What Is STEL TWA On Gas Detector ?

In gas detectors, STEL and TWA are two metrics used to assess the risk of exposure to toxic gases based on time-weighted average concentrations. Simply put, they address the same fundamental question: how long is it safe to remain in an environment with a given gas concentration?

1.STEL: Short-Term Exposure Limit

Definition: The maximum average concentration to which a worker may be exposed over a 15-minute period.

Purpose: To prevent acute poisoning. It addresses "short-duration, high-concentration" exposures-for example, during a sudden gas leak or brief exposure while performing emergency repairs.

Alarm Logic: The device calculates a rolling average over the most recent 15 minutes; an alarm is triggered if this value exceeds the STEL.

2.TWA: Time-Weighted Average

Full Term: Time-Weighted Average

Definition: The average concentration to which a worker may be exposed over a standard 8-hour workday.

Purpose: To prevent chronic poisoning.

Alarm Logic: The device calculates an 8-hour rolling average (starting from the moment the unit is powered on or the beginning of a work shift); an alarm is triggered if this value exceeds the TWA.

3.Example

Suppose the limits for a specific gas are:

TWA = 20 ppm (8-hour average)

STEL = 30 ppm (15-minute average)

Scenario Concentration Duration Result

Scenario A 25 ppm 15 minutes Exceeds STEL (30) - Alarm triggered

Scenario B 25 ppm 8 hours Exceeds TWA (20) - Alarm triggered

Scenario C 10 ppm 8 hours Within TWA limits - Safe

Scenario D 35 ppm 5 minutes High concentration for a short duration; whether it exceeds the STEL depends on the specific 15-minute average calculation.

4.Our company's latest single gas detector and multi-gas detectors feature audible, visual, and vibration alarms. They display real-time TWA and STEL values, providing enhanced risk warnings. And more other features like;

1)Sensitive sensor;

2)LCD colorful display in English;

3)Alarm Type: Audible, visual and vibration;

4)Low and high alarm points,can be set by user ;

5) Rechargeable 2500mAh lithium battery;

6) C type USB interface,can connect with computer for data records(optional);

7) Can keep 3000 alarm records and 990000 real time data,view on detector;

8)Displaying battery capacity, low battery indication and automatic turn-off;

5.STEL/TWA Limits for Common Gases