Calculating STEL and TWA

Updated: June 18, 2020

Time weighted average (TWA)

Time weighted average (or TWA for short) is the average allowable amount of gas exposure during an eight-hour period.

There are three different regulatory bodies that have different ways of calculating TWA: OSHA, ACGIH and EH40. Depending on the regulatory body your organization follows, your TWA value may be calculated one of two ways.

OSHA: TWA is calculated based on an 8-hour moving window. The window is fixed at 8 hours and can not be changed in Blackline Live.

ACGIH/EH40: TWA is calculated by taking the total exposure divided by a fixed interval regardless of exposure time. TWA values can be customized in G7's configuration profile, where each individual gas has a customizable TWA period (between 4 and 16 hours).

Short term exposure limit (STEL)

Short term exposure limit (or STEL for short) is the limit of gas concentration that you can be continuously exposed to for a 15-minute time frame without suffering adverse health effects.

STEL values can be customized in G7's configuration profile, where each sensor has a customizable STEL period (between 5 and 15 minutes).

Resetting STEL and TWA values on G7

STEL/TWA values remain on the device and are also sent to the Blackline Live portal. These values are reset on G7 if:

  • G7 is powered off
  • G7 begins charging (unless G7's configuration has operational while charging enabled)
  • Configuration properties change. This includes a unit change, gas type change , the sensor being disabled or re-enabled or STEL period change.

Please note that resetting your peak readings from G7's gas options menu does not reset TWA or STEL.

STEL and TWA in configuration modes

As a default, STEL/TWA is paused when entering certain modes, and resumed when leaving them. These modes are: pre-entry, leak check and SCBA. STEL/TWA readings can be enabled in these secondary modes if desired, please contact our Customer Care team to do so.

 

SENSOR DRIFT AND NEGATIVE READINGS

Over time sensor readings can drift slightly causing negative values to be displayed. These negative values will not be applied to the TWA or STEL calculations. Instead they are zeroed out when applied to the calculation so that the false negative readings do not inappropriately lower the result.