area and identify the source.
- Increase ventilation.
- Prepare to leave or stop work if the reading rises.
- On fixed systems, send a warning to the control room and start preventive actions.
Typical low alarm examples (always confirm against your site procedures and local regulations):
| Gas | Typical low alarm |
|---|---|
| Flammable gas (combustible) | 10% LEL |
| Hydrogen sulfide (H2S) | 10 ppm |
| Carbon monoxide (CO) | 25 to 35 ppm |
| Oxygen (O2), deficiency | 19.5% vol |
Note that oxygen is a special case: it alarms in both directions. Below 19.5% is considered oxygen-deficient and above 23.5% is oxygen-enriched, which is a fire and explosion risk.
What Is a High Alarm?
The high alarm (Alarm 2 or A2, and sometimes a third level, A3) means the situation is serious. Gas has reached a concentration where action is required now, not later.
What it should trigger
- Immediate evacuation of the area for portable detectors.
- Automatic actions in fixed systems: shutting down equipment, closing valves, starting emergency ventilation, activating sirens and strobes.
- Notification of the emergency response team.
Typical high alarm examples:
| Gas | Typical high alarm |
|---|---|
| Flammable gas (combustible) | 20% LEL (some sites use 40% LEL for fixed systems) |
| Hydrogen sulfide (H2S) | 15 ppm |
| Carbon monoxide (CO) | 100 to 200 ppm |
| Oxygen (O2), enrichment | 23.5% vol |
The high alarm should always sit comfortably below levels that are immediately dangerous to life or health (IDLH) and, for flammable gases, well below the lower explosive limit (100% LEL). The point of the high alarm is to give people time to get out.
What Is TWA in Gas Detection?
TWA stands for Time-Weighted Average. It is the average concentration a worker is exposed to over a standard reference period, normally 8 hours for a workday.
TWA exists because the health effects of many toxic gases depend on total dose over time, not just on instantaneous peaks.
A worker can walk through a spike without a problem, but sustained moderate exposure can still cause harm. TWA captures that.
The TWA Formula
TWA = (C1 × T1 + C2 × T2 + … + Cn × Tn) ÷ 8 hours
Where C is the concentration during each period and T is the duration of that period in hours.
A Simple Example
A worker is exposed to CO as follows during an 8-hour shift:
- 4 hours at 10 ppm
- 2 hours at 40 ppm
- 2 hours at 0 ppm
TWA = (10 × 4 + 40 × 2 + 0 × 2) ÷ 8 = (40 + 80 + 0) ÷ 8 = 15 ppm
The worker saw a 40 ppm peak, but the 8-hour average is 15 ppm, which is lower than the typical occupational limits for CO.
How Portable Detectors Calculate TWA
Most portable multigas detectors keep a running record of exposure from the moment they are switched on and compute the TWA automatically.
Many instruments treat the unmonitored remainder of the 8-hour period as zero exposure, so the number depends on how long the detector was actually worn.
Check your manufacturer’s manual for how yours handles this, and reset or clear the TWA at the start of each shift according to site procedure.
TWA vs STEL vs Ceiling Limits
These three terms are often confused. They answer different questions.
| Term | Full name | Averaging period | What it protects against |
|---|---|---|---|
| TWA | Time-Weighted Average | 8 hours | Chronic or cumulative exposure over a shift |
| STEL | Short-Term Exposure Limit | 15 minutes | Acute effects from short, higher exposures |
| Ceiling | Ceiling limit | Instantaneous | A value that must never be exceeded |
Exposure limit values depend on the authority: ACGIH publishes TLVs, OSHA publishes PELs in the US, NIOSH publishes RELs, and in Mexico the reference is NOM-010-STPS-2014, which defines limits as LMPE-PPT (time-weighted average), LMPE-CT (short-term) and LMPE-P (ceiling). Limits for the same gas can differ between these sources, so use the one that applies to your site and jurisdiction.
Low Alarm vs High Alarm vs TWA: Side-by-Side
| Feature | Low alarm | High alarm | TWA alarm |
|---|---|---|---|
| Purpose | Early warning | Emergency | Cumulative exposure |
| Based on | Instantaneous reading | Instantaneous reading | Running average over time |
| Typical response | Investigate, ventilate | Evacuate, shut down | Limit further exposure, rotate workers |
| Used in fixed detectors | Yes | Yes | Rarely |
| Used in portable detectors | Yes | Yes | Yes, for toxic gases |
The key difference is that low and high alarms respond to what is happening right now, while TWA responds to what has been accumulating over the shift.
Fixed vs Portable Detectors: How Setpoints Differ
Portable detectors protect the individual. They are worn in the breathing zone, so alarm levels are usually tied to occupational exposure limits and include TWA and STEL for toxic gases.
Fixed detectors protect an area or process. They typically use two or three alarm levels tied to plant response actions through relays, 4-20 mA signals, a PLC, or a fire and gas system. They rarely calculate TWA because nobody is standing next to them for eight hours.
How to Choose Alarm Setpoints
Do not simply accept the factory default and move on. Work through these steps:
Start with the regulation and the site standard
Use the applicable exposure limits and your company’s procedures as the baseline.
Consider the hazard
A highly toxic gas like H2S needs setpoints that give people time to escape. Flammable gas setpoints must stay far below the LEL.
Consider response time
Setpoints must include the time the sensor takes to respond (T90) plus the time needed for people and systems to act. If it takes five minutes to evacuate, the alarm must trigger early enough to allow that.
Consider sensor performance
Measurement accuracy, drift and background noise determine how close to zero you can set a low alarm without constant nuisance alarms.
Match each alarm to a defined action
Every setpoint should have a clear response written down. If the low alarm and high alarm trigger the same action, one of them is not doing its job.
Document and review
Record setpoints, the reasons behind them and who approved changes. Review them after incidents, process changes and calibration findings.
If you work with safety-rated systems, setpoints also feed into your risk assessment and safety requirement specification, so changes should go through proper management of change.
Common Mistakes with Alarm Setpoints
Leaving factory defaults unreviewed
Defaults are generic and may not fit your gas, process or jurisdiction.
Setting the low alarm too close to background noise
This creates false alarms, and crews learn to ignore the detector.
Setting the high alarm too high
By the time it sounds, people may already be in danger.
Changing setpoints to stop nuisance alarms
Instead of finding the real cause, such as drift, interferents, or condensation.
Ignoring TWA and STEL on portables
Workers in areas with low but persistent gas levels can exceed the 8-hour limit without ever triggering a high alarm.
Skipping calibration and bump tests
Correct setpoints mean nothing if the sensor reads low. Regular calibration and functional tests are what make setpoints trustworthy.
Not training the crew
Workers must know what each alarm sounds like and what to do when they hear it.
Practical Tips from the Field
- Make low and high alarms audibly and visually distinct, for example with different beep patterns and flashing rates.
- Keep a setpoint register for every detector type on site, so replacement units are configured identically.
- When a detector alarms repeatedly at the same place and time of day, investigate the process before touching the setpoint.
- For personal detectors, confirm the TWA is cleared at the start of the shift so the previous day’s exposure is not carried over.
- After any firmware update, configuration restore or sensor replacement, verify the alarm settings on the display.
Frequently Asked Questions
What is the difference between a low alarm and a high alarm on a gas detector?
A low alarm is an early warning that gas has reached a level needing attention. A high alarm indicates a dangerous concentration that requires immediate action such as evacuation or shutdown.
What does TWA mean on a gas detector?
TWA means Time-Weighted Average. It is the average gas concentration a worker has been exposed to over a standard 8-hour shift, and it is used to check exposure against occupational limits.
What is the difference between TWA and STEL?
TWA averages exposure over 8 hours to evaluate cumulative exposure. STEL averages over 15 minutes to catch short, higher exposures that could cause acute effects.
What are typical alarm setpoints for H2S?
Many portable detectors use 10 ppm for the low alarm and 15 ppm for the high alarm, but you should confirm against your site procedures and local regulations, since exposure limits differ between authorities.
What are the standard alarm levels for flammable gas?
A common configuration is 10% LEL for the low alarm and 20% LEL for the high alarm. Some fixed systems use 20% and 40% LEL. All should be well below 100% LEL.
Can I change the alarm setpoints on my gas detector?
On most detectors, yes, but only authorized personnel should do it, following your management of change process and the manufacturer’s instructions. Never raise a setpoint just to silence a nuisance alarm.
Why does oxygen have both low and high alarms?
Because both too little and too much oxygen are hazardous. Below 19.5% is oxygen-deficient, and above 23.5% is oxygen-enriched, which increases fire and explosion risk.
Key Takeaways
- Alarm setpoints define how early a gas detector warns people, so they are as important as the sensor itself.
- Low alarm is the early warning, high alarm is the emergency, and TWA tracks cumulative exposure over a shift.
- Use STEL alongside TWA to catch short-term spikes.
- Choose setpoints based on regulations, hazard, response time, and sensor performance, then document them.
- Never raise setpoints to hide nuisance alarms. Find and fix the root cause.
- Calibrate and bump test regularly so the numbers you set can be trusted.
Disclaimer
The values in this article are typical examples for educational purposes. Always follow your site procedures, the manufacturer’s instructions and the regulations that apply in your jurisdiction.