Calculating STEL and TWA

Updated August 26, 2022

This article describes how TWA and STEL are calculated for G7c and G7x devices.

Time weighted average (TWA)

TWA is the average allowable amount of gas exposure during a given time interval. 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 cannot 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).

In both cases, G7 provides TWA based on second-by-second gas readings, with the overall calculation updated every 15 minutes.

Short term exposure limit (STEL)

STEL is the limit of a toxic gas concentration that you can be continuously exposed to for a short time interval without suffering adverse health effects.

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

G7 provides STEL based on second-by-second gas readings, with the overall calculation updated every minute.

STEL is calculated as follows:

Resetting STEL and TWA values on G7

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

• G7 is powered off.
• Configuration properties change. This includes a unit change, a gas type change, the sensor being disabled or re-enabled, or a STEL interval change.
• STEL and TWA are manually reset on the device.

NOTE: If you are likely to power cycle your device during your shift and your device is running firmware version 3.422 or higher, you can configure your device to retain TWA and STEL values through power cycles in Blackline Live. When this setting is turned on:

• If your device is off for less than 15 minutes, both STEL and TWA are retained.
• If your device is off for more than 15 minutes but less than 8 hours, TWA (and peak readings) are retained, and STEL is reset.
• If your device is off for more than 8 hours, STEL and TWA (and peak readings) are reset.

Resetting peak readings using 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
• SCBA

NOTE: STEL/TWA readings can be enabled in these secondary modes if desired. Contact the Blackline Safety Customer Care team for more information.

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.