Gas Detector Alarm Settings: Baixo, Alto, STEL and TWA

A gas detector alarm setting is a decision point. It tells the instrument when to warn the user, but the number alone does not explain what the worker should do next. That response has to come from the site’s safety procedure.

Low and high alarms usually respond to the current gas reading. STEL and TWA alarms use exposure averages calculated over defined periods. Oxygen alarms work differently again because both a falling and a rising concentration can be hazardous.

Factory defaults are useful references, not universal safety settings. The approved values for a portable monitor should reflect the gases, units, applicable exposure limits, detector capability, workplace rules, and response plan. Copying another site’s numbers can create late warnings, unnecessary alarms, or inconsistent exposure records.

Gas detector alarm settings at a glance

Alarm type	Measurement basis	What it tells the user	Typical application
Baixo	Current reading	The first configured concentration threshold has been reached.	Toxic gas or combustible gas warning
Alto	Current reading	A higher concentration threshold has been reached.	Withdrawal, isolation, or shutdown response
DEFINIR	Short-period exposure average	The average exposure over the configured short period has reached its limit.	Toxic gas exposure management
TWA	Longer exposure average	The accumulated average exposure has reached its configured limit.	Worker exposure across a shift or defined workday
Oxygen low	Current oxygen reading	Oxygen has fallen below the configured level.	Oxygen deficiency
Oxygen high	Current oxygen reading	Oxygen has risen above the configured level.	Oxygen enrichment and increased fire risk

A standard portable four-gas monitor commonly combines oxygen, gás combustível, monóxido de carbono, and hydrogen sulfide channels. Our guide to what a four-gas monitor measures explains the purpose and limitations of that configuration.

What is a low gas alarm?

The low alarm is commonly the first instantaneous threshold for a toxic or combustible gas channel. When the displayed value reaches the setpoint, the detector starts its low-alarm sequence.

“Baixo” does not mean that continued exposure is safe. It only identifies the first configured action point. The written procedure might require the worker to stop the task, leave the area, notify a supervisor, increase ventilation, or start another control. If nobody knows the required response, the setpoint is only a number in a menu.

A low setting that repeatedly alarms should be investigated. Raising it to stop the noise may hide a real release, poor ventilation, an interfering gas, or a detector problem. Review our gas detector false-alarm troubleshooting guide before changing an approved setting.

What is a high gas alarm?

The high alarm is a second instantaneous threshold set above the low alarm for the same rising gas channel. It normally signals a more serious condition and may use a faster or more urgent alarm pattern. The response could include immediate withdrawal, process isolation, shutdown, or emergency notification.

Many instruments latch high alarms. The alarm remains active until the gas falls below the relevant threshold and an authorized user completes the reset procedure. Pressing an acknowledgment button may silence part of the warning without clearing the event. Acknowledgment, silence, and reset are different functions and should be covered in training.

Oxygen channels require special attention. On a typical monitor, the low oxygen alarm responds when the reading falls, while the high oxygen alarm responds when it rises. The terms “low” e “high” describe oxygen concentration, not the severity of the hazard.

What is a STEL alarm?

STEL means short-term exposure limit. A detector calculates an average reading over a specified short period and compares it with the configured STEL value. STEL alarms are normally used for toxic gases, not for oxygen or combustible gas channels.

The time window matters as much as the concentration. The NIOSH Pocket Guide explains that a NIOSH STEL is generally a 15-minute time-weighted average unless a chemical entry states otherwise. The same resource also explains that exposure definitions differ between NIOSH and OSHA. See the official NIOSH Pocket Guide introduction before assigning a time basis.

A STEL alarm may remain after the worker reaches clean air because the instrument is still calculating the average from earlier exposure. A lower current reading does not erase the previous minutes. Clearing the exposure history or restarting the detector can break the record that connects the monitor to the worker.

What is a TWA alarm?

TWA means time-weighted average. It represents accumulated exposure over a longer defined period, commonly a work shift or workday. The detector combines concentration and time, so moderate exposure over several hours may eventually trigger a TWA alarm even if the current reading never reaches the high-alarm setpoint.

The averaging period is not universal. NIOSH recommended exposure limits may use a workday of up to 10 horas, while OSHA permissible exposure limits commonly use an 8-hour workshift. The applicable chemical, jurisdiction, employer program, and worker schedule all need to be checked.

Assign personal monitors consistently when TWA is used. Passing one detector among several workers makes it difficult to connect the accumulated exposure value to one person. Docking and fleet software can preserve records, but only if detector assignment and clock settings are controlled.

Alarm setting, exposure limit, range, and calibration value

Several numbers appear in a detector program, and they answer different questions. Mixing them up is one of the easiest ways to create a bad configuration.

Term	Meaning	Example question
Alarm setpoint	The configured value that starts an alarm.	When should the worker be warned?
Exposure limit	A regulatory, advisory, or company exposure criterion.	What exposure basis applies to this worker and gas?
Sensor range	The concentrations the sensor is designed to measure or display.	Can the sensor measure around the intended setpoint?
Calibration concentration	The certified reference-gas concentration used to check or adjust the detector.	What known gas value is used to verify accuracy?
Over-range value	A concentration above the measurement or display capability.	Has the sensor been exposed beyond its usable range?

UM 25 ppm calibration-gas component does not automatically require a 25 ppm alarm. The calibration value is chosen to verify or adjust sensor response. The alarm value is chosen from the hazard and exposure program. Nosso bump test and calibration guide covers how certified test gas is used for functional and accuracy checks.

Example H2S, CO, O2, and LEL alarm settings

The following values are factory examples published for one MSA portable monitor configuration. They show a common pattern, but they should not be copied into another instrument or site without review. Sensor options, country settings, regulations, and company policies can change the values.

Canal	Unit	Example low	Example high	Example STEL	Example TWA
Hydrogen sulfide	ppm	10	15	15	10
Monóxido de carbono	ppm	25	100	100	25
Oxigênio	% volume	19.5	23.0	N/A	N/A
Gás combustível	%LEL	10	20	N/A	N/A

The source table tells users to check the monitor or calibration certificate because national and corporate requirements vary. Consult the current MSA ALTAIR 4XR factory setpoint documentation for the full manufacturer context.

Combustible gas readings require the correct unit, sensor, calibration gas, and target-gas assumptions. Ten percent LEL is not the same as 10 percent gas by volume. The difference is covered in our combustible gas detector selection guide.

How to choose gas detector alarm setpoints

1. Identify each hazard and measurement unit

List the target gas, expected concentration, release pattern, task duration, possible oxygen displacement, and known interfering gases. State whether the required unit is ppm, mg/m3, percent volume, or percent LEL. Do not assume that a four-channel label covers every vapor at the site.

2. Select the applicable exposure or action basis

Identify the source behind each proposed setpoint. It may be an OSHA permissible exposure limit, NIOSH recommended exposure limit, local regulation, corporate occupational hygiene limit, process action level, or emergency-response criterion. The OSHA Air Contaminants standard lists enforceable federal exposure limits for covered substances, but the alarm-setting decision still has to account for the detector and the workplace response.

3. Confirm the detector can support the setting

Check the installed sensor’s range, resolution, accuracy, response time, environmental limits, and cross-sensitivities. Confirm whether that channel supports STEL and TWA calculations. A threshold inside the display range may still be unsuitable if the sensor lacks the required resolution or response for the application.

4. Define the action for every alarm

Alarme	Example response to define in the site procedure
Baixo	Stop work, move as instructed, notify the responsible person, and assess the source.
Alto	Withdraw, isolate, shut down, or start the emergency action specified for the task.
DEFINIR	End further exposure, preserve the worker record, and follow the occupational hygiene response.
TWA	Remove the worker from additional exposure and review the full exposure history.
Fault or over-range	Leave the hazardous area as required and remove the detector from service.

5. Review latching, silence, acknowledgment, and reset

Document what happens after the gas falls below the setpoint. State which alarms latch, who may silence or acknowledge them, and what must happen before reset. Include local sound, vibração, screen messages, relays, wireless alerts, and control-room displays.

6. Approve and record the configuration

Keep the detector ID, configuração do sensor, units, low and high settings, STEL and TWA values, averaging periods, source documents, approver, effective date, and reason for the revision. Use password protection or fleet templates when available, but keep a readable approved record outside the instrument.

7. Verify the complete warning chain

After configuration, perform the manufacturer’s required test with suitable gas. Check sensor response and every warning output the program relies on. A detector with configurable exposure alarms, such as the Otywell OT139 multi-gas detector, still needs the correct gas channels, settings, test procedure, and user training for its application.

Common alarm-setting mistakes

• Copying factory defaults from another model, country, or sensor configuration
• Confusing ppm, percent volume, and percent LEL
• Treating the low alarm as proof that lower concentrations are safe
• Choosing a setpoint without checking sensor range, resolution, and response time
• Using an exposure limit without confirming its averaging period
• Clearing TWA or STEL history when a detector changes hands
• Raising thresholds to reduce nuisance alarms without investigating the cause
• Allowing uncontrolled changes in the instrument or docking software
• Training workers on gas alarms but not on pump, sensor, bateria, and over-range warnings
• Changing settings without testing the complete warning chain afterward

When should alarm settings be reviewed?

Review the approved configuration when:

• A new chemical, process, or confined-space task is introduced.
• The detector model, sensor type, range, or firmware changes.
• An exposure limit or company rule is revised.
• Repeated alarms, near misses, or an incident show that the response is unclear.
• Ventilation, work duration, protective equipment, or emergency arrangements change.
• The detector moves to another jurisdiction, customer site, or worker group.
• Fleet software or a configuration template is updated.

A periodic review should compare the approved record with instruments in service. It should also check training, event logs, detector assignments, and the actions taken after alarms.

Gas detector alarm-setting checklist

Frequently asked questions

What is the difference between low and high gas alarms?

Both normally respond to the current reading. Low is the first configured threshold. High is a second, higher threshold tied to a more urgent response. The site procedure defines the action required at each level.

What is the difference between STEL and TWA?

STEL uses a short-period average, commonly 15 minutes when the applicable source defines it that way. TWA uses a longer average, often an 8-hour or 10-hour workday depending on the authority. Neither is the same as the current instantaneous reading.

Should gas detector alarms equal OSHA exposure limits?

Not automatically. OSHA limits may provide part of the basis, but the setting also depends on the gas, time period, jurisdiction, sensor capability, work method, internal limits, and required response. The reason for the selected value should be documented.

What should the oxygen alarm be set at?

Use the approved workplace procedure, applicable rules, and manufacturer instructions. Published portable-monitor examples often use a low setting around 19.5 percent oxygen and a high setting around 23.0 ou 23.5 percent, but these values still need site approval.

Why is combustible gas commonly displayed in percent LEL?

Percent LEL shows how the measured response relates to the lower explosive limit of the gas used for the detector’s scale or calibration assumptions. It is intended for fire and explosion risk, not low-level toxic exposure. Target-gas correction and sensor limitations still apply.

Why does a TWA alarm remain after moving to fresh air?

The alarm reflects accumulated average exposure, so a clean current reading does not erase the earlier exposure. Follow the site procedure for worker response, recordkeeping, acknowledgment, and reset.

Can workers change portable gas detector alarm settings?

Only when they are authorized by the site’s change-control procedure. Uncontrolled changes can make detectors inconsistent and disconnect the alarm from the approved worker response.

Should alarm settings be checked during a bump test?

A bump test should confirm sensor response and the warning outputs required by the manufacturer’s procedure. It does not prove that the approved setpoints are suitable for the hazard or that the displayed concentration is accurate. Those questions require configuration review and, for accuracy, a calibration check.

Final rule for alarm configuration

Every alarm needs four documented details: a gas and unit, a setpoint source, a detector that can measure it, and an action the user understands. If one is missing, the configuration is incomplete.

Review factory values instead of copying them. Confirm STEL and TWA time bases, test the warning chain after changes, and keep the approved record.